A 11 10 3 07Db22

/

NBS SPECIAL PUBLICATION 526

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

QC

100

U57

NO. 526

1978
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

! Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
National Bureau of Standards OCT i <-> i^/dj

12th Informal Conference on Photochemistry

.—.

Proceedings of the

12th Informal Conference on Photochemistry

Held at the National Bureau of Standards

Gaithersburg, Maryland v

June 28-July 1, 1976

Edited by

Michael J. Kurylo

and

Walter Braun

Center for Thermodynamics and Molecular Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

Sponsored by

National Bureau of Standards Department of Transportation

Department of Energy

National Science Foundation

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1978
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901 . The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
v ?;;ticnal Bureau of Standards OCT i w> i^/jj

12th Informal Conference on Photochemistry

^

Proceedings of the

12th Informal Conference on Photochemistry

Held at the National Bureau of Standards

Gaithersburg, Maryland

June 28-July 1, 1976

Edited by

Michael J. Kurylo

and

Walter Braun

Center for Thermodynamics and Molecular Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

Sponsored by

National Bureau of Standards Department of Transportation

Department of Energy

National Science Foundation

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1978
Library of Congress Catalog Card Number: 78-600105

National Bureau of Standards Special Publication 526

Nat. Bur. Stand. (U.S.), Spec. Publ. 526, 411 pages (Oct. 1978) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402- Price $6 Stock Number 003-003-01984-4
ABSTRACT

The 12th Informal Conference on Photochemistry was held at the National Bureau of

Standards Laboratories in Gaithersburg, Maryland, from June 28-July 1, 1976. Nearly 300

members of the international scientific community attended the 123 individual research

papers presented. Extended abstracts of these presentations (many revised after the

Conference) comprise the present proceedings. The wide range of highly specialized topics

include Environmental Chemistry (both Photochemical Smog and Upper Atmosphere/Ozone Layer),

Laser Photochemistry, Photochemical Isotope Separation, Photochemical Conversion of Solar

Energy, Actinometric and Radiometric Measurements, Chemil uminescent Processes, Primary

Photochemical and Photophysical Processes, Inorganic Photochemistry, and Elementary Reaction Processes.

Key words: Chemical kinetics; energy transfer; fluorescence; lasers; photochemistry;

solar energy; spectra

111
CONTENTS

Section Page

Abstract iii

M. J. Kurylo and W. Braun

SESSION A: ACTINOMETRIC AND RADIOMETRIC MEASUREMENT

A Modern Approach to Accurate Radiometry 1 E. Zalewski

Temperature Dependence of the Fluorescence Quantum Yields of Rhodamine Derivatives . 3 R. E. Schwerzel and N. E. Klosterman

NBS Ultraviolet Radiometric Standards 5 W. R. Ott

New Methods of Measuring Light Intensities 8

D. G. Taylor, J. N. Demas, W. D. Bowman, E. W. Harris and R. P. McBrlde

Infrared Photography at 5 and 10 pm^ 11 G. F. Frazier, T. D. Wilkerson and J. M. Lindsay

SESSION B: CHEMILUMINESCENT PROCESSES

Kinetics of the Chemiluminescence Accompanying Metal Atom Oxidation Reactions ... 14

W. Felder and A. Fontijn

A Study of the Chemiluminescence of the Pb + 0^ Reactions 18

M. J. Kurylo, W. Braun, S. Abramowitz and M. Krauss

Chemiluminescence in Reactions of Ozone . 21 5. Toby, F. S. Toby and B. Kaduk

Dioxetane Chemistry in the Gas Phase UV-Visible Chemiluminescence from the

Reactions of 0

2

( 1 A ) with Olefins 24

D. J. Bogan and J? L. Durant, Jr.

SESSION C: PHOTOCHEMICAL ISOTOPE SEPARATION

Molecular Dissociation by High-Intensity Infrared Laser Radiation 31 K. H. Welge

Photochemistry of Formaldehydes: Past and Present 32

E. K. C. Lee, R. S. Lewis and R. G. Miller

Decomposition of Formic Add Vapor by Infrared Laser Radiation 35

R. Corkum, C. Willis, M. H. Back and R. A. Back

The Laser Augmented Decomposition of D^B Adducts 37

K. R. Chien and S. H. Bauer

Infrared Laser Isotope Separation 40

J. L. Lyman and S. D. Rockwood

C0

? TE Laser Induced Photochemical Enrichment of Carbon Isotopes 43

6. J. Ritter and S. M. Freund

Measurement of Mean Lives in Atomic Uranium 45

J. Z. Klose

i v
Principles of Photochemical Separation of Isomeric Nuclides 49

G. C. Baldwin, H. M. Clark, D. Hakala and R. R. Reeves

SESSION D: PRIMARY PHOTOCHEMICAL PROCESSES

(LARGE MOLECULES AND ENERGY TRANSFER)

The Early Years of Photochemistry in the Vacuum Ultraviolet 51 W. Groth

Fluorescence and Decomposition of Tertiary Amines in a Glow Discharge 53

F. Lahmani and R. Srinivasan

Photochemistry and Flash Photolysis of 5-Nitroquinol ine 55

A. C. Testa and A. Cu

Photochemistry and Photophysics of Nitrogen Heterocycles 57

M. S. Henry and M. Z. Hoffman

The Photophysics of Several Condensed Ring Heteroaromatic Compounds 60

F. S. Wettack, R. Klapthor, A. Shedd, M. Koeppe, K. Janda, P. Dwyer and K. Stratton

Molecular Weight Dependence of Triplet-Triplet Processes in Polyo(2-Vinylnaphthalene). 63

N. F. Pasch and S. E. Webber

Singlet and Triplet Precursors of 0

2 ^A 66

B. Stevens and J. A. Ors 9

The Heavy Atom Effect on the Photochemical Cycloaddition Processes of Acenaphthylene. 70

B. F. Plummer and L. J. Scott

SESSION E: PHOTOCHEMISTRY WITH LASERS

Laser Induced Fluorescence Emission Spectroscopy of H

? C0(A, A?

) 72

K. Y. Tang and E. K. C. Lee

c c

Laser Excited No

2 Fluorescence Lifetime Studies 1n the 600 NM Region 75

V. M. Donnelly and F. Kaufman

Laser Fluorescence Studies, including the B

3 n(0

+

) States of BrF, IF and I CI 77

M. A. A. Clyne, A. H. Curran and I. S. McDermid

Laser-Induced Fluorescence of CN Radicals Produced by Photodissociation of RCN ... 79

R. J. Cody, M. J. Sabety-Dzvonik

Photofragment Spectroscopy of the A-^-X Transition in CN Using Two Pulsed Lasers ... 82

A. Baronavski and J. R. McDonald

Spectroscopic and Photochemical Studies of Gaseous Ions Using Tunable Dye Lasers . . 86

J. R. Eyler

Photodissociation Spectra of Positive Ions with Time-Of-Fl ight Analysis 88

T. F. Thomas and J. F. Paulson

Vibrational States of Molecules 1n the Visible Region by Thermo-Optical

Spectroscopy and the Local Mode Model 92

A. C. Albrecht

Mechanism of High Intensity I.R. Photodecompos1t1on of Molecules 94

W. Braun and W. Tsang

v
SESSION F: ENVIRONMENTAL PHOTOCHEMISTRY (UPPER ATMOSPHERE)

The In-Situ Measurement of Atoms and Radicals in the Upper Atmosphere 99

J. G. Anderson

The oCs) Airglow - New Laboratory Results 102

T. Slanger and G. Black

A Quantum Yield Determination for 0(^D) Production from Ozone Via Laser Flash

Photolysis 105

D. L. Philen, R. T. Watson and D. D. Davis

Absence of N2O Photolysis in the Troposphere 108

D. H. Stedman, R. J, Cicerone, W. L. Chameides and R. B. Harvey

Temperature Dependence of 0(^D) Reactions of Atmospheric Importance Ill

J. A. Davidson and H. I. Schiff, G. E. Streit, A. L. Schmeltekopf and

C. J. Howard

Modeling Stratospheric Photochemistry and Kinetics 114

J. McAfee and P. J. Crutzen

The Rate Constant for 0 + NO + M from 217-500K in Five Heat Bath Gases 115

J. V. Michael, W. A. Payne and D. A. Whytock

Determination by the Phase Shift Technique of the Temperature Dependence of the

Reactions of 0( 3 P) with HCI, HBr, and HI 117

D. L. Singleton and R. J. Cvetanovic

SESSION G: INORGANIC PHOTOCHEMISTRY

High Energy Pulsed Laser Photolysis of Some Chromium (III) and

Cobalt (III) Complexes 120

A. W. Adamson, A. R. Gutierrez, R. E. Wright and R. T. Walters

Studies Using a Combination of Flash Photolysis and Pulsed Magnetic Induction:

Application to the NO3 Radical in Aqueous Acid Solution at 25°C 126

T. W. Martin and M. V. Stevens

Luminescence of Some Metal Trisdithioacetylacetonate Complexes 130

M. K. DeArmond and J. T. Merrill

Luminescent Transition Metal Complex Photosensitizers 134

J. N. Demas, J. W. Addington, R. P. McBride, W. E. Harris and S. Peterson

Picosecond Studies of Transition Metal Complexes 137

P. E. Hoggard, A. D. Kirk, G. B. Porter, M. G. Rockley and M. W. Windsor

Electron Transfer Properties of Excited States of Transition Metal Complexes .... 139

V. Balzani, F. Bolletta, M. Maestri, A. Juris and N. Serpone

2+

A Comparison of the Excited-State Electron-Transfer Reactions of RU(BIPY)j

and 0S(BIPY)3

+ 142

P. Fisher, E. Finkenberg and H. D. Gafney

vi
SESSION H: PRIMARY PHOTOCHEMICAL PROCESSES (SMALL MOLECULES)

Photodissociation of Simple Polyatomic Molecules 145

J. P. Simons

Energy Distribution in the Photodissociation of Methylketene at 215 NM 147

M. E. Umstead, R. G. Shortridge and M. C. Lin

! The Production and Reactions of Vibrationally Excited 1 ,1 ,2 ,2-Tetrachloroethane . . 150

M. H. J. Wijnen

Measurement of Branching Ratios for the 0 + CS

2 > OCS + S Reaction 152

R. E. Graham and D. Gutman

Photodissociation of Molecular Beams of Metallic Iodides 155

M. Kawasaki, H. Lltvak, S.-J. Lee and R. Bersohn

The Photochemistry of 2-Furaldehyde 1n the Gas Phase 157

A. Gandini, P. A. Hackett, J. M. Parsons and R. A. Back

Photoinitiated Decomposition of Monosilane 160

E. R. Austin and F. W. Lampe

Excitation of HNO by 0

2

(

] A

g

) 163

T. Ishiwata, H. Akimoto and I. Tanaka

Solar Photodissociation Rates for C10N0

2

; An Estimate of Current Levels

of C10N0

2 in the Stratosphere .... 7 167

J. P. Jesson, L. C. Glasgow and P. Meakin

The Tropospheric Lifetime of CFC1, 174

J. P. Jesson, P. Meakin and L. C. Glasgow

The Gas Kinetic and Photochemical Degradation of Chlorine Nitrate 176

A. R. Ravishankara, G. Smith, G. Tesi and D. D. Davis

SESSION I: PHOTOCHEMICAL CONVERSION OF SOLAR ENERGY

Photogal vanic Cells 177

M. D. Archer, M. I. C. Ferreira, W. J. Albery and W. R. Bowen

The Importance of Intermediate Partitioning in Energy Storing Photoreactions .... 180

G. Jones, II, W. R. Bergmark and M. Santhanam

Optimization of the Iron-Thionine Photogal vanic Cell; Photochemical Aspects .... 183

P. D. Wildes, N. N. Lichtin and M. Z. Hoffman

A Biomimetic Approach to Solar Energy Conversion 186

T. R. Janson and J. J. Katz

Light Energy Conversion via Photoredox Processes in Mlcellar Systems 189

M. Gra'tzel

Photoelectrolysis of Water by Solar Energy 191

. D. I. Tchernev

SESSION J: PHOTOPHYSICAL PROCESSES

Recent Work in Exciplex Photophysics 193

D. V. O'Connor and W. R. Ware

vii
Charge Transfer and Hydrogen Atom Transfer Reactions of Excited Aromatic

Hydrocarbon-Amine Systems 196

N. Mataga, T. Okada and T. Mori

Protolysis Equilibria of Lumichrome (6,7-Dimethyl-Alloxazine) in the Lowest

Excited Singlet State 199

B. Holmstrbm, S. Bergstrbm and H. B. Larson

Photochemistry of Pheophytins and Porphyrins 204

D. C. Brune, J. Fajer and S. P. Van

SESSION K: ENVIRONMENTAL PHOTOCHEMISTRY (PHOTOCHEMICAL SMOG)

A Kinetic Study of CH^ and (CH

3

) 3 C0

2 Radical Reactions by Kinetic

Flash Spectroscopy 212

M. R. Whitbeck, J. W. Bottenheim, S. Z. Levine and J. G. Calvert

The Reaction of OH Radicals with C^H, and CpH

4 215

R. Overend and G. Paraskevopoulos

On the Triplet State(s) of Sulphur Dioxide 219

J. W. Bottenheim, F. Su, D. L. Thorsell, J. G. Calvert and E. Damon

The S0

2

( B.j ) Photosensitized Isomerization of Cis and Trans-1 ,2-Dichloroethyl ene . . 224

F. B. Wampler and J. W. Bottenheim

Gas Phase Photochemical Synthesis of Peroxyacyl Nitrates via Chlorine-Aldehyde

Reaction 227

B. W. Gay, Jr., R. C. Noonan, J. J. Bufalini, P. L. Hanst

Chemical Control of Photochemical Smog 230

J. Heicklen

Photochemically Induced Free Radical Reactions in Nitrogen Dioxide-Acetaldehyde

Mixtures 233

E. R. Allen

Photo-Oxidation of Toluene-NO?-^-^ System in Gas Phase 243

H. Akimoto, M. Hoshino, G. Tnoue, M. Okuda and N. Washida

Effects of Solar Energy Distribution on Photochemical Smog Formation 247

G. Z. Whitten, J. P. Killus, H. Hogo and M. C. Dodge

SESSION L: PHOTOCHEMICAL AND PH0T0PHYSICAL PROCESSES (CONDENSED PHASES)

Photophysics of Bound and Dissociative States of Small Molecules in Condensed Phases. 251

L. E. Brus and V. E. Bondybey

Spectroscopy and Photochemistry of Matrix Isolated Metal Haxafluorides 253

W. F. Coleman, R. T. Paine, R. B. Lewis, R. S. McDowell, L. H. Jones and

L. B. Asprey

Mercury Photosensitized Production of Trapped Radicals in Organic Glasses at <77 K 256

N. Bremer, B. J. Brown, G. H. Morine and J. E. Willard

Magnetic Field Effects on Triplet-Triplet Annihilation in Crystals 259

S. H. Tedder and S. E. Webber

The Contribution of the Physical and Chemical Defects to the Photochemistry

of Crystalline Durene at Very Low Temperatures 262

A. Despres, V. Lejeune and E. Migirdicyan

viii
Photochemistry of Electrons Trapped in Organic Glasses 267

G. C. Dismukes, S. L. Hager, D. P. Lin, G. H. Morine and J. E. Willard

Spin Labels and the Mechanism of the S-j^T, Nonradiative Process in

Duraldehyde; Possible Manifestation of Pseudo Jahn-Teller Forces on Nonradiative Processes 271

A. Campion and M. A. El-Sayed

Photodesorption from Metals: Measured Desorption Rates in Comparison with a Mo-Treatment 274

N. Trappen

Matrix Isolation Spectroscopic Studies of Free Radicals and Molecular Ions Produced by Collisions of Molecules with Excited Argon Atoms 276

M. E. Jacox

SESSION M: ENVIRONMENTAL PHOTOCHEMISTRY (UPPER ATMOSPHERE)

Atmospheric Chemistry of Chlorofl uorocarbons 280

M. J. Molina

Ultraviolet Photcabsorption Cross-Sections of CFgC^ and CFCL^ as a Function

of Temperature 282

A. M. Bass and A. E. Ledford, Jr.

Photodissociation of CCl^ 285

R. E. Rebbert and P. Ausloos

The Photolysis of C£0

2 287

S. Jaffe and W. B. DeMore

Stratospheric Reactions of Chlorofluoromethanes . 290

R. J. Donovan, H. M. Gillespie, J. Wolfrum and K. Kaufmann

SESSION N: ENVIRONMENTAL PHOTOCHEMISTRY (PHOTOCHEMICAL SMOG)

Some Fundamental and Applied Aspects of the Atmospheric Reactivity of

Organic Molecules 294

J. N. Pitts, Jr., R. Atkinson, A. M. Winer, K. R. Darnall, A. C. Lloyd

and R. A. Perry

IR Fourier-Transform Spectroscopic Studies of Atmospheric Reactions 300

H. Niki, P. Maker, C. Savage and L. Breitenbach

Photochemically Generated Ozone from Isolated Strong Point Sources 303

D. D. Davis and W. Keifer

Evidence for Alkoxy Radical Isomerization in C^-Cg Alkanes in N0

x ~Air Systems . . . 305

W. P. L. Carter, K. R. Darnall, A. C. Lloyd, A. M. Winer and J. N. Pitts, Jr.

SESSION 0: PH0T0PHYSICAL PROCESSES

Collisional Destruction of Rovibronic Levels in S-| Glyoxal : Electronic,

Vibrational and Rotational State Changes 309

L. G. Anderson, A. E. W. Knight and C. S. Parmenter

The Vibronic Dependence of Glyoxal Photodissociation 311

G. H. Atkinson and C. G. Venkatesh

fx
Behavior of Benzene in Low Vibrational Levels 314

S. A. Lee, J. M. White and W. A. Noyes, Jr.

Dual Lifetime Fluorescence from Pyrimidine 316

K. G. Spears and M. El-Manguch

Time Resolved Emission Spectra of Low Pressure Aromatic Molecules 318

M. D. Swords, A. W. Sloman and D. P. Phillips

Studies in the Mechanism of Radiationl ess Conversion of Electronic Energy 321

T. A. Gregory and S. Lipsky

Photosensitization of the 2-Butenes by Benzaldehyde in the Gas Phase 323

A. J. Yarwood, G. R. De Mare and M. Termonia

SESSION P: ELEMENTARY REACTION PROCESSES (THERMAL SYSTEMS)

Near Infrared Detection of Peroxyl Radicals in Mercury Photosensitized Reactions . 326

H. E. Hunziker

Reactions of Hydrogen Atoms with Fluorinated Ketones 327

D. W. Grattan and K. 0. Kutschke

The Reaction of NH

2 with Olefins Studied by Flash Photolysis 331

R. Lesclaux and P. V. Khe

4

3 Detection and Reactions of NH(X Z ) Radicals in the Vacuum UV Flash Photolysis

of NHg Using Resonance Fluorescence 334

I. Hansen, K. Hoinghaus, C. Zetzsch and F. Stuhl

Interpretation of the Arrhenius Plots for Reactions of Oxygen Atoms with Olefins . 337

R. J. Cvetanovic

3 Absolute Rate of the Reaction of 0( P) with Hydrogen Sulphide 339

D. A. Whytock, R. B, Timmons, J. H. Lee, J. V. Michael, W. A. Payne and

L. J. Stlef

Kinetics of the Reaction: OH + N0

2 t+M) * HN03 (+M) Over a Wide Range

of Temperature and Pressure 342

C. Anastasi, I. W. M. Smith and R. Zellner

Vacuum UV Flash Photolysis of Phosphlne: Rate of the Reaction H + PH^ and

Implications for the Photochemistry of the Atmosphere of Jupiter 345

J. H. Lee, J. V. Michael, W. A. Payne, D. A. Whytock and L. J. Stlef

SESSION Q: ENERGY TRANSFER

2 ,2 Energy-Dependent Cross Sections for Quenching of Li ( P) and Na( P) 348

J. R. Barker, S.-M. Lin and R. E. Weston, Jr.

Electron1c-to-V1brational Energy Transfer Reactions: Na(3

2 P) + C0(X] S +

, v=0) ... 351

D. S. Y. Hsu and M. C. Lin

3 Energy Transfer in the Collision of Metastable Excited Ar P 2 Atoms with Ground

State H

2

S Atoms 354

P. B. Monkhouse, K. D. Bayes and M. A. A. Clyne

3 3 3 Quenching Rate Constants for Ar( P2

), Kr( P

2

) and Xe( P

2

) by Halogen-Containing

Molecules, and Branching Ratios for XeF and KrF Formation 359

J. E. Velazco, H. E. Kolts and D. W. Setser

x
V-V Energy Transfer in - Additive Gas Mixtures Using a Stimulated Raman

Excitation Technique 364

R. G. Miller and J. K. Hancock

SESSION R: ELEMENTARY REACTION PROCESSES

(EXCITED STATE DYNAMICS AND ENERGY TRANSFER)

Experiments Concerning the Laser Enhanced Reaction between O^t and NO 368

K.-K. Hui and T. A. Cool

Infrared Laser Enhanced Reactions: Chemistry of N0(v=l) with 0

3 370

J. C. Stephenson and S. M. Freund

Kinetic Energy - and Internal State Dependence of the NO + Oj -* N0

2 * + O2 Reaction. 373

A. E. Redpath and M. Mensinger

The Effect of Infrared Laser Excitation on Reaction Dynamics: 0 + C^H^^ and

0 + 0CS+ 376

R. G. Manning, W. Braun and M. J. Kurylo

Reaction of Flash Photolytlcal ly Produced CN(X

2

E +

, v) Radicals with 0(

3 P) Atoms . 380

K. J. Schmatjko and J. Wolfrum

Vibrational Photochemistry: The Relaxation of HCl(v=l) and DCl(v=l) by Bromine

Atoms 384

R. D. H. Brown, I. W. M. Smith and S. W. J. Van der Merwe

Vibrational Relaxation of HF(v = 1,2,3) in the Presence of H

? , N?> and C0? .... 388

J. F. Bott

Quenching of N0(B

2

ll r

) v

, =Q Produced by the Reaction of N(

2 D) with r^O ...... . 390

G. Black, R. L. Sharpless and T. G. Slanger

CW Infrared Laser Isotope Separation: CI + CH^Br (v

g

) + HC1 + C^Br 395

T. J. Manuccia, M. D. Clark and E. R. Lory

APPENDIX I

Author Index ..... 397

x1

A MODERN APPROACH TO ACCURATE RADIOMETRY

Edward Zalewski

National Bureau of Standards

Washington, DC 20234

The many recent advances in electro-optic technology have produced several new and very

powerful tools for accurate photon flux measurements. The two advancements that have the

greatest immediate impact on photochemistry are the electro-optic amplitude stabilization

of high-powered cw lasers and the electrically calibrated pyroelectric null radiometer. The

first has provided a high intensity radiant power source that is monochromatic, highly collmated and very stable—long-term drifts of less than 0.05 percent have been obtained. Such

amplitude stabilized lasers facilitate precise control of the number of photons entering a chemical reaction. The second advancement, the electrically calibrated pyroelectric null

radiometer, has brought a new order of accuracy, sensitivity and convenience to the measurement of radiant power, thereby facilitating the accurate measurement of photon flux.

The tremendous improvement in amplitude stabilization of cw lasers has been accomplished by using a stable solid state photodetector to monitor the laser beam and, in turn

control the transmittance of an electro-optic modulator. A block diagram of a laser amplitude stabilization system is shown in figure 1. The box labeled "beam uniformity optics"

refers to apertures, a spatial filter and a beam expanding telescope used to obtain a uniform photon density in the cross section of the beam. Such a beam shaping system does

result in a considerable loss of radiant power but is sometimes necessary to avoid saturation effects in certain experiments.

FEEDBACK CONTROL

SILICON MONITOR

0ETECT0R

BEAM

ELECTRO-OPTIC cw LASERS UNIFORMITY • MODULATOR

OPTICS BEAMSPLITTER

Figure 1: Block diagram of a cw laser stabilization system. The silicon monitor detector

supplies a sense signal to a feedback loop. This controls the transmittance of the

electrooptic modulator to reduce the fluctuations in the amplitude of the cw laser beam.

1
The fundamental principle of the electrically calibrated pyroelectric null radiometer

is illustrated in figure 2. The idea of electrically calibrating a thermal detector is not

new. The new features are the pyroelectric detector material and the ac null-balance

approach to the measurements. The pyroelectric, like a thermopile or a bolometer, is a thermal detector. However, it is an ac device rather than dc, responding to temperature

changes instead of the steady-state value. This property permits the use of synchronous

amplification, increasing the signal-to-noise of the measurements, and consequently improving the sensitivity over that of previous types of electrically calibrated detector

radiometers. It also allows the electrical heating pulses to be applied to the detector

alternately with the radiant heating pulses. Alternate pulses in the output signal are electronically inverted and amplifed by a wide-band, lock-in amplifier. The amplitude of

the electrical heating pulses are adjusted to match the effect of the radiant heating

pulses in order to obtain a zero value for the dc component of the amplifer output. That

is, when the null conditon is reached the areas under the electrical and radiant heating

pul ses are equal

GOLD BLACK SURFACE PYROELECTRIC

ELECTRICAL INPUT

Figure 2. Schematic diagram of the principle of operation of the electrically calibrated

pyroelectric null radiomater. The pyroelectric material is alternately heated by radiant

and electrical input pulses. The areas under the two types of pulses in the resulting wave train are equal when the heating from the two modes is equivalent.

The question of whether the electrical and radiant heating modes in the detector are really equivalent can be ansered without recourse to blackbody measurements. From a study

of the physics of these detectors it is found that several types of inequivalences occur

between the two different heating modes. From the physics, one also obtains the methods

whereby the magnitudes of the inequivalences can be evaluated in auxiliary experiments.

With a knowledge of the magnitudes of these effects, it is then possible to apply the

appropriate correction factors to assure the accuracy of the measurement. With presently

existing electrically calibrated pyroelectric null radiometers the best accuracies obtainable

are between 0.5 and 1 percent. This is over a range of power from 10 yW to 100 mW and a wavelength range from 200 to 10,000 nm.

2
TEMPERATURE DEPENDENCE OF THE FLUORESCENCE QUANTUM YIELDS OF RHODAMINE DERIVATIVES

Robert E. Schwerzel and Nancy E. Klosterman

Battel le Columbus Laboratories

505 King Avenue

Columbus, OH 43201

The measurement of absolute fluorescence quantum yields is often a difficult and

tedious task which can pose a number of potential sources of error [1.2] 1

. While the

fluorescence yields of powdered samples or monolayer assemblies can best be obtained by

absolute measurements (and several relatively straghtforward procedures have recently been

developed for this purpose [1-4], the fluorescence yields of solutions are most easily

obtained by relative measurements in which the fluorescence intensity of the sample solution is compared to that of a standard solution. The convenience and widespread popularity

! of the relative measurement technique make it essential that fluorescence standards be well

characterized, both in terms of purity and fluorescence quantum yield under the experimental

conditions used. This paper examines the fluorescence behavior of one commonly used fluorescence standard, rhodamine B, and two closely related derivatives, rhodamine 6G and rhodamine 101

.

Rhodamine B Rhodamine 6G Rhodamine 101

Rhodamine B is an attractive candidate for use as a fluorescence standard for several

reasons. It has a constant quantum yield over a wide wavelength range and is not quenched

by oxygen [5]. Further, it strongly absorbs the mercury 546 nm line, making it particularly

convenient for measurements in the yellow and red portions of the visible spectrum.

The quantum yield of rhodamine B is poorly defined, however, with reported values

ranging from 0.69 [6] to 0.97 [5]. Some of this spread may result from calibration errors

[1]. It appears likely, however, that an additional contributing factor is the change of

the quantum yield as a function of temperature and solvent viscosity [7-9]. Drexhage [9]

has reported that the fluorescence quantum yield of rhodamine B in ethanol is approximately

0.40 at 25 C, and that the value increases to nearly 1.00 when the temperature is lowered.

Conversely, the quantum yield drops to only a few percent when the solution is heated to 78

C. This behavior has been explained by an activated radiationless decay mechanism involving the hindered rotation of the diethylamino groups. This view is supported by the lower

temperature dependence of rhodamine 6G and the even lower temperature dependence of rho-

• damine

i | Figures in brackets indicate the literature references at the end of this paper.
101 (which has a quantum yield of nearly 1.00 over a wide temperature range) [9]. Our

results are generally consistent with those of Drexhage. The paper will elaborate on these

results and their relevance to the selection of a suitable yellow-red fluoresence quantum

yield standard.

References

[ 1] Demas, J. N., and Crosby, G. A., J. Phys. Chem., 75, 991 (1971) references cited

within.

[ 2] Billon, D., Laine-Boszdrmenyi , M., and Fallot, P., Meures Du Ren dement Quantique De Fluorescence Des Poudres Et Des Solutions Au Moyen De Spectrofl uorimetres Associes Ou

Non A Une Sphere Integrante, Commissariat A L'Energie Atomique Rapport CEA-R-4466,

July 1973 (Available from the Service de Documentation, C.E .N .-Saclay , B.P. n° 2, 91-

GIF-sur-YVETTE, France).

[ 3] Wrighton, M. S., Ginley, D. S., and Morse, D. L. , J. Phys. Chem., 78, 2229 (1974).

[ 4] Mobius, D., and Debuch, G., Chem. Phys. Lett., 28, 17 (1974).

[ 5] Weber, G., and Teale, F. W. J., Trans. Faraday Soc, 53, 646 (1957); 54, 640 (1958).

[ 6] Parker, C. A., and Rees, W. T., Analyst (London), 85, 587 (1960).

[ 7] Huth, B. G., Farmer, G. I., and Kagan, M. R., J". Appl. Phys., 40, 5145 (1969).

[ 8] Drexhage, K. H. , Laser Focus, 9 (3), 35 (1973).

[ 9] Drexhage, K. H., in Dye Lasers, Schafer, F. P., ed., Springer-Verlag, New York, 1973,

Chapter 4.

4
NBS ULTRAVIOLET RADIOMETRIC STANDARDS

W. R. Ott

National Bureau of Standards

Washington, DC 20234

1. Introduction

In the wavelength region between 5 and 400 nm, the National Bureau of Standards

offers a variety of standard sources and detectors which may be used to calibrate the

radiant power of unknown sources or the response of radiation detectors and spectral

radiometers. The energy range corresponding to these wavelengths spans from about 3 eV to 300 eV. This energetic radiation is of great interest not only in basic photochemistry

research, e.g., in photo-absorption and fluorescence studies, but also in industrial applications of photochemistry and and photobiology. For example, extreme doses of ultraviolet

radiation can cause skin cancer; controlled doses can heal. Continuous exposure to ultraviolet radiation results in the fading of paints and dyes; controlled exposures in specific

processes are used to produce protective coatings on materials. Some documented examples

of the various applications of ultraviolet radiation as both a natural and artificial

element in our home and work environment are shown in table 1.

Table 1. List of applications requiring ultraviolet radiation measurements

Photochemistry Fusion Research

Thin film production

Tooth decay prevention

Textile dyes

Paint curing

Industrial finishes

Cast hardening

Instant oscillographs

Material degradation

UV photosensitive paper

Counterfeit money detection

Communications

Astrophysics

Skylab

Space shuttle

Reentry and rocket exhaust

Space Science

National controlled thermonuclear

fusion efforts

Bacteriological VUV and X-Ray Lasers

Germicidal lamps in hospitals,

schools, and offices

Germicidal lamps in industry

Integrated circuits

Plasma probes

Isotope separation

Molecular synthesis

High resolution holography

Tumor therapy

Laser fusion

Environmental Studies

Atmospheric sciences

Ecology and the ozone layer

Oil spill identification

Water purification

Smog gauge NBS benchmark experiments

Photobiology

5
Medical and Therapeutic Plasma Chemistry

Jaundice treatments

Calcium deficiency

Skin disease treatments

Wrinkl ing

Drug detection

Thin film deposition

Plasma arc steel furnace

Recycling of alloys in steel

production

2. Radiometric Quantities

In general, there are two ways to determine the radiant power of an unknown light

source: (a) through the use of standard sources, and (b) through the use of standard de- tectors. Standard sources are most useful when it is desired to know the emission characteristics of an unknown source. Standard detectors are most useful, on the other hand,

when it is desired to know the radiant power at the location of a detector. For example,

if the quantity of interest is spectral radiance [watts cm~ 2 nm" 1 sr~ 1 ], that is, if one is

concerned with the power radiated by a specified emitting surface (cm 2

) in a certain wavelength band (nm) at a given solid angle (sr), then the light source to be investigated as well as the standard source may be set up in such a manner that the radiation from both

passes through the same optical-spectrometric arrangement, thereby eliminating all geometric and other specific factors of the instrumentation. In short the calibration is effected by a simple substitution of sources in the experimental arrangement. On the other

hand, if the quantity of interest is spectral irradiance [watts cm~ 2 nm that is, if one

is concerned with the radiant power incident on a specified surface area (cm 2

) in a certain

wavelength band (nm), then ideally a standard detector and filter arrangement is placed at the location of the irradiated surface to measure the power. Altenrati vely, if such a standard radiometer is not available, a standard source of spectral irradiance may be used

in conjunction with a suitable diffusing element (to account for variations in the geometries of the standard and the unknown sources) to determine the response function of the

user's diffuser- radiometer system.

At NBS, standard sources of both ultraviolet spectral radiance and spectral irradiance are available. In the case of NBS standard detectors for the ultraviolet, the

calibrated quantity is the absolute quantum efficiency [photoelectrons per incident photon]

as a function of wavelength.

The three primary source standards being used at NBS are the gold point blackbody

cavity for which the spectral radiance is given by Planck's law, the wall -stabilized

hydrogen arc for which the spectral radiance is given by accurately known quantum mechanical absorption coefficients for atomic hydrogen, and the electron storage ring facility for

which the spectral radiance is given by the theory of electron synchrotron radiation. The

selection of one of these standards for a calibration application is influenced mostly by

the specified wavelength region.

The primary standards are most often used to calibrate secondary or transfer stan- dards of spectral radiance which can then be issued to customers. The secondary radiance

standards are also used to generate spectral irradiance standards. The following ultraviolet standard sources, listed in order of decreasing wavelength, are available from NBS: the tungsten filament quartz-halogen lamp (above 250 nm); the tungsten strip lamp (above

225 nm); the low pressure mercury vapor lamp (253.7 nm); the deuterium arc lamp (165-

350 nm); the argon "mini-arc" (115-400 nm); and the synchrotron radiation source, SURF-II

(5-400 nm). The relative strengths and limitations of these radiometric standards with

respect to accuracy, reliability, convenience, and intensity and wavelength range will be

discussed in the presented paper.

3. Standard Sources

6
4. Standard Detectors

The primary standard for ultraviolet detector calibrations at NBS is a double-ionization chamber, a gas-filled detector in which each photon absorbed produces one electron-ion

pair which is collected by a simple arrangement of parallel plates used to set up the

collecting field. The transfer to the actual photodiodes, which are available to customers,

is accomplished through the use of a uniformly grey (percent of radiation absorbed is independent of wavelength) thermopile, whose efficiency has been calibrated with the ionization chamber at short wavelengths and checked with a spectral irradiance standard source at 253.7 nm. Windowed photodiodes calibrated in this manner have been used to evaluate the

response of other detectors, such as photomul itpl iers , and the response of radiometers,

such as "hazard meters", which are designed to have a spectral response equivalent to a specified erythemal curve. Calibrated photodiodes are available throught the wavelength

region 20-320 nm.

7
NEW METHODS OF MEASURING LIGHT INTENSITIES

D. G. Taylor, J. N. Demas, W. D. Bowman, E. W. Harris and R. P. McBride

Chemistry Department

University of Virginia

Charlottesville, VA 22901

An inexpensive, easily constructed large area bolometer has been developed. The

proto-type has a 1 inch x 1 inch sensitive surface, but smaller sizes could be easily built

to give greater sensitivity and faster response times. The 1 inch x 1 inch version has a maximum deviation of sensitivity of 3 percent from the center to the edge and is considerably more uniform over the central 3/4 inch diameter. Sensitivity is ^ 5 yW (< 1 uW/cm 2

) with an ^ 45 second time constant. The detector uses a positive temperature coefficient

thermistor in a dc bridge; it is mounted on the back of a carefully designed copper plate

which smooths out temperature anisotropy on the sensing plate before detection. To minimize drift, the detector is held in an insulated massive aluminum head. The aluminum head

is stabilized to ^ 100 p K by means of a solid state proportional controller. Although our current version is only a relative bolometer, we have also made an absolute version using a manganin resistance wire calibration heater. A better arrangement would be to use a distributive heater over the entire surface of the plate, and it could be easily fabricated.

A simple, inexpensive but extremely precise luminescence quantum counter comparator

has been developed. The system gives part-per-thousand reproducibility and accuracy for

the intercomparison of optically dense quantum counters. It can easily detect the small,

but significant variations in spectral sensitivity between rhodamine B under the following

conditions: 8 g/1 in ethylene glycol; 8 g/1 , 5 g/1 and 2 g/1 in methanol. We have also

begun calibration of nile blue A and methylene blue. These dyes promise to give spectral

sensitivity to ^ 700 nm, although in the 340-590 nm region they are not as flat in response

as rhodamine B. We have calibrated rhodamine B (5 g/1) in methanol against our large area

bolometer. The absolute sensitivities of a number of systems are given in table 1; the

data are references to the large area bolometer via the 5 g/1 rhodamine B calibration. The

relative

Table 1. Absolute spectral sensitivity of quantum counters

3

Rhodamine B Rhodamine B Methylene Blue Nile Blui

5 g/l 8 g/1 1 g/l 2 g/

ethylene MeOH MeOH MeOH

glycol

, nm

590 1.001 .973 .983 *1.010

.999 1.001 .992 1.012

570 .996 1 .005 .996 1.019

1.000 1.010 .997 1 .028

550 .996 .999 .994 1.021

1.000 1 .002 .992 1.014

530 .990 .995 .987 1.003

.999 1.000 .996 1.001

8
Table 1. Absolute spectral sensitivity of quantum counters

3 (continued)

Rhodamine B Rhodamine B Methylene Blue Nile Blue A

5 g/1 8 g/1 1 g/1 2 g/1

ethylene MeOH MeOH MeOH

glycol

X nm

510 1 .001 .999 1.002 .988

.998 .997 1 .005 .970

490 1 .007 1 .005 1 .016 .963

1 .015 1 .008 1 .023 .957

470 1 .016 1 .001 1 .024 .948

1 .006 .996 1 .029 .970

450 1 .003 .982 1 .042 1 .002

1 .002 .982 1 .053 1.008

430 1 .005 .996 1 .053 1 .016

1,.012 1 .001 .984 1.015

410 1 .007 .999 .921 1.019

1 .004 1 .000 1.006 1.011

390 .999 .991 .991 1 .002

.987 .980 .980 .988

370 .989 .982 .982 1 .001 .977 .980 .965 1 .000

a Relative to bolometer, measured via rhodamine B (5 g/1) MeOH calibrated counter.

sensitivities of the quantum counter systems are given in table 2; the detector was an RCA

7164R equipped with a Corning C.S. 2-62 filter.

Table 2. Relative sensitivity of quantum counters

3 '*5

Rhodamine B (2 g/1) MeOH 1

,71 Rhodamine B (5 g/1) MeOH 1

,00

3

Rhodamine B (8 g/1 ) MeOH 0. 49

Rhodamine B (5 g/1) Ethylene Glycol 0. 92

Rhodamine B (8 g/1) Ethylene Glycol 0. 61 Rhodamine 6G (5 g/1) MeOH 1. 67

Methylene Blue (1 g/1) MeOH 0. 064

Nile Blue A (2 g/1) MeOH 0. 15

a Mean sensitivity 360-590 nm compared to standard (Rhodamine B (5 g/1) MeOH) counter.

^All counters viewed from rear.
Based on our photooxygenation work with transition metal complexes [1.2] 1 we have

developed the first chemical actinometer specifically designed for use with high power

laser sources [3,4]. The system is the Ru(bipy) 3 2+ (bipy = 2,2*-bipyridine) photooxygenation of tetramethyl ethylene (TME) in methanol. The key reactions are

D hv *D (1)

*D + 0, D + V (2)

1 L OOH

0

2 + (CH

3

) 2 C=C(CH

3

) 2

y CH

2 =C(CH3 )C (CH

3

) 2 (3)

where D is Ru(bipy) 3 2 + and l 0 2 is singlet oxygen. The net effect of absorption of radiation by by Ru(bipy) 3 2 is consumption of 02 . The actinometer system is closed and 0 2 consumption is monitored on a gas buret.

The effective quantum yield for oxygen uptake, Vg^* is given by

k sv[o

2

] TtmeT

0'.

c = 0.855 bV t (4)

° bS

1 + K sv [0

2

] [TME] + 6

where K SY is the Stern-Volmer quenc hing cons tant for 0 2 quenching of *Ru(bipy) 3 2 and

equals 1400 M" 1

, e = 0.0027 M, and [0 2 ] and [TME] are the average 0 2 and TME concentrations

during the photolysis. The actinometer is intrinsically quantum flat and usable for

x <_ ^ 520 nm, which makes it ideal for use with the ionized Ar laser lines.

An apparently previously unreported artifact with the ferrioxalate actinometer has

been discovered [5]. With aged phenanthrol ine developing reagent and following the re- commended procedure, intensity errors of -40 to + 20 percent can result if the phenanthrol ine is added before the buffer. If the buffer is added before the phenanthrol ine, the errors are smaller but can still be significant 5 percent). If fresh (<_ 3 weeks)

phenanthrol ine solutions are used, however, development is instantaneous (< 5 min) and

independent of the order of addition of reagents.

A preliminary calibration of the 0.15 F ferrioxalate at the 458 nm ionized Ar laser

line indicates that the true yield may be * 10 percent lower (0.86) than the accepted one

(y 0.96) [6]. A cooperative effort now in progress with the NBS (Gaithersburg) radiometry

group will supply a definitive answer to this question by the time of the conference.

References

[ 1] Demas, J. N., Harris, E. W., Flynn, C. M., Jr., and Diemente, D., J. Amer. Chem.

Soc, 97, 3838 (1975).

[ 2] Demas, J. N., Harris, E. W., and McBride, R. P., in preparation.

[ 3] Demas, J. N., Harris, E. W., and McBride, R. P., Lasers in Physical Chemistry and

Biophysics, p. 477 [Elsevier, 1975).

[ 4] Demas, J. N. , McBride, R. p., and Harris, E. W., submitted for publication.

[ 5] Bowman, W. D. , and Demas, J. N., submitted for publication.

[ 6] Hatchard, C. G. , and Parker, C. A., Proc. Roy. Soc. (London), 235A , 518 (1956).

T Figures in brackets indicate literature references at the end of this paper.

10
INFRARED PHOTOGRAPHY AT 5 AND 10 ym

Gene F. Frazier, T. D. Wilkerson 2 and J. M. Lindsay 3

Versar Incorporated

Springfield, VA 22151

A new technique for infrared photography is described involving the sensitization

and desensitization of silver halide films by IR irradiation. Since a visible exposure

follows the IR exposure, the time order is opposite to the well known Herschel effect.

Also, we record images at much longer wavelengths, 5 and 10 ym, even though the films are conventional commercial types such as Polaroid 55 P/N. This film is particularly useful

for quantitative densitometry, some of which is reported here. For the image power density

range 0.1-1 W/cm 2

, IR exposure times are about 10-1 second, respectively. CO and C0 2 lasers (cw) have been the principal IR light sources, and the visible flash is variable

(.001-1 s) and is chosen to fog the film to a modest level of density. Exposure times of

1/10-1/100 second are possible at higher power density, opening up the possibility of

inexpensive IR cinematography of the transient behavior of high power laser mode patterns.

Exposure time sequences are indicated in figure 1. The long IR exposure (a) yields

desensitization of a negative and blackened print. The short IR exposure (b) can give the

opposite or the same effects, depending, for example, on the color of the visible flash V. A delay of order 5 seconds or more (c) yields no IR effect on the latent image subsequently

formed by V.

1

(a)

(IR)

to

(V)

(b) (IR)

*0

(V)

(c) (IR) 5

"SEC" (V)

Resarch supported by AF0SR under Contract No. F44620-72-C-0076.

2 Permanent address: Institute for Fluid Dynamics and Applied Mathematics, University of

Maryland, College Park, MD 20742.

3 Permanent address: Chesapeake Biological Laboratory, Solomons, MD 20668.

11
Figure 2 shows a type of characteristic curve (2 cases) for which IR exposure is

varied by changing its duration at constant power density (0.3 W/cm 2

). Curve (a) shows

only desensitization, relative to the imposed visible fogging, when the visible light is

green (0.56-0.57 pm). Curve (b) for white light shows reversal from sensitization to desensitization as IR exposure is increased.

IR exposure (sec)

2 4 6 8 10 12 -0.3

-0.2

-O.I

Dr 0

09 iZ 1.4 1.5 15 1.7 Log It

In ongoing work, we want to develop a better understanding of the mechanisms of IR influence on the latent image and, if possible, improve the sensitivity and dynamic range

of the IR response. Two main lines of inquiry seem to be the most promising: (A) better

delineating the interaction between IR radiation and the gelatin {i.e. non-silver halide

materials) contained in the emulsion, and (B) estimating the net influence of IR irradiation on the drift mobility and lifetimes of photogenerated species within the grains.

Progress on these topics will be discussed in the context of prior work [1-4] 4

.

T+ Figures in brackets indicate literature references at the end of this paper.

12
References

[ 1] Malinowski, J., Photogr. Soi. Eng., 14, 112 (1970).

[ 2] Brown, F. C, and Kobayashi, K., J. Phys. Chem. Solids, 8, 300 (1959).

[ 3] Wei, J. S., and Brown, F. C, Photogr. Soi. Eng., T7, 197 (1973).

[ 4] Babcock, T. A., Lewis, W. C, McCue, P. A., and James, T. H., Photogr. Soi. Eng.,

16, 104 (1972).

13
KINETICS OF THE CHEMILUMINESCENCE ACCOMPANYING METAL ATOM OXIDATION REACTIONS

William Felder and Arthur Fontijn

AeroChem Research Laboratories, Inc. P.O. Box 12 Princeton, NJ 08540

The search for electronic transition chemical lasers has led to major interst in the

chemiluminescence accompanying many metal atom oxidation reactions. Since most of the

metals of potential interest are highly refractory, the AeroChem high-temperature fastflow reactors (HTFFR) (see, e.g., references [1-3] 1 ), which allow for independent control

of temperatures, total pressure, reactant and quencher concentration, are uniquely suited

to the study of their kinetics. These reactors are designed for study of metal atom and

metal monoxide species over the 300 to near 2000 K temperature range. Independent control

of heating zones allows a high temperature [e.g., ^ 1700 K for Sn) source section to be

combined with a reaction zone which can be at any desired temperature within this range.

Here, we discuss excited state formation and quenching kinetics in the Sn/N 2 0 system.

1. Experimental

Figure 1 shows the HTFFR used in this work which was modified to cover the intermediate temperature 400 - ^ 1400 K) range; it consists of three sections, an upstream ^

30 cm.

Figure 1. HTFFR for kinetic studies in the intermediate {aa 400 - 1400 K) temperature

range.

long heated alumina flow tube source section, a short temperature transition region tube

and a reaction zone section which is a ^ 30 cm long heated alumina flow tube. Sn is vaporized from a separately heated source placed in the upstream end of the reactor. The vapor

is entrained in a bath gas flow and transported to the reaction zone where the second rei Figures in brackets indicate literature references at the end of this paper.

14
agent (e.g., N 20) is added through a traversable inlet (fig. 1). Fixed viewing ports at

the downstream end of the reaction zone allow optical observations [3,4] of chemi 1 lumin- escence intensity and spectral distribution and of [Sn] as functions of the reaction time

(flow velocity and distance of inlet from observation port), [N

? 0], total pressure, and

temperature. Figure 2 shows the optical layout and a cross-section of the reactor in the

observation plane.

DETECTOR C

Figure 2. Optical arrangement and cross-section of HTFFR. Detectors A and B are spectrometers used for metal atom absorption and chemi luminescence spectral measurements,

respectively. Detector C is a PMT filter combination for chemi luminescence intensity

and titration measurements. Reaction tube 2.5 cm i.d., reaction tube observation ports

1 cm diam.

Photon yields are obtained by measuring: (i) the absolute chemi 1 uminescence intensity, integrated both over wavelength and the extent of the glow in the flow tube, and

(ii) the absolute [Sn]. Absolute wavelength-dependent intensity is determined by calibrating the detection system against the 0/NO glow [5] and integration is accomplished by

traversing the N 2 0 inlet to allow construction of intensity vs. distance (time in the flow

tube] plots, the area of which gives the integrated intensity. Absolute [Sn] is calculated from absorp-tion measurements at x = 286.3 nm using a hollow cathode lamp and the best

literature value [6] for the absorption f-number. Experimentally, this measurement is

limited to 3 x 10 10 < [Sn] < 3 x 10 11 ml" 1 where the absorption intensity can be accurately

measured; the calculated [Sn] abs(olute) are probably accurate to within a factor of ^2.

Work is in progress to increase the accuracy of these [Sn] determinations by a titration

technique using N0 2 as titrant and N 20 as chemi 1 uminescence indicator. Such titrations,

at the >_ 10 m s

~

1 flow velocities required for Sn transport, demand [Sn] > 10 13 ml" 1

. We

have already successfully titrated [Sn] in the 10 13 - 10 14 ml" 1 range as part, of the Sn quenching measurements.

The overall rate coefficient of the Sn/N 2 0 reaction is obtained from 286.3 nm absorption measurements of the rate of change of the relative [Sn] as a function of re- action time and [N 2 Q] using standard techniques which are reviewed in detail in references

[2] and [3]. Quenching rate coefficients are obtained from the observed variations in

hoton yield as a function of quencher concentrations via Stern-Volmer type analyses.

uenching by reagents is a problem inherent to excited species produced by chemi-excitation

and required the development of methods suitable for the corresponding rate coefficient

measurements, which will be briefly described.

15
2. Results

The reaction

Sn + N

2 0 SnO (all states) + (1)

has been found to be strongly temperature dependent. A cursory determination yielded ki = (1.0 ^ 4) x 10" 11 exp[(-3000 + 1000/T] ml molecule" 1 s" 1 over the 500-390 K range. In this

range, only SnO (a-X) and (A-X) band system emission could be definitely established. The

total photon yield, $, defined as the fraction of Sn oxidation events that leads to emission of a photon, increases by a factor of only ^ 1.25 between 500-930 K, i.e., from ^ 0.54 to 0.69. This increase is essentially all due to an increase in the A-X system

intensity ($(A-X) increases from 0.04 to 0.16 over this range), while $(a-X) remains

constant at ^ 0.5. The rate coefficient for photon emission --defined by dh

v/dt = k nv [Sn]

[No0]--i s a strong function of T, since khv(T) = k 1 (T)$(T). The large $(a-X) makes SnO

(a^z) particularly attractive as a candidate for the upper state in a potential electronic

transition chemical laser.

Quenching experiments have concentrated on Sn0(a 3 i). The following upper limit rate coefficients (based on the estimate [7] radiative lifetime of ^ 1 x 10~ 3 s) were measured:

k^

11 < 1 .2 x 10" 12

;

k N2 < 0.6 x 10 16

N

20

QAr

Q

< 1 x 10 -14.

< 1 .0 x 10

-16

in ml molecule"^" 1 units.

3. Discussion

The magnitudes of the observed rate coefficients differ strongly from those observed

in several other simple metal atom oxidation reaction systems, e.g., Ba/N 20 [8-12]. Probabl reasons for these differences will be discussed. It will be shown that these differences

make Sn/N 2 0 a very attractive electronic transition chemical laser candidate system.

Work sponsored by the Defense Advanced Research Projects Agency (ARPA) and monitored

by the Air Force Weapons Laboratory, Air Force Systems Command, United States Air Force,

Kirtland Air Force Base, New Mexico 87117.

References

[ 1] Fontijn, A., Kurzius, S. C, Houghton, J. J., and Emerson, J. A., Rev. Sci. Instr.,

43, 726 (1971).

[ 2] Fontijn, A., Felder, W., and Houghton, J. J., Fifteenth Symposium (International) on

Combustion, p. 775 (The Combustion Institute, Pittsburgh, 1975).

[ 3] Fontijn, A., AIM Paper 76-131 (January 1976).

[ 4] Felder, W., and Fontijn, A., Chem. Phys. Lett., 34, 398 (1975).

[ 5] Fontijn, A., Meyer, C. B., and Schiff, H. I., J. Chem. phys., 40, 64 (1964).

[ 6] DeZafra, R. L., and Marshall, A., Phys. Rev., A170, 28 (1968).

16
[ 7] Meyer, B., Smith, J. J., and Spitzer, K., J. Chem. Phys., 53, 3616 (1970).

[ 8] Jonah, C. D. , Zare, R. N. , and Ottinger, Ch., J. Chem. Phys. y 56, 263 (1972).

[ 9] Jones, C. R. , and Broida, H. P., J. Chem. Phys.y 60, 4369 (1974).

[10] Palmer, H. B., Krugh, W. D., and Hsu, C. J., Fifteenth Symposium (International) on

Combustion, p. 951 (The Combustion Institute, Pittsburgh, 1975).

17
A STUDY OF THE CHEMILUMINESCENCE OF THE Pb + 0

3 REACTIONS

M. J. Kurylo, W. Braun, S. Abramowitz , and M. Krauss

National Bureau of Standards

Washington, DC 20234

1. Introduction

Recent studies on laser enchanced reactions have shown that vibrational energy in a reactant triatomic molecule can appear as excitation in a product triatomic molecular

species. Because of the spectroscopic complexities of triatomic molecules, we are investigating a chemi luminescent reaction producing a diatomic product. The metal atom-oxidant

systems represent a class of such reactions which have been investigated because of their

potential as chemical lasers. We report herein some observations on the reaction

Pb + 0

3 - Pb0

2

. (1

While the information obtained from studying the infrared laser enchanced reaction com- ponent is minimized by the overall reaction complexity, it does nevertheless provide some additional insight into this reaction system. This information coupled with new high

pressure spectroscopic results complement the detailed low pressure investigation by

Oldenborg, Dickson, and Zare (ODZ).

2. Experimental

The furnace, reaction cell, fast flow pumping system, C0 2 laser, and spectrometer

are shown schematically in figure 1. Lead vapor, produced from a resistively heated

crucible

-Figure 1. Schematic of the chemi luminescent flow reaction apparatus.

18
containing lead metal, enters the glass reaction chamber in an Ar diluent stream. The 4 percent 0 3 in 0 2 effluent from a commercial 0 3 generator is mixed with Ar and flows past

the cell windows (to eliminate window deposits) into the cell. There it diffusively mixes

with the lead-argon flow. The temperature of the crucible (as measured by a thermocouple

probe) ranges from 900-1000 K while the temperature in the flame reaction zone varies from

500-600 K. The total pressure in the cell varies from 1 to 5 torr with the flow through

the furnace being anywhere from 20 to 50 percent of the total flow.

The 0.5 cm diameter beam from a C0 2 laser tuned to the 9.6 pm P(30) transition is square wave chopped and traverses the flame exciting v 3 , the asymmetric stretching mode of

0 3 (1 043 cm" 1 )- The chemi luminescence from the Pb + 0 3 reaction is monitored through a spectrometer-photomultipl ier assembly. The photomultiplier output is fed thorugh a series

of pulse amplifiers and voltage discriminator into a dual counter, one channel of which

records the "laser-on" signal and the other the "laser-off". A printout from these

counters is synchronized with the spectrometer wavelength-scan-drive thereby facilitating

the recording of the modulation spectrum vs wavelength. The normal spectra ("laser-off")

emission is automatically obtained from the "laser-off" counter.

3. Results

In contrast to the spatially sharp diffusion flame observed for the Ba + 0 3 (or N 2 0)

reaction, the Pb - 0 3 flame is quite diffuse (apparently reaction limited). It is brightest in the high temperature zone at the furnace nozzle, diminishes with decreasing temperature (increasing distance from the nozzle), and persists for some time two or three feet

into the pumping system. These observations suggest a reaction rate for Pb + 0 3 which is considerably slower than gas kinetic. The spectrum which we observe at several torr total

pressure partly resembles the low pressure spectrum obtained by 0DZ and reproduced by us

in a similar quasi-beam apparatus. The short wavelength end of the more complete spectrum

(fig. 2) can be identified from the low (submicron) pressure spectrum in that it is less

diffuse. The long wavelength portion between 480 nm and 595 nm is almost entirely the new

state recently characterized by QDZ. At still longer wavelengths, our spectrum differs

from that of 0DZ in that we observe a series of strong lines which do not agree with a >

X, A X, or B -> X, but rather appear to originate from a new state observed via six weak

lines by 0DZ and identified as b by them. This b state is seen more intensely in our high

pressure spectrum. This series of lines in our spectrum can be fit to the expression:

V. v")

= 16315 + 441.0 v' - [717.7 3.53 v"]

where the lower state constants have been taken from Rosen's compendium.

Figure 2. Chemiluminescence spectrum for the Pb(v) + 0

3 reaction,

19
Part of the modulation spectrum is reproduced in figure 3. This spectrum, which has

been smoothed, represents only that component of the chemi luminescent emission which

varies with the laser excitation of 0 3 [i.e., only that component which either increases

or decreases due to a change in reaction rate with reactant vibrational excitation).

Thus, if the emission originated only from a single electronic state, the modulation

spectrum would appear as a quasi-continuum with possibly some small structure due to sharp

changes in population of one vibrational line relative to an adjacent one. The large

amount of structure in figure 3 indicates the presence of emission from at least two electronic states. The peaks (or valleys) in the spectrum do not agree well with .either

the a -> X or b > X states, but rather with the A -> X transitions.

i i i i i ' i i—i—i—i—i—i—

170 3J0 330 310 490 470 430 WAVELENGTH ("")

Figure 3. Smoothed modulation spectrum (the component of the emission which varies with

laser excitation of 0

3

) of the Pb(v) + 0

3 reaction.

The results will be discussed and possible mechanisms proposed taking into account

both our results and other investigations of this system.

20
CHEMI LUMINESCENCE IN REACTIONS OF OZONE

S. Toby, F. S. Toby and B. Kaduk

School of Chemistry

Rutgers University

New Brunswick, NJ 08903

Chemi luminescence (CL) can provide highly sensitive analytical methods for the estimation of reactants, it serves as a unique probe for the identification of intermediates,

and it yields data which provide information on energy transfer and partitioning. Gas

phase ozone systems have been particularly rich sources of CL, and much work has been done

on O3 + alkenes as an adjunct to kinetic studies. Pitts and co-workers [l] 1 using ozonized

oxygen obtained CL from a variety of alkenes and tentatively identified the emissions as from excited formaldehyde and a-diketones. A more extensive investigation [2] confirmed

emissions from CH 2 0 C

1 /^) and (CH0) 2 C3 Au 1 and reported that in the absence of 0 2 Meinel

bands (0HCX 2tt)) were also seen. The presence of 0 2 had no effect on the CH 2 0( 2 A2 ) emission

in the alkenes studied [2] nor in the case of O3 + tetramethylethylene and the quenching

has been shown to be 1Q times as rapid by 0 2 as by N 2 £4], An intriguing parallel is the

fact that added 0 2 reduces the reaction rate of O3 + alkenes by about an order of magnitude

under conditions normally employed £5J, A similar effect has been noted in the 0 3 + C 2 F tt system: added 0 2 quenches the CL £6J and also slows the reaction [7].

Reported here are six new CL spectra, measured with apparatus previously described

[3, 6], The spectra were measured at room temperature using a Jarrell-Ash 0.25-m monochromator with a spectral slit width of 3 nm and a cooled EMI 9683QKB photomultiplier. The 0 3 flowed tn a helium carrier gas at a total pressure of approximately 50 torr.

The total intensities uncorrected for spectral response and relative to C 2 H k = 1 are

as follows: trans-C 2 F 2 H 2 30, C 2 F 3 C1 350, C 2 F 3 H 45, thiophene 50, CS 2 540, HCCCN 22.

1. Trans-C 2 F 2 H 2 , C 2 F 3 C1 and C 2 F 3 H

The CL spectra from the reactions of these fluoroalkenes with 0 3 are shown below

_J 1 1 1 i I 1 1 { 1 ! 1 1 1 1—!—1—1—1—1—! 2qq S$o SoOhft

T

Figures in brackets indicate the lieterature reference at the end of this paper.

21
Soo fins

300 WO GOO 7CO urn

All three substrates give a band from about 350 to 510 nm with a broad maximum at 430 nm and observable fine structure. The species responsible for this emission has not thus

far been identified. There is an additional band from O3 + C 2 F 3 H in the ultraviolet which we identify as due to singlet excited CF 2 and which is also emitted strongly from 0 3 +

C 2 F 4 [6].

2. Thiophene

The CL spectrum from O3 + thiophene is as follows

4* ThiophCftG

1 l 1 300 $00 800m

Two features are seen. The structureless band with a maximum at 370 nm is due to singlet excited S02 which has been found in the CL of 0 3 with several sulfur-containing

compounds [1,8] and which probably arises from the reaction SO + 0 3

-* 0 2 + S0 2 *. The

banded feature from 520 to 810 nm is identical to the longer wave emission from 0 3 + H 2 S and which has been identified as due to HSO [8], most likely from SH + 0 3

-> HSO* + 0 2

.

3. Carbon Disulfide

The strong emission from 0 3 + CS 2 appears as follows:

22
_1 I ( I J i- 300 wo zoo ico 7qo Soomr

The emission appears to be entirely due to excited S0 2

. The kinetics of this reaction

have been studied by Olszyna and Heicklen [9] who did not report CL but postulated the

formation of excited S0 2 from the reaction CS + 0 3 * CO + S02 *.

4. Cyanoacetylene

This unusual spectrum from 0 3 + HC 3 N extends from 340 to 780 nm and the emission has

been identified as due to CH(A 2 A > X

2 n), CN(A 2 n > X

2 i) and CN(B 2

E -> A

2 n).

The relationship of CL with the kinetics and energetics of some of these reactions

is discussed.

References

[ 1] Pitts, J. N., Jr., Kummer, W. A., Steer, R. P., and Finlayson, B. J., Adv. in Chem.

,

(ACS Symposium) No. 113, 246 (1972).

[ 2] Finlayson, B. J., Pitts, J. N., Jr., and Atkinson, R., J. Am. Chem. Soc, 96, 5356

(1974) .

[ 3] Toby, S., J. Luminescence, 8, 94 (1973).

[ 4] Schurath, U., Gusten, H., and Penzhorn, R. D. , J. Photochem. , 5, 33 (1976).

[ 5] Toby, F. S., Toby S., and O'Neal, H. E., Int. J. Chem. Kinet., 8, 25 (1976).

[ 6] Sheinson, R. S., Toby, F. S., and Toby, S., J. Am. Chem. Soc, 9_7, 6593 (1975).

[ 7] Toby, F. S., and Toby, $., J, Phys. Chem. , 80, 0000 (1976).

[ 8] Becker, K. H., Inocencio, M., and Schurath, U., int. J. Chem. Kinet., Symp. 1_, 205

(1975) .

E 9] Olszyna, K. J., and Heicklen, J., J, Phys. Chem., 74, 4188 (1970).

23
DIOXETANE CHEMISTRY IN THE GAS PHASE UV - VISIBLE CHEMI LUMINESCENCE FROM THE REACTIONS OF 0 2 ( 1 A

g

) WITH OLEFINS

Denis J. Bogan

Chemical Dynamics Branch

Code 6180

Naval Research Laboratory

Washington, DC 20375

and

Joseph L. Durant, Jr. Chemistry Department

The Catholic University of America

Washington, DC 20064

1. Introduction and Background

Dioxetanes (4 membered cyclic peroxides) are the most notable of a small number of

strained cyclic molecules which are know to undergo thermal unimolecular reactions yielding

electronically excited products [l^] 1

.

0 0 0 0

R C C R" C C

(E

fl ^ 25)

R

1 R"

1 R R' R

1 " R"

The quantum yields for production of one excited carbonyl product range from 0.01 to perhaps 0.5 with a preferential production of triplet (nn*) states [1]. Such quantum

yields are orders of magnitude greater than the statistically expected yields based upon

equipartitioning of energy to all accessible product states. Recent pervasive interests

in chemi 1 uminescence, chemical lasers and electronically non adiabatic reactions have

combined to make dioxetane chemistry the subject of considerable attention by both experimentalists [1,2] and theoreticians [3,4],

Previous to our work, studies have involved synthesis of the dioxetane of interest

followed by thermal decomposition. The latter has almost always been done in solution

where it is complicated by solvent interactions and hence inability to obtain high resolution spectra of the primary excited products, More common than direct chemi luminescence

are studies involving energy transfer to dyes which fluorescence efficiently {e.g., 9, 10 dibromo anthracene), and product analysis, Much important information has been gained,

for example the singlet-triplet titration experiments of Turro and coworkers [5]; however,

the condensed phase techniques, by their very nature, preclude knowledge of the disposition

of energy in the primary excited products.

i Figures in brackets indicate the literature reference at the end of this paper.

24
Direct 1, 2 cyclo addition of 0 2 (

1 a) to olefins in solution has been used to prepare

jioxetanes from mono olefins lacking allylic hydrogen [6,7]. This technique is known as

low temperature photosensitized ozygenation and involves production of 0 2 (

l h) via energy

transfer from an organic dye, D.

D + h> D*

3 D* + 0

2

( 3 E) * D + 0

2

(

1 z) or 0

2

(

] A)

0

2

(

1 z) + Quencher -> 0

2

(

1 a)

Dnly the dioxetanes of electron rich olefins have been made by this technique.

We have found that the reaction of olefins with C^Oa) in a low pressure discharge

flow system result in the observation of electronically excited products which are expected

from the corresponding dioxetanes. The experimental apparatus is shown as figure 1. A

nicrowave discharge through pure 0 2 or 3 percent 0 2 in He doped with a trace of Hg was

used to produce C^Oa) free of atomic oxygen. The olefin was added downstream (following

2 light traps) and chemi luminescence was observed axially in a cell of 30 cm path length,

with a 1/4 meter grating spectrometer and cooled S-5 photomul tipl ier in a single photon

counting configuration. There is no evidence of complications from atomic oxygen or

ozone. Typical pressure and residence time were 4 torr and 0.1 sec. respectively. The

results, of our experiments with ethyl [8], methyl and n butyl vinyl ethers [8,9], and

ethylene [10] have been reported. These and other experiments now underway have centered

on two major areas. First, and discussed in Section II, is the dynamics of the reaction,

particularly orbital symmetry considerations governing the correlation to excited products

and the vibronic motions which serve to promote the reaction. The second main thrust,

discussed in Section III, is the production and study of emission spectra which have not

been observed previously.

NLET

I

Y

MICROWAVE

CAVITY

R

PUMP

Figure 1. Discharge flow system; R = heated flow reactor with removable quartz windows,

M = 1/4 meter monochromator with detector described in text.

25
2. Dynamics, Capsule Summary

The initial motivation for doing gas phase studies was to obtain high resolution

spectra under conditions which would minimize energy transfer and other quenching pro- cesses. We soon learned, however, that there is an important fundamental difference between

gas phase addition of Oa^A) to olefins and liquid phase addition effected by photosensitization. Thermochemistry and chemi 1 uminescence activation energies (Table I) showed that

the initially formed adduct of olefin plus (Oa^A) contains 45 kcal, or more, of excess vibrational energy [8,10]. Experiments were done measuring the rate of collisional stabilization versus the gas kinetic collision frequency; no stabilization was found and we concluded that the mean lifetime of the vibrational ly excited dioxetane was less than 10~ 8 sec. [8,9].

Table I. Data summary for reactions of O^A^) with Olefins

Olefin Ejkcal/moleJ Emission -AH D-( kcal /mole)

—a— K

H

2 C=CH0Me

H

2 C=CH0Et 12 + 1 H

2 C0 107

H

2 C=CH0nBu

H2C = CH2 21 + 1 H

2 C0 90.5

Et0HC=CH0Et

H

2 C=C=0 15 + 1 H

2 C0 129

H

2 C=CHF H

2 C0 136

H

2 C

= CF

2

H

2 C0 122

HFC=CF

2 HFCO, F

2 C0 175^

F

2 OCF

2 F

2 C0 171

H

2 C=CHC1 H

2 C0 129

H

2 C=CC1

2 H

2 C0 103

CI

2 C=CC12 none— 124

H

2 C=CHCH0 none

MeHC=CMe

2 none- 104

Me

2 C=CMe

2 none^- 109

aHd is calculated for ground state carbonyl products. Thermochemical data are from

reference 16, reference 8 for vinyl ethers.

HCOOEt* is believed to rearrange quantitatively to C 2 Hi+ plus HC00H via a Norrish Type II process, see reference 16b.

The formation of allylic hydroperoxides has E ^ 3-6 kcal and is favored over dioxetane

formation. a

Our estimate. eWe infer that C1 2 C0* is lost completely by predissociation and/or

other non radiative processes.

26
Thus the overall reaction is a chemical activation process (la), followed by rapid fragmentation (lb).

0=0 (

] a)

(E ^ 10„ 20)

0 — 0 0 * 0

R r r R" ^ R C C R" -prv-x C + C

R' L=L R"1 (la)

R1 R

„. UD; R R' R"' R"

The chemically activated dioxetane must have a lifetime > 10~ 12 sec, since dioxetanes have

been isolated and characterized from low temperature solution photosensitization [7].

Many reactions give formaldehyde (A *A 2

-> X emission. A hot band representing

excitation in (the out of plane bend) occurs with intensity which correlates positively

with available energy and negatively with the number of vibrational modes of the expected

dioxetane. The correlation crudely approximates an RRK relationship. The principal geometry difference between H 2 C0 in dioxetane and H 2 C0*( 1 A 2 ) is in v^, which suggests that

chemical Franck-Condon factors are operative and that the hot band intensity is inversely

related to the lifetime of the vibrational ly excited dioxetane.

In contrast to the lifetimes, electronic energy partitioning of chemically activated

dioxetanes is non statistical. Methyl, ethyl and n-butyl vinyl ethers give only H 2 C0*

emission and exhibit equal quantum yields for H 2 C0* emission at all temperatures of study

[9]. Many other unsymmetrical olefins give only one of the two possible product emissions.

The only apparent exception is C 2 F 3 H which is discussed in the following section.

The energy partitioning and hot band information (which could not have been obtained

from condensed phase studies) are being used to formulate an orbital symmetry - vibronic

interaction theory of the dynamics of these reactions [4],

3. Previously Unobserved Emissions

The study of A - X (nn*-ground) electronic transitions of carbonyl compounds has been

a subject of major and continuing interest in both photochemistry and molecular spectroscopy. Neither F 2 C0 nor HFC0 (A-X) emissions have been characterized. The A (nn*) state

of HFC0 predissociates very readily and HFC0 is the basis of a photodissociation chemical

laser [12]. The A (nn*) state of F 2 C0 is of interest to atmospheric modellers because F 2 C0

is a likely secondary product of fl uorochlorocarbon photodissociation in the stratosphere

and it can in turn, photodissociate to give F atoms which can then destroy ozone [13].

Figure 2 shows spectra which were obtained from the reactions,

0

2

(

1 A) + C

2

F4 + F

2 C0* + F2 C0 (2)

0

2

(

] A) + C

2 F

3H HFC0* + F2 C0 (3)

F o C0* + HFC0

We have a complete vibrational assignment of the F 2 C0 (A -> X) emission; we have not yet

assigned HFC0.

The barrier to inversion in the A state of F 2 C0 has been estimated [14] as 4000" 1

, hence we expect the behavior of an undoubled well in vi since the lower vibrational levels

.of 2 deep wells will not communicate effectively. On this basis the reported origin in

the absorption spectrum [15] at 42084 cm" 1 is the energy of the transition 4j.

27
I I I I 250 300 350 400

nm

Figure 2. Chemil uminescence spectra from reactions of 0 2 (

l hg) with fluorinated olefins.

Bottom panel is C 2 F 3 H only, middle panel is C^Fi, only, top panel is C 2 F 3 H plus C 2 F 1+ . Conditions were; P(total) = 4.1 torr (approx. 0.2 of olefin and 3.9 of oxygen), T = 800

deg K, reactor residence time = 0.1 sec, spectral slit = 0.4 nm., scan rate = 12.5 nm./

min., time constant = 1 sec, full scale intensity = 1 OK counts/sec

28
Plausible assignments have bee obtained for 65 lines out of the total of ca. 70 lines

in the spectrum. The spectral slit width was 4A and line positions have an uncertainty of

ca. +25 cm" 1

. The observed progressions all obey the vibronic symmetry selection rules for

the point group c 2

. The fundamental ground state assignments of Neil sen et al [17] (mode

numbers redefined fn Herzberg convention) were used. These are

Vj = 1942 cm" 1 carbonyl stretch

v 2

= 965 cm" ] (ai) totally symmetric stretch

^3 = 626 cm" 1 (ai) F-C-F in plane bend

Vl» 8< 774 cm" 1 (bi) out of the plane bend

^5 == 1249 cm" 1 (b 2 ) assymetric F-C-F stretch

^6 = 584 cm" 1 (b 2 ) wagging

All but 2 (hot band) progressions originate from an electronic origin at 41396 cm" 1

, hence

in the excited state v£ = 688 cm" 1

. Only and v 5 appear as bare progressions; suprisingly

vi appears only in concert with promoting modes. Some of the observed progressions are <

n = 1, 3, . . . . 15

n = 1, 3, . . . . 9

n = o, 2, . . . . 14

2l4l6n

n = o, 2, . . . . 10

o 0&

Oc 0-, 0

n = o, 1, • • . . 4

3i4 6 3iD

n

n = o, 2, . . . . 8

Reasonably convincing indirect evidence that the emission is indeed F 2 C0 (A + X) is; a) our emission begins at 235 nm as compared to the known origin of absorption at 237.6, [15]

b) the observed emission intensity is kinetically first order in both 0 2 (

1 a) and C 2

Fit. The spectrum becomes diffuse and steadily diminshes in intensity from 350 to 520 nm., much

like the long \ bands of H 2 C0 (A-X).

The spectrum from the reaction of C 2 F 3 H shows clear evidence that the F 2 C0* emission

is present as well as a new very complicated emission. The emission at A = 400 +25 nm is first order in C 2 F 3 H. A spectrum was obtained using C 2 Fit and C 2 F 3 H both at the concentrations used for their respective pure spectra and this a simple sum of the pure component

spectra. This establishes that the postulated HFC0* emission from reaction 3 is not excited

! by energy transfer from F 2 C0*.

As a result of these experiments, this chemi luminescence technique is established as

a means of characterizing previously unobserved carbonyl (A-X) emissions. The region of

the excited state phase space which can be reached by light excitation of a polyatomic

molecular is severely restricted by the Franck-Condon principle. The volume of phase space

accessible by chemical reaction is much less restricted and it is not surprising that re-

' gions can be reached from which radiative transitions can be observed. This is particularly

significant for excited states which predissociate near the electronic origin.

29
References

1] Turro, N. J., et al, Accounts Chem. Res., 7_, 97 (1974).

2] Mumford, C, Chemistry in Britain, Y\_, 402 (1975).

3] Dewar, M. J. S., and Thiel, W. s J, Am. Chem. Soc, 97, 3978 (1975).

4] a) Chiu, Y-N., J. Chem. Phys., in press, 64, number 7, (1976).

b) Chiu, Y-N., Bogan, D. J., and Sheinson, R. S., to be submitted.

5] Turro, N. J., and Lechtken, P., J. Am. Chem. Soc, 94, 2886 (1972).

6] Adam, W., Chemiker Zeitung, 99_, 142 (1975).

7] Bartlett, P. D. , and Schaap, A. P., J. Am. Chem. Soc, 92, 3223 (1970).

8] Bogan, D. J., Sheinson, R. S., Gann, R. G., and Williams, F. W., J. Am. Chem. Soc,

97, 2560 (1975).

9] Bogan, D. J., Sheinson, R. S., and Williams, F. W., paper F-9, 8th International

Conference on Photochem. , Edmonton, Canada, August 1975; abstract including spectra to

be published in J. Photochem.

;i0] Bogan, D. J., Sheinson, R. S., and Williams, F. W., J. Am. Chem. Soc, 98, 1034 (1976).

;il] Berry, M. J., Chem. Phys. Lett., 29, 329 (1974).

;i2] Klimek, D. E. , and Berry, M. J., Chem. Phys. Lett., 20, 141 (1973).

[13] a) Rowland, F. S., and Molina, M. J., Revs. Geophys. and Space Phys., 1_3, 1 (1975).

b) Chou, C. C, Ruiz, H. V., Crescentini, G., and Rowland, F. S., paper PHSC 5,

First Chemical Congress of North America (and 171st National ACS), Mexico City, Mexico,

December 1975.

;i4] Condirston, D. A., and Moule, D. C, Theor. Chim. Acta, 29, 133 (1973).

[15] Workman, G. L., and Duncan, A. B. F., J. Chem. Phys., 52, 3204 (1970).

.16] a) Benson, S. W., in Thermochemical Kinetics, Wiley, New York, 1968.

b) Benson, S. W., and O'Neal, H. E., Kinetic Data on Gas Phase Unimolecular Reactions,

NSRDS-NBS 21, U. S. Dept. of Commerce, 1970.

;i7] Nielsen, A. H. , Burke, T. G., Woltz, P. J. H., and Jones, E. A., J. Chem. Phys., 20,

596 (1952). —

;i8] Bogan, D. J., and Durant, J. L. , Jr., J. Mol. Spectroscopy, to be submitted.

30
MOLECULAR DISSOCIATION BY HIGH-INTENSITY INFRARED LASER RADIATION

K. H. Welge

Fakultat fiir Physik

Universitat Bielefeld

Bielefeld, West Germany

The dissociation of molecules by high-intensity laser radiation in the infrared has

recently gained substantial interest, particularly since it has been found that the process

is highly state and isotope selective. However these processes have not yet been investigated in any detail and are not yet well understood.

This paper will be concerned with a review of this subject with emphasis on our own

studies in this area. We have been concerned with three kinds of experiments using line

selected pulsed C0 2 -laser radiation at energies £ 100 Joule/pulse and intensities £ 10

GWatt/cm2:

T. Dissociation and photolysis of molecules at pressures down to collisionless

conditions using laser fluorescence spectroscopy for in-situ diagnostics

of fragments. Experiments have been concerned with NH 3 and its isotopic

compounds;

2. Crossed molecule/C0 2 -laser beam experiments on the kinetics and dynamics

of the high-intensity field dissociation process by photodissociation

fragment spectroscopy;

3. Experiments on isotope selectivity by measurement of the isotope

separation factor as function of parameters such as number of pulses per

sample, gas pressure, laser energy etc.. Also, the quantum yield of

separation has been investigated.

31
PHOTOCHEMISTRY OF FORMALDEHYDES: PAST AND PRESENT

Edward K. C. Lee, Roger S. Lewis and Richard G. Miller

Department of Chemistry

University of California

Irvine, CA 92717

In the past, photodecomposition of formaldehyde has been studied extensively because

of general interest in understanding the primary photochemical processes for the simple

carbonyl compounds. Recent interest in its study certainly stems from the need to evaluate

the role that formaldehyde plays in photochemical air pollution, particularly due to the

formation of H-atoms and consequent reactions in the photooxidation processes [1,2] 2

. More recent and current interest is generated from the suggested practicality of photochemical laser isotope separation of deuterium and other isotopes [3,4]. Since quantum

yields of the fluorescence, the radical products (H and HCO) and the molecular elimination

products were not known at various single vibronic levels (SVL) earlier, we have undertaken a systematic study in which the mechanism and rates of these primary processes in

H 2 C0, HDCO, and D 2 C0 can be established by various photophysical and photochemical techniques.

We have recently reported the observation of a large variation of the radiative lifetimes (tr) and the non-radiative lifetimes (t^) of selected SVL's of the first excited

state of H 2 C0 (Si, A *A 2 ) [5]. New results on the fluorescence quantum yields ($p) of

H 2 C0, HDCO, and D 2 C0 vs. excitation wavelength (X ex ) have been obtained by fluorescence

excitation spectroscopy of these isotopic formaldehydes at low pressures [6], and they are shown in figure 1. The values of $p for the first excited vibrational level (4 1

) having

one quantum excitation of the out-of-plane wagging mode (vt*

1 = 1) are M3.035 for H 2 C0,

^0.05 for HDCO, and 1,0 for D 2 C0, indicating the absence of photochemical activity at the

4

1 level of D 2 C0; the fluorescence decay times (jpl at low limiting pressures are 0.082,

0.141 and 4.5 ysec, respectively. r

In general, the rates of non-radiative transitions increase exponentially with the

increasing excitation energy. The fact that the values of $p and tp are slightly higher

in HDCO than in H 2 C0 predicts similar rates of deactivation for this isotopic pair, whereas very different photochemical rates are expected for D 2 C0. The rate of non-radiative

transition in D 2 C0 (4 1 ), showing an extremely large deuterium isotope effect. Furthermore,

operational complications in photochemical isotope separation due to collisional electronic quenching and vibrational energy transfer in H 2 C0 and HDCO should be negligible

below 1 torr pressure.

This work has been supported by the Office of Naval Research and the National Science

Foundation.

Figures in brackets indicate the literature references at the end of this paper.

32
1.0

0.5

0.2

0.1

F 2

10

-2

I0"

3 -

5

-

350 340 330 320 310 300

HDCO

0.

10"

10-3

360 350 340 330 320 310 300

X ex ( nm)

: igure 1. Fluorescence quantum yield vs. excitation wavelength: 0.045 torr D 2 CO (filled);

0.20 torr HDCO (half-filled); 0.098 torr H^O (open), SVL assignment: squares = 2V

progression; circles = 2 4

3 progression; triangle = Z^ 1 progression where n = 0, 1, 2...

It has been well recognized that the radical formation process could seriously lower

the isotope separation efficiency [3,4]. In order to evaluate the quantum yield of hydrogen atom formation from various SVL's, we have tried to devise a sensitive, quantitative

and convenient spectroscopic method for detecting photochemical H-atoms by observing the

near infrared chemi 1 uminescence emission from the HNO* formed by recombination of

H + NO(+M) [7]. We have found that the chemiluminescence photoexci tation spectrum^observed in the mixture of H 2 CO and NO shows the electronic absorption features of H 2 C0(A *A 2 ) *

'X ^i) transition, since the photoexcited H 2 C0* gives off H-atoms by radical decomposition

process. The preliminary results are illustrated in figure 2 [8]. The chemiluminescence

emission from HN0*/DN0* was confirmed by the analysis of the 760 nm emission band from

the mixture of HDCO and NO excited with a cadmium laser (325.0 nm) [8]. The threshold

for the H-atom formation from H 2 CO appears to be near 330 nm. The quantitative implications of the above observation to the atmospheric chemistry and the laser isotope separation will be presented at the conference.

33
1 torr H2 CO

10 torr NO

1 torr H2 CO

10 torr N2

"550" 270 280 290 300 310 320 330 340

X (nm) ex

x

'

Figure 2. Excitation spectra run with a long pass emission filter (700 nm cut off).

References

[ 1] a) DeGraff, B. A., and Calvert, J. G., J. Amer. Chem. Soc, 89, 2247 (1967).

b) McQuigg, R. D. , and Calvert, J. G., J. Amer. Chem. Soc, 9J_, 1590 (1969).

[ 2] Calvert, J. G., Kerr, J. A., Demerjian, K.-L., and McQuigg, R. D., Science, 175 , 751 (1972).

[ 3] a) Yeung, E. S., and Moore, C. B., J. Chem. Phys., 58, 3988 (1973).

b) Moore, C. B., Accounts Chem. Res., 6_, 323 (1973).

[ 4] Marling, J. B., Chem. Phys. Lett., 35, 84 (1975).

[ 5] Miller, R. G. , and Lee, E. K. C, Chem. Phys. Lett., 33, 104 (1975).

[ 6] Miller, R. G., and Lee, E. K. C, Chem. Phys. Lett., 00, 0000 (1975).

[ 7] a) Clyne, M. A. A., and Thrush, B. A., Vise. Faraday Soc, 33, 139 (1962).

b) Ishiwata, T., Akimoto, H., and Tanaka, I., Chem. Phys. Lett., 21_, 322 (1973).

[ 8] Lewis, R. S., Tang, K. Y., and Lee, E. K. C, unpublished results.

34
DECOMPOSITION OF FORMIC ACID VAPOR BY INFRARED LASER RADIATION

R. Corkum , C. Willis, M. H. Back and R. A. Back

PIE Group Chemistry Division

National Research Council of Canada

The infrared induced decomposition of formic acid vapor has been studied using the

3 18 and P 2 s lines from a line tunable, pulsed HF laser. The study was undertaken both to investigate the parameters involved in the induced decomposition and to assess the potential of the system for photochemical isotope enrichment.

The beam from the laser was focused into a photolysis cell fitted with infrasil

quartz windows. After irradiation the cell contents were condensed at -196°C and the non-

:ondensible products measured by gas burette and analysed either by gas chromatography or

Uiass spectrometry. The only non-condensible product observed was CO and in a few experilents it was demonstrated that an equivalent amount of H 2 0 was formed. Less than 3% of

:he decomposition led to C0 2

HCOOH + I.R. H

2 0 + CO > 97% (1)

HCOOH + I.R. H

2 + C02

< 3% (2)

:nd H 2. Earlier studies have shown (1) that both channels are important in the pyrolysis

if formic acid vapor.

The laser beam was focused into the cell with an f = 7.5 cm lens. If this was

emoved no decomposition was detected. Using a long focal length lens, f = 150 cm, to

'ocus the beam into the cell gave some decomposition but with a yield of only about 1% of

hat with the f = 7.5 cm lens. These observations are indicative that the decomposition

[as a relatively sharp intensity threshold. This was confirmed using neutral -density

i Iters in conjunction with the f = 7.5 cm lens.

The effect of temperature, pressure, added inert gas and cell path length were

nvestigated. Increasing temperature in the range 25-180°C led to a slight increase in ield but this could be attributed to a shift in the dimer-monomer equilibrium, with only

he monomer absorbing the HF radiation. Increasing the pressure in the range 10 - 40 Torr

ed to an almost proportional increase in relative quantum yield. Addition of N 2 0 or C0 2 id not affect the amount of IR absorbed but dramatically reduced the observed yield of

0. This effect is interpreted as collisional deactivation of vibrational ly excited

ormic acid. Photolysis was performed in cells 10 cm and 1 cm long. The amount of CO ormed in the short cell was only slightly smaller than that formed in the long cell,

Ihich is consistent with decomposition occurring only close to the focus of the lens. A

ower limit of <j>co 1 6 x 10~ 2 could be set for the quantum yield of the decomposition

ased on the light absorbed in the short cell. This is a surprisingly large value partiularly when it is noted that the activation energy of 29 + 1 kcal/mole found in the pyroysis is equivalent to about 3 photons of the incident radiation, and that the quantum

ield is still increasing with increasing pressure.

Department of Chemistry, University of Ottawa, Ottawa, Canada.

Figures in brackets indicate the literature references at the end of this paper.
Unfortunately, the formic acid system offers little promise for isotope separation.

Since H 2 0 and not H 2 is the hydrogen-bearing product, any separation in the photolysis

would be rapidly lost by re-equilibration with the formic acid substrate. Various mixtures

of H

12 C00H and H

13 C00H have been irradiated but in each experiment the isotopic ratio in

the CO produced faithfully reflected the ratio of the starting material, despite the fact

that H

12 C00H absorbs the P 18 line nearly twice as effectively as H13 C00H.

The laser radiation is absorbed by the OH stretching mode of monomeric formic acid

and two limiting mechanisms for the subsequent decomposition can be considered. In the

first, individual molecules are excited to levels above their dissociation energy by

stepwise absorption of quanta in the 0-H stretching vibration. Collisional or power

broadening of the higher levels must be invoked to circumvent energy mismatch due to anharmonicity, although since only 3 or 4 quanta are required to exceed the dissociation

energy, this problem is not serious. Unimolecular dissociation of vibrationally excited

formic acid molecules would follow the usual internal reorganization of vibrational energy

into the mode leading to dissociation to CO + H 2 0 in collisional deactivation.

In the second limiting mechanism collisional relaxation of the initially excited OH stretching vibration is fast enough that stepwise excitation to higher levels is not required. Collisional relaxation into other modes results in a small volume of hot vapor

which decomposes through a normal thermal decomposition mechanism in competition with

cooling by loss to the surrounding cold vapor.

Either mechanism could probably account for the relatively high quantum efficiency

of the decomposition and both would show a strong dependence on intensity as observed.

The lack of isotopic selectivity and the pressure dependence suggests that the second

mechanism is operative, but uncertainties in the details of the thermal decomposition make

it impossible to make a clear choice between the two. The mechanisms suggested are limiting cases, the first favored at low pressure and the second at high pressure. The present

experiments may correspond to a mixture of the two with an intermediate degree of relaxation of the OH stretching excitation into other vibrational modes.

References

[ 1] Blake, P. G. , and Hinshelwood, C, Proa. Hoy. Soo. (London) A255 , 444 (1960).

36
THE LASER AUGMENTED DECOMPOSITION OF D^B ADDUCTS

K. R. Chien and S. H. Bauer

Department of Chemistry

Cornell University

Ithaca, NY 14853

We have completed the investigation of the laser augmented decomposition of H 3 BPF 3 and extended these studies to D3BPF3. The low power, low pressure absorption coefficients

(aio) of D3BPF3 for many lines in the C0 2 laser are somewhat larger than the corresponding

magnitudes for H3BPF3, and the entire band is shifted to higher frequencies. The band

shapes and re-lative positions follow the trends predicted from a normal mode analysis.

The P(J) dependence of the augmentation factor is most interesting. For the tri-deutero

compound, the highest photolytic yield is produced by the P(24) line; here oti 0 is approximately 1/5 as large as that at the absorption band peak, which occurs at P(4); for the

tri-hydrido compound, the most effective line is P(32), where the absorption coefficient

(am) 1S approximately 1/4 of that at the band peak, which occurs at P(22). The H/D

separations are readily achieved by line selection, With P(24) the augmented decomposition

of D3BPF3 preferentially selects the n B isotope by ^35% in a 5 minute run. Also, D3BPF3

is decomposed preferentially, relative to HD 2 BPF 3

; the H/D separation is substantial, and

increases with irradiation time. The (10/11) specificity did not show up in H3BPF3, in

agreement with the normal mode analysis.

The augmented rate [H3BPF3 at P( 32) ] is linearly dependent on the pressure from 2-15

mtorr. At 25 mtorr, and more so at 50 mtorr, the degree of augmentation is increasingly

greater. This may be due to the decrease in the mean free path within the reaction cell

and consequent v-T heating of the reagent, such that a thermal component is superposed on

the photolytic rate. The rate is clearly not linearly dependent on the power. Plots of

log (% conversion) vs log (laser intensity) gave slopes ^3.1, indicating that at least

three photons are required per molecule to augment decomposition. The time dependence of

the decomposition, at low % conversion, proved to be unexpected. Whereas the % decomposition is linear with time at the high power levels (120 watts), or for lower powers at higher temperatures (60 w at 36°C), at low powers (50 watts) the % decomposition is approximately quadratic with time but becomes linear at longer time periods. This is strongly

suggestive of a multi -photon process.

For runs below 15 mtorr the rate constants at several laser powers were calculated

on the basis of a first order rate law since these rates are independent of pressure; the

reaction time was kept constant at 12 minutes. "Activation energies" (Eb) were deduced

from pairs of rate constants at two wall temperatures: 25°C and 38°C. The magnitudes of

Elj varied from 2.5 to 5.9 kcal/mole, with a mean value of 3.5 kcal/mole. At this state we assumed that the increment in average gas temperature (which must be somewhat higher than

the wall temperature) is equal to the increment in wall temperature.

To determine the photon conversion efficiency we measured changes in laser output

levels due to intra-cavity power absorption by the sample, at laser power levels used for

the moderate photolysis runs (approximately 60 watts). The mean absorption coefficients

<a> were calculated following the analysis of Kaufman_agd Oppenheim. They found that

laser output decreased essentially linearly with (1-e"01 ) until lasing was quenched.

Write

\/J° = l-e(l-e" aD

); aD = miCe-i) + J L /J[

3

-] where 3 = 3.33 for our laser and a = <a> p at

37
these low pressures. The <a> coefficients for selected lines were thus obtained. The

gross photochemical efficiency was then estimated. The laser power extracted by the

sample in the cavity per passage through the cell is 20 J[(l-e <cl> pD) , where J|_ is the

power read with the monitoring meter, through the output mirror (95% reflecting). The

number of photons absorbed is 40 JL«<a>pD'10 7 At/hv. The ratio of photons absorbed/

molecule decomposed is ^4 x

'\0 k for 60 w of laser power and p = 3 mtorr (298°K). Then

- 30% is decomposed in 12 minutes. This low efficiency and the observed low value for Eg

(mean: 3.5 + 1.1 kcal/mole), in contrast to the overall thermal value of 29.3 kcal/mole,

may be accounted for by considering: (a) the geometry of the reaction cell, (b) the

previously estimated thermal activation energy, E^ n (lb) ^ 5 kcal/mole, and (c) the slope

of log (% dec) vs log (power) line. A semi-quantitative account follows.

The two elementary steps which account for the thermal reaction are:

DoBPF, ~ D-B + PF, AH? ^24.5 kcal/mole

D

3

B + D

3 BPF3 B

2 Dg + PF3 AH° ^-13.4 kcal/mole (lb)

It is plausible to assume that the vibrationally excited adducts do dissociate to a greater

extent than via the thermal reaction. Under high flux irradiation, the D 3 BPF 3 (v 3 ;v 9 ) leave the illuminated core and collide with unexcited adducts and dissociation fragments

in the larger encompassing volume. However, since BD 3 's recombine to B 2 D 6 and D 3 BPF 3 with essentially kinetic cross-sections, its concentration level in the encompassing

volume is but an order of magnitude higher than under thermal conditions. The upper limit

temperature in the encompassing volume is - 4b°C because of the absence of detectable

decomposition of admixed H 3 BC0 (in a 12 min. run). The augmented rate we observe is due

to D 3 BPF 3 which enter the annular region around the illuminated core and there encounter

BD 3 's.

A conventional steady-state condition on [BD 3 ] leads to:

[BD

3

] ss

=

(iT7} H +

1 (k

b /k_l )(1 " s)

where £ is the fraction of D 3 BPF 3 decomposed at the specified time, and y ranges from 2 [at early times, when [BD 3 = [PF 3 ]] to unity, when [PF 3 ]>> [BD 3 ].

Under laser irradiation both ki and k. increase. Thus, for those adducts that

accumulate 3 ,-<-about 3 kcal/mole reside in B-P extension. Since Ej$ n = 4.8 kcal/mole and

Eta = 24.5 k«l/mole, we postulate that:

k, (3v3

} = 2.4 x 10

15 exp[- 21*500] (3)

1 RT

- 3 x 10

12 exp[- TjjW]

(4)

Application of Eq. (2) gives [BD 3 ]J

% 1.2 x 10" 11 moles/cc, when ? * 0.1, and T = 318°K for

one mfp thick annular region. (Volume = 1.3 x 10 3 cm 3 ).

38
The number of adducts that enter and leave the illuminated core (per sec) is 2.8 x 1020 when the molecule density is 10 11+ cm~ 3 (3 mtorr) [derived from NcA/4]. Their mean ex- posure time (met) to the radiation is 4 x 10~ 5 sec. During one such period 1.12 x 10 16 D3BPF3 enter and leave; concurrently, 1.37 x 10 15 photons are absorbed; i.e. on the average

one out of 8 adducts picks up a single photon. Goodman calculated that 17 of these pick up two, and one out of 79 picks up three in a coherent sequential process. Thus, ^1.8 x 10 13

D 3 BPF 3 (3v 3

) enter the annular region (1 mfp = 1.5 cm) per met. These either get de-excited

or react with a D 3 B prior to entering the remaining volume.

The total moles of B 2 D 6 produced during a 12 min. run, at the 60 watt level, via

D

3 BFP

3

< 3 *3> BD

3 B2 D

6 PF3

* <rrwJ e.i » w> mo ^ * '° u ^d3

] ss(5)

If we accept the above estimate for [BD 3 ]j $

, M = 9.9 x 10" 7 moles, compared with 3.9 x 10" 6 moles of the adduct initially present; that is, a conversion of ^25%, close to that observed.

39
INFRARED LASER ISOTOPE SEPARATION

John L. Lyman and Stephen D. Rockwood

Los Alamos Scientific Laboratory

P.O. Box 1663

Los Alamos, NM 87545

It has been established in previous work [1-5] 1 that when SF 6 is irradiated by intense

C02 laser radiation isotopically selective dissociation occurs. The following points have

been established:

1. The reaction is selective when the pressures is less than about 10 torr. [1,2,5]

2. Both reaction yield and selectivity decrease with increasing pressure. [1,2,3,5]

3. A fluorine atom scavenger such as H 2 enhances reaction yield and selectivity

[1,2,3,5].

4. The reaction yield increased more rapidly than linear with pulse energy and there

appears to be an intensity threshold. [4,5]

5. The reaction yield depends on the laser frequency, but it does not follow the

infrared band contour [4].

6. Other molecules also undergo isotopically selective reactions [3].

In a typical experiment a mixture of SF 6 and H 2 is irradiated by focussing the C0 2 pulses into the reaction cell with a short focal length BaF 2 lens [2,3]. If it is assumed that 40 laser photons are required for rapid dissociation (the bond strength is 28

photons), then the photon utilization efficiency is 2.5 x 10

-1+ for a sample of 0.2 torr of

25% SF 6 in H 2 and 1.0 Joules/pulse.

It has been suggested that this efficiency could be improved by irradiating the gas in reflective cylinders [6]. Several cylindrical cells have been constructed and tested. (See

Fig. 1 for cell construction and dimensions.)

1 Figures in brackets indicate the literature references at the end of this paper.

40
REFLECTIVE CELL

Figure 1. Reflective cell schematic. Light enters from the left through an 8 cm focal

length BaF2 lens. Three types of cyl inderical sections (right side) were used. Cell (1),

0.95 cm x 61 cm long, copper. Cell (2), 50 cm i.d. x 20 cnflong, gold. Cell (3), 0.23

cm i.d. x 61 cm long, stainless steel.

For any meaningful evaluation of laser-induced chemical reactions occurring in these

cells the distribution of laser flux (or intensity) must be known. If, it is assumed that

geometrical optics is applicable to this problem then the intensity in a cylinder at a point

x centimeters from the entrance end and r centimeters from the cylinder axis is approximately

given by

*^ - *0 2r7

R" TM

where 1S tne fl ux °f tne laser light at the focussing lens and the laser spot is assumed

to be of uniform flux with radius b. The cylinder has radius a and wall reflectivity R and

n is the mean number of times a ray strikes the wall before reaching the point x. n is

given by

n = 1/3^ (2)

where f is the lens focal length.

The following table lists some preliminary results for the three cells listed in Fig.

1. All experiments were performed with 25% SF6 in H 2 irradiated with C02 laser pulses at

10.6 ym (P(20) line). The equivalent reaction volume, v, is the fraction of the SF 6 that

reacts per laser pulse times the total cell volume and is therefore a measure of the

reaction yield. The selectivity factor, a, is defined by [3]

d [ 32SFfi ] = o I^SFfil

d[ 34SF

6

]

[ 34

SF

6

]

or as the ratio of equivalent reaction volumes for the two isotopic species.

(3)

41
The isotopic enrichment factor [3] for the residual gas, B, will be given by

In 6 = (1-1/a) Nv/V (4)

where N is the number of laser pulses and V is the cell volume. Equation 3 was used to calculate a from b, v, and N. In experiments where the amount reacted was not determined,

a was assumed to be five and Eq. 3 was used to calculate v.

Note from the table that enhancements in reaction yield as high as 50 were obtained,

this corresponds to a photon utilization effiency of 0.5%. This is certainly an improvement over the single focus efficiency. The highest efficiency was obtained with cell 2 which has the highest wall reflectivity (> 0.97). The reflectivity is of cells 1 and 3 respectively 0.87 and 0.70. Experiments are continuing with these cells.

P(torr) E(J)

Table 1 a v(cm )

Relative

Yield

0.1 3.5 5* 6.0 14.2

0.5 2.5 6.4 3.1 13.0

1.0 3.5 4.5 1.4 6.7

2.0 3.9 1 .7 2.3 15.6

0.1 2.1 5* 16.1 ^50

0.2 3.4 5* 8.9 13.2

1.0 2.8 5* 1 .3 3.8

0.5 2.7 5* 0.4 1.6

*Assumed; see text

The pulsed C0 2 TEA lasers normally give mode locked pulses. That is, the 200 ns laser pulse is really a train of closely spaced subnanosecond spikes. This mode locking

can be supressed by adding a cw gain cell to the laser cavity. Experiments performed with

and without mode locking indicated that the selectivity of the SF 6 laser induced dissociation is not significantly altered by mode locking. However, the reaction yield is greater

by 25 to 50% for a mode locked pulse than a non-mode locked pulse of the same total energy.

References

[ 1] Ambartzumian, R. V., Gorokhov, Y. A., and Letokhov, V. S., JET? LETT. 21_, 171 (1975).

[ 2] Lyman, J. L. , Jensen, R. J., Rink, J. P., Robinson, C. P., and Rockwood, S. D., Appl.

Phys. Lett. 27, 87 (1975).

[ 3] Lyman, J. L., and Rockwood, S. D. , J. Appl. Phys., 47, 595 (1976).

[ 4]. Ambartzumian, R. V., Gorokhov, Y. A., Letokhov, V. S., Makarov, G. N. , and Puretskii,

A. A., Zh. ETF. Pis. Red., 23, 26 (1976).

[ 5] Lyman, J. L., and Rockwood, S. D., Proceedings of the Laser-Optics International Laser

'75 Conference Nov. 11-13, 1975, Anaheim, CA (to be published).

[ 6] Cotter, T. P., (private communication).

42
C0

2 TE LASER INDUCED PHOTOCHEMICAL ENRICHMENT OF CARBON ISOTOPES

Joseph J. Ritter and Samuel M. Freund

National Bureau of Standards

Washington, DC 20234

Several groups have previously reported on the C0 2 laser photolysis of gaseous C1 2CF 2

i [1,2,3] 2

. In one case [3], carbon isotopic perturbations were noted in samples subjected

to high energy pulses from a C0 2 transverse excitation (TE) laser. We have studied the C0 2 TE laser-induced chemical reactions of C1 2CF 2 with several reagents and have noted substantial carbon isotopic enrichments along with evidence for the participation of a reactive

intermediate, difl uorocarbene

The systems studied along with the principal products are given below:

Reaction Reactants Principal Products

I CI

2 CF

2 + 0

2 C0F2

, (C1CF

2

) 2

, Cl 2

II C1

2 CF2 + N0 C2

F4' C0F 2' C1CF0 ' N0C1 ' ^ C1CF

2

) 2

III CI

CF

2 + ( H

3 C

)

C=CH

2

C

2

F4 + comP 1ex array of other products

IV C1 2 CF

2 + HC1 HCF2 C1, (C1CF

2

) 2

, Cl 2

The reaction mixtures were contained in a 1 i stainless-steel cell and subjected to focussed 929 cm" 1 (P-36) radiation from a C0 2 TE laser delivering 300 ns wide pulses, with

0.20 J/pulse at 2 pps.

Chlorine (identified by color and mass spectrometry) was removed by treatment of the

crude reaction mixtures with Hg at 25°. The residual gases were then subjected to GLC

(10 m, 20% squalane on firebrick). Carbonyl fluoride-containing mixtures were passed over

uncoated firebrick to effect a quantitative C0F 2

-> C0 2 conversion, in order to avoid this

conversion by the firebrick support within the GLC column. Separated components were

identified by ir and isotope ratios determined by mass spectrometry with a typical relative

standard deviation of < 3%. Estimates of halocarbon product recoveries were made from GLC

peak area measurement, assuming equivalnet TC detector response for all compounds. Other,

generally less volatile materials, in amounts too small for unequivocal identification, were

noted in all of these systems.

Mass spectrometri c examination of the products of reaction I, indicated a depletion of

13 C while the analysis of recovered, unreacted C1 2CF 2 indicated its 13 C content to have

changed from an initial 1% to the 16-20% range. This alternation in 13 C content is

sufficient that it is manifested in the ir spectrum of the recovered material with the

appearance of two new bands (1131 and 1077 cm" 1

) as shown in figure 1. Additional

experiments performed under identical conditions but with the R-18 (1077 cm" 1

) line of the

TE laser resulted in recovered C1 2CF 2 which was depleted in 13 C. Thus the results of the

isotopically specific reactions were used to confirm the assignment of the 1077 and 929 cm" 1 absorptions to 13 C and 12C species of the C1 2CF 2 respectively. For II and III, unreacted

C1 2CF 2 enriched in 13 C, was recovered.

T^Present address: Los Alamos Scientific Laboratories, Los Alamos, New Mexico.

Figures in brackets indicate the literature references at the end of this paper.

43
I 1

I I I 1200 1100 1000 900 800 CM"'

Figure 1. TOP: C1 2CF 2 prior to laser photolysis. Pressure approximately 10 torr. BOTTOM: CI 2 CF 2 recovered after laser photolysis in the presence of 0 2

. Pressure

approximately 10 torr.

The appearance of 12C enriched COF 2 from reaction I and 12C enriched C 2 F lf from reactions II and III constitutes evidence for the laser induced, isotopically specific formatioi

of di fluorocarbene (:CF 2 ).

Difluorocarbene is also implicated in the formation of HCF 2C1 from reactions with HC1 [4]. The fact that C 2F1+ is not also noted in the HC1 and 0 2 reactions is probably due to the facility of the 0 2 + :CF 2 + COF 2 + 0, CI 2 + :CF 2

-> C1 2CF 2 and HC1 + :CF 2-> HCF 2 C1 re- actions as compared to the dimerization of :CF 2

. On the other hand, our results indicate

that when C1 2CF 2 is laser-irradiated in the presence of compounds very reactive toward

chlorine such as NO or an olefin, the Cl 2 + :CF 2 + C1 2CF 2 reaction is accordingly less

favored and the probability for dimerization of :CF 2 thus improved. Other workers [3] have

observed C 2?k from the laser photodissociation of C1 2CF 2

.

Further studies designed to affirm the role of laser-produced difluorocarbene and an assessment of its potential as a reagent in isotopically specific syntheses are currently

in progress.

References

[ 1] Freeman, M. P., and Travis, D. N. , J. Chem. Phys. y 60_, 231 (1974).

[ 2] Zitter, R. N., Lau, R. A., and Wills, K. S., J. Amer. Chem. Soe., 97, 2578 (1975).

[ 3] Lyman, J. L., and Rockwood, S. D. , J. Appl. Phys., 47, 595 (1976).

[ 4] Mahler, W., Inovg. Chem., 2, 230 (1962).

44
MEASUREMENT OF MEAN LIVES IN ATOMIC URANIUM 1

Jules Z. Klose

National Bureau of Standards

Washington, DC 20234

During the past several years the availability of tunable dye lasers had led to attempts to develop inexpensive methods of producing isotopically enriched uranium to meet

the needs of the expanding nuclear power industry. One of these methods involves isotope

separation through stepwise photoionization processes in which selected levels of the isotope of interest are excited with tuned laser light, and the final step of ionization is

achieved with a laser or an ultraviolet lamp. The isotope ions are then separated electrostatically from the neutral atoms. Using this method, groups at the Lawrence Livermore

Laboratory [1 ,2] 2 and the Avco Everett Research Laboratory [3] have done experiments

demonstrating the separation of 235 U and 238 U on a small scale.

In selecting a transition to be used in separating 235 U from 238 U, the overriding

requirement is that the isotope shift must be large enough to avoid overlapping the un- shifted line with the hyperfine structure of the isotope line. Once this requirement has

been met, the further criteria of strong absorption and suitable level lifetime must be

applied. Thus, selecting transitions requires knowledge of both the absorption cross

sections, i.e., transition probabilities, and the excited state lifetimes. Although these

two quantities are related, their relationship is usually not a simple one since most

excited atomic states have several decay modes. However, in some cases decay branching

ratios, i.e., relative transition probabilities, are available from other sources such as arc measurements. In such cases the absolute transition probabilities can be derived from

the level lifetimes. To help provide some of this needed lifetime data, the National

Bureau of Standards has been carrying on a program of measurements of mean lives of excited

levels in the uranium atom [4], In a complementary program at NBS, relative transition

probabilities in uranium are being determined using a stabilized arc [5,6].

At NBS uranium atomic lifetime determinations have been made using low-energy electrons

for excitation and a method of delayed coincidence for detection [4]. IN order to provide

free atoms for the measurements, a Knudsen effusion device especially designed to produce

diffuse beams of atoms of heavy elements was constructed and placed in operation [4]. At

this writing aQ

result of 7.3 ns has been obtained for the mean life of the upper level

of the 3584.9 A resonance transition in natural uranium [7]. For 238 U this level has an energy of 27886.99 cm" 1

. This lifetime was used to place the recently obtained NBS relative

transition probabilities and those of Croliss and Bozman [8] on absolute scales. In addition efforts have been made to measure the mean life of the 16900.39 cm" 1 level using

the 5915.4 A resonance transition. These efforts were largely thwarted by the very highintensity scattered light from the source at ^ 2000°C, which gave rise to very poor signalto-background ratios.

i This work was supported in part by the U.S. Energy Research and Development Administration

through the Lawrence Livermore Laboratory under Interagency Agreement LLL SANL 384-001.

2 Figures in brackets indicate the literature references at the end of this paper.

45
Cm"

50000 r IONIZATION LIMIT 49250 t 160 Cm-1

40000 -

30000 -

20000 -

10000 -

-.2 2 000

2 8 650

-16000

34659 , J=7

J z 5

I 10

17261

j 116900

J = 7

620 V5 0 5L0«

Figure 1. Examples of two- and three-stage ionization processes for separating 235 U from

235U. The energies of the levels and transitions are given0 in wavenumbers (cm" 1 ), and

the wavelengths of the transitions are given in angstroms (A). The values are for

238U, but differences in energies and wavelengths between *35U and 238 U are generally

not discernible within the precision of the quantities given in the figure. This figure

was reproduced with changes from reference (10).

HIGH -TEMPERATURE

SOURCE PHOTO- MULTIPLIER

AMPL FIER DISCRIM INATOR

Figure 2. Block diagram of the delayed coincidence experimental setup used for measuring

mean lives in heavy elements.

46
In an attempt to overcome this difficulty of low singla-to-background ratios, a tunable

dye laser pumped by a pulsed nitrogen laser is being installed as the source of excitation.

The tunable laser will provide such strong decay signals that much more efficient means of

detecting the decays can be used. Also, the problem of cascading into the level of interest

from simultaneously excited higher levels will be eliminated by selective level excitation

with the tunable laser.

The groups at the Avco Everett Research Laboratory [3] and the Lawrence Livermore

Laboratory [9] have reported measuring mean lives in UI using their experimental isotope

separation setups. In their methods the lifetime of an intermediate level is obtained by

observing the yield of isotope ions as a function of the delay between the exciting and

ionizing laser pulses.

HIGH-TEMPERATURE

SOURCE

I

M0N0CHR0MAT0R

SAMPLING

SCOPE

SIGNAL

AVERAGER

PHOTOMULTIPLIER

I INPUT SWEEP

TRIGGER OUTPUT

ADDRESS

ADVANCE

PULSE

PULSE

NITROGEN

LASER

Figure 3. Block diagram of a system for measuring atomic mean lives utilizing the strong

decay signal produced by a pulsed tunable dye laser as the source of excitation. Systems

similar to this have been described in references (11) and (12).

References

[ 1] Tuccion, S. A., Dubrin, J. W. , Peterson, 0. G., and Snavely, B. B., IEEE J. Quantum

Electron. QE-10 , 790 (1974).

[ 2] Tuccion, S. A., Foley, R. J., Dubrin, J. W. , and Krikorian, 0., IEEE J. Quantum

Electron., QE-11 , 101D (1974).

[ 3] Janes, G. S., Itzkan, I., Pike, C. T. , Levy, R. H., and Levin, L. , IEEE J. Quantum

Electron., QE-12 , 111 (1976).

[ 4] Klose, J. Z. , Proceedings of the SPIE/SPSE Technical Symposium East, March 22-25,

1976, Vol. 76 (in press).

[ 5] Voigt, P. A., Phys. Rev., A 11_, 1845 (1975).

47
[ 6] Voigt, P. A., and Kornblith, R. L. , J. Opt. Soc. Am. , 66, 492 (1976).

[ 7] Klose, J. Z., Phys. Rev., A V\_, 1840 (1975).

[ 8] Corliss, C. H. , J. Res. Nat. Bur. Stand. (U.S.), Sect. A 80, 1 (1976).

[ 9] Carlson, L. R. , Paisner, J. A., Worden, E. F., Johnson, S. A., May, C. A., and

Solarz, R. W. , Lawrence Livermore Laboratory Report UCRL-77832, 1976 (unpublished)

[10] Radziemski, L. J., Jr., Gerstenkorn, S., and Luc, P., Opt. Commun. , 1_5, 273 (1974)

[11] Haroche, S., Paisner, J. A., and Schawlow, A. L., Phys. Rev. Lett., 30, 948 (1973)

[12] Schenck, P., Hilbron, R. C. , and Metcalf, H., Phys. Rev. Lett., 3J_, 189 (1973).

48
PRINCIPLES OF PHOTOCHEMICAL SEPARATION OF ISOMERIC NUCLIDES 1

G. C. Baldwin 2

, H. M. Clark, D. Hakala, and R. R. Reeves

Rensselaer Polytechnic Institute

Aside from intrinsic scientific interest, photochemical separation of isomeric states

of a single nuclide offers one possibility of achieving a nuclear population inversion,

essential to the development of gamma ray lasers [1,2] 3

. Just as for isotope separation,

photochemical methods of isomer separation depend upon the existence and magnitude of

spectral differences between the species to be separated exceeding the resolution of tunable

optical sources.

Mercury has already been shown to be capable of isotopical ly-selective photochemical

reaction using 254 nm radiation [3]. These studies have been performed on the even mass

number stable isotopes {i.e. 198 and 202) which have zero nuclear spin and one hyperfine

component per isotope within the hyperfine structure of the 254 nm line. Mercury 197 m, the excited nuclear state, has a half-life of 24 hours, with 3 hyperfine components due

to the nuclear spin splitting effect [4,5]. The ground state, Hg 1979, is also radioactive

with a half-life of 64 hours, but the nuclear spin splitting results in a doublet [4,5].

Mass, spin, quadrupole, and nuclear volume effects give rise to shifts and splittings

of the optical energy levels of atoms and molecules. In the case of mercury, its optical

spectrum has been shown to contain several lines having well -resolved hyperfine structures

characteristic of the different isotopic species. The hyperfine structure of the 254 nm

has been measured for Hg 197m and Hg 197 9[5]. Isotopes have been separated by chemical

reaction of mercury atoms selectively excited at 254 nm by means of a mercury lamp

incorporating a single isotope (or enriched in a single isotope). The excited atoms can react with oxygen to produce isotopically enriched HgO which may be recovered [3].

In a similar manner it should be possible to excite and separate the Hg 197m . However,

a different source of radiation is needed. The specific wavelength required may be

obtained by means of a nitrogen laser coupled to a tunable dye laser. The 254 nm line can then be scanned using a nonlinear potassium pentaborate (KPB) doubling crystal. Resolution

is in the order of .0008 nm using an incavity etalon with such a laser. One absorption

line of the triplet of the Hg 197m at 254 nm is almost 0.002 nm from the nearest Hg 197 9 component and about 0.0026 nm from the nearest hyperfine component of any of the stable

isotopes [5]. Therefore it should be possible to recover the Hg 197m enriched with respect

to Hg 197 9 and the other isotopes. The degree of isomer enrichment may be determined by

measurement of the characteristic radiations emitted in the decay of the Hg 197m and Hg 197 9. It is noted also that the use of recoil-based radiochemical isotope enrichment (the SzilardChalmers process) during the preparation of the isomeric species would facilitate the

subsequent photochemical enrichment of the isomeric state.

1 Partially supported by the National Science Foundation.

20n leave to Los Alamos Scientific Laboratory, 1975-1976.

3 Figures in brackets indicate the literature references at the end of this paper.

49
Another element which has been studied for photochemical isotope enrichment [6] and

has isomeric states of reasonable lifetime is bromine. Selective excitation of the molecule,

Br , results in the formation of bromine atoms. Reaction of these atoms with HI can produce isotopic enrichment in the product formed initially. Bromine-80 has a metastable

state with a half-life of 4.4 hours and a ground state with a half-life of 17.6 minutes.

It should be feasible to separate these isomers by a photochemical process if the molecules

containing the isomers has not been measured, but isomer shifts should exist for these

species compared to the normal bromine.

References

[ 1] Baldwin, G. C, and Khokhlov, R. V., Physios Today, 28, (2), 32 (1975).

[ 2] Letokhov, V. S. , Soienoe, 180, 451 (1973), 190, 344 (1975).

[ 3] Gunning, H. E., and Strausz, 0. P., in Advances in Photochemistry, ed. by Noyes,

Hammond, and Pitts, Vol. I, p. 209 (1963).

[ 4] Lederer, C. M. , Hollander, J. M., and Perlman, I., Table of Isotopes, John Wiley (1967).

[ 5] Mellissinos, A. C, and Davis, S. P., Phys. Rev., 115 , 130 (1959).

[ 6] Leone, S. R. , and Moore, C. B. , Phys. Rev. Letters, 33_, 269 (1974).

50
THE EARLY YEARS OF PHOTOCHEMISTRY IN THE VACUUM ULTRAVIOLET

Wilhelm Groth

Institut fur Physikal ische Chemie der Universitat Bonn

o o The wavelength region between 1850 A and about 1250 A was opened to spectroscopy

around 1900 through the pioneering work of V. Schumann. Photochemistry, which is the

younger sister of spectroscopy, reached the Schumann region only after 1930. After shortly

describing the importance of photochemistry for the kinetics of thermal reactions,

especially for the mechanism of chain reactions, the first spectroscopic investigations in

the Schumann ultraviolet with the light of condensed sparks between metal electrodes and

with quartz optic are mentioned. Difficulties originated in the transparency of the window

materials, the absolute energy measurements and above all in the lack of suitable monochromatic light sources.

Starting with the development of the high-current low-voltage xenon lamp of Harteck and

Oppenheimer the technical improvements of the rare gas lamps are described: the application

of low-current glow discharges, micro-wave discharges, and the use of other rare gases xenon,

krypton and argon.

In the beginning, the absorption and primary photochemical and total reactions of simple

gases were investigated: oxygen, nitrogen, hydrogen, ammonia, carbon dioxide, carbon monoxide water vapour, and some organic compounds like methane, ethane, and propane.

Remarkable are some experiments carried out by the author and Hans Suess in 1937 who

illuminated mixtures of water vapour and carbon dioxide by the xenon resonance wavelengths.

The following primary reactions occur:

C0o + h v -> CO + 0 (1)

A

H

2 0 + hv * OH + H (2)

H atoms react with CO to give formaldehyde or glyoxal according to

H + CO (+M) - HC0 (+M) (3)

2HC0 —H

2 C0 + CO (4)

(HC0),

This process was assumed to have played an important role in the earth's atmosphere. It would give an explanation for the first appearance of free oxygen in the primitive atmosphere and for the formation of certain carbon compounds that were probably the pre-requisite

for the evolution of organic life.

51
Later on the hypotheses of Oparin, Urey, Kuhn, and Bernal assumed that the earth's

primitive atmosphere contained hydrogen, methane, and water vapour. Since the greater part

of the energy available in the earth's atmosphere would be contributed by the ultraviolet

radiation of the sun, the earlier photochemical experiments were resumed in 1957 by the

author and Hanns von Weyssenhoff, using mixtures of mehtane of ethane with ammonia and water

vapour in mercury-sensitized as well as digect photolysis experiments. For irradiation the resonance wave lengths of mercury at 2537 A and 1849 A, of xenon at 1469 A and 1296 A, and

of krypton at 1236 A and 1165 A were used. Amino acids (glycine, a-alanine, and a-aminobutyric acid), and fatty acids (formic acid, acetic acid, propionic acid), and hydrocarbons

could be detected in several experiments. These results were confirmed in 1959 by Terenin,

in 1961 by Dodonova and Siderova, and in 1966 by Ponnamperuma and Flores.

Special photochemical reactions in the Schumann ultraviolet which were carried out in the 1 930 ' s with a xenon lamp were concerned with the reactions of 0 atoms. It could be

shown that even at extremely small oxygen partial pressures in mixtures with hydrogen and

carbon oxide the ozone formation in a triple collision predominates the primary attack on hydrogen or carbon oxide molecules. It was assumed that the primarily formed energy rich ozone at low 0 atom concentrations reacts with the molecules before it loses its energy by

col 1 isions

.

Also the nitrogen oxidation was investigated in the light of the xenon lamp. N 20 could be detected mass-spectrotnetrical ly; about 10"** of the 0 atoms formed N 20. The small

amount of N 20 which is found in the earth's atmosphere can at least partially be produced

by this photochemical reaction.

52
FLUORESCENCE AND DECOMPOSITION OF TERTIARY AMINES IN A GLOW DISCHARGE

Francoise Lahmani 1 and R. Srinivasan

IBM Thomas J. Watson Research Center

Yorktown Heights, New York 10598

1. Introduction

Although electric discharges in gases have been extensively used to produce the excited

states of many atoms and small molecules which were subsequently studied by spectroscopy,

little attention has been given to large polyatomic, fluorescent molecules (with the exception of benzene and toluene). Tertiary aliphatic amines which have been shown to be strong

emitters in the near ultarviolet [l] 2 should offer an interesting class of compounds for

study. We have conducted an investigation of the fluorescence and decomposition of such

tertiary amines in a continuous electric discharge in a flow system. In this communication,

i we shall confine our remarks to diazabicyclo[2.2.2]octane (DABCO) which seems to be typical

of these systems.

2. Experimental

The flow system and discharge that were used were unexceptional. The flow rates were

about 0.02 mole/hr at the vapor pressure of DABCO which ranged from 0.03 to 0.3 torr. The

light emitted from the discharge was analyzed by a 1 -meter, Jarrell-Ash spectrometer and

detected by a EMI photomultipl ier (S-20 response).

3. Results

The following is a summary of the results that were obtained:

1) In pure DABCO vapor (0.3 torr), at the lowest voltage at which a discharge could

be sustained (60 V/cm) a purple glow was observed which on spectroscopic analysis was found

: to be identical to the fluorescence of DABCO vapor that has been reported [1].

2) With an increase in the energy of the discharge, the intensity of the light that

was emitted increased. A new emission which consisted of the CN* violet system at 421.6,

388.3, and 359.0 nm was observed. With an increase in voltage, the intensity of CN*

emission increased rapidly while the fluorescence of DABCO remained steady or decreased

slightly.

3) Addition of helium sharply increased the current as well as the emission from CN*.

• Argon produced qualitatively the same effect as helium. At a given voltage and a constant

ratio of helium to DABCO, the intensity of CN* emission was roughly proportional to the current up to a 20-fold increase. At the same time, the fluorescence of DABCO was only

slightly increased or not at all.

i On leave from Laboratorie de Photosphysique Moleculaire University de Paris Sud, Orsay,

France.

2Figures in brackets indicate the literature references at the end of this paper.

53
4) The CN*(B 2 z) * (X 2 e) violet bands Av=0 Av=-1 were studied under high resolution in

order to obtain information on the vibrational and rotational intensity distributions. The

weakness or absence of anomalous distributions in the intensities indicated that the first

excited electronic state A

2 n of CN was of minor importance relative to B

2

i in this study.

5) The intensity distribution pattern was the same (within experimental uncertainty)

in the presence of absence of helium or argon, with one exception.

6) Under conditions in which only the fluorescence of DABCO was observed, no decomposition products were detected. When CN* emission was also present, the products

that were detected were C 2 hydrocarbons, HCN, and a reactive small molecule which may be

C 2 H 3N.

4. Discussion

Present observations suggest that there are two independent routes to the excitation

of DABCO in an electric discharge. These consist of

1) One that leads to excitation to the fluorescent electronic state which has been

tentatively identified by Halpern [1] as a low-lying optically forbidden singlet state with

a lifetime of 1040 nsec.

2) A second path which induces the dissociation of the molecule. The mechanism by

which DABCO dissociates and gives rise to CN* B

2

e cannot be deduced from the present experiments, but it is obvious that CN* cannot be a direct dissociation product of DABCO.

The observation that the distribution of vibrational and rotational energy in the B

2

z state appears to be normal (without any population inversion) is consistent with other

studies on electric discharges. It suggests that either a thermally equilibrated ground

state of CN is the precursor to CN*(B 2 e), the excitation being due to collisions between CN

and electrons, or that CN* is produced in a dissociation process from unstable intermediates

without much change in the equilibrium C-N bond distance. This kind of intensity distribution in CN*(B 2 e) has been obtained in electron impact induced dissociation of cyanides [3]

and in the photodissociation of HCN [4]. It is markedly different from results obtained in

the reaction of active nitrogen with hydrocarbons [5] or dissociative excitation of halogen

cyanides by collision with metastable argon atoms [6].

The enhancement of the decomposition of DABCO relative to its fluorescence on the

addition of helium or argon confirms the existence of two parallel pathways for the excitation of DABCO.

References

[ 1] Halpern, A. M. , Mol. Photochem., 5, 517 (1973).

[ 2] Pearse, R. W. B., and Gaydon, A. G., The Identification of Molecular Spectra, John

Wiley and Sons, Inc., New York (1963).

[ 3] Tokue, I., Urisu, T., and Kuchitsu, K. , J. of Photochem., 3, 273 (1974-1975).

[ 4] Mele, A., and Okabe, H., J. Chem. Phys., 51_, 4798 (1969).

[ 5] Kiess, N. H., and Broida, H. P., 7th Symposium on Combustion 1958, Butterworth (1959),

p. 207.

[ 6] Coxon, J. A., Setser, D. W. , and Duewer, W. H., J. Chem. Phys., 58, 2244 (1973).

54
PHOTOCHEMISTRY AND FLASH PHOTOLYSIS OF 5-NITROQUINOLINE

A. C. Testa and A. Cu Department of Chemistry

St. John's University

Jamaica, NY 11439

As an extension of our photochemical studies on aromatic nitro compounds into heterocyclics we have investigated the photochemistry of 5-nitroquinol ine, which in the presence

of 50% isopropyl alcohol -water solutions containing HC1 results in photoreduction to a chlorinated 5-amino-quinol ine. The quantum yield of 313 nm photolysis, which proceeds via

the triplet state, increases with HC1 concentration and levels off at a maximum value of

5.4 x 10-2 when (HC1) _> 1 M. Photochemical disappearance is attenuated in sulfuric acid

solutions as well as in near isopropyl alcohol, which indicates that the chloride ion serves as an electron donor to the triplet state of the aromatic nitro compound. Measured

triplet yields for 1- and 2-nitronaphthal ene are 0.63 and 0.83, respectively, and it is

reasonable to expect a comparable value for 5-nitroquinol ine.

In order to evaluate the importance of the chloride ion as an electron donor we have

carried out a flash photolysis sutdy of this molecule in a variety of solvents with the aim

of trying to identify triplets, anions and netural radicals that may account for its photochemical behavior. Although the absorption spectrum of neither the triplet nor the radical

anion is known we felt that a comparison of photochemical and flash photolysis data might

provide the needed correlation to assign any observed transients.

Flash photolysis of 5-nitroquinol ine does not result in any triplet-triplet absorption

for any of the solutions studied, which suggests that the triplet is short lived (<40 usee).

That the triplet is populated can readily be discerned from the phosphorescence yield of

5-nitroquinol ine in EPA at 77 °K, which was measured and found to be 0.27. The distinct

0-0 band, vibrational structure and similarity to naphthalene and nitronaphthalene phosphorescence provide evidence that the lowest triplet is tt,tt*. The photochemically active medium

where reduction occurs with 313 nm photolysis is 50% isopropyl alcohol-water containing HC1

,

and flash photolysis of these solutions do result in the production of two species absorbing

at 550 and 410 nm with lifetimes in the millisecond time range and exhibiting first order

decay kinetics. The optical density of the 550 transient increases up to (HC1) ^0.5 M, and

then decreases rapidly for increasing acidic solutions. In contrast, the optical density

of the 410 transient in 50% isopropyl alcohol-water increases continuously from 0.02 M to

6 M HC1, and is also observed in 12 M HC1

In view of the necessity of HC1 and isopropyl alcohol for formation of the 550 transient

we assign its absorption spectrum to the radical anion generated by electron transfer from

the chloride ion. We have already reported a similar event from nitrobenzene and 4-nitropyridine. Since the first order decay constants for the 550 and 410 transients each remain

constant in the acid range 0.5-3 N HC1 , we believe that an equilibrium is operating between

the anion and its protonated form. Thus, we consider it an attractive possibility to consider the 410 transient as the protonated anion.

Despite the uncertain assignment of the 410 transient, flash photolysis data support

the view that the photoreduction of 5-nitroquinol ine occurs via two transients. It appears,

however, that alcohol is not necessary for the photoreduction of 5-nitroquinol ine since the

quantum yield in 12 M HC1 has been determined to be 5.7 x 10" 2

. A summary of the photochemical quantum yields, optical density of the 550 and 410 transients, as a function of

HC1 concentration are summarized in figure 1. It is seen that the maximum optical density

for the 550 transient occurs where the disappearance quantum yield has been observed to achieve its upper limit of 5.4 x 10" 2

. That this transient is not present in near isopropyl

55
alcohol or 50% isopropyl alcohol with 6 N sulfuric acid underlines the importance of the

chloride ion as an electron donor. This transient is absent in air-saturated solutions and

further supports the view that it is the radical anion generated via electron transfer, i.e.,

ArN02* 3 + Cl" + ArN0 2 ~ + Cl«. The lifetime of this transient is 5.3 msec. Since the 410

absorption can be observed in 12 M HC1 as well as in 50% isopropyl alcohol -water with

6 N HC1 , it appears that the alcohol may not be the predominant species responsible for its

formation. The lifetime of the 410 transient is 1.1 msec.

Figure 1. Absorption data for the 410 (a) and the 550 nm ( ) transient as a function of HC1 concentration. (0) = 313 nm quantum yields.

It is evident that for solutions in which (HC1) > 0.5 M, the quantum yield remains constant, but the optical density of the 550 nm transient decreases while that of the 410 in- creases. In the acid range less than 0.5 M the intensity of both species is increasing as

is the photochemical quantum yield. It thus seems reasonable to conclude that both transients

contribute to the observed photoreduction. The relatively large phosphorescence yield for

5-nitroquinol ine suggests that its triplet-triplet absorption should be readily observed in

laser flash photolysis.

56
PHOTOCHEMISTRY AND PHOTOPHYSICS OF NITROGEN HETEROCYCLES

Marian S. Henry and Morton Z. Hoffman

Department of Chemistry

Boston University

Boston, MA 02215

Aromatic N-heterocycle compounds, such as 2,2 1 -bipyridine (bipy), provide an opportunity

to examine the effects of varying molecular structure on the energetics and reactivities of

the excited states. The nitrogen heteroatom is an intimate part of the aromatic structure

and, being a Lewis base, is subject to protonation and coordination to metal ions. These

latter processes, as well as substitution on the aromatic system are expected to have a significant effect on the excited states and their observed photochemistry and photophysics.

This research is directed specifically to the establishment of structural -energeticreactivity relationships for the excited states of these molecules.

The initial phase of this research has involved the characterization of the photophysics

and photochemistry of bipy. The ground and flash photolysis transient absorption spectra,

and fluorescence excitation and emission spectra have been examined as a function of pH.

Acid dissociation constants have been calculated for the ground state (So), the fluorescent

state (Si), the flash photolysis transient (Ti), and the species generated from Jy, upon the

absorption of light (T 2 ). The results are given in Table I. Results for So are in agreement with those reported by McBryde [I] 1

. Relative fluorescence intensities are 1000:1:5

for bipy, bipyH+

, and bipyH£+ respectively. Fluorescence excitation and emission spectra

show a good mirror-image relationship for bipy and bipyH£+

, but not for bipyH+

.

Table I. pK, values for bipy

a Equil ibrium S

a

s

b

b

l T

C

'2 bipyH+ * bipyH

+

+ H+ -0.5 -7.8 -0.5 -1 .4

bipyH

+ % bipy + H + 4.5 3.8 5.0 6.7

a. Calculated from pH dependence of absorption spectra.

b. Calculated from pH dependence of emission spectra.

c. Estimated from Tx absorption and Forster cycle [9,10].

• i Figures in brackets indicate the literature references at the end of this paper.

57
In the ground state, bipy is known to be either cis- planar (bipyH ) or trans- planar

(bipy H£+ and bipy) [2,3]. The mirror-image relationship between absorption and fluorescence

spectra and the small Stokes' shift found for bipy H<? + (3.5 kK) and bipy (2.1 kK) suggest a small geometry change upon excitation. Inasmuch as the ground state extinction coefficients

are similar [4], the radiative rate constants are expected to be similar. Therefore, the

decreased fluorescence intensity of bipy, relative to bipy H£+

, is attributed to much faster

nonradiative processes, either inter-system crossing or internal conversion. A significantly

larger Stokes' shift (5.8 kKO and the lack of a mirror-image relationship for bipyH+ point

to a change in geometry in the S 0 —

x process. The effect may be due to twisting of the two pyridine rings.

The absorption spectrum of the flash photolysis transient Ti is very similar to that of

the biphenyl triplet [5] and is assigned to that of the free base triplet state. Its decay

is second-order with no ionic strength dependence. By measurement of the rate of hydrogen

atom abstraction from isopropyl alcohol the lowest free base triplet state has been found

to be of TT,ir* character, in agreement with Gondo and Kanda [6]. In acidic solution, the

decay of triplet bipyH+

is slower but also second-order with an ionic strength dependence

consistent with the excited state having a +1 charge. In very acidic solution, tirplet

bipyH+ decays via first-order kinetics with k = 3 x 10 3 sec

-1

. It is apparent that a competition exists between uni- and bimolecular decay under the relatively high concentration of triplet produced by the flash. Triplet bipy does not react with halide ions but

triplet bipyH+ reactions with I" and Br", but not CI". Finally, we have found that triplet

bipyH+

is quenched by Mn 2+ but not by Zn 2+.

The pKa values of Ti and T2 are similar to those for So while those of Si, particularly

for bipyH|+

, are highly shifted to less positive values. The sensitivity of the Si equilibrium bipyH+ * bipy + H + towards buffer concentration indicates short fluorescence

lifetimes [7,8]. Thus the position of the equilibrium is set by pH and the extent to which

equilibrium is reached before fluorescence occurs is set by the buffer concentration. It

is therefore possible to select the state of protonation of the two excited states, Si and

Ti , which most determine the photochemistry (Table II).

Table II. State of Protonation of Excited States

pH range S-j T-|

< (-8) bipyH 2 * bipyH2/

(-7) - (-2) bipyH

+ bipyH£

+

0-3.5 bipyH

+ bipyH

+

^4.8 bipy bipyH

+

> 6 bipy bipy

58
References

[ 1] McBryde, W. A. E. , Can. J. Chem., 43, 3472 (1965).

[ 2] Nakamoto, K. , J. Phys. Chem., 64, 1420 (1960).

[ 3] Spotswood, T. McL., and Tanzer, C. I., Aust. J. Chem., 20, 1227 (1967).

[ 4] Linnell, R. H., and Kaczmarczyk, A., J. Phys. Chem., 65, 1196 (1961).

[ 5] Porter, G. , and Windsor, M. W., Proa. Roy. Soc, 24 5A , 238 (1958).

[ 6] Gondo, Y. , and Kanda, Y. , Bull. Chem. Soo. Jap., 38, 1187 (1965).

[ 7] Lasser, N. , and Feitelson, J., J. Phys. Chem., 77, 1011 (1973); 79, 1334 (1975).

[ 8] Schulman, S. G., and Capomacchia, A. C, J. Phys. Chem., 79, 1337 (1975).

[ 9] Ireland, J. F., and Wyatt, P. A. H. , J. C. S. Faraday I, 68, 1053 (1972).

[10] Jaffe, H. H., and Jones, H. L., J. Org. Chem., 30, 964 (1965).

59
THE PHOTOPHYSICS OF SEVERAL CONDENSED RING HETEROAROMATIC COMPOUNDS

F. Sheldon Wettack, Robert Klapthor, Allan Shedd, Mary Koeppe, Ken Janda,

Patti Dwyer, and Kathleen Stratton

Department of Chemistry

Hope College

Holland, MI 49423

As a part of our interest in the photophysics of aromatic compounds, we have examined

the series of condensed-ring heteroaromatic compounds, indene, indole, benzofuran and

benzothiophene. The ring system of these compounds can be considered to be isoel ectronic

with naphthalene, but with less symmetry and a variable heteroatom. Previous work on absorption [I] 1 and fluorescence [2] and the variation in such properties as weight,

aromaticity, and vibrational frequencies combine to yield an ideal series for a photophysical

investigation. In particular we have been interested in learning of the role of the

heteroatom in radiative and non-radiative processes from both the singlet and triplet states. Although work directed toward this end has been carried out in both the condensed and gas

phases, the results reported in this paper will be confined to the condensed phase.

Results at 298 K

In the liquid phase at room temperature the fluorescence of the series varies

significantly. Quantum yields are shown in Table I. Also shown are fluorescence lifetimes

measured for the same solutions and calculated values of the radiative (k f

) and nonradiative

(k nr

) rate constants from the excited singlet state.

Table I 7 7 7 Compound <(. f

i f (nsec)

4> isc

k f (x 10

_/

) k nr

(x 10

_/

) k isc

(x 10"')

Indene 0.073 12 0.67 0.61 7.7 5.9

Indole 0.41 9.0 0.07 4.6 6.6 0.8

Benzofuran 0.23 7.5 0.07 3.1 13 0.9

Benzothiophene 0.014 ^0.3 0.73 4.7 340 240

We have also measured the intersystem crossing yield UjcC

) for these compounds using

the sensitized 1 ,3-pentadiene isomerization technique of Lamola and Hammond [3]. The

results are shown in Table I. The triplet yield measurements were carried out with benzene

and cyclohexane solutions at concentrations of 10-2 - 10" 3 M whereas the fluorescence

measurements were performed at 10-5 - 10~ 6 M in cyclohexane.

j

Figures in brackets indicate the literature references at the end of this paper.

60
Results at 77 K

Phosphorescence yields at 77 K in 2,2,4-trimethylpentane (isooctane) solvent and lifetimes in EPA are shown in Table II.

Table II

Compound Tp(sec)

p isc

Indole 0.40 5.6 7.1 x 10-2

Benzofuran 0.34 1.2 28 x 10" 2

Benzothiophene 0.23 0.44 52 x 10" 2

It was found that the most reproducible yields were obtained with isooctane solvent.

Cyclohexane, methanol and EPA were also used but problems with solubility, fractured cells

and nonreducibil ity were encountered. The yields shown in Table II are based on naphthalene

as a standard (<j> = 0.051).

Experimental

In all of the work, spectra corrected for emission detector sensitivity were used to calculate the yields. In addition, the effective optical density over the excitation bandwidth was used to account for the relative absorption by standard and unknown. The

phosphorescence spectra and lifetimes were obtained using a Farrand spectrofl uorometer

modified to accept cylindrical cells at liquid nitrogen temperatures. The fluorescence

spectra were taken on the same instrument and all absorption measurements were made on a Cary 14. Fluorescent lifetimes were determined by the time-correlated single photon counting technique. Each quantum yield was determined several times in independent experiments.

Reproducibilities in the fluorescence yields are better than 10% and the phosphorescence

1 yields are reproducible to 10%.

Discussion

At 298 K sufficient data are available to calculate k n

- S c> the rate constant for intersystem crossing, using <j>j sc

= k-j sc Tf. The results are included in Table I. Clearly the

sulfur heteroatom influences the rate of intersystem crossing in benzothiophene. Otherwise

Msc 9°es approximately as the degree of aromaticity in the compounds. It should be noted

that the <f>-j sc values were determined at higher concentrations than the fluorescence yields

and lifetimes. Preliminary work with indole indicates that if is concentration dependent

and hence the intersystem crossing yield is a minimum value. It is possible that k-j sc for

indole and, perhaps benzofuran, may be as much as an order of magnitude higher than the

j values shown in Table I.

At 77 K the relationship <f> p

= k p

T p

<j)-j sc holds. If one assumes that kn

3r

= 0, the

relationship <j>p/<j>f = k-j sc /kf, known as Kasha intersystem crossing ratio, is obtained.

If (j>f is taken to be 0.6 for indole [4], Msc is calculated to be 3 x 10 7 sec

-1

. If this

same value for k-j sc exists in the liquid phase at 298 K, <Hsc = 0-3.

The relationship <f>p/xp = k p

<t>i sc leads to the values of kp<)>j sc shown in Table II. These

values imply that kp and/or <j>-j sc are increasing in going from indole to benzothiophene at

77 K. Since the phosphorescence yields indicate that a significant increase in <t>-j SC is unlikely, one concludes that k is increasing.

61
We find no evidence of phosphorescence from indene at 77 K in isooctane. Phosphor- escence has been reported for EPA solutions. Whether this is an indication that the singlet

state photophysics of indene at 77 K is isooctane does not include intersystem crossing or whether all of the energy dissipation from the triplet state is of a radiationless nature

is yet unknown. An exhaustive search for phosphorescene in isooctane is continuing.

Benzothiophene, indole, and benzofuran show relatively high phosphorescence yields

compared to other hydrocarbon aromatic compounds and the values are consistent with emission

yields found for dibenzo condensed-ring heteroaromatic molecules. The fact that indene

shows no phosphorescence is consistent with the low yields found in compounds which strictly

involve tt-tt* excited triplet states (e.g., naphthalene, benzene).

References

[ 1] Hollas, J. M., Spectvoohimioa Acta, 19, 753 (1963).

[ 2] Van Duuren, B. L., Anal. Chem., 32, 1436 (1960).

[ 3] Lamola, A. M. , and Hammond, G. S., J. Chem. Phys., 43, 2129 (1965).

[ 4] Eisenger, J., and Navon, G. , J. Chem. Phys., 50, 2069 (1969).

[ 5] Birks, J.B., Photophysics of Aromatic Molecules, Wiley-Interscience, New York, 1970.

62
MOLECULAR WEIGHT DEPENDENCE OF TRIPLET-TRIPLET PROCESSES IN P0LY0(2-VINYLNAPHTHALENE)

Nicholas F. Pasch and S. E. Webber

Department of Chemistry

University of Texas

Austin, TX 78712

In 1969 Cozzens and Fox [I] 2 demonstrated the occurence of triplet-triplet annihilation

leading to delayed fluorescence in polymers of 1 -vinyl naphthalene (P1VN). Since that time

a number of observations of delayed fluorescence in polymer solutions and films for both

homopolymers and copolymers have been reported [2-5]. Kl offer and Fischer [5] have shown

previously that the intensity of delayed fluorescene in poly(vinylcarbazole) increases

with P, the degree of polymerization. It is our purpose to report some similar experimental observations of delayed fluorescence for solutions of poly(2-vinylnaphthalene)

(= P2VN) at 77 °K with P values from ^100 to <3000.

All polymers were synthesized using sealed ampules containing freshly sublimed

2-vinylnaphthalene in "spectro" grade benzene (^1.5 M) and initiated with AIBN (2 x 10" 3 to

1 x 10-2 M). The highest MW samples were obtained by eliminating the solvent from this

method. All MW's were determined by viscometry using the results of L. Utrecki, R. Simha,

and N. Eliezer [6].

After the initial set of experiments described herein was completed we received from

Professor R. Simha two samples of anionically polymerized poly(2-vinyl naphthalene) used

in reference 6. These samples were used as received and gave no evidence of any spectroscopically signficant contamination.

For all experiments we used a 1:1 THF:Et 2 0 (tetrahydrofuran :diethyl ether) glass at

77 °K. The THF was freshly distilled from lithium aluminum hydride with triphenylmethyl

radical as indicator. Diethyl ether usually was found to give a sufficiently weak phosphorescence (near 500 nm) to be used without further purification. For more recent experiments

the diethyl ether was freshly distilled from lithium aluminum hydride.

A solution approximately 10" 3 M in naphthalene groups was made up with the 1:1 THF:

Et 2 0 mixtures using quartzware that had been recently pyrolyzed with a Mekker burner to remove any organic residues. Successive dilutions were made in several cases and the

spectra of the 10" 3

, 10_lt and 10" 5 M solutions were compared. For the highest molecular

weight polymers there was a significant (^15%) increase of the delayed fluorescence

intensity relative to the naphthalene phosphorescence in the 10_1+ M solution compared to

the 10" 3 M. No significant change occurred in the next 10-fold dilution except for a diminished signal to noise ratio. Hence the results we report are for ^10-1+ M solutions

in naphthalene chromophores.

i Supported by the Robert A. Welch Foundation.

2 Figures in brackets indicate the literature references at the end of this paper.

63
A phosphorimeter of our own design was operated in the traditional way. A single

chopper alternately exposed the sample to excitation and then exposed a McPherson model

218 monochromator fitted with an EMI 6255S photomul tipl ier to the delayed emission.

Excitation and observation times were approximately 2.35 msec. For long time decay curve experiments a pair of Vincent Associates Uni-blitz shutters was used to cut off excitation

and expose the photomul tipl ier to the long lived emission at a given wavelength, selected

on the monochromater. For all decay curve measurements the photomul tipl ier current was amplified by a Keithley model 427 current to voltage converter and signal averaged by a Fabri-Tek model 1060 instrument computer (manufactured by the Nicolet Instrument Co.).

Sample excitation was effected by a 200 watt high pressure Hg lamp filtered by either a Corning 7-54 filter or a 7-54 plus an Oriel 313.0 nm interference filter.

The delayed emission spectrum of these polymers consists of two features: (1) a region

to the long wavelength side of 480 nm, typical of naphthalene phosphorescence, and (2) a peak around 340 nm, typical of naphthalene fluorescence. Hence triplet-triplet annihilation

leading to delayed fluorescence is present in P2VN. The relative intensity of the delayed

fluorescence to the phosphorescence is dependent on the molecular weight (MW) of the polymer.

In general the higher the MW the larger the ratio_I[)F/Ip (intensity of delayed fluorescence

to phosphorescence). We have plotted Ipp/Ip vs. P in figure 1. The choice of P for the

abscissa is for convenience of display.

-i 1 1 1i i 10 20 30 40 50 60

Figure 1. The ratio iQp/Ip vs. P. The solid line is from a Poisson distribution of excitons. The closed circles are for our original samples, the open circles for newer samples

of our polymers, and the open triangles are the samples provided by Professor Simha.

While the trend of the results is clear, individual differences between polymers of

approximately_the same MW prevent an unambiguous determination of the functional dependence

of Idf/ I P_on P* Two features worth noting are: (1) the apparent extrapolation of l^/lp to zero for P f 0, and (2) the apparent "saturation" of the Idf/ 1 ? ratio for the four polymers

with the highest MW. Observation (1) follows if one treats the number of triplets on the

polymer as obeying a Poisson distribution where the average number of triplets per polymer

chain is smaVI . This treatment yields the solid line in figure 1. Observation (2) is ex- pected when P exceeds the average diffusion length of the triplet exciton. However, since

two of the high molecular weight polymers were bulk polymerizations, it is not clear if they

should be regarded as typical of the solution synthesized polymers.

64
Certain general features of the decay curves for all polymer samples can be summarized:

(1) the delayed fluorescence decays much faster than the phosphorescence for the first 0.5

sec. After the delayed fluorescence has decayed to approximately 0.1 to 0.05 of its initial

value, the rate of decay of the del_ayed fluorescence is approximately twice that of the

phosphorescence for polymers with P greater than 700. This does not seem to be the case for

shorter polymers but poorer signal to noise ratios for delayed fluorescence in these samples

prevents a firm statement from being made. (2) Both delayed fluorescence and phosphorescence

display highly nonexponential decay for approximately 0.5 to 1.5 sec after cessation of

excitation, at which point the former is very weak and the latter appears to be nearly exponential. The non-exponential ity for phosphorescence decay is much more pronounced for

longer polymers than short polymers. (3) Only a slight dependence of the decay curves on I ex

is observed. This implies that the kinetic behavior of the triplets is dominated by uniexcitonic processes (emission, trapping, and radiationless transitions). (4) In general

the decay of delayed fluorescence is lower as the MW of a polymer increases, although

differences in individual samples preclude a clear-cut relation from being derived.

References

[ 1] Cozzens, R. F. , and Fox, R. B., J. Chem. Phys., 50, 1532 (1969).

[ 2] Fox, R. B. , Price, T. R., Cozzens, R. F., and Echols, W. H., Macromoleoules, 7_, 937

(1974); Fox, R. B., Price, T. R., Cozzens, R. F., and McDonald, J. R., J. Chem. Phys.,

57, 2284 (1972); Fox, R. B., Price, T. R., and Cozzens, R. F., Ibid, 54, 79 (1971).

[ 3] Helene, C, and Longworth, J. W. , J. Chem. Phys., 57, 399 (1972).

[ 4] David, C, Lempereur, M., and Geuskens, G., Europ. Polymer J., 8, 417 (1972) and

references therein to earlier work.

[ 5] Kloffer, W. , and Fischer, D. , J. Polymer Sci. } Symposium No. 40, 43, (1973).

[ 6] Utracki, L., Simha, R. , and Eliezer, N. , Polymer, 1_0, 43 (1969).

65
SINGLET AND TRIPLET PRECURSORS OF 0

2 'Ag

B. Stevens and J. A. Ors

Department of Chemistry

University of South Florida

Tampa, FL 33620

The quantum yield YMO2 of photosensitised addition of molecular oxygen to an organic

acceptor M is the product of the quantum yield yA of 0 2

1A formation and the efficiency

<(> Mq^ of 0 2 ^ addition to the acceptor (process 1) where

>2

M + 0

2

(

] A) + M0

2 (1)

0

2

(

] A) - 0

2

( 3z) (2)

'f'MO? ~ kiLM]/(k 1 LM] + k 2 ). Measurements of 7^02 as a f unct ion °f dissolved oxygen concen- tration have been analysed [I] 1 to identify the operative oxygen quenching process of

singlet (S^ and triplet (Tj) states of the sensitiser of which processes 3-6 are spinallowed and exothermic

s

l + o

2

( 3z) + T, + 0

2

(

] A) (3)

s

l + 02

( 3E) > T, + 0

2

( 3E) (4)

T

l + 0

2 (\) > S Q + 0

2

(

] A) (5)

T

l + 0

2

( 3z) -> S

Q + 02

( 3E) (6)

if both the singlet-triplet splitting aE$j (process 3) and triplet energies Ey (process 5)

exceed the excitation energy of at 800 cm" 1

. In terms of the parameters

a = k 3 /(k 3 + k^) and e = k 5 /(k 5 + k 6 ) the overall reaction quantum yield is given by eq.

(I) where y T

<- is

YM0

2

= *M0

2 {eY

IS + P 0

2 [a + e(1 -^IS ):iJ

T

Figures in brackets indicate the literature references at the end of this paper.

(I)

66
the sensitiser intersystem crossing yield and the probability that Si is quenched by O^a,

pr> = K[0 2 ]/(1 + K[0 2 J) is available from independent measurements of the Stern-Volmer

u 2 fluorescence quenching constant K.

Linear plots of the data yMg 2

( po 2

) Provide the quotients (Table 1)

^M0

2

(P 0

2

= 0)/YM0

2

(P0

2

= 1} = e W(a + e

> U

which are close to independent measurements of y (tetracene 2

) or to values estimated as

1 - Yp (DMBA) indicating that a << e for these compounds or that 4 and 5 are the dominant

quenching processes. However this conclusion is not unambiguous for those sensitizers

with high fluorescence quantum yield (rubrene and DMA) where the estimate of ytc as

1 - Yr is exceeded by the uncertainty in measurement of yf>

Table 1. Photoperoxidation Parameters in Benzene at 25°C

Sensitiser 10

3 e(M)a

Y

F Y IS lO^k^S"1

1 a + £

Rubrene 1.0 0.98 £ 0.02 < 3.0 1 9 + 0.4

DMA 1.2 0.90 < 0.10 £ 1.5 2 0 0.4

Perylene 0.89 < 0.11 < 7.1 2 2 + 0.3

Tetracene 2.4 0.19 0.68

b 120 1 2 + 0.2

DMBA 1.6 0.36 <_ 0.64 27 1 0 + 0.2

a ref. 5 b ref. 2

We have used competitive techniques to determine values for e = k 2 /ki for the

compounds listed in Table 1. This permits direct evaluation of yA as yMn Hun and eq. (I) in the form

M° 2 M° 2

YA = yMo

2

(1 + 3/CM]) = £Y

IS + Po^ct + e(l - Y

IS )] (III)

is used to present the experimental data ya(Pq ). Quantum yields y^q of self-sensitised

photoperoxidation of rubrene, tetracene, 9, 10-dimethylanthracene (DMA) and 9,10-dimethyl-1 ,2,-benzanthracene (DMBA) were estimated from recordings of the optical density

as a function of time at the actinic wavelength and the source intensity at this wavelength

determined by ferrioxalate actinometry. In the case of perylene, this was used to sensitise the photoperoxidation of DMA and DMBA at 436 nm where these acceptors are transparent at the concentrations used.

The data presented in figures 1 and 2 illustrate two behavioral patterns, viz

a) yA(Pq = 1) = a + e ^ 1.0 for those sensitizers with high intersystem crossing

. yield. Since y^(Pq = 0) = ey

is * Yj$ for these compounds (tetracene and DMBA), it

must be concluded that e £ 1 and a * 0 consistent with 4 and 5 as the dominant oxygen

quenching process.

67
b) ya(Pq = 1 ) = a + e 2.0 for rubrene, perylene and DMA which exhibit high fluor- escence quantum 2 yields (_> 0.9) and low intersystem crossing efficiencies. Since neither a nor c as defined can exceed unity, it must be concluded that a ^ e ^ 1 or that processes 3 and 5 are largely responsible for oxygen quenching of the singlet and triplet states of

these sensitisers. The reverse of process 3 represents the energy pooling process responsible [4] for the oxygen enhancement of fluorescence under conditions where triplet state relaxation is relatively slow.

2.0

1 0

• Rubrene 0 Tetracene

/• ^

Fig. 1

i 0 5 1.0

Figure 1

Since aE st exceeds 8000 cm" 1 for the sensitisers examined here, the nature of the

singlet quenching process is essentially determined by the relative rates of non-radiative

transitions from the complex formed initially 3 r.(Si 3 z) to lower energy complex states

3 r^(T 11 A) and 3 r (T 3 e). If processes 3 and 4 operate by an exchange mechanism these rates

will largely depend on the relative energies of these states viz

k 3 » k4

k 3 » k4

which reflect the ordering of molecular states

T

n

> S

1 > T

1 y IS - 0

S

l > Tn

> T

l Y

IS » 0

expected for sensitisers of high and low fluorescence yields and intersystem crossing rate constants kr<. which differ by an order of magnitude (Table 1). The quenching processes 3 and 4 should not therefore be mutually exclusive particularly for sensitisers where

3

r g (T

p

3 E) > \(S^z) > 3 T f

(T

} \)

3 v.(S

}

3 i) > 3

r g (T

n

3

E) > hf (J^L)

68
References

[ 1] Stevens, B., and Algar, B. E., Ann. New York Acad. Sci. } 171 , 50 (1970).

[ 2] Kearvell, A., and Wilkinson, F., Chem. Phys. Letters 11_, 472 (1972).

[ 3] Stevens, B., Perez, S. R. , and Ors, J. A., J. Amer. Chem. Soa. 96, 6846 (1974)

[ 4] Kenner, R. D. , and Khan, A. U., Chem. Phys. Letters 36, 643 (1975).

[ 5] Stevens, B., and Algar, B. E., J. Phys. Chem. 72, 2582 (1968).

69
THE HEAVY ATOM EFFECT ON THE PHOTOCHEMICAL CYCLOADDITION PROCESSES

OF ACENAPHTHYLENE

B. F. Plummer and Lou Jean Scott

Trinity University

San Antonio, TX 78284

Modification of photochemical processes through the influence of heavy atom perturbation continues to be a fruitful area of study [1] . Quantum mechanical interpretations

satisfactorily rationalize the effect of heavy atom perturbations. However, predictions of

specific molecular compounds that will show a measurable change in photochemistry under

heavy atom perturbation have not yet materialized [2].

The discovery by Cowan and Drisko that the photodimerization of acenaphthylene (A)

showed a dramatic heavy atom effect (HAE) [3] stimulated us to explore the potential of

this perturbation when applied to the cycloaddition reactions of other molecules to A. Compounds such as acrylonitri le, a-chloroacrylontri le, cyclopentadiene (CP ) and cis and

trans- 1 ,3-pentadiene were found to cycloadd photochemically to A when HA perturbation

in the form of brominated solvents was used [4].

The triplet nature of these direct irradiation processes was verified through S-V

quenching studies using cyclooctatetraene and ferrocene as quenchers. The triplet energy

of these quenchers lies in the range of 30-40 kcal/mole and thus establishes a lower limit

for the triplet energy of excited triplet A as near 40 kcal/mol. The product ratios from

direct and triplet sensitized radiations were compared for A reacting with CP and foand to be similar. The sensitizer used for these studies was the dicyclohexyl 18-crown-6-ether

complex of disodium Rose Bengal.

The influence of heavy atom solvents such as bromoethane, 1 ,2-dibromoethane , and

bromobenzene as compared to cyclohexane and acetonitrile was studied. The quantum yield

of the reaction was found to vary in a predictable way with a change in the concentration

of CF\ The mechanism of this reaction was found to conform to the general kinetic ex- pression in equation 1.

1 =1 +

1 k d1

v nsc nsc r

L J

[D] + [CP]

The quantum yield of reaction is 0

r

, a = the fraction of intermediate diradicals forming

product, k , = the rate constant of excited triplet state decay of A and k r

= the rate constant or bimolecular formation of intermediate from A and CP.

Using the reasonable assumption that 0. for A is unity in heavy atom solvents, a value for a of 0.2 was obtained from the kinetic data. Thus, about 20% of the diradicals

produced continue on to form products, the remainder apparently dissociating to form A and

CP.

i Figures in brackets indicate the literature references at the end of this paper.

70
A comparison of the external perturbation of bromine was made to internal perturbation by studying the photocycloaddition of 5-bromoacenaphthylene ( ABr ) to CP. Similar

kinetic expressions were obeyed for A and ABr reacting with CP. However, it was learned

that the internal heavy atom exerts a larger effect upon than does the external heavy

atom.

The unique capability of A to magnify heavy atom perturbation in conjunction with

its relatively poor fluorescence quantum yield < 0.001) and phosphorescence quantum

yield (0 ^ 0) has led us to explore the photochemistry of the acetyl derivative of A. The new molecule 5-acetoacenaphthylene (aA) was synthesized by acetylating acenaphthene

with acetic anhydride and magnesium perchlorate and then subsequently dehydrogenating the

saturated precursor with DDQ. Single photon emission studies of aA_ showed only a very

weak fluorescence near 400 nm and no detectable phosphorescence. The absorption spectrum

of aA resembled that of A except for a larger absorptivity coefficient at similar regions

of maximum absorption. A weak shoulder at 499 nm (e = 18) in cyclohexane that shows a bathochromic shift in ethanol suggests that the n -> tt* transition may occur at this

wavelength.

We have studied the excited state cycloaddition of aA to C£ in regular and heavy

atom solvents. The photoproducts formed are analogous to those formed between A and CP,

as verified by gc product distributions, nmr and ir measurements, and mass spectroscopic

analysis.

We have studied the direct and sensitized product distributions of the reaction of

aA with cP_ and find that they are similar. Additionally, the direct reaction can be

quenched with ferrocene. The kinetic studies of aA reacting with CP_ have been investigated

and data will be reported that suggest an intermediate role of the carbonyl gorup in aA

in effecting spin-orbital perturbation of the reactions of aA .

References

[ 1] Plummer, B. F., and Schloman, W. W., Jr., J. Amer. Chem. Soo. 98, 3254 (1976).

Cowan, D. 0., and Koziar, J. C, Ibid., 98, 1001 (1976).

Gutierrez, A. R., and Whitten, D. G., Ibid. , 96, 7128 (1974).

[ 2] Giachino, G. A., and Kearns, D. R., J. Chem. Phys. 52, 2964 (1970).

[ 3] Cowan, D. 0., and Drisko, R. L., Tetrahedron Lett., 1255 (1967).

[ 4] Plummer, B. F., Ferree, W. I., Jr., and Schloman, W. W., Jr., J. Amer. Chem. Soo.

96 , 7741 (1974) and referneces therein.

71
LASER INDUCED FLUORESCENCE EMISSION SPECTROSCOPY OF HgCOfA.Ag) 1

Kenneth Y. Tang and Edward K. C. Lee Department of Chemistry

University of California

Irvine, CA 92717

The single vibronic level (SVL) radiative lifetimes (ip) of the first excited

singlet state of H 2 C0(A 1 A 2 ) have been studied recently [l] 2

. The most interesting finding

was a large variation of the radiative lifetimes showing a general trend of increasing

radiative lifetime with increasing vibrational energy (E - b

) and expectedly shorter

values of t r for the SVL levels with one quantum excitation of the asymmetric C-H stretch

mode (v 5 ). More specifically, the radiative lifetime of the 5

1 level at E ., = 2968 cm" 1

, ipCS 1

) was found to be ^ 0.7 x 10" 6 sec, several times shorter than in(2 2 4') = 11 x 10" 6 sec at E ., = 3343 cm" 1

. Since the non-totally symmetric, out-of-plane puckering mode

(vij is rlnuch more active than v 5 in the absorption and in the emission spectra for electric

dipole forbidden but vibronically allowed transition ( 1 A2~ 1 Ai)> this behavior of the

radiative lifetime is very suprising. In order to establish the extent of the HerzbergTeller type vibronic coupling in the radiative transitions from the 5

1 level, we have

decided to determine quantitatively the activity of the v 5 mode in the molecular resonance fluorescence spectra from several single vibronic levels at low pressures (nearly

collision free). The most interesting results [2] have been obtained with the 5

1 and the

lM 1 levels which are nearly degenerate (2.7 cm T

) and perturbed by Coriolis coupling.

For the purpose of illustration, the spectra taken at 3207.0 ft (vac) and 3215.1 8

(vac) excitation with 0.5 torr of H 2 C0 at 23°C are shown in figure 1. The collisionally

relaxed spectrum obtained at A (vac) = 3207.0 A and 5.0 torr of added N 2 is shown in

the middle for comparison. Thepumping was achieved with a flashlamp-pumped dye laser

equipped with frequency doubling crystals and fine tuning etalons, and the fluorescence

was detected with an optical multichannel analyzer.

i

This research has been supported by the National Science Foundation and the Office of

Naval Research.

2 Figures in brackets indicate the literature references at the end of this paper.

72
5 level

X = 3215.1 fi (vac) ex

H

2 CO =0.5 torr

X = 3207.0 S (vac) ex

H

2C0 =0.5 torr

N

2 = 5.0 torr

,• •

• »..; , ».-. ' ' v.

l

1

^ 1 level

X = 3207. 0 S (vac! ex

H

2 C0 0.5 torr

Emission spectrum.

"1

3 :'0 1+00 U30

1

I

1 h60 (rim)

Despite our attempt to obtain an SVL spectrum from the "selective" excitation of the

o-vibronic level of either 5

1 or lU 1

, we were unable to obtain a "level" purity better

han 80% as Dyou can see from figure 1. We believe that the 3207.0 A excitation corresponds

n +h Q Uai kq 3 (j' = ig s k' = 4 * J" = 13, K" = 3) transition and 3215.1 A excitation

= 2 + J" = 7, K" = 3) transition and the overlapping

3) transition.

13 = K' J :ogresponds to the 5

1 'Q

3 (J' = 7, K

i lK

P 3 (j

1 = 2, K' = 2 J" = 3, K"

73
In examining the SVL emission spectrum we find that the intense bands in the emission

involve the odd quantum number change in the out-of-plane bending mode (vj and the progressions are formed by the C-0 stretch mode (v 2). Hence, the emission spectrum from the

5

1 level is similar to the one from the 4° level, showing always the 5l contribution as if

v 5 is the "frozen" mode. Likewise, the emission spectrum from the lH 1 level is also

similar to the one from the 4

1 level, again showing the 1 \ contribution as if vi is the

"frozen" mode. The most intense bands from the 5

1 level are 4^5 j, 2"J4?5} and 4§5}, where

as the most intense bands from the m1 level are 1^4^ and 112^4^.

Fluorescence gives spectra whose distinctive features characterize the emission from

the levels 5

1 and This makes us to believe that the intramolecular energy flow

between these two vibronic levels is slow compared to the time scale (10_8 sec) of the

unimolecular decay processes (mostly non-radiative), although these two states are nearly

degenerate with 2.7 cm

-1 of separation.

The fluorescence intensity Ip from an SVL is proportional to the radiative decay rate constant (k^) and the population (N). Since the 5

1 and the iM 1 levels are ^2.7 cm" 1 apart, the Boltzmannized population ratio at room temperature should be very close to unity:

N(5 1 )/N(1 M 1

) = 1.013. In the presence of 10 % 100 torr of N 2 , the 3207.0 % (5 1) and the

3215.1 A (l 1^

1

) excitations give an identical "equilibrated" spectrum consisting of the

emission from the two SVL's as shown in figure 1 (middle). Considering the lifetimes

(^15 nsec) and the pressures used, the collision induced vibrational relaxations to other

vibronic levels are negligible and that the 5

1 lty 1 relaxation is considerably more efficient as in rotational relaxation . The intensity ratios, Ip(5 1 )/Ip(l 1 4

1

) , obtained

from the two excitation wavelengths as a function of N 2 pressure gives a limiting value of

^1.8 at high pressures. Therefore, k^(5 1

) is 1.8 times greater than kR(l 1 4

1 ). Using the

value of kR(5 }

) = 1.3 x 10 6 sec-1 obtained previously [1] we calculate kp (1M 1) to be

^0.7 x 10 6 sec-1

. This value is greater than koH 1

) = 0.5 x 10 6 sec" 1 obtained previously

[1].

In conclusion, the present data clearly indicate that the radiative transitions (even

from the 5

1 level) show insignificant involvement of the v 5 mode for the Herzberg-Tel ler

type vibronic coupling but yet the overall radiative rate from the 5

1 level is 1.8 times

greater than that from the 1*4* level. Also, it appears that kR(l 1 4

1

) is also 1.4 times

as great as kR(4 x ). We believe that further theoretical advance should shed more light

on the significance of the "frozen" mode in enhancing the radiative transition rates.

References

[ 1] Miller, R. G., and Lee, E. K. C, Chem. Phys. Lett., 33, 104 (1975).

[ 2] Tang, K. Y., and Lee, E. K. C, submitted for publication to Chem. Phys. Lett.

74
LASER EXCITED N0

2 FLUORESCENCE LIFETIME STUDIES IN THE 600 NM REGION

V. M. Donnelly and F. Kaufman

Department of Chemistry

University of Pittsburgh

Pittsburgh, PA 15260

A partial analysis of the absorption and fluorescence spectra of N0 2 indicates that

the 2 Bi state is responsible for much of the absorption above 23,000 cm" 1 [l] 1

, while

below 19,000 cm" 1 the 2 B 2 state carries virtually all the oscillator strength [2,3].

The visible absorption spectrum is extremely complex due mainly to extensive perturbations

of the excited states by high vibrational levels of the ground state. This factor also

accounts, at least in part, for the fact that both excited states exhibit fluorescence

lifetimes much longer than the 0.3 ysec order of magnitude value calculated from the

integrated absorption coefficient. Broad band modulated excitation studies [4] have

reported fluorescence lifetimes of 50 to 90 ysec in the 400-600 nm region. Narrow

bandwidth pulsed laser experiments have shown that in the 451-461 nm region [5], decays

are slightly non-exponential with average lifetimes of about 80 ysec, while at 593 nm, decays are very non-exponential due to at least two components with lifetimes of 30 and

115 ysec [3]. We report here the results of laser excited_fluorescence lifetime experiments in the 578-612 nm region. At pressures as low as 10" 5 torr decays are highly

non-exponential for some excitation wavelengths, and nearly exponential for others.

These results can be explained in terms of a single electronic state ( 2 B 2 ) vibronically

coupled to high vibrational levels of the ground state.

Fluorescence was excited by a Nd-YAG pumped tunable dye laser with a bandwidth of

either 1.5 or 0.15 cm" 1. The fluorescence was detected through cutoff or interference

filters by a photomultipl ier tube which observed the central portion of a 111 spherical

fluorescence cell. Fluorescence decays were summed over lO 4 to 10 5 laser pulses on a multi-channel analyzer with 10 ysec per channel time resolution. Nonexponential decays

were fit to the expression

I(t) = Re" t/TS + e" t/xL

(1)

Very good fits were obtained in all cases. However this does not prove that decays are

simple biexponentials, so the parameters t s

, t. and R should be interpreted with caution.

Decays were recorded as a function of excitation energy and pressure (P < 25 mtorr). For excitation coincident with the strong 2 B 2 * 2 A!_absorption at 585, 593, and 612 nm, the

decays are highly non-exponential. Plots of t/" 1 and x," 1 vs. P extrapolated to zero

pressure give "lifetimes" in the 22-75 ysec range for t s and 170-260 ysec for x. . The

slopes of the t s _1 vs. P and x." 1 vs. P plots are typically 5 x 10" 10 and 0.75 k 10" 10 cm 3 sec" 1

, respectively. When the laser wavelength does not correspond to strong 2 B 2 features (578, 594, 603 nm) the decays are nearly exponential at all pressures. The

slopes and intercepts of the t" 1 vs. P plots at these wavelengths are in the same range

as the Ti~ 1 vs. P plot slopes and intercepts at 585, 593, and 612 nm. R in eq. (1)

varies from ^0 to 3 over the 578-612 nm region. Other experiments show that and

especially R vary strongly for small changes (0.15 cm" 1

) in the excitation energy. The

largest R and the shortest t<. correspond to strong features in the fluorescence excitation

spectrum.

i Figures in brackets indicate the literature references at the end of this paper.

75
The overwhelming spectroscopic evidene [2,3] against the existence of more than one strongly absorbing electronic state in this wavelength region leads us to believe that

these results are due to variable vibronic coupling between the 2 B 2 state and upper

vibrational levels of the ground state as suggested by Douglas [6]. Excitation at 578,

594, and 603 nm is due to high levels of the ground state which lie close enough to levels

of the 2 B 2 state so that they can borrow oscillator strength and make transitions possible.

Absorption to and fluorescence from such levels are weak since they are mostly 2 Ai in character and the transition is forbidden; hence the fluorescence lifetime will be much

longer than that calculated from the integrated absorption coefficient. At 585, 593, and

612 nm the upper state has more 2 B 2 character due to the coincidence of these wavelengths

with positions of zero-order 2 B 2 levels. Even so, the state populated is still mostly

2 Ax because of vibronic coupling. So at these latter wavelengths the average lifetime

would be expected to be shorter than at 578, 594, and 603 nm, but still much longer than

the integrated absorption coefficient value. Previous attempts to explain the lifetime

anomaly by electronic state mixing were not successful because the ratio of the density

of ground state to excited state vibrational levels is only about a factor of ten (at ^600

nm) leaving an unexplained factor of about 30. However, this calculation was based on a single excited state, whereas it has been demonstrated that over the entire absorption

region, two electronic states are excited. In order to get a more reasonable estimate we have assumed, first, that the 2 Bi state is responsible for all absorption at energies

above 25,000 cm" 1

; second, that all absorption at energies less than 18,000 cm" 1 is due to the 2 B 2 state; and third, that absorption by the 2 B 2 state is symmetric about a maximum at 20,000 cm" 1 where it is responsible for 70% of the total absorption. We thereby calculate

a lifetime of roughly 3 psec for the 2 B 2 state. When this is multiplied by a level density

ratio of 10 the lifetime anomaly for the 2 B 2 state is essentially removed. If this

argument is valid, then our data are qualitatively consistent with the thoery of Bixon and

Jortner [7]. This theory predicts that the type of vibronic coupling described above

results in many additional vibronic states, each with a different lifetime, the magnitude

of which is inversely related to the amount of excited electronic state character the state possesses. Due to the thermal distribution of rotational energy in the ground state, these

states are seen as overlapping vibronic bands in the room temperature absorption spectrum.

The rotational structure is very dense, so that even a narrow band width laser excites

more than one vibronic state, resulting in non-exponential fluorescence decays. The

strongest absorbing states near 585, 593 and 612 nm possess the shortest lifetimes, so exciting here one finds the most highly non-exponential decays. At wavelengths which do not correspond to these states (at 578, 594 and 603 nm) decays are more nearly exponential.

This explanation should help in an understanding of that part of N0 2 absorption due to the perturbed 2 B 2 electronic state.

References

[ 1] Douglas, A. E., and Huber, K. P., Can. J. Phys. 43, 74 (1965); Hardwick, J. L. , and

Brand, J. C. D., Chem. Phys. Lett. 21_, 458 (197377 Senum, G. I., and Schwartz, S. E.,

Chem. Phys. Lett. 32, 569 (1974).

[ 2] Abe, K., Myers, F. , McCubbin, T. K., Jr., and Polo, S. R. , J. MoT. Spectrosc. 50,

413 (1974); Brand, J. C. D., Hardwick, J. L. , Pirkle, R. J., and Seliskar, C. J.,

Can. J. Phys. 51, 2184 (1973); Smalley, R. E. , Wharton, K., and Levy, D. H., J. Chem.

Phys. 63, 4977~0975).

[ 3] Stevens, C. G. , Swagel , M. W., Wallace, R., and Zare, R. N., Chem. Phys. Lett. ^8,

465 (1973).

[ 4] Keyser, L. F., Kaufman, F., and Zipf, E. C, Chem. Phys. Lett. 2, 523 (1968);

Schwartz, S. E., and Johnston, H. S., J. Chem. Phys. 5J_, 1286 (1969); Keyser, L. F.,

Levine, S. Z., and Kaufman, F., J. Chem. Phys. 5^, 355 (1971).

[ 5] Sackett, P. B., and Yardley, Y. T., J. Chem. Phys. 57, 152 (1972).

[ 6] Douglas, A. E., J. Chem. Phys. 45, 1007 (1966).

[ 7] Bixon, M. , and Jortner, J., J. Chem. Phys. 50, 3284 (1969).

76
LASER FLUORESCENCE STUDIES, INCLUDING THE B

3 n(0

+

) STATES OF BrF, IF AND IC1

Michael A. A. Clyne, Alan H. Curran and I Stuart McDermid

Department of Chemistry

Queen Mary College

Mile End Road, London El 4NS, England

A number of new laser excitation spectra, mostly of transient molecules, are reported.

These include: (a.) the B

3 n(0 ) - X

1 ! transitions of BrF, IF and IC1 (440-630 nm); and

(b) the A

3 n - X1 ! transition of SO (259-261 nm). We also report that laser excitation of

CIO radicals is unobservably weak in the most favourable (263-280 nm) region of its

A

2 n -X 2 n system; fluorescence is therefore unlikely to be useful as a monitor for stratospheric CIO.

In this paper, we emphasize results of spectroscopic studies of the B

3 n(0

+

) - visible transitions of BrF, IF and IC1 , using laser fluorescence. Rotational constants

for the excited B state of BrF (8 >_ v

1 >_ 3) have been measured for the first time (in

collaboration with J. A. Coxon). An interesting predissociation at v' = 8, J

1 - 29 of

BrF B

3 n(0 ) has been identified. Results on this predissociation, and hopefully also on the lifetimes of the excited state, will be presented.

Strong fluorescence by the transient molecule IF also was observed over a wide

range of excited B state vibrational levels, up to v' = 11, J' = 45, where the onset of

predissociation is confirmed. The source of the transient BrF and IF molecules were the

rapid reactions (1) and (2) carried out in a discharge-flow system [E. H. Appelman and

M. A. A. Clyne, JCS Faraday I, 1975, 71_, 2072]:

F + Br

2 + BrF + Br

F + IC1 > IF + CI [2)

Observation of fluorescence from the B state of IC1 , which is very

+weakly bound,

was surprising. Because of the unexpectedly short lifetime of the 3

3 n(0 ) state,

fluorescence to the ground state from the normally dominant, but long-lived 100 ysec)

A

3 n(l) state of IC1 , could be quenched out almost entirely by using IC1 pressures near

5 Torr. In this way, the overlapping strong A-X fluorescence could be virtually

completely suppressed. Energy levels up to v' = 2, J

1 - 70, of I 35 C1 B

3 n(0 ) were

observed in fluorescence in the B-X transition. The dominant transition at 300 K were the

2,3; 1,3; 2,4 and 1 -,4 bands. Absence of fluorescence from v' = 3, which is sharp in

absorption, indicates that the B state of IC1 is completely predissociated above v' = 2.

RKGC potential energy curves and Franck-Condon factors for the B and X states of

IF, ICJ and IBr will be presented. The absence of detectable fluorescence from the

B

3 n(0 ) state of IBr will be discussed in the light of these results.

The non-collisional lifetimes of the B

3 n(0

+

) states of the halogens and interhalogens will be discussed, in relation to the corresponding B-X absorption spectra.

77

LASER-INDUCED FLUORESCENCE OF CN RADICALS PRODUCED BY PHOTODISSOCIATION OF RCN

Regina J. Cody and Michael J. Sabety-Dzvonik 1 Astrochemistry Branch

NASA/Goddard Space Flight Center

Greenbelt, MD 20771

Laser-induced fluorescence spectroscopy has become a useful tool in probing free

radical processes in photodissociation, flames, electric discharges, etc. With this

technique measurements have been made of radical properties and formation processes and

i! of the internal energy content of product radicals. In addition to laboratory dissociative processes, radical emissions are also widely observed in astronomical spectra,

including those originating from comets. Generally, the first radical emission observed

i as a comet comes toward the sun is the Violet Band system of CN. The source mechanisms

formany of these cometary radicals are unknown. It has been postulated that photodissociation of appropriate parent molecules by solar radiation could be a source for

these radicals. In this work we will report our observations on the CN radicals formed

by the VUV photodissociation of cyanoacetylene (C 2 HCN), which has been seen in interstellar space, and of methylcyanide (CH 3 CN) and HCN, both of which were observed in the

atmosphere of Comet Kohoutek.

The flash photolysis-laser induced fluorescence apparatus has been described

previously in detail [1-3] 2

. Two flashlamps with different spectral outputs are coupled

with various UV filters to dissociate the parent compounds in different absorption regions

above 135 nm. One flashlamp is a commercial xenon lamp with a spectral output above

190 nm, while the other is an argon lamp 0.6 ps FWHM, 0.8 - 1.0 joule, 50 pps, pressure

> 1 atm) with a spectral output above 135 nm (short wavelength cut-off of SrF 2 ). The

spectral outputs of both lamps were measured with a 1 m normal incidence spectrometer using

la sodium salicylate coated photomul tip! ier tube. The time delay between the firing of

the flashlamp and the

+ triggering of the N 2 laser pumped dye laser can be varied from 2 ps

to 20 ms. The CNjX 2 z ) fragments produced by the photolysis are excited by the dye laser

output to the B

2

e state via the Violet Band system, Au = 0 sequence. The fluorescence

radiated by these CN(B) radicals is detected by a photomultipl ier tube after passage

through an holographic grating monochromator which acts essentially as a broad band

interference filter (FWHM ^ 10 nm). The number of collisions which the radicals undergo

between formation and detection is controlled by the time delay and the total gas pressure.

Both pure parent gas samples and mixtures of RCN in N 2 or an inert gas are photolyzed.

Pure samples of C 2 HCN were photolyzed at pressures of 0.02 - 0.04 mm Hg and time

delays of 2-3 us, which correspond to a collision number of < 2 for a kinetic temperature

of 300 °K. The argon lamp coupled with either a SrF 2 windon (x > 135 nm) or a sapphire

window (x > 145 nm) was used for these photolysis experiments. As shown in fig. 1, the

spectrum of the CN radicals showed predominantly the 0-0, 1-1, and 2-2 bands with strong

bandheads for the 0-0 and 1-1 bands. After correction for the variation in laser intensity with wavelength, the intensities of the rotational lines in the R branches of

the 0-0 and 1-1 bands were computer fitted to the Maxwell -Bol tzmann equation to yield

rotational temperatures. The areas under the rotational distribution curves were

I.NAS/NRC Research Associate.

2 Figures in brackets indicate the literature references at the end of this paper.

79
CN FROM CYANOACETYLENE Ptotw=0.03? mm

t d = 2.0ms. &y = o Argon Lamp, Sr Fj

P0 a Kq.0

ftlllllll M il lllllllll l l

I J ill IN '

I I'' '

'

' J.' ' ' ' ' ' '

1 '

' ' '

1 0 10 5 I 0 5 It . IS 20 25 30 35 B| , 40 «

50

11 1 I II II llll ll l " I I I I II I I I I I I I I I I I I I I I I I

I I I I - 10 5 1 0 5 10 15 20 25 30 35 40 45

368 0 387.0 383.0 382-0

2 + Figure 1. Laser-Induced Fluorescence Spectrum of CN(X Z ) Radicals

integrated and ratioed to give the relative vibrational population distributions. Since

only a small number of the rotational lines in the 2-2 band could be measured due to the

low signal intensity, Boltzmann graphs for this band were not made. To obtain the vibrational population ratio for the o" = 2 level, individual line intensities were ratioed to the appropriate 0-0 lines, and these ratios were averaged. The results for the low

collision number experiments are summarized in Table I.

Table I. Summary of CN(X) from cyanoacetylene

EXCITATION VIBRATIONAL ROTATIONAL VIBRATIONAL VIBRATIONAL

WAVELENGTH LEVEL TEMPERATURE POPULATION TEMPERATURE

(nm) (K) RATIO (K)

> 135 0 1822 + 166 1.0

(SrF

2

) 1 1667+198 0.50+0.05 4300

2 0.32 + 0.07 5100

> 145 0 1368 + 123 1.0

(Sapphire) 1 1130+236 0.40+0.06 3200

2 0.18 + 0.05 3400

Previous work [2-4] has shown that CN formed in the A( 2

JTj) state, u = .0 level, can

be collisionally induced to transfer energy into the X state, u" = 4 level, through near energy resonances between certain rotational states in the A and X states. This energy

transfer results in an enhancement of the intensity of the 4-4 band at high collision

number. The vibrational population distributions resulting from the photolysis of high

pressure (P = 50 mm) mixtures of ^ 0.2% C 2 HCN in N 2 or He are shown in figure 2. For photolysis at X > 145 nm (sapphire) the vibrational curve shows that very little, if any,

CN is formed in the A state. However, for x > 135 nm (SrF 2 ) the vibrational distribution

indicates that a small amount of CN is formed in the A state. It is not possible to form

CN(B) in the photolysis, since the threshold wavelength for the production of B state is

<_ 131.8 nm [5], and a SrF 2 window was the shortest wavelength cut-off (a ^ 135 nm) used.

80
CN(X) FROM CYANOACETYLENE

V"

P

T < 0.040 MM C2 HCN, TD < 3 /J. S P

T

= 50 MM (~0.2 %C2HCN/M) , TD

= 10/J.S

Figure 2. Vibrational population distributions of CN(X 2

E +

) radicals.

Photolysis of HCN and CH 3 CN samples confirm that the CN radicals are formed predominantly in the A state. Photolysis at X > 135 nm of high pressure (P = 50 mm) mixture

of ^ 1% CH 3 CN in Ar (delay time = 3 ys) yielded a vibrational population distribution of

N 0 :Ni :N 2 :N 3 :N 1+ :N 5 = 1.00:0.78:1.21:4.63:5.80:1.47.

References

[ 1] Jackson, W. M. , J. Chem. Phys. 61_, 4177 (1974).

[ 2] Jackson, W. M. , and Cody, R. J., Ibid. , 4183 (1974).

E 3] Cody, R. J., Sabety-Dzvonik, M. J., and Jackson, W. M., submitted to J. Chem. Phys.

(1976).

j[ 4] Savety-Dzvoni k, M. J., Cody, R. J., and Jackson, W. M., submitted to Chem. Phys. Lett.

(1976).

[ 5] Okabe, H., and Dibeler, V. H., J. Chem. Phys. 59, 2430 (1973).

81
PHOTOFRAGMENT SPECTROSCOPY OF THE A ^ X TRANSITION IN CN USING TWO PULSED LASERS

I Sim

Andrew Baronavski 1 and J. R. McDonald

Chemistry Division

Naval Research Laboratory

Washington, DC 20375

1. Introduction

The CN radical has been studied extensively for many decades [1-4] 2

; still little is known of the dynamics of its formation and decay. Ling and Wilson [5] studied the photofragment spectroscopy of ICN with a high power (70 mjoule) laser photolysis source and mass spectrometric detection to determine the translational energy and angular dependence of the

CN radicals. Ling and Wilson inferred from their results that 60% of the CN is formed in the first excited electronic state (A 2 R\/ 2*3/1) anc' 40% is formed in the ground state with

a vibrational energy inversion as high as v =3 and a rotational temperature of a few thousand degrees Kelvin. More recent work by Savety-Dzvonik and Cody [6] uses flashlamp photolysis of ICN and fluorescence detection following excitation to the B %

+ state of CN. This work indicates that for photolysis at A _> 220 nm, the vibrational distribution of the

CN fragments maximizes at v" = 0 with very little v" = 1. They also report rotational

temperatures as high as 2700°K. In their work they were limited to detection at times

^3 psec after the photolysis pulse and to pressures at which several gas kinetic collisions

take place before detection. These workers report no detectible yield of CN A state under

these conditions. Clearly a severe conflict exists between the observations of the two groups of workers.

2. Experimental

We have designed a photof ragment spectrometer based upon fluorescence excitation detection of the fragment species. In our experimental system, (see fig. 1) a moderate power

Nd 3+ YAG laser is frequency quadrupled to 266 nm providing pulses with typically 80 joules

energy at 35 nsec FWHM. This laser is used as the photolysis source; excitation of the

fragments is accomplished by synchronizing this laser with a tunable flashlamp pumped dye

laser (Chromatix Model CMX-4). The probe laser is characterized by 0.06-2.6 mjoule pulse

energies depending upon wavelength and ^1 ysec FWHM pulses. Briefly, the internal trigger

circuitry of the CMX-4 laser is defeated; the sync out pulse from the CMX-4 triggers a double pulse generator which sends one pulse to trigger the YAG laser. Since there is a 250 ysec delay between the YAG lamp trigger and the internally generated Q-switch the second:

trigger pulse from the pulse generator must be delayed by a corresponding or greater amount.

j jj The second pulse from the pulse generator then goes back to trigger the sparkgap of the CMX4 laser. The result is a photolysis pulse which can be followed from 0 to approximately 250

ysec. laser by the probe pulse. The temporal jitter between pulses is +0.5 nsec. The

repetition rate is up to 30 Hz. In the experiment to be described a 30 cm diameter cell

with windows one meter apart and with internal baffles to reduce scattered laser light is

used. Fluorescence is detected at 90° with either an RCA 31000A or a cooled RCA 7102 photomultiplier tube. The input gate of a PAR 160 Boxcar Integrator is matched to the

*NRC Postdoctoral Fellow.

2 Figures in brackets indicate the literature references at the end of this paper.

I<0,

fell

Vttt

82
•fluorescence signal and the output sent to a strip chart recorder or a Nicolet Model 1070

Signal averager. The signal averager digitizes at such a rate that each channel on a single

?can accumulates the fluorescence signal from several laser pulses. When it is desired to neasure fluorescence lifetimes, the fluorescence signal from the photomultiplier goes to a Siomation Model 8100 transient digitizer (10 nsec/point) and then to the Nicolet Model 1070

signal averager.

532 nm

YAG Laser

igure 1. Block diagram of the photofragment experimental apparatus,

3. Results

Excitation spectra were recorded for ICN pressures in the range of 8-35 mtorr and for

|CN + He mixtures with He pressures of 0.200-100 torr. Figure 2 shows a typical spectrum

"btained for 18 mtorr ICN in 2.4 torr of He buffer gas. This excitation spectrum encom-

'asses the v"=0 + v'=4 and v"=l v'=5 vibronic bands of the A X CN transition and is

uncorrected for laser power. Analysis of this and other spectra indicates a vibrational

emperature of 700°K. We find a vibrational distribution of ^99% in v"=0, ^1% in v"=l

nd <0.1% in v"=2. These values are for the collision free initial vibrational population

istributions . In spectra of ICN without buffer gas rotational structure is evident out to

|" ^50 which indicates that the rotational temperature in the v"=0 level is several

housand decrees Kelvin. Spectra such as that in Fiqure 2 show that helium is very

ffective in equilibrating the rotational distribution.

The number density of CN for the ICN pressures employed in our experiments was in the

ange of 10 9 -10 u molecules/cm 3

. A signal to noise of approximately one is observed for

xcitation i>n the 0-3 band of CN at an ICN total pressure of 1 mtorr, which indicates the

ensitivitv of the experiment; i.e. we can routinely make experimental measurements on CN amples at the equivalent of 1 ytorr pressure.

83
Wavelength (nm)

Figure 2. Fluorescence excitation spectrum of 18 mtorr ICN and 2.4 torr He. The Qi , Ri,

and R2 band heads of the 0-4 transition are evident at 619.2 nm, 620.5 nm, and 621.4 nm The Qi band head of the 1-5 transition appears at 635.5 nm. This spectrum has not been

corrected for laser power.

An attempt was made to observe fluorescence from the CN (A 2n) state which is reportedly formed [5] by phot '"S is of ICN at 266 nm. A cooled RCA 7102 P.M. was used for

detection at several mtorr .ssure. A fluorescence signal from excitation of the 0-3 band

by the probe laser was easily observable, however, when the probe laser is blocked no 0-0

band fluorescence is observable. Based upon these and other observations we estimate that

an upper limit of 0.1% of the CN formed by photolysis is created in the A state.

Lifetimes of various A

2n single vibronic levels will be reported as will the electroni

bimolecular quenching rates of A state CN by ICN.

4. Conclusions

Our results essentially agree with the conclusions of Savety-Dzvonik and Cody even though their photolysis was carried out at higher energies. We find that CN is formed

primarily in v"=0 in the X

2

e + state with a rotational temperature of several thousand

degrees an' find no direct formation of CN A

2 n state; both of these results are in contrast to Ling and Wilson's observations. We have shown that photofragment spectroscopy

utilizing two pulsed lasers can be a very sensitive and general technique for probing

the dynamics of photodissociation processes. Number densitites of 10 10 molecules/cm 3 can

be routinely probed with sufficient resolution and at low enough total pressures to determine the initial electronic, vibrational and rotational distributions in the absence

of collisions.

84
References

[ 1] Jenkins, F. A., Roots, Y. K. , and Mulliken, R. S., Phys. Rev., 39, 16 (1932).

[ 2] Herzberg, G. , and Phillips, J. G. , Astrophysical J., 108 , 163 (1948).

[ 3] Davis, S. P., and Phillips, J. T., The Red System (A 2n-X %

+

) of the CN Molecule,

University of California Press, Berkeley and Los Angeles (1963).

[ 4] Coxon, J. A., Setser, D. W., and Duewer, W. H., J. Chem. Phys., 58, 2244 (1973).

[ 5] Ling, J. H., and Wilson, K. R. , J. Chem. Phys., 63, 101 (1975).

[ 6] Savety-Dzvonik, M. J., and Cody, R. , to be submitted to J. Chem. Phys.

85
SPECTROSCOPIC AND PHOTOCHEMICAL STUDIES OF GASEOUS IONS USING TUNABLE DYE LASERS

John R. Eyler

Department of Chemistry

University of Florida

Gainesville, FL 32611

In recent years tunable laser sources have been used in ever-increasing numbers for

both spectroscopic and photochemical studies of neutral species. However, there have been

few applications of these powerful, essentially monochromatic light sources to the study of

ionic species. Our work involves a wide ranging study of the irradiation of gaseous ions by

tunable organic dye lasers, with the belief that results of importance to both ionic spectroscopy and ionic photochemistry will be forthcoming. Investigations to date_have been

carried out in the areas of photo_detachmen£ of negative ions (A + hv * A- + e~) and photodissociation of positive ions CA + hv -> B + C).

These studies use a pulsed ion cyclotron resonance (icr) mass spectrometer [1.2] 1 to generate either positive or negative ions by several mechanisms and to trap them for periods

as long as several seconds in duration. During this time the ions are subjected to tunable

irradiation from a flashl amp-pumped dye laser. The effect of this irradiation upon the

trapped ions is greatly enhanced by use of an intracavity technique [3] which places the icr

cell inside the laser cavity. Such a technique gives improved sensitivity when examining

processes of low cross-section and threshold behavior.

Recent experimental work in the area of negative ion photodetachment has been carried

out by Brauman and co-workers using an icr spectrometer and_a conventional arc_lampmonochromator_i 1 1 umination source [4]. Species such as NH 2 ~, PH 2 ~, AsH 2 ", SeH", C 5 H 5 ",

Ci+Hi+N" , C 6 H 5 0", and C 6 H 5 S" have been investigated and improved values of electron affinities

and related thermochemical quantities for these species and associated neutrals have been

obtained. A value for the spin-orbit coupling constant of SeH* was_determined from an analysis of the relative cross-section vs. wavelength curve for SeH" photodetachment.

Tunable laser irradiation has been used to photodetach negative ions in a beam experiment

[5] by Lineberger and co-workers. Use of laser irradiation allows much higher precision

in electron affinity determinations, and makes possible observation of two-photon effects.

Our initial research using the laser-icr technique involved the photodetachment of

SH~ ions [6] C§H" + hv •> SH- + e~). This work demonstrated that fine structure unobserved

in previous SH" photodetachment studies could be obtained when using this technique which

combines production of ions with relatively low rotational "temperatures" with the inherently high resolution of the tunable dye laser. Current work involves study of photodetachment of species such as SD", Cl 2 ~, and C 2 H~.

A majority of the ionic photodissociation studies reported in the past several years

have been carried out by Dunbar and co-workers [7] using an icr spectrometer and arc lamp

illumination. These have produced much valuable information abou.t the £hermochem.istry,

energy, levels, and structures of various gaseous ions such as NO , C 4 H 8 , C 10 H 14 , and

CH 3 C1 . In addition, these workers have studied the chemistry of photo-produced ions in selected systems. Recently, other groups have also reported photodissociation results

[8,9].

i Figures in brackets indicate the literature references at the end of this paper.

86
. Our first experiments in the area of dye laser- induced photodissociation involved

C7H8 (parent) ions in gaseous toluene (C 7 H 8 + hv -> C 7 H 7 + H«) [10]. The observed onset

and general shape of the dissociation spectrum were in quite good agreement with an earlier

study [11] of much lower resolution. The ease of studying both negative ion photodetachment and positive ion photodissociation with the laser-icr technique allowed a comparison

of the cross-section for C7H8 photodissociation versus that for SH" photodetachment under

quite similar experimental conditions. This resulted in+

a value of a (490 nm) = 1.01 +

0.24 x 10" 18 cm2 for the absolute cross-section for C 7H 8 photodissociation at 490 nm.

No significant fine structure was seen in the photodissociation spectrum of C 7 Hq

+

in

spite of the high energy resolution of dye laser irradiation. We are currently investigating a number of positive ions of simpler structure, in hopes that fine structure related to vibrational energy levels in the pertinent electronic s_tates of the. ions involved will be

observed. Two of the species being studied are C 2 H 5 C1 and CH0CH0 .

References

[ 1] Baldeschwieler, J. D. , and Woodgate, S. S., Accounts Chem. Res. 4, 114 (1971);

Beauchamp, J. L., Ann. Rev. Rhys. Chem. 22_, 527 (1971 ); and Hem's, J. M. S., in

Ion-Molecule Reactions, Volume 2, Ed. by J. L. Franklin (Plenum Press, New York,

1972), Ch. 9.

[ 2] Mclver, R. T. , Jr., and Dunbar, R. C, Int. J. Mass. Spectrom. Ion Phys. ]_, 471 (197*).

[ 3] Eyler, J. R., Rev. Sci. Instrum. 45, 1154 (1974).

[ 4] Richardson, J. H., Stephenson, L. M. , and Brauman, J. I., J. Amer. Chem. Soc. 9_7, 296 (1975), and referneces contained therein.

[ 5] Lineberger, W. C, in Chemical and Biochemical Applications of Lasers, Volume I, Ed. by C. B. Moore (Academic Press, New York, 1974), Ch. 3.

[ 6] Eyler, J. R., and Atkinson, G. H. , Chem. Rhys. Lett. 28(2), 217 (1974).

[ 7] Dunbar, R. C, in Interactions Between Ions and Molecules, Ed. by Pierre Ausloos

(Plenum Press, New York, 1975), p. 579.

[ 8] Freiser, B. S., and Beauchamp, J. L. , J. Amer. Chem. Soc. 96, 6260 (1975).

[ 9] Vestal, M. L., and Futrell , J . H. , Chem. Phys. Lett. 28, (4), 559 (1974).

[10] Eyler, J. R., J. Amer. Chem. Soc, in press.

[11] Dunbar, R. C, Ibid., 95, 472 (1973).

87
PHOTODISSOCIATION SPECTRA OF POSITIVE IONS WITH TIME-OF-FLIGHT ANALYSIS

Timothy F. Thomas and John F. Paulson

Air Force Geophysics Laboratory (LKB)

Hanscom AFB Bedford, MA 01731

One of the many new opportunities provided photochemists by the development of tunable

lasers in the visible and UV regions is the study of very dilute systems such as welldefined ion beams. Recent applications include measurement of photodetachment spectra of numerous negative ions [I] 1

, photo-destruction spectra of polyatomic negative ions such as C0 3 ~ and 0a" [2], and determination of kinetic energy spectra upon photodissociation of

H 2+ and D 2 [3,4]. We have assembled an apparatus capable of determining both absolute

photodissociation cross-sections of ions as a function of wavelength (the photodissociation

spectrum) and the kinetic energy spectrum of product ions by time-of-fl ight analysis.

Figure 1 shows the apparatus used. A pulsed laser beam, ^2 1/2 mm in diameter,

crosses a mass-selected ion beam whose energy can be varied from 10 to 200 eV by the

decelerating plates shown. The quadrupole mass analyzer is set to pass only the product

ion formed during a photolysis, or to monitor the primary beam intensity intermittently.

A photo-diode triggers the time-of-fl ight logic at the start of each laser pulse so that the

time between the pulse and arrival of a product ion at the electron multiplier can be

measured to an accuracy of 10 nanoseconds, if desired. The polarization rotator shown in the laser beam permits determination of the direction of the electronic transition moment,

as well as checking whether product ions ejected at large angles to the flight axis are accepted by the quadrupole mass analyzer.

Ion Source

>-Somple

Pyroelectric

Energy

Meter

Decelerating Lens System n„ 1

iiiii

6" Sector

Magnetic

Analyzer

Pulse ft Trigger

Generator

Stort-.|

Time of Flight

Logic

Multi- Channel

Analyzer

Events*

Photodiode

Amplifier

Discriminator

Q«] Electron Multiplier

|

< ij Forodoy Cup

50 cm Quadrupole

Polarization „ _ , . r i Mass Analyzer

Rotator L '

Tunable

Dye

Laser

Vibrating Reed

Electrometer

Figure 1. Apparatus for photo-dissociation of ions with time-of-fl ight analysis of product

ions.

T"Figures in brackets indicate the literature references at the end of this paper.
Partial results obtained for three reactant ions are given below.

^2_. The only transition accessible at the available wavelengths is lso a-*2Pa,j. Figure 2 confirms that the dipole moment of the transition lies along the internuclear axis Figure

3 shows the time-of- flight spectrum obtained at primary ion energies so low that only D+ ions ejected hi the forward direction are accepted by the quadrupole mass analyzer. Product ions

produced from different vibrational levels of the lsaq state posses different kinetic

energies and thus show different flight times as indicated on the figure

30 60

Figure 2. Effect of direction of laser beam polarization on cross section for photodissociation. 9„,_._ = 0 corresponds to horizontal polarization. Primary ion energy:

23 V. polar

36 r

30 -

« 24 -

i 3

• O

o

V"

hv - D

+ 4 II 8

I I 1

+ D

6

I

• ••

« •

• • • • • m • m

JL il 20 25 30

Time (microseconds)

,e • mtem mm

1 >—

35. 40

Figure 3. Time-of-fl ight spectrum for D 2 . Aj

r source pressure = 0.64 Torr, ionizing voltage

* + 8.5 V.

= 580.0 nm, primary ion energy = 17.5 V,

200 V. Quadrupole rods were biased at

89
To obtain absolute cross sections the primary ion energy was increased to 173 V so that all product ions were accepted by the quadrupole analyzer. Average values obtained

are shown in Table I, along with the photodestruction results of von Busch and Dunn [5] who

used a band width of 200 A versus <_ 1 A used here.

Table I. Cross Sections for D£

+

+ hv -* D++ D

A (nm) = 445 585 590 595 600

a x 10

19 (cm

2

) = 9.5+0.7 5.3+0.3 5.1+0.2 5.7+0.3 5.2+0.3

von Busch & Dunn: + 9.9 5.8 5.7 5.6 5.5

(interpolated)

HD

+

. Following the same electronic transition as in D 2 +

, two dissociative processes are possible:

HD

+

+ hv -> H + D+

, a = 2.0 x 10" 19 cm

2

-> H +

+ D, o = 2.0 x 10" 19 cm

2

The cross sections shown are average of several results at 590 nm, using a source pressure

of 0.62 Torr, ionizing voltage of 200 V, and primary ion energy of 173 eV.

N?0. Table II shows a few features of the highly-structured photo dissociation spectrum

that has been obtained for the processes:

N

2 0

+

+ hv -> N0

+ + N.

Table II. Photodissociation Cross Sections for ^O*9

A(nm) 340.1 338.5 324.9 323.6 311.0 308.5

a x 10

19 (cm2

) 15.2 25.7 3.5 7.9 1.1 1.2

(1,0) (1,0) (2,0) (2,0) (3,0) (3,0)

a Source pressure = 0.82 Torr, ionizing voltage = 70.4 , primary ion energy = 173 eV.

Analysis of the emission spectrum of N 2 0

+

, at high resolution [6], indicates that the

transition involved here must be

^2 2

X ^3/2,1/2 "* A Z '

with tjie pairs of peaks in Table II representing a progression in the symmetric stretching

mode (vi = 1345.5 cm" 1

) of the upper electronic state. The splitting between members of

each pair corresponds reasonably well to the reported spin-orbit coupling constant of

132. cm" 1 [6] in the 2 n state.

Photo-dissociation cross sections have also been measured at 445 (5 x 10 21 cm2

) and 640 (1.3 x 10" 21 cm 2

) nm. These low values, and the failure to observe a significant

change in 0321-8 u Pon. addition of N 2 to increase the source pressure to 1.9 Torr, indicate

that most of the N 2 0 ions enter the photolysis region in their ground vibrational states. However, several peaks in the photo-dissociation spectrum, e.g. at 336.5 and 321.8 nm, may be due to the Renner effect operating in 2

tt state ions possessing one or more quanta

in the bending mode.

90
Recent photoelectron-photoion coincidence spectroscopy of N 2 0 indicates the onset

of predissociation in the A state lies at the Vi ' = 1 level, where ^40% of the ions

dissociate and the remainder presumably fluoresce [7]. Our preliminary analysis of the

TOF spectra at low

+ primary ion energies (17.4 eV) clearly shows that a significant

fraction of the NO are produced with kinetic energies in excess of 1 eV, requiring that the

nitrogen atom be produced in its ground electronic state ( k S) . This_can only occur if predissociation proceeds via a state of higher multiplicity, such as

k

i.~ [8],

References

[ 1] Lineberger, W. C, et al . , J. Chem. Phys. 61_, 1300 (1974), and references therein.

[ 2] Peterson, J. R. , et al . , J. Chem. Phys. 62_, 4826; 63, 1612 (1975).

[ 3] van Asselt, N. P. F. B., Maas, J. G. , and Los, J., Chemical Physics 5, 429 (1974).

[ 4] Ozenne, J. B., Pham, D. , and Durup, J., Chem. Phys. Lett. 1_7» 422 (1972).

[ 5] Busch, F. v., and Dunn, G. H., Phys. Rev. A5_, 1726 (1972).

[ 6] Callomon, J. H., Pvoc. Chem. Soc. 13 (1959); Callomon, J. H., and Creutzberg, F., Int. Conf. on Spectroscopy, Bombay, 1967, p. 143.

[ 7] Eland, J. H. D., J. Mass Spectvom. Ion Phys. 12, 389 (1973); 13, 251 (1974).

[ 8] Lorquet, J. C, and Cadet, C, Int. J. Mass Spectry. ion Phys. 7_, 245 (1971 ).

J

91
VIBRATIONAL STATES OF MOLECULES IN THE VISIBLE REGION BY THERMO-OPTICAL SPECTROSCOPY

AND THE LOCAL MODE MODEL 1 ' 2

A. C. Albrecht

Department of Chemistry

Cornell University

Ithaca, NY 14853

When a very small fraction of an incident laser beam is absorbed by a liquid, a steady state temperature gradient on the order of millidegrees can appear transverse to the propagation direction of the incident light. This produces an index of refraction

grddient causing the transitting beam to diverge. When a tunable laser is used for the

heating beam, this thermal lens effect provides a basis for an extremely sensitive new spectroscopy [l] 3

. We have explored the absorption characteristics in the red (Rhodamine

6-G) region of the visible spectrum of several characteristic organic molecules. These

include several aromatic hydrocarbons, methanol, acetone, methylenedichloride, and several

alkanes. Isotope studies show that absorption by these molecules is derived from stretching excitations of bonds containing the hydrogen atom. The excitation energy is more than

fifty percent of that needed for bond dissociation. Thus this spectroscopy offers an opportunity to explore molecular potential energy surfaces along reaction coordinates and

at energies approaching those required for breaking bonds.

The spectroscopy itself has been developed both in a single beam transient mode as well as in a dual beam, automatic scanning mode. In the latter case a fixed wavelength

laser is used to probe the lens which is formed by a chopped, tunable, heating beam.

Synchronous detection is used. In the single beam mode the time course of the developing

lens is followed from the first moment the sample is illuminated to the point where the

stationary temperature gradient has appeared. An absolute measure of the extinction

coefficient can be made in this case. Extinction coefficients nine orders of magnitude

weaker than those of typical allowed electronic transitions can easily be measured.

Samples having optical thicknesses of kilometers give easily detected thermal lensing

effects in centimeter path lengths. Singlet-triplet transitions are candidates for such

spectroscopy. One such transition has been identified in anthracene.

The theoretical understanding of vibrational molecular states responsive to light in the visible region appears to be greatly facilitated by the local-mode model (LM). Here

the zeroth order description of an excited state is not given by the usual overtone or combination (normal mode) approach but rather by expressing the occupation number of local

mode anharmonic vibrators. Thus in benzene for example [2], the LM model pictures the C-H

stretching states to be built up of six identical, uncoupled, anharmonic C-H oscillators.

The LM model can incorporate any degree of anharmonicity in its zeroth order framework.

Since it is an uncoupled system model, and since the perturbation due to the light wave is also separable, the LM model predicts that multiple quantal excitation can be one-photon

allowed only when they take place within one bond. The model predicts the very simple

spectroscopy of a one dimensional anharmonic oscillator for all degrees of excitation even though the state density rises abruptly. The number of states for j identical oscillators

l

Supported in part by a grant from the National Science Foundation and the Materials Science

Center of Cornell University.

2This paper includes work found in references 1 and 2 as well as more recent studies carried

out in our laboratory by Dr. R. Swofford and Mr. M. Burberry.

3 Figures in brackets indicate the literature references at the end of this paper.

9?
containing altogether v quanta is just the binomial coefficient ((j + v-l)/(j-l)). Thus

for the six dimensional C-H stretching space in benzene (j=6) there are 462 states at v=6.

The observed line however is that of a single state - that (of Ei symmetry) which carries

all six quanta in one bond. This is by far the most anharmonic or the 462 states. The LM model can enumerate all levels appearing at any v once a two parameter fit is made to the

observed 'overtone' spectrum. The corresponding energy level diagram for the deuterium

isotope is automatically determined by the mass effect. Although only two parameters are

needed for the energy fit, an effort to predict intensities requires an expansion of the

potential energy function at least to the fifth power in displacement. Thus the attempt to match intensities of these very weak transitions provides considerable analytical insight

into the potential energy surface. As even higher transitions are observed it is expected

that a serious challenge to theory may emerge, for now, as dissociation is approached, the

zeroth order Born-Oppenheimer approximation must be questioned.

The LM model predicts that in a polyatomic molecule having several equivalent oscillators there is a remarkable harmonic multi-photon channel that can provide a rapid route

for the deposit of intense monochromatic infrared laser radiation in a manner having

photochemical significance. The fact that pulsed infrared light can bring about very

energetic molecular excitation, as seen by visible luminescences as well as by photodecomposition, has been the subject of intense interest. The basic question concerns the

mechanism of how a large number of photons of a single frequency can build up in a sinqle

molecule, especially when its vibrational levels are known to be anharmonic . We will show

how in benzene, for example, the LM model predicts a highly allowed six photon, sequential,

transition among vibrational states which are harmonic . (At v = 6 this harmonic state is

the one where one excitation appears in each of the six uncoupled C-H oscillators). The

implications of this to examples in the literature will be touched upon.

References

[ 1] Long, M. E. , Swofford, R. L., and Albrecht, A. C, Science 191_, 183 (1976).

[ 2] Long, M. E. , Swofford, R. L., and Albrecht, A. C, J. Chem. Phys. 65_, 000 (1976).

93
MECHANISM OF HIGH INTENSITY I.R. PHOTODECOMPOSITION OF MOLECULES

W. Braun and W. Tsang

National Bureau of Standards

Washington, DC 20234

There has been considerable recent interest in the photodecomposition of polyatomic

molecules using resonant infrared radiation from TEA lasers [I] 1

. Most of the emphasis

has been on the separation of isotopes, with the smaller polyatomic molecules as the

absorbent [2]. Except for empirical observations for use in optimizing isotope separations

very little has been done to describe the nature of the decomposition process. A greater

understanding may well provide essential inputs for the selection of molecules to be used

for isotope separations as well as dictate the conditions for optimizing such processes.

Furthermore, this is a new and unique means of molecular excitation and the possibility that

it can be used to probe fundamental properties of molecules is certainly worthy of

investigation. This, in turn, can lead to solutions for a wide range or problems in gas kinetics, combustion and analytical chemistry.

In the present investigation we will deal specifically with describing the nature of

the photodecomposition process and we will look in detail at the products derived from the

photodecomposition of somewhat larger polyatomic organic molecules. We focus specifically

on the i.r. high intensity photolysis of isobutyl and ethyl chlorides, bromides and iodides.

The experimental arrangement is shown in fig. 1. laser radiation, P(36) level, 10.6

band, from a CO -TEA laser (approximately 0. 300/joules/pul se) is focused by means of a spherical mirror and a 10 cm NaCl lens (effective focal length 5 cm) into the 1.0 cc reaction cell. From 10 to 100 pulses are used for each experiment. With a pulse rate of 1 to

2 pulses/sec, mixing of the products in the cell after each pulse is assured. Pressures

in the reaction cell ranged up to 50 torr He and 0.5 to 2-torr of the organic compound of

interest. In this pressure and concentration range the extent of alkyl halide decomposition

per flash is invariant. Gas chromatography with flame ionization detection is used for

analysis. The columns used are 8 ft. Porosil "C" (1/8"), and 6 ft. Poropak Q (1/8"). The

former makes excellent separation but appears to be somewhat reactive. The latter is a boiling point column and was used initially in order to get a rough idea of the product

distribution. Except for isobutyl iodide the compounds were pumped and degassed in the

usual manner. No other attempts at further purification were made.

NaCI LENS . 50* TRANSMITTING MIRROR

Figure 1. Experimental setup.

figures in brackets indicate the literature references at the end of this paper.

94
Chromatographic results of laser pyrolysis on the three isobutyl halides as well as

i:hose of isobutene and propylene are presented in Figure II. With respect to the halides

It cursory examination reveals striking similarities. This is particularly the case with

-espect to isobutyl chloride and bromide and suggests that the photolysis of a common perbursor may be responsible. The most logical precursor candidate is isobutene. It is a product, and the compounds formed from its laser photolysis must in any case be accounted

for. This hypothesis is supported by the results on the laser photolysis of isobutene.

: igure 2. Gas Chromatograms for the photolysis of isobutyl chloride, isobutyl bromide,

i isobutane, isobutyl iodide, and propylene, 10.6 y (34) C0 2 laser line. Isobutyl

chloride - 2.5 torr C^HgCl , 47.5 torr He, 120 pulses; sensitivity x 2. Isobutyl bromide - 2.5 torr Ci+HgBr, 47.5 torr He, 180 pulses; sensitivity x 1. Isobutane - 1.2-5 torr Ci+H 8 , 48.75 torr He, 20 pulses; sensitivity x 1. Isobutyl iodide - 2.5 torr C^Hgl, 47.5 torr

He, 210 pulses; sensitivity x 2.5. Propylene - .625 torr C 3 H 6

, 49.37 torr He, 20 pulses;

sensitivity x 1

Figure II shows clearly that there is a definite correspondence (from the peaks

iown) between the "cracking" pattern for isobutene and isobutyl chloride and bromide. More

quantitatively, the important products from isobutyl chloride and bromide photolysis (in

)rder of relative concentration) are isobutene, methyl acetylene, acetylene, ethane and

jllene, while for isobutene photolysis the order of important products is also methyl

acetylene, acetylene, ethane and allene. The carbon/hydrogen ratio for the products are

1.9 for isobutyl chloride, 1.9 for isobutyl bromide and 2.0 for isobutene. With respect

to isobutyl iodide, comparison with the bromide and chloride spectra indicate that for the

former there is a relatively larger amount of ethane, isobutane (most prominent), iso-

,oentance and propylene (less prominent) superimposed upon the basic bromide or chloride

'cracking" pattern.

The chromatograms (not shown) form the ethyl halide photolysis are cleaner and thus

less striking. For the bromide and chloride the main products are ethylene and acetylene.

The product ethylene, is however also partially photolyzed by the laser light, with acetylene, the dominant product. Thus, as in the isobutyl system, it is not unreasonable to assume that photolysis of the initial product will account for a significant if not all of

the other compounds formed.

95
For isobutyl chloride and bromide photolysis the observed "primary" products are consistent with a photolytic mechanism as given in Figure III. After the laser pulse,

assuming high extent of reaction, there will be significant quantities of radicals in the

reactive zone (cone) and one can assume that a large amount of combination and addition

CH, H

1 3 1 CHj-C-C-H

H X

CH3 H

,C = Cs CH3 H

CH, H 3C=C'

' H

ntw -CH nh>s.-H

C2 H

2

CH, H

7C=c'

•CH 2

. >H

C

3 H4

Figure 3. Mechanism of isobutyl chloride and bromide decomposition.

will occur. These processes are summarized in Figure IV and account for all of the observec

"secondary" products. Such a mechanism showns why there is quantiative agreement between

isobutyl chloride and isobutyl bromide "cracking" patterns. The isobutyl iodide "cracking"

pattern shows some differences which lead us inevitably to the conclusion that some C-I fission occurs. The newly formed isobutyl radical can then isobutane and isopentane via

the reactions

CH

3 + i C4

H

g

C

5H 12 ( 1S0Pentane )

and

iC.HQ + HI CA

H ln (isobutane) + I.

These products are minor or absent in isobutyl chloride and isobutyl bromide photolysis.

The mechanism for ethyl halide decomposition can be considered to be similar to that

for the isobutyl halides. That is direct formation of ethylene and the hydrogen halide,

the photolysis of ethylene, and in the case of the iodide some contributions due to C-I bond splitting.

96
• • * Ob *

at, » «b « * «• e a i -

« " / "

** * "K ''

Ce> * < • C » <! o Ob • «V e* « » «* » * # a • < »- • . «' «% .Ob *"

* * en, .1 ear,

«H c*e ——

c - "cin. 1. "•*/-•'-?-•." *.*»"" or, °>, "

en-c.e-H *H m e»,c- c.» ;

• # *

* i i

Figure 4. Combination processes after laser pulse.

Having defined a fairly plausible mechanism for the overall process, it is interesting

to note that these are what one would expect if the reactions are in each case proceeding

via its lower thermal channel (s). The thermal channels for alkyl halide dissociation are displayed in Figure V and dissociation channels for several olefins are included as well

[3,4]. The increased important of C-I bond splitting is due to its close proximity to the

HI elimination channel. On the other hand the similarity between isobutyl bromide and

chloride chromatograms may be indicative of the fact that a 20 kcal difference between

decomposition channels is of significance. In carrying out the discussion solely on the

basis of energetics we are of course neglecting entropy effects.

eoC-C cleavage e-c cteHMf cleavage ""cleavage

=C-C 5££ cleavage

cleavage lllylC-H «"X C-H *"C) ' H

c. c

cleavage cleavage cleavage

70- C-Br

5 , US , > ag—ellm.

laomerliatlon to methyl -^-H

crtyrtne J-

-100

-so

—BO -70

cleavage c W

Figure 5. Thermal decomposition channels,

C3H4

alien* . ailyi methyfaiiyl

A remaining important question that must now be settled is whether mechanisms that we

have outlined occurs thorugh specific excitation by the laser pulse or by a rapidly following thermal pulse. Indeed, the product distribution is in fact closely akin to what can

be expected from a high temperature thermal reaction. This question can be unambiguously

•settled from the chromatograms in Figure VI. They represent experimetns with propylene

and isobutyl chlorides alone and in a mixture. The key and self-evident observation is that

if one adds together the pyrolysis chromatograms of the individual compounds the new chromatogram is exactly that of the mixture. Thermally, there is no question that isobutyl

97
chloride is vastly more labile than propylene over all temperature conditions. Thus the

failure of propylene to induce any isobutyl chloride decomposition provides extraordinarily

strong evidence for the specific, non-thermal, nature of the laser excitation.

"Figure 6. Gas chromatogram from the photolysis of isobutyl chloride, propylene and in

combination. 10.6 p P(34) CO 2 laser line. Propylene - .31 torr C 3 H 6

; 49.69 torr He,

60 pulses. Isobutyl chloride - .94 torr C4H9CI; 49.1 torr He, 60 pulses. Propylene + Isobutyl chloride - 31 torr C 3 H 6 ; .94 torr C^HgCl; 48.75 torr He, 60 pulses.

The overall picture that can be derived from these data and analysis is that there

is a rapid vibrational excitation of an absorbing molecule to a level at which it is un- stable (within the time frame of the laser pulse). Inasmuch as the specific rate constant

for the decomposition of (an unstable very highly vibrational ly excited) molecule is almost

exponentially dependent on the energy above any reaction threshold [5], it is easy to see that, in general, a molecule will seek to escape from the laser excitation by decomposing

through its lowest energy dissociation channel. Indeed such a dependence on lifetime or energy suggests that rather large increments in energy must be added into the system if one wishes to activate higher decomposition channels. All of these observations fit well

within the framework of our present day understanding about thermal stability of molecules.

[ 1] Freeman, M. P., and Travis, D. N. , J. Chem. Rhys., 60, 231 (1974).

[ 2] Lyman, J. L. , and Rockwood, S. D. , J. Appl. Rhys., 47, 595 (1976).

[ 3] Benson, S. W. , and O'Neal, H. E., Kinetic Data on Unimolecular Reaction, NSRDS-NBS-21,

U.S. Government Printing Office, 1970.

[ 4] O'Neal, H. E. , and Benson, S. W. , in Fvee Radicals, Ed., J. K. Kochi, Vol. II, Wiley

Interscience, NY, 1973, pp. 275.

[ 5] Robinson, P. J., and Holbrook, K. A., Unimolecular Reaction, Wiley Interscience, NY,

1972.

isoevTYL-a .A

References

98
THE IN-SITU MEASUREMENT OF ATOMS AND RADICALS IN THE UPPER ATMOSPHERE

James G. Anderson

Department of Atmospheric and Oceanic Science

Space Physics Research Laboratory

University of Michigan

Ann Arbor, MI 48109

Although upper atmospheric photochemistry has become rapidly more complicated in the

past five years as our appreciation for the multitude of gases present in the stratosphere

has grown, the basic reaction pattern displayed by each of the chemical cycles involving

oxygen, hydrogen, nitrogen and chlorine remains simple. Figure 1 generalizes this pattern

into three common parts:

(1) Chemical source terms, which represent the upward flow of stable polyatomic

molecules from the earth's surface and troposphere;

(2) Linking radical

or molecular fragments which are formed from the chemical source

terms either by photolysis or by chemical reaction;

(3) Reservoir or sink terms which are formed by the recombination of the radicals and

are recycled into the radical system by photolysis and chemical reaction and removed by

downward and meridional transport.

The source and sink terms are relatively stable chemically, having chemical lifetimes

on the order of weeks to months so that their global distribution is, in general, governed

by transport processes rather than by details of the local chemical environment. In contrast, the radicals have chemical lifetimes on the order of minutes and they thus reflect,

with considerable alacrity, the chemical conditions in their vicinity. A composite of the

hydrogen, nitrogen and chlorine systems, including only the major reaction paths, is shown

in Figure 2 which can be subdivided by inspection into three cycles similar in form to Figure 1

Figure 1. System overview. Figure 2. Composite oxygen, hydrogen,

nitorgen, and chlorine photochemistry.

99
Significant progress has been made over the past five years in the measurement of

stratospheric source and sink terms most notably by infrared absorption and emission techniques, whole air samples and filter trapment. These techniques have proven to be powerful

for the determination of absolute concentrations in the part per million to part per billion

range where extended regions of spatial integration were acceptable. Because of the long

chemical lifetime of these species, details of their local structure were not relevant and

there developed a very direct and beneficial exchange between field measurements and

theoretical calculations.

In order to hope for such a exchange in the case of radicals, an experiment must

(a) have a detection sensitivity in the part per trillion range in a defined volume element

in which other interacting radicals are simultaneously measured, (b) possess a means for accurate absolute calibration and (c) be capable of measuring the diurnal behavior of

several species.

A technique which quite naturally satisfies these requirements is atomic and molecular

resonance fluorescence in combination with a high velocity sample flow which eliminates

heterogeneous interaction between the atmospheric sample and the instrument. This geometry

is familiar to laboratory gas phase kinetics studies and its adaptation for this work is shown in Figure 3. A beam of photons, resonant with a preselected electronic transition of

an atom or molecule is passed across the flowing gas sample confined to the interior of a pod or "nacelle" which establishes laminar flow around and through the device as it is lowered through the atmosphere on a stabilized parachute platform. Photons scattered out

of the lamp beam are collected and counted by a photomultiplier with associated optics which

observes in a direction perpendicular to both the incident photon beam and the gas flow

direction.

Absolute calibration is accomplished using a laboratory system shown in Figure 4 which

provides a known quantity of the atom or radical in a flow at the appropriate total pressure, temperature and flow velocity. As Figure 4 shows, the flight instrument occupies the central portion of the reaction zone so that a known concentration of the atom or radical,

formed either by discharge alone or by chemical reaction in the upstream portion of the reaction zone, flows through the instrument's interior. After calibration, the instrument

is removed from the system, fitted with airfoil sections and flown optically unaltered from

its configuration during calibration.

TELEMETRY

CASE

PHOTOMULTIPLIER TUBE

PULSE COUNTING ELECTRONICS

COLLECTION

OPTICS

SCATTERING VOLUME

CARRIER

GAS ADDITION

1,

TRACE ADDITION SYSTEH

OBSERVATION

LIGHT BAFFLE

GAS ADDITION VALVE

LOOP FIGURE 3

PUMPING

STATION

DETECTOR' LAMP BAFFLE'

UMBILICAL CORD'

INSTRUMENT GROUND SUPPORT

STATION FIGURE 4

'Sir

Figure 3 Figure 4

100
Helium filled research balloons capable of lofting the instrument package to 45

<ilometers from where the measurement commences upon deployment onto the stabilized parachute platform have thus far been used exclusively although rocket or aircraft deployment

is equally possible. Results of two flights for the measurement of ground state atomic

oxygen 0( 3 P) are shown in Figure 5 and two flights investigating the vertical profile of

the OH radical are shown in Figure 6. These results will be discussed as well as the

development of instruments for the measurement of CI, CIO, NO, N0 2 and H 2 0.

1-

0( 5 P) IN THE earth's stratosphere

50 OH (Art IN the earths stratosphere • 25 NOVEMBER , 1974

x = 56° 10 30 A M CST

<

o

1-

5

5£

20

[0(

3 Pl] [cm 3

J

Figure 5 Figure 6

101
THE 0( 1 S) AIRGLOW - NEW LABORATORY RESULTS

Tom SI anger and Graham Black

Stanford Research Institute

Menlo Park, CA 94025

Ever since it was postulated [l] 1 by Sidney Chapman forty-five years ago that the

energy source for the production of O^S) in the lower thermosphere (^100 km) was the

recombination of ground state oxygen atoms, it has been realized that the intensity of

the 5577 A 0{ 1 S) * 0( 1 D) transition in the airglow is a measure of the 0( 3 P) density. Not

until relatively recently has any kinetic data become available to make it possible to transform the 5577 a intensities, which have now been measured from the ground [2], from

rockets [3], and from satellites [4], into oxygen atom concentrations.

In order to perform this transformation, one needs to know the production and loss rate coefficients of 0( 1 S) in the atmosphere at a temperature of 'v200°K. There are two alternate production schemes, the one linked with Chapman's name, involving a single threebody interaction,

0(

3 P) + 0(

3 P) + 0(

3 P) + 0(

] S) + 0

2 + 0.9 eV , (1)

and the one proposed by Barth and Hildebrandt [5], in which there is an intermediate 0 2 * state,

0(

3 P) + 0(3 P) + M + 0

2 * + M (2)

0

2 * + 0(

3 P) - 0

2 + 0(

] S) , (3)

0

2 * + M * 02 + M , (4)

No laboratory measurements have been able to choose between these alternatives. The only

significant 0( 1 S) losses in the atmosphere are radiation and quenching by 0( 3 P) and 0 2 ; is now generally believed that 0( 3 P) quenching is dominant in the region of peak 0( 3 P)

densities

.

In the present work, we have measured the 0( 1 S) production rate coefficient in a flowing afterglow, and the 0( ] S)-0( 3 P) interaction rate coefficient in a combination

photolysis-afterglow system, in both cases as a function of temperature. In the production

rate measurements, 0{ 1 S) is generated through recombination in an 0( 3 P) stream, from a conventional NO-titrated N 2 afterglow. The absolute 0{ l S) intensity is determined by 0( 3 P) + NO photometry, while the 0( 3 P) concentration is determined by N0 2 titration. The 0{ 1 S)

is then quenched with N 2 0, to observe the competition between the O^S) reaction with N 2 0

and the other loss processes. Accurate measurements of the temperature-dependent rate coefficient for the 0( 1 S)-N 2 0 interaction were needed and carried out [6]. From these data,

it is possible to extract a value for the overall production rate coefficient, but not to state whether it refers to reaction (1) or the more complex reaction scheme embodied in

i Figures in brackets indicate the literature references at the end of this paper.

102
reactions (2), (3), and (4). The reason for this ambiguity is that it was not possible to work at total pressures low enough for the [M]-dependence of the complex mechanism to become

apparent1 The resultant overall rate coefficient [7] is 1.4 x 10" 30 exp(-1300/RT)

cm6 molec~ 2 sec

_1

, a figure considerably larger than that used until now. The positive

activation energy suggests that the mechanism is not the same for the usual 3-body reaction.

The 0( 1 S)-0( 3 P) quenching was studied by generating 0( 1 S) from N 2 0 photolysis at 1300 A in a pulsed mode, and observing its decay time as a function of added 0( 3 P), from a partially titrated nitrogen discharge. We were able to show that the observed increase in

0( 1 S) loss rate was due only to 0( 3 P), and a quenching rate coefficient of 5.0 x 10" 11 exp(-610/RT) cm 3 molec

-1 sec"^ was established [8]. This is slightly more than a factor of

two larger at 300°K than the value determined by Felder and Young [9], but orders of

magnitude larger than the recent theoretical prediction of Krauss and Neumann [10]. An Arrhenius plot is shown in figure 1.

Application of these new rate coefficients to the atmosphere leads to some interesting

results. If we ask whether the atmospheric observations correlate better with the laboratory results through reaction (1) or through reactions (2)-(4), the answer is quite clear.

Based on current 0( 3 P) profiles, the assumption that our laboratory rate coefficient pertains to the simple mechanism, reaction (1), results in calculated 5577 A intensities that

are far too high. To utilize the complex scheme, we need to make assumptions about the

efficiencies of reactions (2) and (3) in giving the necessary electronic states, but with

this flexibility [which involves identifying 0 2 * as 0 2 (A 3 E )], a very good correlation can

in fact be made [11], which indicates that typical mid-latitude peak 0( 3 P) densities are

'in the neighborhood of 6 x 10 11 cm" 3

. The work that

+ remains is to test these assumptions,

which involves making kinetic measurements on 0 2 (A 3 z ) and also, if possible, on the other

electronic states of 0 2 in the same energy region. These states are difficult to produce

and detect because of their metastable nature.

103
References

[ 1] Chapman, S., Proa. Roy. Soc. (London) A132 , 353 (1931).

[ 2] Hernandez, G. , Planet. Space. Sci. 19^, 467 (1971).

[ 3] Greer, R. G. H., and Best, G. T., Planet. Space Sci. 15., 1857 (1967).

[ 4] Donahue, T. M. , Guenther, B., and Thomas, R. J., J. Geophys. Res. 78, 6662 (1973).

[ 5] Barth, C. A., and Hildebrandt, A. F. , J. Geophys. Res. 66, 985 (1961).

[ 6] Slanger, T. G., and Black, G., Temperature dependence for quenching of 0( J S) by

N 20, to be published in J. Chem. Phys., Sept. 1, 1976.

[ 7] Slanger, T. G., and Black, G., J. Chem. Phys. 64, 3767 (1976).

[ 8] Slanger, T. G., and Black, G., J. Chem. Phys. 6±, 3763 (1976).

[ 9] Felder, W., and Young, R. A., J. Chem. Phys. 56_, 6028 (1972).

[10] Krauss, M., and Neumann, D. , Chem. Phys. Lett. 36, 372 (1975).

[11] Slanger, T. G., and Black, G., 0( 1 S) in the lower thermosphere - Chapman vs Barth,

to be published in Planet. Space Sci. (1976).

104
A QUANTUM YIELD DETERMINATION FOR 0(D) PRODUCTION FROM OZONE VIA LASER FLASH PHOTOLYSIS

Dan L. Phi 1 en , Robert T. Watson 1 and Douglas D. Davis

Department of Chemistry

University of Maryland

College Park, MD

1. Introduction

It is now generally accepted that the OH free radical is one of the single most

important species which controls the chemistry of numerous trace gases in both the troposphere and stratosphere. The principal source of OH is also now believed to be the result

of the reaction of 0( 1 D) with atmospheric water vapor (0( 1 D) + H 2 0 -> 20H).

The source of 0{ l D) in turn is the photolysis of ozone in the Hartley continuum

(<3000A) and possibly in the Huggins bands between 3000A and 3500A.

Thus, it is very important to know the production rate of 0( 1 D) from the photolysis of ozone

in the region 3000A to 3400A in order to better understand both tropospheric and stratospheric chemistry.

The absorpton cross section has been measured for the wavelength region of tropospheric

interest by Wanatabe [I] 2

, Inn and Tanaka [2] and Vigroux [3]. These data have been compiled

by Ackerman [4] and in addition, the absorption coefficients of ozone in the UV and visible

region have been compiled by Griggs [5].

On thermochemical grounds, one would expect reaction (1) to occur at wavelengths shorter

than 3100A and extensive studies [6] of ozone photolysis have established that reaction (1)

is indeed the dominant process at wavelengths shorter than 3000A. The reaction

has also been shown to be the only process occurring at wavelengths equal to or greater

than 3400A. Unfortunately, conflicting results have been obtained in the 3000-3200A region,

which is the fall off region for O^D) production.

0

3 + hv (\<3000A) -> 0('D) + 0

2

C Ag)

0

3 + hv (3000A <A< 3500A) ->- O^D) + 0

2

( 3 zg)

(1)

(2)

2. Previous Work

Jet Propulsion Laboratory, Pasadena, CA.

2 Figures in brackets indicate the literature references at the end of this paper.

105
The gas phase photolysis of ozone has been studied by several groups in the region

of 3130A. Jones and Wayne [7] measured the variation of 0 3 disappearance, <f>[0 3 ], with 0 3 pressure and determined a value of 0{ 1 D) production of 0.1. However, Castellano and

Schumacher [8] report a value of 1.0 based on results of similar experiments. Simonaitis

et al . [9] studied the reaction of 0( 1 D) with N 20 and concluded that the quantum yield

of 0( 1 D) was 0.5, from the amount of N 2 formed; and recently Kajemoto and Cvetanovic [10]

have studied the temperature dependence of O^D) production in the region of 3130A using the same techniques.

Lin and DeMore [11] measured the yield of isobutanol from the photolysis of 0 3 with

isobutane and reported a value of less than 0.1 for 4>[0 3 ] relative to the values below

3000A which is assumed to be unity. Thus it is evident that a careful study is needed to determine the quantum yield of OpD) from the photolysis of ozone.

3. Experimental

Moorgat [12] and Martin [13] have both studied the photolysis of ozone with CW arc

lamps using the reaction of 0( 1 d) with N 2 0 to subsequently produce excited state N0 2

. This

technique is used in the present investigation and involves the reaction of 0( l D) with

N 2 0 according to the following scheme:

0

3 + hv -y 0(

] D) + 0

2

0(

] D) + N

2 0 2N0

NO + 0

3 N0

2* + 0 + N0

2 + 02 N0o * + N0 o + hv

Thus, the amount of N0 2 * formed is directly related to the amount of 0( 1 D) produced. The

quantum yield of O^D) can now be determined by measuring the fluorescence of N0 2 * as a function of wavelength for the photodissociation of ozone. N 20 does not absorb in the

photodissociation region of approximately 3000A, and the N0 2 fluorescence occurs at wavelengths greater than 6000A so that conditions exist for an inherently clean experiment.

The experimental arrangement consists of a flashlamp pumped dye laser which is frequency

doubled to provide the photodissociation energy in the region of 2900A to 3200A (see

figure 1). The use of a tunable dye laser allows a high photon flux and a high spectral

resolution to give better definition to the nature of the quantum yield. Doubling is

accomplished by the use of an ADA and an RDP temperature tuned crystal and the N02 fluore- scence is monitored by standard photon counting techniques.

Results of this investigation will be discussed in light of previous experiments

and newly acquired data.

106
Flash Lamp Pumped

Tele Dye Laser

BS SHG a.

Trigger

Trigger

Figure 1

SHG Second Harmonic Generation

BS Beam Splitter

PD Photodiode

PM Photo Multiplier

MCA Multi-Channel Analyzer

Tele Beam Reducing Telescope

References

[ 1] Wanatabe, K. , Inn, E. C. V., and Zelikoff, M. , J. Chem. Phys., 21_, 1026 (1953).

[ 2] Inn, E. C. Y., and Tanaka, Y., J. Opt. Soo. Am., 43, 870 (1953).

[ 3] Viqroix, M., Contribution a

1 'etude experimental de 1 'ozone, Marson et oie, Paris

(1953).

[ 4] Ackerman, M., in Mesopherio Models and Related Experiments, D. Reidel, Ed., Dordrecht

(1971), p. 149.

[ 5] Griggs, M.. , J. Chem. Phys., 49, 857 (1968).

[ 6] Hampson, R. F. , ed., J. Phys. Chem. Ref. Data, 3 (1973).

[ 7] Jones, I. T. N. , and Wayne, R. P., Proa. Roy. Soo. A., 319 , 273 (1970).

[ 8] Castellano, E., and Schumacher, H. J., Z. Physik. Chem., 6_5, 62 (1969).

[ 9] Simonaitis, R., Braslonsky, S., Heicklen, J., and Nicolet, M., Chem. Phys. Lett., 19^, 601 (1973).

[10] Kajimoto, 0., and Cvetanovic, R. J., Chem. Phys. Lett., 37, 533 (1976).

[11] Lin, C. L., and DeMore, W. B. , J. Photoohem., 2, 161 (1973-74).

[12] Moorgat, G. K. , and Warneck, P., Z. Natruforsek, 30A, 835 (1975).

[13] Martin, D., Girnan, J., and Johnston, H. S., 167 ACS National Meeting, March 31 - April 5, Los Angeles, 1974.

107
ABSENCE OF N

2 0 PHOTOLYSIS IN THE TROPOSPHERE

D. H. Stedman, R. J. Cicerone, W. L. Chameides and R. B. Harvey

Department of Atmospheric and Oceanic Science

University of Michigan

Ann Arbor, MI 48109

Since the early studies of Bates and Hays (1967) it has been accepted that N 2 0

photolyzes in the troposphere. Thus, it was thought that not all of the N 2 0 released at ground level (by whatever natural source) could be transported intact to the stratosphere.

Once in the stratosphere a small fraction of the N 2 0 is attacked by 0( 1 D) to yield NO molecules, and this process furnishes the NO to conterbalance much of the natural production

of 0 3 [Crutzen , 1970; McElroy and McConnell , 1971]. From theoretical numerical models

(e.g., Crutzen , 1974; McElroy et al . , 1974) , an N 2 0 source of about 3.5 x 10 7 metric tons per year was needed to balance the altitude-integrated photochemical loss of N 2 0, almost

half of which loss was tropospheric.

Using the photodissociation cross section shown by Bates and Hays , the photodissociation rate, j(N 2 0)

N,0 + hv * 0( D) + N

when Rayleigh scattering is included, is calculated to be about 3 x 10" 10 sec

-1 in the lower

troposphere. Recently, however, Johnston and Selwyn (1975) have shown that the absorption

cross section is negligible in the A > 260 nm spectral region important for tropospheric

photochemistry. Their work would imply a zero value for j(N 2 0) in the troposphere.

While their work was in progress we decided to use an actinometer, previosuly used

to measure j(N0 2 ) (Jackson et al . , 1975), to directly measure j(N 2 0) at ground level. In the experiment for j(N 2 0) , pure N 2 0 at latm was used as the flowing gas in the quartz tube_

[Jackson et al . , 1975] with a flow just enought for the downstream NO detector (^30 cm 3 min _1

). The residence time in the tube was about 100 sec. If the N 2 0 were photolyzed then the product

0( 1 D) reacts according to:

0( 'D) + N

2 0 -> N2 + 02

-> 2N0

at equal rates [see data survey edited by Hampson and Garvin, 1975]. Thus a quantum yield

of one NO per N

2 0 photolyzed is expected. If j(N 20) ^ 3 x 10" 10 sec

_1 then the yield of NO would be 3 x 10 10 x 100 = 3 x 10~ 8 atm. Since the NO detector has a noise level ^10" 9 atm. then quantitive detection of j(N 2 0) was possible if Bates and Hays' cross sections applied.

When the experiment was first performed, exposure of the N 2 0-containing tube to sun- light generated significant quantities of NO. At this point, the calibrated NO detector was switched to the NO (NO + N0 2 ) mode, and an impurity 0.2 ppm N02 in the N 2 0 was determined.

The photolysis of this impurity was generating the NO, and when the impurity was removed

with an ascarite trap the NO generated by sunlight on N 2 0 became undetectable. Figure 1 shows the trace from such an experiment, performed at a time when j(N0 2 ) was also being

measured.

108
Results of twenty-seven measurements argund_solar noon gave an upper limit for

N 20) determined experimentally as <1 x 10

-1 *sec

_1

. Thus, tropospheric photolysis of N 2 0

as been shown to be negligible by direct experiment, as predicted by the concurrent specroscopic study of Johnston and Selwyn. With the Bates and Hays (1967) cross sections a (N 2 0) = 10" 11 sec~ 1 could only be observed with an 0 3 column of >3 x 10~ 19 cm~ 2

, three or

our times larger than the normal ozone content.

0.007

- 0.005

o2

0.003

PURE N20

A

\

N20 + Qlppm M02 PURE N20

/ Yv I \ f

/ V

« 0c+ 21, 1975

1=00 p.m. EOT

20 10 20 0 —TIME min

4=

20

XIQQ. 10 o

z

10

igure 1. The lower trace and right hand axis shows a record of NO formation in the solar

photolysis of N 2 0, with and without a small impurity of N0 2. Three traces are shown with

sunlight intercepted by a black cloth at the beginning and end of each trace (dotted

lines). The upper trace shows the concurrent measured value of the solar photolysis rate

of N0 2 , j(N0 2 ), on the left hand axis. NO production in the absence of impurities is not distinguishable from the background.

References

Bates, D. R., and Hays, P. B., Atmospheric nitrous oxide, Planet. Spaoe Soi. 1_5, 189-197 (1967).

Crutzen, P. J., The influence of nitrogen oxides on the atmospheric ozone content,

Quart. J. Roy. Met. Soc. 96, 320-327 (1970).

Crutzen, P. J., A review of upper atmospheric photochemistry, Can. J. Chem. 52_, 1659-1581 (1974).

Hampson, R. F., and Garvin, D., (editors), Chemical kinetic and photochemical data

for modelling atmospheric chemistry, NBS Technical Note 866 National Bureau of

Standards, Washington, DC, 113 p., 1975.

Hunten, D. M. , Residence time of aerosols and gases in the stratosphere, Geophys.

Res. Lett. 2, 26-28 (1975).

Jackson, J. 0., Stedman, D. H., Smith, R. G. , Hecker, L. H., and Warner, P.O., Direct

N0 2 photolysis rate monitor, Rev. Sci. Instrum. 46_, (4) 376-378 (1975).

Johnston, H. S., and Selwyn, G., New cross sections for the absorption of near ultraviolet radiation by nitrous oxide (N 20), submitted to Geophys. Res. Lett., 1975.

109
MC

u 1r°y,MnB " and McConnell > J- C., Nitrous oxide: a natural source of strato spheric NO, J. Atmos. s&i. 28, 1095-1098 (1971).

stratoMcElroy, M. B Wofsy, S. C, Penner, J. E. , and McConnell, J. C, Atmospheric ozone possTble 1mpact of stratospheric aviation, J. Atmos. Sai. |l_, 287-303 (1974)!

110
TEMPERATURE DEPENDENCE OF 0('D) REACTIONS OF ATMOSPHERIC IMPORTANCE

J. A. Davidson and H. I. Schiff

York University, Toronto, Ontario, Canada

and

G. E. Streit, A. L. Schmeltekopf and Carleton J. Howard

Aeronomy Laboratory

NOAA Environmental Research Laboratories

Boulder, CO 80302

The reactions of O^D) with N 2 0, H 2 0, H 2 » and CH^ are now well established as the

principal sources of the radical species (NO, OH) responsible for the natural destruction of

stratospheric ozone. Because of the importance of these reactions, there have been many

determinations of the relative rates of the reactions of O^D) with these and other species

[1_2] 1

. On the other hand, systematic investigations of absolute reaction rates have been

limited to those conducted in Husain's [3-6] lab and our own [7].

In order to assess the relative importance of O^D) deactivation and reaction to produce radical species under stratospheric conditions we have undertaken an investigation

of the temperature dependence of the reactions of O^D) of stratospheric importance.

The apparatus used in these studies has been described in detail previously [7] and

hence will only be discussed briefly here. A frequency quadrupled Nd-YAG laser, yielding

1-4 mj at 266 nm in a 10 nsec pulse, was employed to photolyze O3 to produce 0( ! D) in a temperature regulated cell. The time evolution of the 0( : D) concentration was then

followed by means of the emission at 630 nm. Because 0( 1 D) has a long radiative lifetime

(t = 100 sec) and reacts at near collision frequency with most gases, signal averaging

techniques were required to obtain a statistically meaningful decay trace. Our treatment

of this data in order to extract rate constants has been previously reported [7].

In Table I our room temperature rate constants are compared with the results from

other laboratories. With the exception of 0 3

, the results obtained in Husain's lab using

time time resolved attenuation of resonance absorption are significantly higher than our

results. We have previously suggested that this discrepancy may be due to the use of a rather large correction to Beer's law employed by Husain. Subsequently, Phillips [13] has

calculated that in the case of the 0 3 rate determination this correction was appropriate,

while in all other cases the correction should have been much smaller, hence leading to a reduction of the reported rates by about a factor of 2.

T

Figures in brackets indicate the literature references at the end of this paper.

Ill
Table 1. Summary of 0( D) absolute rate constant measurements (298 + 2°K)

0

3 o

? N

2 C0

2 N

20

H

20 CH4

H

2

D

2 HC1 NH3 Reference

(10"^°cm-V molecule-sec)

5.3+2.6 0.74 + .15 0.54+.15 1.8+.4 2.2+ A 3.5+. 6 4.0+.9 2.9+.5 1 ,2

3.3+ 0.003 0.22+.1 0.3+.13 8,9,10

0.63+.3 11

2.5+1.0 12

2.7+.2 0.70+.05 0.69+.06 2.1+.2 2.2+.2 3.0+.3 3.1+.4 2.7+. 3 1.S+.2 3,4,5,6

2 4+ 1 0 41+ 05 0.30+.01 1 .2+.09 1 .4+.1 2.1+1.0 1.3+.3 1.3+.05 1.3+.05 1.4+.3 3.4+.3 This Work*

*Error limits imply precision only. Accuracy estimated to be +15%.

The results of our temperature dependence studies are summarized in Table II. Those

molecules which deactivate 0( 1 D) primarily through a reactive channel (N 2 0, H 2 0, CH^, H 2 , NH 3

, HC1 and O3) exhibit rate constants which are invariant with temperature. The rate constants for those molecules which deactivate Q( l d) through an energy exchange process

(N 2 , 0 2

, and C0 2 ) increase with decreasing temperature. In the case of C0 2 this increase

extends only to about 200K below which rate constant remains independent of temperature.

Table 2. Arrhenius parameters for the deactivation of 0( D) (k = Ae ) Molecule A(cm /molecule sec) E(cal/mole) Range(°K)

N

2 0 1 1 X ID"

10 204 - 359

H

2 0 2 3 X 10- 10

253 - 353

CH

4

1 4 X 10- 10 198 - 357

H

2

9 9 x ID"

11 204 - 352

NH

3

2 5 x io10 204 - 354

HC1 1 4 x ID"

10

199 - 379

°3 2 4 x IO"

10 103 - 393

N

2 2 0 X 10" 11 -214 + 15* 104 - 354

°2 2 9 x IO"

11 -134 + 20* 104 - 354

6 8 x IO"

11 -233 + 47* 200 - 354

c and

1 2 X IO"

10 139 - 200

* One standard deviation Accuracy is estimated to be + 20%

112
This research was conducted in the NOAA Aeronomy Laboratory and was supported in part

by NASA contract NGR 52-134-005.

References

Cvetanovic, R. J., Can. J. Chem. 53, 1452 (1974).

Garvin, D. , and Hampson, R. F. , editors, NBSIR 74-430 (1974).

Heidner, R. F. , III, Husain, D., and Wiesenfeld, J. R., Chem. Phys. Lett. ]6_, 530 (1972)

Heidner, R. F., Ill and Husain, D. , Nature Phys. Sai. 241 , 10 (1973).

Heidner, R. F., Ill and Husain, D. , Int. J. Chem. Kinet. 5, 819 (1973).

Heidner, R. F., Ill and Husain, D. , Int. J. Chem. Kinet. 6_, 77 (1974).

Davidson, J. A., Sadowski, C. M., Schiff, H. I., Streit, G. E., Howard, C. J.,

Jennings, D. A., and Schmel tekopf , A. 1 . , J. Chem. Phys. 64, 57 (1976).

Biedenkapp, D., Hartshorn, L. G., and Bair, E. J., Chem. Phys. Lett. 5_, 379 (1970).

Snelling, D. R., and Bair, E. J., J. Chem. Phys. 47, 228 (1967).

Snelling, D. R., and Bair, E. J., J. Chem. Phys. 48, 5737 (1968).

Noxon, J. F., J. Chem. Phys. 52, 1852 (1970).

Gilpin, R., Schiff, H. I., and Welge, K. H., J. Chem. Phys. 55, 1087 (1971).

Phillips, L. F., Chem. Phys. Lett. 37, 421 (1976).

TT3
MODELING STRATOSPHERIC PHOTOCHEMISTRY AND KINETICS

John McAfee and Paul J. Crutzen

National Center for Atmospheric Research

Upper Atmospheric Research Branch

Boulder, CO 80303

An analysis will be presented of our chemistry. The effect of uncertainties in

sis on the impact of human activity on the

present understanding of stratospheric photorate coefficients will be discussed with emph ozone layer.

114
THE RATE CONSTANT FOR 0 + NO + M FROM 217-500 K IN FIVE HEAT BATH GASES

J. V. Michael 1

, W. A. Payne and D. A. Whytock 2 Astrochemistry Branch

Laboratory for Extraterrestrial Physics

NASA/Goddard Space Flight Center

Greenbelt, MD

The absolute rate constant for 0 + NO + M has been measured previously be a variety

of techniques [3] 3 and is of interest in atmospheric modelling [4]. ost workers have

used the discharge flow method with chemi 1 uminescent detection of [0]; however mass spectrometric and e.s.r. detection have also been employed. Values relative to a known reaction

have also been reported from steady state photochemical experiments. There are six real

time studies. Three utilize phase sensitive detection and two make direct measurements

from chemi luminescence [3,4]. There has been only one direct measurement with the 0 atom

resonance fluorescence technique, at room temperature in Ar [5]. The currently accepted

value for the temperature dependent rate constant is based primarily on the work of Klein

and Herron and Clyne and Thrush and is given from 200-500 K in Arrhenius form as k. = 3.0 x 10" 33 exp(940/T) cm 6molecule" 2 sec 1 (M = 0 2 , Ar) [3,4].

z

The apparatus used in the present study has_been described previously [6]. Binary

mixtures of NO in heat bath (2.5 x 1

-tf

<_ X NQ <_ 10" 2

) were repetitively flash dissociated

(A >_ 142 nm, x = 2.5 usee) in a flowing system yielding N( 4 S) and 0( 3 P) after the method of

Stuhl and Niki [7]. Further 0( 3 P) is produced by the fast reaction, NC*S) + NO +

N 2 + 0( 3 P). This reaction is effectively complete before the first data are collected

(delay time _> 300 usee). Flash energies, and therefore concentrations, are sufficiently

low so that the secondary reaction, 0 + N0 2 * 0 2 + NO, is totally negligible.

Fluorescent photons are repetitively counted and stored with a multichannel analyzer

under a given set of experimental conditions. The resulting decay constant can be expressed

as,

"observed

= k tTM + (D

where L is the termolecular rate constant for 0 + NO + M, and k . is the diffusional loss

rate constant out of the viewing zone. At a given pressure and temperature, at least three

decay constants are obtained as a function of [NO]. Plots of k . agains [NO] yield k. . = k.[M] as the linear least squares slope, and the intercept gives R , . Plots of k. . against

[M] then yield k

f as the linear least squares slope for a given temperature. In nearly all cases the least squares standard deviation in the intercept is within one standard deviation of zero. Determinations at six temperatures (217 K, 252 K, 298 K, 355 K, 418 K, and

500 K) are then carried out for any given heat bath gas. The heat bath gases used are N 2 , He, Ne, Ar, and Kr.

i NAS/NRC Senior Resident Research Associate.

2

' Catholic University, on leave from the Chemistry Department, The University of Essex,

Colchester, England.

3 Figures in brackets indicate the literature references at the end of this paper.

115
The results for M = N 2 are shown in Table I and can be expressed in Arrhenius form

as k. = (15.5+2.0)xl0" 33 exp(582+37)/T) cm 6molecule~ 2 sec

_1

. This result can be directly

compared to_that_of Klein and Herron [8] who give (3.97+0.55)xl0~ 33 exp( (971+50)/T) cm6 molecule~ 2 sec

-1 (M = N 2 ). The room temperature values are in good agreement, but discrepancies occur for all other temperatures above room temperature, the largest being at 500°K where the present results are 1.8 times larger. The temperature dependence of k t with the other heat bath gases appears to be similar.

Table I. Rate data for 0 + NO + N, -* N0o + N

T/K k^/10" cm molecule

217 20.8 + 1.2

a

252 17.2 + 2.3

298 11.5 + 0.9

355 7.40+ 0.68

418 6.24+ 0.23

500 4.92+ 0.25

a Quoted error limits are the standard deviation.

References

[ 1] NAS/NRC Senior Resident Research Associate.

[ 2] Catholic University, on leave from the Chemistry Department, The University of Essex,

Colchester, England.

[ 3] Reviewed by D. L. Baulch, D. D. Drysdale, D. G. Home, and A. C. Lloyd, Evaluated

kinetic data for high temperature reactions, Vol. 4, Butterworths , London, 1972.

[ 4] Hampson, R. F., Jr., and Garvin, D., editors, Chemical kinetic and photochemical data

for modelling atmospheric chemistry, NBS Technical Note 866, U.S. Government Printing

Office, Washington, DC, 1975.

[ 5] Slanger, T. G. , and Black, G., J. Chem. Phys. 53, 3717 (1970).

[ 6] Klemm, R. B., and Stief, L. J., J. Chem. Phys. 61_, 4900 (1974).

[ 7] Stuhl, F., and Niki, H., J. Chem. Phys. 55, 3943 (1971).

[ 8] Klein, F. S., and Herron, J. T., J. Chem. Phys. 4J_, 1285 (1964).

116
DETERMINATION BY THE PHASE SHIFT TECHNIQUE OF THE TEMPERATURE DEPENDENCE OF THE REACTIONS

OF 0(

3 P) WITH HC1 , HBr, AND HI

D. L. Singleton and R. J. Cvetanovic

Division of Chemistry

National Research Council of Canada

Ottawa, Canada

A few studies have been reported recently on some specific aspects of the reaction of

ground state oxygen atoms with hydrogen halides, 0 + HX * OH + X, namely, the effect of HX vibrational energy on the reaction rate [I] 1 and the relative yields of excited products

[2], 0H(v=0,l) and X( 2 P

3 / 2 ,V2) • The temperature dependence of the bulk rate constants has

received little attention, and, in the case of HC1 , where more than one determination has

bee reported, the results are in poor agreement.

We have used a phase shift technique to determine the rate constants for the reactions

of 0(.

3 P) with HC1, HBr, and HI between 298-554 K. Also, as an incidental part of this work,

the rate constant of the reaction 0( 3 P) + NO + M > NO2 + M CM = HC1 ) has been determined

between 298-398 K.

2. Experimental

As described previously [3] ground state oxygen atoms are formed by modulated photosensitized decomposition of nitrous oxide in a flowing mixture of N 2 0, NO, HX, and Hg

according to the simple reaction scheme:

Hg^SoJ^jmUHgt3 ^)

HgC

3

P-|) + N

20 v Hg(

] So) + N

2 + 0(

3 P) (1)

0(

3 P) + HX OH + X (2)

0(

3 P) + NO + M N0

2 + M (3)

The expression for the phase shift, 4>, between the incident 254 nm light, modulated at frequency v, and the chemiluminescence from reaction (3) is

f» - k2CHCl] k

3[N0]M .

The phase angles are measured with appropriate filter - photomultipl ier combinations

and a lock-in amplifier.

T Figures in brackets indicate the literature references at the end of this paper.

117
Two methods of modulation were used. The first, for the HC1 reaction below 500 K where frequencies of 30-90 Hz were required, involved mechanically chopping the light

from a rf powered low pressure mercury lamp. In the second method, for all the other

reactions, which required frequencies of 1.3-6 kHz, the amplitude of the rf that powered the

lamp was modulated.

Total pressues were between 30-70 Torr and were varied by a factor of two at each

temperature except for HC1 at 298 K where the total pressure was 16-18 Torr. Approximate

relative concentrations [N 2 0] : [NO] : [HX] were: HC1 (T < 500 K) , 1:0.001-0.01:0.1-1;

HC1 (T > 500 K) , 1:0.1:0.1; HBr, 1:0.1: 0.01-0.1; HI, 1:0.1: 0.001-0.01. The residence

time of the gases in the reaction cell was < 0.4 sec for the HI experiments; < 2.5 sec for HBr and HC1 (T_> 500 K);_and < 0.2 sec for HC1 (T < 500 K). The total flow rates were betweenJO" 14 to 10" 3 mole sec" 1

. The rate of production of 0-atoms was about 1 x 10 12 atoms

cm" 3 sec

_1 as determined by gas chromatographic analysis of the products of the 0 + 1-butene

reaction.

3. Results

For the reactions with HC1 (T > 500 K), HBr, and HI, the rate constants k 2 and k 3 were determined from intercepts and slopes of plots of 2Trv/[HX]tani}> vs. [N0][M]/[HX] . The values

of k 3 (M = N 2 0) were within about 10% of the previously determined values. The standard

deviation of each value of k 2 was 7-9% for HC1 and 2-5% for HBr and HI. The values of k 2 are shown in figure 1.

2.0 2.5 3.0

1000/ T(K~')

3.5

Figure 1. Arrhenius plots of k 2 ; • this work; + Ref. 4; Ref. 5; ' Ref. 6.

118
The rate constants plotted in figure 1 cover a range of reactivity of four to five

orders of magnitude. The reactions of ozygen atoms with HC1 (T > 500 K) and with HBr and

HI are sufficiently fast to be measured by the procedure described previously [3]. The

reaction with HC1 at temperatures smaller than about 500 K is much slower and required a modified procedure.

The reaction with HC1 (T < 500 K) required lower frequencies, but at these longer

time scales, diffusional loss of 0( 3 P) occurred from the viewing zone of the photomultiplier that detected the chemiluminescene. In this case, eq. (1) becomes

2TTv/tan<|> = k 3 [N0][M] + kp/[M] in the absence of HC1 and assuming a first order diffusional

loss of oxygen atoms. Tne first order diffusion rate constant, kg, was obtained in ex- periments without HC1 present from the interecept of plots of 27rv[M]/tan<t> vs. [NO]. [M] 2 at each temperature below 500 K. The slopes gave values for k 3 (M = N 2 0) which were in

good agreement with previously determined values. An additional complication arises because

the higher HC1 concentrations (which were even as high as the N 2 0 concentrations at 298 K and 330 K) meant that HC1 was a significant third body in reaction 3. Incorporating k D and a, (the third body efficiency for HC1 ) , into the simple mechanism, we obtain, in place

of eg. (1), 2irv/tan$ = k 2 [HCl] + k 3 [N0]([N 2 0] + o[HCl]) + k p/[M]. Plots of 2TM/tan*-k

D /[M]

vs. [NO], in experiments where [NO] was varied while all other concentrations were helH constand, gave k 2 [HCl] as the intercept. From the slope, the value of a was obtained at each

temperature (298-443 K) using the known concentrations of N 2 0 and HC1 , and the previously

determined value of k 3 (M = N 20).

The results are given in figure 1. The Arrhenius expression for the 0 + HBr reaction

is (8.09 + 0.86) x 10 9 exp(-3.59 + 0.08 kcal mole'VPJ) 1. mole^sec" 1. The plot of lnk 2 vs. 1/T for 0 + HC1 is not linear. A one parameter empirical expression for the results

over the temperature interval of these experiments is k 2 = (3.7 +2.1) x 10" 13 tC7

' 11 -' 09

)

1 .mole

_1 sec"^ , which was obtained from a least squares treatment of lnk 2 vs. InT. All the

indicated uncertainties are one standard deviation.

At room temperature, HC1 is 1.7 times as efficient as N 2 0 as a third body in reaction

3. The Arrhenius expression for reaction 3, with M = HC1 , is (3.3 +1.0) x 10 10 exp

(0.6 + 0.2 kcal mole 1 /R1) between 298-398 K, which compares with the previously determined

expression for M = N 2 0, 6.12 x 10 9 exp(1.23 kcal mole

_1 /PJ) 1

2mole" 2 sec~ 1

.

A comparison is made with some literature values for k 2 in figure 1. The results for

HBr are in good agreement with Smith's values [4] and with Glass's room temperature value

[2], 2.7 x 10 7

1 .mole

_1 sec-1

. For HI, the only available comparison is_with the approximate room temperature value reported by Glass [2] (^1 x 10 9 l.mole _:L sec 1 ), which is in

close agreement. For HC1 , the present values agree well at higher temperatures with Smith's

values but are somewhat larger at lower temperatures.

Some recent experiments in which HC1 was replaced by argon indicate that the rate constants for the 0 + HC1 reaction below 500 K may have to be revised downward. However,

further experiments are required on this part of the present work.

References

[ 1] Karny, Z., Katz, B., and Szoke, A., Chem. Phys. Lett. 35, 100 (1975); Arnoldi, D.,

and Wolfrum, J., Chem. Phys. Lett. 24, 234 (1974); Quigley, G. P., and Wolga, G. J.,

J. Chem. Phys. 63, 5263 (1975).

[ 2] Takacs, G. A., Glass, G. P., J. Phys. Chem. 77, 1182 (1973).

[ 3] Singleton, D. L., Furuyama, S., Cvetanovic, R. J., and Irwin, R. S., J. Chem. Phys.

63, 1003 (1975).

[ 4] Brown, R. D. H., and Smith, I. W. M. , Int. J. Chem. Kinet. 7, 301 (1975).

[ 5] Wong, E. L., and Belles, F. E., NASA Technical Note D-6495 (1971 )

[ 6] Balakhnin, V. P., Egorov, V. I., and Intezarova, E. I., Kinetics and Catalysis ]2_,

258 (1971).

119
HIGH ENERGY PULSED LASER PHOTOLYSIS OF SOME CHROMIUM (III) AND COBALT (III) COMPLEXES

Arthur W. Adamson, A. R. Gutierrez, R. E. Wright and R. T. Walters

A current interest of this Laboratory is in the study of excited state processes

through the use of a high energy pulsed laser system. Results of three types of experiments

will be reported. Figure 1 allows illustration of the first two. The photochemical literature for Cr(III) complexes so far consists just of quantum yield measurements [l] 1

. The

yield for photochemistry from the first thermally equilibrated excited (thexi) quartet

state C

4 !-! 0

) is given by i>i = rj> k x where <f> is the yield for producing h Ti°, k is its

chemical rate constant, and l/x|z (vQ + k g + k ), k and k being the nonradiative

and radiative deactivation rateq constSnts respectively. Arternatf vely, photochemistry may

derive from reaction of the first doublet thexi state ( 2 D in 0. ), with i>2 = <t

> d

l< cd

T d' wnere

k . is the chemical rate constant and 1/t . = (k , + K , + k ,/. In addition, Tt has been

postulated that the 2 D and 4 L!° states may therffiSlly ThtercoHVert [2].

Department of Chemistry

University of Southern California

Los Angeles, CA 90007

Ligand lield terms lor On symmetry

Chemical

reaction

Internal conversion

Chemical

reaction — Radiadonless

deactivation

f by medium

Chemtca I reaction

Energy Distortion

Figure 1

Figures in brackets indicate the literature references at the end of this paper.

120
Clearly, a full elucidation of such systems requires determination of the separate

rate constants. It is certanly insufficient to find ^ + <t> 2 or even the separate yields.

These last have been inferred from quenching experiments [1,2].

The first type of laser study is that of the determination of ligand and solvent

effects on through studies of emission lifetimes. Weak room temperature emission has

been reported tor several Cr(III) complexes [3,4] and additional results are included in

Table 1. These demonstrate that emission lifetimes of various complexes vary greatly with

the type of ligation. We attribute these variations more to changes in k nr (the rate constant for non-radiative decay of the first doublet excited state) thant<j> changes in ke (the emission £ate constant). Thus, at 77 °C, the lifetimes for Cr(NH 3 ) 6

3 and

Cr(NH 3 ) 5 (H 2 0) 3 are 56 and 70 psec, respectively, presumably approximating the l/k

e values

[5]. At room temperature, x for the first complex has dropped to 1.8 ysec or 31 fold, while

that for the aquopentaamine has dropped to less than 20 nsec, or more than 3500 fold. We infer that coordinated water enhances the non-radiative deactivation rates and speculate

that this is due to hydrogen bonding into the solvation shell which establishes good vibrational communication with the medium.

Table 1. Emission lifetimes for Cr(III) complexes at 25 °C

Complex t, ns Complex t, ns (in water) (in water)

Cr(bipyr)

3 3+

45,000 Cr(NH

3

) 5 NCS

2+

140

(148 in 60% AN)

Cr(NH

3

) 6 3+

1,800 t-Cr(NH

3

) 2 (NCS)

4

" 7

(.120 in AN)

t-Cr(en)

2 )(NCS)

2 + 1,490 Cr(NCS)6

3 "

< 20

(30 in AN)

t-Cr(en)

2

F

2+ 1,360 Cr(acac)

3

) 1.5 (Windsor)

Cr(en)

3 3+

1,300

AN = acetonitrile. Cr(en)

2 (H2 Q)F

2+

, Cr(NH

2 )r(H

2 0)

3+

, cis-Cr(2,3,2-tet)Cl

2

+

, cis-a-Cr(trien)

Cl

2

: t < 20 ns. Cr(urea)g3. no observable emission

Figure 2 shows a point by point emission spectrum for Cr(NH 3 ) 6 3+

. The long wavelength

peak is shifted by 160 cm" 1 on deuteration, in essentially mirror image behavior to that

found for the doublet adsorption band. Clearly, it would be better to have more detailed

emission spectra, but the general effect seems clear. We are, however, very interested in

obtaining such spectra for other complexes, and in other solvent media.

Table 2 shows the marked solvent dependence that we have found for the emission lifetimes of trans- Cr(NH 3 ) ? (NCSK~ (reinecke salt). Note that water and hydrogen bonding

solvents materially shorten the lifetime relative to that in less polar and non-hydrogen

bonding media. Figure 3 shows the data for water-acetonitrile mixtures plotted vs. mole

fraction and according to solvation sphere composition as estimated by Langford [6]. This

plot is linear, obeying Stern-Volmer kinetics. Thus, k nr(j behaves as a bimolecular rate constant, first order in complex and first order in solvent. The yields for thiocyanate

.release reported by Langford increase with water content, contrary to expectation were

reaction occurring from the doublet or emitting state. This is an acceptable finding, however, since it is generally recognized that most of the reaction occurs from the first

quartet excited state. This interpretation does imply, however, that the doublet and

quartet states are not in rapid thermal equilibrium.

121
122
Table 2. Emission lifetimes of trans-Cr(NH 0 ) «(NCS) ^20 °C

Solvent t ,ns Sol vent t ,ns JU 1 VCII L t n c

CH,CN *(AN)

3

K

' 177 CH o N0o

3 2 105 y 1 Jr v-c i U 1 CO

CH

2 C12** 175 DMSP 89 ate Lit, aL I

acetone 158 pyridine 87 (C H ) NM

* 2 5'2

40% AN + <f> 153 DMF 78 CH^OH

40% AN + CHC1

,

3 135 THF 39

AN 120 C

2 H5 0H 31 H

2 0 7 *deaerated

** solubilization by 18-6 crown ether

MOLE FRACTION OR n/nQ

Figure 3. Trans-Cr(NH

3

) 2 (NCS)

4

" in acetonitrile-water mixtures.

123
Several of the systems have been studied over a range of temperatures; some data are sown in tables 3 and 4. Fairly high activation energies are found. The values are in

general agreement with those obtained from phosphorescence yields by Kane-Maguire and Langford. We also have I 0 values, that is, emission intensities immediately following the

stimulating flash. Preliminary results indicate that these also show a temperature dependence, contrary to simple expectation. We need, however, to make sure that the emission

spectra are not changing with temperature, perhaps an unlikely effect, but one which could

account for the I 0 changes.

Complex

Cr(en;

3+

Cr(bipyr)

3

3+

Table 3. Emission parameters for Cr(III) amines

Solvent

H

2 0

D

20

H

2 0

E*, kcal/mole

Emission Relative I

10.2

12.0

7.5

6.8

0-

Table 4. Emission parameters for trans- Cr(NHg)

2 (NCS)4

'

Solvent E*, kcal/mole

Emission Relative I Q Viscosity

CH

3 0H 12.5 -1.0 2.6

CH

3 CN 11 .4 -7.3 1 .6

(C

2 H5

) 2 NH 9.7 -4.6

Acetone 12.7 -3.1 1.5

The second and third types of study are ones for which the laser system was designed.

The system comprises a Q-switched, gated, amplified, and frequency doubled Nd oscillator;

maximum output is 1 to 1.5 J at 530 nm with 20 nsec halftime. If the beam is collimated to

3 mm diameter, the nominally absorbed number of photons (i.e. assuming no ground state

bleaching) can be made greatly to exceed the number of molecules of compound.

This capability makes possible the investigation of Ca) two (successive) photon

processes and (b) single pulse photolyses in which primary product formation is monitored.

As an example of Ca), we obtain T<j for trans-Cr(NH 3 ) 2 CNCS) lt in acetonitrile both from

emission decay and from excited state absorption (for which the spectrum has been reported

[7]). For several systems excited state absorption has also been observed as an excess attentuation of the transmitted pulse, the pulse shape being skewed if the lifetime of the

absorbing excited state is greater than that of the pulse. A third experiment of type (a)

is that of two-photon processes involving Co C 1 1 1

) ammines. As illustrated in figure 4,

adsorption of a second 530 nm pulse should populate a charge transfer excited state, the

evidence being the appearance of Co(II) as photolysis product. The effect has been re- ported for Co(NH 3 )fi 3 using a highly focussed, multiply pulsed N 2 laser (337 nm) [8]. We find it for Co(NH 3 ) 5 Br2 after a few unfocussed pulses at 530 nm.

124
100

POSSIBLE TWO PHOTON PROCESS

FOR A Coin) COMPLEX

530 nm

80

310 nm 'CT —co(m

3 60 CT <f>~0.Z

UJ \5 40

(B) — oquotion

<f> « I 20

0

Figure 4. Possible two photon process for a Co(III) complex.

Type (b) experiments are in progress with various Cr(III) ammines and acidoammines.

We observe the rise time of primary photo-product formation by means of a suitably monochromated monitoring beam. The <j> 2 process should lead to product with an appearance time

tj, which typically is in the short microsecond range. The ^ process may produce product

much more quickly; the rise time in this case should yield the so far inaccessible t .

[ 1] Zinato, E., Concepts of Inorganic Photochemistry, A. W. Adamson and P. F. Fleischauer,

Wiley, 1975.

{ 2] Ballardine, R., Varani , G., Wasgestian, H. F., Morri , L., and Balzani, V . , J. Phys.

Chem. 77, 2947 (1973).

|[ 3] Kane-Maguire, N. A. P., and Langford, C. H., Chem. Commun. 895 (1971).

,'[ 4] Adamson, A. W., Geosling, C, Pribush, R., and Wright, R. , Inorg. Chim. Acta 16,

L5 (1976).

[ 5] Chatterjee, K. K. , and Forster, L. S., Spectrochim. Acta 20, 1603 (1964).

[ 6] Sastri , V. S., Henwood, R. W., Behrendt, S., and Langford, C. H., J. Amer. Chem.

Soc. 94, 753 (1972).

[ 7] Ohno, T., and Kato, S., Bull. Chem. Soc. Japan, 43_, 8 (1970).

[ 8] Cunningham, K. M. , and Endicott, J. F., Chem. Comm., 1024 (1974).

q References

125
STUDIES USING A COMBINATION OF FLASH PHOTOLYSIS AND PULSED MAGNETIC INDUCTION:

APPLICATION TO THE NO, RADICAL IN AQUEOUS ACID SOLUTION AT 25 °C

T. W. Martin and M. V. Stevens

Department of Chemistry

Vanderbilt University

Nashville, TN 37235

1. Introduction

The photolysis of (NH 4 )2Ce(N0 3 ) 6 in acid solution is a convenient means of studying

the chemistry of the NO3 radical, one of the strongest oxidants known. Over the years we

have learned a few things about its absorption spectrum [I] 2

, decay kinetics [2], and

competitive electron transfer with various Ce(III) species [3]. More recently, by using

a pulsed laser and kinetic spectroscopy, we have obtained deeper insight into the comprehensive mechanism of the generation and decay of the N0 3 radical in nitric and perchloric

acid solutions at room temperature [4]. It was found that the 20 ns pulse from a 4-joule

frequency-doubled ruby laser was virtually instantaneous in comparison to the thermal rate processes involved. Hence, it was possible to make a kinetic analysis over the complete

lifetime of NO3 in these solvents. These studies revealed that an excited-state metastable complex, [Ce(III) . . .N0 3 ]*, is a precursor to the N0 3 radical. Thus, the earlier

published mechanism [5] must be enlarged as follows to account for this complex:

Ce(IV)N0

3

" + hv [Ce(III). . .NO3]* (1)

[Ce(III). . .NO3]* — Ce(IV)N0

3

" (2)

[Ce(III)...N0

3 ]* —2— Ce(III) + N0

3 (3)

Ce(III) + N0

3 — Ce(IV)N0

3

" (4)

N0

3 + N03 —

N

2 06 (5)

N

2 06 + 2 Ce(IV) 2 N0

2

+

+ 0

2 + 2 Ce(III) (6)

N0

2

+ + N0

3

" + H

2 0 2 H0N0

2

2 H+

+ 2 N0

3

" (7)

The measured rate constants and quantum yields derived from our analyses, together with a new value of the extinction coefficient for N0 3 are summarized in table 1. From these

data, with regard to the generation of N0 3

, we infer that k 2 increases on dilution of

H0N02 because of increased coordination of H 2 0 with Ce(IV); whereas the opposite is true

for k 3 which increases with increased coordination by N0 3 ~. From these trends and since

^03 = k 3/(l<2 + k 3 ), it is not surprising that $nq 3 becomes constant and equal to unity

when [H0N0 2 ] >. 6M. As to the decay modes, ki+ follows similar trends to k 3 , but k 5 is essentially constant and independent of [HONO2].

Duckworth Pathology Laboratory, Methodist Hospital, 1265 Union Avenue, Memphis, TN 38104.

2 Figures in brackets indicate the literature references at the end of this paper.

126
Table 1. Summary of rate constants and quantum yields for the photolysis of

CNH

4

) 2 Ce(N0

3

) 6 in aqueous acid solutions at 25 + 1 °C

Solvent

(k ? + a)

X 10 ^s

1

(k +

L x 10

a)

(k 4 ±

x 10

6M

1

a)

s _1

(k

5 ±a) b

x lO'V^" 1

( *N0 3

) C

1.00 M H0N09 5.12 + .37 4.36 + .37 .600 + .089 d .46 + .04

2.00 M H0N0,

c 2.14 + .34 6.41 + .34 .743 + .059 d .75 + .04

3.00 M H0N0 9 .63 + .05 6.76 + .21 1.08 + .04 .91 + .11 .92 + .02

5.00 M H0N0

2

<v> 0 7.74 + .20 1.78 + .10 .93 + .09 1 .00 + .01

12.00 M H0N0

2 % 0 8.75 + .22 4.08 + .16 .92 + .08 .99 + .01

3.00 M H0C10

3 6.04 + .62 2.01 + .62 .372 + .050 d .25 + .02

1.00 M H0C10, d d .352 + .070 d .11 £ .02

Initial samples also contained about 1.00 mM Ce(IV) and 3.00 to 10.00 mM Ce(III).

! 3 Calculated assuming eCNO^g.^

nm

= 4.80 + 48 M"^cm"^ and an effective pathlength, £ £

= .55 cm.

'Based on = k^/ ( + k^) and assuming o^q is maximum as well as equal to 1.00 in 6 M H0N0

2

. MU3

J u MU

3

Datum not measured.

2. Magnetic Studies

Concurrent with our NQ 3 work, we developed an apparatus which can combine either

:onventional flash photolysis or pulsed laser techniques with the simultaneous use of a ligh field pulsed magnet capable of generating inducations to 200 kilogauss (kG). The

)urpose was to test if magnetic inductions could in any way truly influence either the

:xcited or the ground states of species in solution at 25 °C. So far, we have measured

:wo bona fide magnetic effects. One had to do with the classic photoisomerization of

til bene [6] where the inducations stimulated the S > T intersystem crossing mode in

:he excited state as efficiently as benzophenone*; the other is the reverse effect of

Simulating the T > S rate determining step monitored by k k in the above mechanism.

Because of the novelty and lack of theory for this experimental approach, we have

lesitated to publish our results. But this second effect involving N0 3 has convinced us

if the possible general applicability of this method for probing mechanism in both the

iround and excited state or organic as well as inorganic systems.

D In table 2, where the [H0N0 2 ] is relatively high, we note that the size of k^ is mly slightly enhanced, about 6 percent, which is probably due only to a small magnetofriction effect on the concentration of paramagnetic species in the ground state. How-

:ver, in perchloric acid (table 3), where the coordination sphere of Ce(III) is richer

n H 2 0 than N0 3 ", we observe a large specific rate enhancement that is quite non-linear

larticularly at B > 75 kG. We suspect this enhancement at B > 150 kG will approach an

ipper limit of 3, because we believe k 4 normally monitors the electron transfer process

nvolving only one of the levels of the ground triplet recombination complex,

127
Table 2. Effect of a magnetic induction of 150 kilogauss on the decay of NO, in aqueous

nitric acid at 25 + 1 °C

J

Ck

4

B + a) x 10'Vs"1 (k4 + a)

a x lO'Vs"1

[H0N0 k^ /k^

2

] at B = 150 kG at B = 0 kG

1.00 M .630 + 0.75 .599 + .089 1.06 + .18

3.00 M 1.08 + .039 .996 + .036 1 .09 + .05

6.00 M 1.86 + .14 1 .78 + .10 1.06 + .10

12.00 M 4.08 + .16 3.96 + .26 1.03 + .08

a Control samples run under identical conditions except B = 0 kG.

Table 3. Effect of magnetic induction on 25+1 °C

the decay of NOg in 3M aqueous perchloric acid at

Magnetic

Induction

B(kG)

(k4

B + o) x 10'W1 (k, + a)a x 10'Vs" 1 H at B = 0 kG k^ /k^

150 .896 + .150 .372 + .052 2.31 + .52

150 .771 + .055 .352 + .071 2.18 + .47

75 .425 + .052 .356 + .022 1.20 + .16

75 .428 + .078 .356 + .022 1.21 + .20

Control samples run under identical conditions except B = 0 kG.

[Ce(III).. .NOa] °, which is active in converting to the ground singlet state,

[Ce(IV)N0 3 ~P°. We postulate that this non-linear magnetic effect either tends to make

all three levels of the triplet complex kinetically active in undergoing T 0

-> S 0 crossing

or it acts to mix those levels in such a way as to enhance the population of the one active level so as to increase ki+ B with respect to the unperturbed specific rate k u . If

either interpretation is correct, this technique represents a new way, independent of

magnetic resonance, for demonstrating and studying CIDEP effects by monitoring shortlived intermediates in both organic and inorganic systems. Its sensitivity to the structure of the ligand coordination sphere is also remarkable.

We are grateful to the National Science Foundation, the U.S. Atomic Energy Commission and Vanderbilt University for sponsoring these studies.

128
References

Martin, T. W., et al . , J. Am. Chem. Soo. 85, 113 (1963)

ibid., 86, 2595 (1964).

ibid., 92, 5075 (1970).

Manuscript in preparation.

J. Am. Chem. Soc, 5084 (1970).

Manuscript in preparation.

12?
LUMINESCENCE OF SOME METAL TRISDITHIOACETYLACETONATE COMPLEXES

M. K. DeArmond and J. T. Merrill

Department of Chemistry

North Carolina State University

North Carolina

1. Introduction

Luminescence of metal complexes can, within the limit of small mixing of ligand and

metal d-orbitals, be classified as "d-d" emission (localized orbital), charge transfer

(delocalized orbital) or ligand localized orbital (devocalized orbital) [1] . Since

emission typically occurs only from the lowest manifold of excited states, only localized

"d-d" orbital emission is observed for first transition series complexes while either

localized and delocalized orbital emission can be observed for heavy d

6 transition metal

complexes dependent upon the ligand. Emission of d

6 Rh(III) and Ir(III) complexes can be

categorized a.s "metal localised orbital" (i.e., Rh(dtc) 3 ) 2 or "delocalized orbital,"

([Rh(bpy) 3 ]

3

, [Ir(byp) 2 Cl 2 ] where bpy = 2,2'-bipyridyl ) . The former type of emission is broad, structureless and exhibits a large Stokes Shift while the latter emission exhibits

vibrational structure and a relatively smaller Stokes Shift.

The e-diketone complexes of Rh(III) and Ir(III) have been reported [4] to exhibit

d-d emission at 11.8 and 11.3 kK, although the spectra was so weak that no lifetime could

be measured. Such an emission is unqiue since no d-d absorption can be identified at energies lower than the intense delocalized orbital transitions. However, large Stokes

Shifts of emission are characteristic of d

6

i localized orbital emitters [2,3,52. Therefore

te d-d absorption may be covered by the intense low energy bands.

In the limit of small mixing of ligand and metal orbitals, the low lying d-d absorption although hidden by the more intense delocalized orbital transitions, may still permit:

typical d-d emission to be observed. But, in the limit of substantial mixing of metal and

ligand orbital, the emission should be of the characteristic "delocalized orbital" type for)

these weak field ligands.

To better understand the interaction between d orbitals and high energy ligand tt- orbitals and the resultant energy transfer processes, netural tris dithioe-diketonate

complexes, M(SacSac) 3 and M(SSSBB) 3 of Cr(III), RhCHI) and Ir(III), were synthesized,

absorption, emission and photoselection determined and lifetimes measured.

All SacSac complexes (Cr(III), Rh(III), and Ir(III)), figure 1 were synthesized

according to literature methods [6], purified by recrystallization and preparative thin

layer chromatography. All compounds were analyzed for C, H, and S by Atlantic Microlab,

Inc., Atlanta, Georgia, and have proper stoichiometry. The Rh(SSSBB) 3 and Ir(SSBB) 3 were synthesized by a method developed in this laboratory and the stoichiometry verified by

analysis.

Figures in brackets indicate the literature references at the end of this paper.

2. Experimental

130
Complex Abbreviation

/

/ s - c

[

) V

'/

% s - c

Rh

+3

, lr

+3

M(SacSac)g

/

6

c

^ CH '/

c

C

g H5 / , M(SSBB).

M = Rh

+3

, Ir+3

ure 1. Formulas and abbreviations of chelates studied.

Absorption spectra were determined at room and liquid nitrogen temperature using a y 14 spectrophotometer. Emission and excitation spectra were determined with an Amincoman Spectrophotofluorometer (SPF) at 77 °K in solvents of EPA, cyclohexane, and a tole, ethanol mixture (30-60 volume percent toluene) with both a Hamamatsu R-136 and RCA

2 red sensitive tube. The photoselection method described by Albrecht [7] and utilized

various workers to relate the relative orientation of emitting and absorbing oscillators

frozen randomly oriented samples was done at 77 K in glassy alcohol solvents. The

nco SPF equipped with Glan-Thompson prisms for excitation and emission was calibrated

h phenanthrene [8] and tested with C 2y d

6 metal chelate complexes [9]. (A maximum

arization value of ^0.4 was obtained).

Emission decay was determined using a Laser Energy Incorporated (LEI) 20 K watt

sed N 2 laser (a \ 337 nm, pulse width = 5 ns) both with the single shot technique and

illoscope detection and using pulse averaging technique with a Princeton Applied Re- rch (PAR) Boxcar Averager Model 160 with an X-Y recorder. Lifetime values were detered from decay curves using a least squared program.

3. Results

The Cr(SacSac) 3 complex gave no emission out to 1.0 pm. The Rh(SacSac) 3 , Ir(SacSac) 3 , SSBB) 3 and Ir(SSBB) 3 gave broad emission bands with maxima in the near IR (table 1).

lifetime values determined from experimental decay curves are tabulated in table 1.

The emission and absorption spectra overlap substantially in the case of the SSBB

plexes of RhlH and Ir^ 11 . The Rh(SacSac) 3 complex enhibits a weak (e ^ 1) absorption

'd at 14.5 kK on the tail of the intense absorption band. The Ir(SacSac) 3 likewise

ibits a weak band (e ^ 75) on the tail of the intense structured absorption at 16.1 kK.

131
Table 1. Absorption and emission data for sulfur chelates.

Absorption Emission (77 K)

Complex

Rh(SacSac).

iKSacSac)^

Rh(SSBB).

Ir(SSBB).

vmax(kK)(83 K)

14.5

19.6

22.3

16.1

19.5

20.1

21.4

17.6

20.2

17.4

19.7

21.6

< 3

7750

76

WkK > T p (psec) v^

2 (cm )

12.8 23.8 1840

12.3 1.1 2090

7000

8220

12.3 30.3 2100

12.5 2.5 2600

7850

Polarization (P) values obtained at 77 K from excitation and emission spectra for t lowest energy absorption bands were small and slightly negative (-0.02 to -0.060), much

greater than the theoretical limit (-0.33) for a planar emitter-linear absorber and much

smaller than the 0.14 limit of a planar emitter-planar absorber combination.

4. Discussion and Conclusions

On the basis of the energy, band shape and lifetime, the orbital and spin character

of the emitting state for the four d

6 complexes are the same and within the ligand field

model best assigned as "d-d" localized orbital emission with the corresponding absorption

transitions overlapped by an intense charge transfer transition. The small magnitude

polarization CP) values could result from a small trigonal splitting of the parent ^(Op)

state such that both the A 2 and the E state are populated at 77 K.

An anomaly is noted if the weak low energy absorption bands for the Rh(SacSac) 3 (14.5 kK) and Ir(SacSac) 3 (16.1 kK) are associated with the respective emission bands.

Then, the experimental Stokes Shift of the emission is much smaller than that for typical

d

6 emitters (as for the S coordinated complexes Rh(dtc) 3 and Ir(dtc) 3 2 ). Such data may

' suggest that: (1) the ligand field is not appropriate, i.e., significant mixing of d and

n orbitals is occurring, or (2) that the weak absorption bands are not associated with th

emission bands. Additional photoselection spectra at low temperature (4 K) and high re solution may clarify these points.

132
The overlap of the absorption and emission spectra for the SSBB chelates of Rh and

IrUl implies that emission is occurring from a state other than the lowest excited state.

Such phenomena are unique but not unknown. Organic emitters such as azulene do exhibit

fluorescence from an upper singlet while some mixed ligand d

6 metal complexes do exhibit

multiple emission [10,11] from energetically close lying states localized in different

portions of the molecule.

The character of the lowest excited states for Cr(SacSac) 3 cannot be determined since

no emission could be observed. The absence of emission may result from either a very low

quantum efficiency and/or it may be beyond 1 pm.

[ 1] DeArmond, M. K. , Accounts Chem. Pes. 7_, 309 (1974).

[2] Hill is, J. E., and DeArmond, M. K. , Chem. Phys. Letts. ]0> 325 (1971 ).

[ 3] Hillis, J. E., and DeArmond, M. K., J. Chem. Phys. 54, 2247 (1971).

[ 4] Crosby, G. A., Watts, R. J., and Westlake, S. J., J. Chem. Phys. 55, 4663 (1971).

[ 5] Thomas, T. R., and Crosby, G. A., J. Mot. Spectrosc. 38, 118 (1971).

[ 6] Heath, G. A., Martin, R. L., and Masters, A. F., Aus. J. Chem. 25, 2547 (1972).

[ 7] Albrecht, A. C, J. Mol. Spectrosc. 6, 84 (1961 ).

[ 8] Azumi, T., and McGlynne, S. P., J. Chem. Phys. 37, 2413 (1962).

[ 9] Carl in, C, Merrill, J. T., and DeArmond, M. D., to be published.

j[10] Halper, W., and DeArmond, M. K. , J. Lumin. 5, 225 (1972).

[11] Watts, R. J., Brown, M. J., Griffith, B. G., and Harrington, J. S., J. Amer. Chem.

Soc. 97, 6029 (1975).

References

133
LUMINESCENT TRANSITION METAL COMPLEX PHOTOSENSITIZERS

J. N. Demas, J. W. Addington, R. P. McBride, W. E. Harris and S. Peterson

Chemistry Department

University of Virginia

Charlottesville, VA 22901

Ruthenium(II) , osmium(II) and iridium(III) complexes with diimine ligands possess

nearly ideal properties as photosensitizers. They absorb intensely, emit strongly in

fluid solution, have long lives, are generally highly photostable and exhibit wavelength

independent efficiencies of population of the emitting state.

Table 1 shows properties of thirteen complexes which have been studied as photosensitizers. The zero point energies of the excited state, E 0

's, the mean lifetimes,

t 0

's, the bimolecular quenching constants for oxygen deactivation, k2*s and the limiting

quantum yields, <t> 0

's, for photooxygenation of tetramethyl ethylene (TME) at infinite con- centrations of 0 2 and TME for each complex are indicated; all values are for methanol at

-v 21 °C.

Table 1. Properties of transition metal complexes photosensitizers ' in methanol at 4, 21 °C

1

Complex Eo

T o

k

2 xl0"

9 *o (kK) (ysec) (M'V 1

) (TME)

[Ru(bipy)

3

] 2+

18.0 0.765 1.8 0.86

[Ru(phen)

3 2+

18.4 0.313 3.3 0.75

[Ru(Clphen)(phen)

2

] 2+

0.947 2.3 0.81

[Ru(Brphen)(phen)

2

] 2+

0.989 2.3 0.80

Ru[ (S02<t>2)phen] (phen >2 17.9 3.98 1.8 0.82

Ru(bipy)

2 (CN)2 18.3 0.40 2.8 0.79

Ru(phen)

2

(CN)-j 18.3 1 .58 5.0 0.68

[0s(bipy)

3

] 2+

14.9 0.049 4.5

[0s(phen)

3

] 2+

15.3 0.183 5.7 0.76

2+

[0s(<f> 2 phen) (phen)

2

] 15.0 0.212 4.6 0.78

0s[(S0

3

<J>) 2 phen](phen)

2 14.8 0.093 6.8 0.74

134
Table 1. Properties of transition metal complexes photosensitizers3 ' in methanol at

-x. 21 °C (continued)

Complex Eo

T o

k 2 xlO"9 *0

(kK) iv'iec) (TME)

>(bipy)

3

] 3+

22.7 2.4 0.34

;ir(phen)

3

] 3+

22.5 2.6 0.28

lose Bengal 0.80

l bipy = 2,

2 '-bi pyridine, phen = 1 ,10-phenanthroline, Xphen = 5-halo,l ,10-phenanthroline,

0? phen = 4, 7-diphenyl-l ,10-phenanthroline, (SO^0) ? phen = disulphonated-4,7-diphenyllflO-phenthroline. J

-

'Data taken in part from references [1-3].

The 4>0

's are all less than unity. The Rose Bengal yield of 0.80 equals its interystem crossing yield of 0.84 within experimental error. Thus, the subunity yields for

:he complexes do not arise from inefficient reaction of 1 02 with TME. Since the interystem crossing/internal conversion efficiencies for complexes of this type has been shown

;o be unity, the inefficiencies of photo-oxygenations are caused by an inefficient energy

ransfer step. We attribute this inefficiency to formation of an exciplex which undergoes

adiationless deactivation before dissociation to form 1 0 2 .

+

From, a combination of intensity and decay time quenching data, it has .been found that

u

2

, CO2 and Ni 2 quench Ru(phen) 2 (CN) 2 and Ru(bipy) 2 (CN) 2 by both static and dynamic

rocesses. The association constant estimated from these data are given in table 2. Since

ther neutral complexes without CN groups do not give static quenching, and since CN can

e a bridging ligand, the asosciation appears to arise from the RuL 2 (CN) 2 complex acting

s a ligand for the metal ion to yield a Ru-CN-M species.

Table 2. Association constants for metal ions with ruthenium(II)-cyanide complexes in

water at «v 22 °C

Ru(bipy)

2 (CN)2 Ru(phen)

2 (CN)

2 Cu

2+

220 270

Ni

2+

10 20

Co

2+

11 17

Ru(bipy) 2 (CN) 2 and Ru(phen) 2 (CN) 2 are also protonated, presumably at the CN. In

he case of Ru(bipy) 2 (CN) 2 , we find evidence for reversible acid-base equilibrium in the

xcited state. Details will be presented.

I 135
References

[ 1] Demas, J. N., and Addington, J. W., J. Amer. Chem. Soo. 3 in press.

[ 2] Demas, J. N., Harris, E. W., Flynn, C. M. , Jr., and Diemente, D., J. Amer. Chem. Soa. 97, 3838 (1975).

136
PICOSECOND STUDIES OF TRANSITION METAL COMPLEXES

Patrick E. Hoggard, Alexander D. Kirk 1

, Gerald B. Porter 2

, Mark G. Rockley 3

, and Maurice W. Windsor 4 Department of Chemistry

Polytechnic Institute of New York

New York, NY

Lifetimes of excited electronic states of a number of transition metal complexes

have been measured in solution at room temperature with picosecond time resolution [I] 5

. Two experimentally distince lifetimes can be retrieved with this technique. The ground

state bleaching (GSB) lifetime is that for the total repopulation of the ground state,

measured by the regrowth in intensity of absorption bands, initially bleached because of

depopulation by the pumping pulse. The excited state absorption (ESA) lifetime is that

for depopulation cf a particular excited state, measured by the decay of the electronic

'absorption spectrum from that state.

Both of these processes are observed only under somewhat special conditions, and in

addition we have thus far been limited by the necessity that the sample absorb significantly at 530 nm, the double frequency of a Nd 3 laser. To observe GSB, the molar extinction coefficient at 530 nm must be higher than about 5000. A similar requirement applies

to ESA - the extinction coefficient must be high, somewhere within the range of the

instrumentation and the continuum pulse used for analysis. Additionally, observation is impossible if the excited state absorption is masked by the ground state absorption.

In spite of these restrictions, a number of transition metal complexes have been

Found for which we can observe GSB and/or ESA. A summary of some of these results is shown in the table.

In the case of the Fe(II) complexes, there are two absorption bands in the visible

^egion, an intense band above 500 nm, representing a CTTL transition, and another much

Weaker' band below 900 nm, assigned to a

3 Ti

q

-<- ^xg ligand field transition. In addition,

".ransitions from 3 CT and ^xg states are probably masked by the spin allowed charge tran-

;fer band. An estimate of the radiative lifetime of the 1 C1 state from the absorption

ipectrum plus the absence of observable luminescence allows us to rule out this state as

:oo short-lived to contribute to the observed GSB lifetime. The 830 ps lifetime is consistent with estimates for the 3 T lq lifetime, but could represent decay into the ground

Department of Chemistry, University of Victoria, B.C., Canada.

Department of Chemistry, University of British Columbia,

Department of Chemistry, Oklahoma State University.

Department of Chemistry, Washington State University.

Figures in brackets indicate the literature references at the end of this paper.

137
Chromium (III) complexes exhibit small extinction coefficients in general, so that

GSB is not expected to be observed. Exited state absorption from the lowest ligand field

states, 4 T2 g and 2 Eg would also be expected to exhibit small extinction coefficients within the manifold of Tigand field states. Observation of ESA in the visible range is thus

to be expected only with ligands which induce charge transfer states in the region of

30-40 kK above the ground state. Thiocyanate and acetyl acetonate complexes exhibit the

required CT bands, and ESA was observed for these complexes.

Complex

Table

Ground state

bleaching

Excited state

absorption

Type of

transition

2+

2+

[Fe(bipy)

3

]

[Fe(phen)

3

]

[Ru(bipy)

2 (CH

3 0H)2

]

[Cr(NH

3

) 2 (NCS

4

)]

_

[Cr(acac)

3

]

2+

[Cr(NCS)

6

] 3-

t = 830 ps

Observed

t = 620 ps Amav < 425 nm max

X„,„ ca 520 nm max —

t = 5 ns

Xma „ ca 500 nm max —

t = 1.5 ns

\ ca 540 nm max —

= 5 ns

probably intraligan

Charge transfer

M * L

Charge transfer

M * L

Charge transfer

M * L

Although the 4 T 2q state is reached by absorption, the ESA spectra of all 3 complexes match the ESA of the 2 Eg state measured at 77 K by Ohno and Kato [2]. Unless the

ESA spectra of l+ T2 g and 2 Eg states are fortuitously the same, it appears that the l+ T2 g state undergoes intersystem crossing to the 2 Eg state in a time less than the duration

of the pump pulse {ca. 5 ps).

References

[ 1] Kirk, A. D. , Hoggard, P. E., Porter, G. B., Rockley, M. G. , and Windsor, M. W.,

Chem. Phys. Lett. 37_, 199 (1976).

[ 2] Ohno, T., and Kato, S., Bull. Chem. Soc. Japan, 43, 8 (1970).

138
ELECTRON TRANSFER PROPERTIES OF EXCITED STATES OF TRANSITION METAL COMPLEXES

V. Balzani, F. Bolletta, M. Maestri, A. Juris, and N. Serpone 1 Istituto Chimico "G. Ciamician" dell ' Uni versita"

Bologna, Italy

Until a few years ago, the attention of inorganic photochemists was entirely focussed

n the intramolecular photoreactions and on the photosol vation reactions of transition

etal complexes [2,3]. In the last few years, however, there has been an increasing

nterest in the intramolecular redox properties of transition metal complexes.

The protagonist of this new trend in inorganic photochemistry is the Ru(bipy)§

+ omplex, whose lowest excited state, commonly indicated by ( 3 CT)Ru(bipy)§ since it has

riplet multiplicity [4] and metal -to-1 igand charge transfer character is able to act as

n energy donor [6], electron donor [7-10], and electron acceptor [11,12] depending on the

pecific nature of the reaction partner:

k

( 3 CT)Ru(bipy)^

+

+ A —- Ru(bipy)f+ *A

( 3 CT)Ru(bipy)^+

t B kred - y Ru(bipy)

3+

+ B~

k

( 3 CT)Ru(bipy)^++ C ^

RuCbipy)* + C +

The ( 3 CT)Ru(bipy)§

+ excited state is luminescent in fluid solution at room temperaure, a very precious property because it provides a simple means of monitoring the, interctions between the excited state and the quencher. The lifetime of ( 3 CT)Ru(bipy)3 is 80 ns in deaerated aqueous solution. The absorption spectrum of this excited state has

ecently been obtained by means of laser flash spectroscopy and shows a maximum at about

60 nm [13]. The quantum yield of triplet formation, which has been measured with diferent techniques, is certainly higher than 0+

5 [13] and probably near unity [6,14,15].

t should also be recalled that the Ru(bipy)^ complex absorbs strongly in the visible,

ith a maximum at 450 nm (e = 15,000).

The "thermodynamic" barriers for reactions 1, 2, and 3 may be evaluated knowing that

he energy difference between (

3 CT)Ru(bipy)§ and ground state Rujbipy)^ is_ 17.1 kK

2.12 eV), and that the reduction potentials of the (

3 CT)Ru(bipy)§ -Ru(bipy) 3 and

j(bipy)r-( 3 CT)Ru(bipy)§ couples are +0.84 V [11] and -0.83 V [11,16], respectively,

3. the NHE. Kinetic barriers have also been found to play an important role in both

nergy [6] and electron [9,11] transfer reactions; unfortunately, systematic studies

re still lacking. It is interesting to note that reactions 2 and 3 provide for the conarsion of light energy into chemical energy. The products of these reactions, however,

re difficult to separate and generally undergo a fast back electron transfer reaction

hi ch dissipates the converted light energy as heat in the reaction medium. Indeed, when

he two products of reaction 2 (or 3) have opposite electric charge the back electron

ransfer reaction is too fast to be seen with conventional flash techniques [16,17].

owever, relatively high stationary concentrations of the products can be obtained in more

avorable cases, and this can be used to produce a photogal vanic effect [18].

r igures in brackets indicate literature references at the end of this paper.

139

(1)

(2)

(3)
Another very interesting complex from the point of view of electron transfer re- actions is Cr(bipy) 3

) . Its lowest excited state, which has doublet multiplicity and

metal -centered character, is relatively long liy.ed (about 50 ysec) and emits luminescence

in fluid solution. As the Cr(bipy)| -Cr(bipy)§ reduction potential is -0.25 V and the

spectroscopic excitation energy

+of ( 2 MC)Cr(bipy)§ is 13.8 kK (1.72 eV), the reduction

potential of the ( 2 MC)Cr(l^ipy) I -Cr(bipy)§ couple is expected to be about +1.45 V [19].

Thus, the ( 2MC)-Cr(bipy) § excited state is expected to act

+as a very strong oxidant.

This expectation seems to be verified since ( 2 MC)-Cr(bipy)^ is quenched at nearly diffusion controlled rates by a number of species which do not possess excited states lower

than 13.8 kK but have E°(Q-Q~) lower than 1.45 V [20]. The quenching of ( 2 MC)Cr(bipy) f

by electron transfer to the quencher is presently under investigation in our laboratory.

One of the most interesting systems we

+ have examined thus far is that given by a solution containing Ru(bipy)§ and Cr(bipy)^ [17,19]. The excited Ru complex is able to reduce the ground state Cr complex, and the excited Cr complex is a.ble to oxidize

+the

ground state Ru complex. In both cases, one obtains the Ru(bipy)^ and Cr(bipy)§ com- plexes which then undergo a back electron transfer reaction to reach their equilibrium

situation:

Ru(bipy)3+ + ( 2 MC)Cr(bipy)^+

hv \ \k

k Ru(bipy)f + Cr(bipy)^

+

< c j

Su(bipy)3+

+ Cr(bipy)*

+

hv + \k b

( 3CT)Ru(bipy)*

+

+ Cr(bipy)

3+

We have here a system in which light absorption leads to the same products, regardless of

the absorbing_species . The rate constants for the two excited state reactions are k = 3.3 x 10 3 molds' 1 and kb = 4.0 x 10 8 mol" 1 s" 1

. The reason for k, > k|j is probably Sue

to kinetic factors which are related to the different electronic distribution in the two excited states.

The redox properties of the excited states of ther Ru(II) and Cr(III) complexes as well as of other transition metal complexes are currently under investigation.

Financial support from the Italian National Research Council is gratefully acknowledged.

References

[ 1] On leave from the Department of Chemistry, Sir George Williams Campus, Concordia

University, Montreal, Canada.

[ 2] Balzani, V., and Carassiti, V., Photochemistry of Coordination Compounds (Academic

Press, London, 1970).

[ 3] Adamson, A. W., and Fleischauer, P. D., eds., Concepts in Inorganic Photochemistry

(Wiley, New York, 1975).

[ 4] Strictly speaking, the spin label is meaningless because of the dominant role

played by spin-orbit coupling [5].

[ 5] Crosby, G. A., Hipps, K. w., and Elfring, Jr., W. H., J. Am. Chem. Soc. 96_, 629

(1974).

140
Balzani, V., Moggi, L. , Manfrin, M. F., Bolletta, F., and Laurence, G. S., Coordination Chem. Rev. 15_, 321 0975).

Gafney, H. D., and Adamson, A. W., J. Am. Chem. Soa. 94, 8238 (1972).

Bock, C. R. , Meyer, T. J., and Whitten, D. G. , J. Am. Chem. soa. 96, 4710 (1974).

Navon, G. , and Sutin, N. , Inorg. Chem. 13_, 2159 (1974).

Laurence, G. S., and Balzani, V., Inorg. Chen. V3» 2976 (.1974).

Creutz, C, and Sutin, N. , Inorg. Chem. 15, 496 (1976).

Juris, A., Gandolfi, M. T., Manfrin, M. F., and Balzani, V., J. Am. Chem. Soa. 98,

1047 0976).

3] Bensasson, R., Salet, C, and Balzani , V. , J. Am. Chem. Soa. 98, 3722 (1976).

4] Demas, J. N., and Crosby, G. A., J. Am. Chem. Soa. 93, 2841 (1971).

5] Bolletta, F., Maestri, M. , and Balzani , V . , submitted for publication.

6] Bock, C. R., Meyer, T. J. , and Whitten, D . G. , J. Am. Chem. Soo. 97, 2909 (1975).

7] Ballardini, R., Varani, G . , and Scandola, F. , to be publ ished.

8] Lin, C. T. , and Sutin, N. , J. Phys. Chem. 80, 97 0976).

9] Bolletta, F., Maestri, M. (1975).

, Moggi, L., and Balzani, V., J.C.S. Chem. Commun. s 901

Maestri, M., Serpone, N., Bolletta, F., Moggi, L., and Balzani, V., manuscript in

preparation.

141
A COMPARISON OF THE EXCITED- STATE ELECTRON-TRANSFER REACTIONS OF

RU(BIPY)g

+ AND 0S(BIPY)3

+

Peter Fisher, Edward Finkenberg and Harry D. Gafney

City University of New York

Department of Chemistry

Queens College

Flushing, NY 11367

The mechanism of quenching of Ru(bipy)| (bipy denotes bipyridine) luminescence is an area of active investigation [I] 1

. Recent studies with organic and inorganic substrates

have established that quenching can occur by an electron transfer or energy transfer

mechanism. For the quenching of the luminescent charge transfer state, LCT, of *Ru(bipy)§+ by the acidopentaamminecobalt(III) complexes, Co(NH 3 ) 5 xn+ , the evidence, at present, supports an electron-transfer mechanism.

A detailed study of the reduction of 13 Co(NH 3 ) 5 X n+ complexes by *Ru(bipy)^+ offers

further support for an excited-state electron- transfer reaction [2]. The reduction potential of *Ru(bipy)|*, 0.8v [1], and small values, 2.0 , of the energies of activation

suggest the reactions are not thermo dynamically controlled. As indicated by the data in

table 1, some degree of correlation exists between <J>g 0 ( II) an(^ tne thermal rates of re- duction of the cobalt(III) complexes. This correlation suggests the reactions are kinetically controlled, that is, the rate of electron transfer must be competitive with the rate of relaxation of the excited state of the electron donor. The correlation is not perfect,

indicating other factors are also important. Since *Ru(bipy)£f

, a strong reductant, is

converted on electron transfer to a strong oxidant, Ru(bipy)^, a necessary criteria for a net electron transfer to occur is that the oxidant undergo an irreversible reduction. For

these cobalt(III) complexes, an irreversible reduction is accompanied by dissociation of

the coordinated ligands. The differences in reactivities of Co(NH 3 ) 5 H 20 3+

, Co(NH 3 )sNCS 2+

, Co(NH 3 ) 5 N§+

, and Co(NH 3 )5S0i+ , only the latter two show a measurable <f>r,o(II)» is attributed to differences in the rate of ligand dissociation and/or intersystems crossing within the reduced cobalt substrate. The latter rate must be competitive with the rate of

reverse electron transfer from the reduced cobalt substrate to Ru(bipy)|+ in order for a net reaction to occur.

To further test these conclusions, a study of the reduction of the same cobalt(III)

complexes by *0s(bipy)§+ under identical conditions has been undertaken. Like *Ru(bipy) 2+

, *0s(bipy)ff is of sufficient potential to reduce all cobalt(III) complexes studied. The

potential of the reaction

*0s(bipy)

2+

Os(bipy)^

+

+

has recently been calculated to be 0.96v [1]; consequently *0s(bipy) is a better re- ducing agent than *Ru(bipy) 32+

, E° = .84v [1]. Although a stronger reducing agent, the

lifetime of *0s(bipy)5+

, 19.2 ns [1], is considerably shorter than that of *Ru(bipy)f+

, 600 ns. A priori, it was expected that *0s(bipy)§+ would reduce only those cobalt(III)

complexes which had thermal rates of reduction > 10 7M

_1

, i.e., Co(NH 3 ) 5 Cl *

,

Figures in brackets indicate the literature references at the end of this paper.

142

Table 1. Comparison of the quantum yields of reduction of various Co(IJiH 3

) g X complexes by

*Ru(bipy)§ with the thermal rates of reduction by Cr(bipy)§ .

Complex

9 0Co(II) 0Co(II)

1 1 h k(M" 1 sec'')

b

3+

RNH,

3 <.001 6.9 x 10

2

2+

RCN <.001

RH

20

3+

<.001 5.0 x 10

4

RF

2+

<.0003 1.8 x 10

3

R00CCH

2+

3 <.001 1.2 x 10

3

2+

RNCS^ <.0001 1.0 x 10

4

RCO3 0.014

< 0.051 0.1 4.1 x 10

4

RS0

4

n n/i "3 u . 1 a c v in^

RC1 2+ - 0.063 0.6 8.0 x 10

5

RBr

2+

0.104 0.7 5.0 x 10

6

RI 2+0.141 1 .0

R'C

2 0j 0.097

= Co(NH

3

) 5

, R

=

= Co(NH

3

) 4

R

Co(NH 3 ) 5 Br|

+

, and Co(NH 3 ) 5

I 2+. As indicated by figure 1, the available data indicates

*0s(bipy)§_ reacts with cobalt(III) complexes which have thermal rates of reduction

<10'M

_1 sec-1

. This suggests that the rate of electron transfer is much faster than the rate of relaxation. Furthermore, the limiting yields of <l>Co(II) obtained with

*0s(bipy)?

+ , figure 1, are essentially unity and much larger than those obtained with

*Ru(bipy)| , table 1. The increased limiting efficiency of *0s(bipy)§+

is thought to reflect a decrease in the rate of reverse electron transfer due to the lower oxidation

potential of Os(bipy)^"1" [1].

References

[ 1] Lin, C. T., and Sutin, N. , J. Phys. Chem. 80, 97 (1976); and references therein.

[ 2] Lam,. P. K. , Adamson, A. W., and Gafney, H. D. , submitted to J. Arner. Chem. Soa.

144
PHOTODISSOCIATION OF SIMPLE POLYATOMIC MOLECULES

J. P. Simons

Chemistry Department

The University

Birmingham B15 2TT, England

Ten years ago, the majority of photochemists were satisfied if they were able to identify the primary products of a molecular photodissociation. Now, with the advent of

the new heavy artillery of techniques for studying energy disposal and angular distribution

they are much more demanding, and as the new results have flowed in, the theoretical

models have burgeoned [2] to codify and rationalize the new data. Until recently, all the

dynamical models for treating the problem of vibrational energy disposal adopted the quasidiatomic approximation, an artificial device that resolves the full intramolecular potential

into two components but has the virtue of allowinq the analysis of a complex process in a relatively simple way. Each of the publications follows the format -- introduction to the

physical model, mathematical analysis, comparison with current experimental data. The last

section is essential, not only because it is the experimental horse that pulls the theoretical cart, but also because none of the models is predictive. This has often led to a "Comedy of Errors" because the current data chosen for comparison (commonly the near u.-v. photodissociation of ICN or the quenching of Hg(6 3 P 0 ) by CO or NO), proved to be inaccurate.

The most recent dynamical models [2a-2d] have chosen the vacuum u.-v. photolysis of the halogen cyanides [3] for comparison, but here, too, experimental re-appraisal of the data

suggests that some of the results may have been in error [4]. Nevertheless,* there is now

a general consensus on the importance of geometry and force constant changes in the transfer from the bound to the repulsive electronic surface during photodissociation and the

utility of a Franck-Condon description for "predicting" vibrational energy distributions in

the separating fragments. The new data for vibrational energy disposal in the vacuum

lu.-v. photodissociation of BrCN at 123.6 nm will be presented and compared with alternative

.theoretical models. The data indicate that vibrational populations calculated from observations of the fluorescence of CN in the B

2

z + state are relaxed at pressures as low as 30 jmtorr, either by collisionally induced transfer from fragments in the a'

+ E + state or by very

^efficient vibrational energy transfer from translationally excited CN(B 2

Z +

) radicals.

,Collisional interchange between the A and B states of CN, previously suggested by Luk and

Bersohn [5] to account for the pressure dependence of the fluorescence decay of CN(B),

may also contribute, since rotational perturbations can be observed in the (o,o) band of

the fluorescence spectrum at the lower pressures.

1 Results obtained using a new variation of the photofluorescence technique, polarized

photofluorescence excitation spectroscopy [6], will be presented. The technique depends on

the formation of rotationally excited fluorescent fragments which retain their memory of

the orientation of the photo-selected parent molecule: the requirement is not restrictive

since rotational excitation following photodissociation in the vacuum u.-v. appears to be

the rule rather than the exception. Measurement of the polarization of the fluorescence

las a function of the absorbed photon energy provides information relating to the symmetry

of the vibronic state initially populated and its lifetime prior to dissociation. The

technique is a member of the family of anisotropic photodissociation experiments developed

by Zare, Bersohn, Wilson and their co-workers [la, 6]. Polarized spectra obtained from the

halogen cyanides, HCN, H 20 and D 20 will be discussed [6,8],

T—

Figures in brackets indicate the literature references at the end of this paper.

145
The rotational excitation in the CN(B 2 E ) produced from the photodissbciation of

BrCN at 147 nm follows a Boltzmann distribution having a temperature of (1100 +250) K [4]. A simple Franck-Condon model will be presented which shows the observed rotational energy

disposal to be consistent with predissociation from a very near linear, photo-excised state

(bond angle 175°). The model also accounts for the rotational energy disposal in the photodissociation of cyanoacetylene, reported at this meeting [9].

References

[ 1] See, for example,

(a) anisotropic photodissociation: Bersohn, R., and Yang, S.-C, J. Chem. Phys.,

61 , 4400 (1974); Bersohn, R. , Dzvonik, M. , and Yang, S.-C, J. Chem. Phys., 6J_, 4408

TT974).

[ 2] (b) photofragment spectroscopy: Busch, G. E., and Wilson, K. R., J. Chem. Phys.,

56, 3626, 3638, 3655 (1972).

(c) photodissociation laser spectroscopy: West, G. A., and Berry, M. J., J. chem.

Phys., 6J_, 4700 (1974); Berry, M. J., J. Chem. Phys. 61_, 3114 (1974).

[ 2] For leading references, see

(a). Band, Y. B. , and Freed, K. F. , J. Chem. Phys., 63, 3382, 4479 (1975).

(b) Mukamel , S., and Jortner, J., J. Chem. Phys., 60, 4760 (1974).

(c) Berry, M. J., Chem. Phys. Letters, 29, 323, 329 (1974): also ref. 1(c).

(d) Simons, J. P., and Tasker, P. W., Hoi. Phys., 26, 1267 (1973)1 27, 1691 (1974).

(e) Quack, M. , and Troe, 0., Ber. Bunsenges., 79_, 469 (1975).

[ 3] Mele A., and Okabe, H., J. Chem. Phys., 51_, 4798 0963).

[ 4] Ashfold, M. R. N. , and Simons, J. p., unpublished work.

[ 5] Luk, C. K, , and Bersohn, R., J. Chem. Phys., 58, 2153 (1973).

[ 6] Chamberlain, G, A., and Simons, J. P., J, Chem. Soc. Faraday Trans. II, 71_, 2043

09751.

[ 7] Van Brunt, R. J., and Zare, R, N, , if, Chem. Phys., 48, 4304 (1968).

[ 8] Macpherson, M, T., and Simons, 0. P., unpublished work.

[ 9J Cody, R. 0., and Savety-Dzyonik, M. 0,, ISth Informaal Conference on Photochemistry,

NBS, 1976.

146
ENERGY DISTRIBUTION IN THE PHOTODISSOCIATION OF METHYLKETENE AT 215 NM

M. E. Umstead, R. G. Shortridge, and M. C. Lin

Physical Chemistry Branch

Chemistry Division

Naval Research Laboratory

Washington, DC 20375

The photolysis of methylketene has been studied by Kistiakowsky and co-workers [1,2]!.

They concluded that the reaction proceeded principally by means of the following paths:

CH3CHCO + hv - CHCH

3 + C0 (1)

CHCH

3 - C2

H 4

+ (2)

C

2

H 4

+ - C

2

H

2 + H2 (3)

C

2H^ + M -> C2

H 4 + M (4)

CHCH

3 + CH3CHCO -> C4

H g+ CO (5)

a mechanism which is consistent with that proposed by Frey for the photodissociation of

diazoethane [3],

We have investigated the photodissociation of methylketene (CH 3 CH=C=0) in a quartz

flash system (x >_ 200 nm) at 300 °K. The products of the photodecomposition were analyzed

by gas chromatograph, and the initial vibrational excitation of the CO formed was measured

by means of a laser resonance absorption method [4]. The results of the product analysis

indicated the presence of CO, C 2 H 2 , and C 2 Hit, with lesser amounts of CHi,, C 2 Hg, C 3 H 6 , (propadiene and propyne) and various C^ hydrocarbons, and are generally in agreement

with those of Chong and Kistiakowsky [2],

In order to measure the Initial vibrational population distribution of the CO, a stabilized cw CO laser preset at the various vibrational -rotational CO lines was directed

along the axis of a quartz flash tube. Mixtures of methylketene in He were flash photolyzed, and the population distribution determined from time-resolved absorption curves

measured for all vibrational levels populated by the photodissociation.

The CO laser resonance absorption measurements showed that the CO formed in the photodfssoctatton of methylketene in the 200^-230 nm region was vibrational ly excited to v=9

and had a Boltzmann vibrational temperature of 3800 + 500 °K. The results of two sets of

C 3 Hi+0 data are plotted in figure 1. One set was obtained from a 1% mixture of CsH^O in

He, flashed at an energy of 0.5 kJ, and the other from a 1.5% mixture flashed at 1.0 kJ.

The fact that the two sets of data led to the same population distribution, even though the

C3H4O concentrations and the flash energies differend considerably, provides evidence that

the extcted CO is indeed produced in the primary step, and not be secondary reactions.

This is also supported by an experiment in which 0 2 was added as a free radical

scavenger. A mixture containing 1% 038^0 and 2% 02 in He was flashed at an energy of 1.0

kJ. The results of the CO excitation measurements were the same as those from the 0 2

- free mixtures, within experimental error.

Figures in brackets indicate the literature references at the end of this paper.

147
kw I O^-==o, .5 kj

*» 1-5^- ==o, |.o kj —- 2.0%=-=o, |.o kj — STAT. CALC.(see the text)

10 20 30 40 50

Ev/kcal-mole

-i

60

Figure 1 Vibrational energy distributions of the CO formed in the photodissociation of methyl ketene and acrolein.

sion

The amount of vibrational energy channelled into CO was calculated from the expres-

<E > = E f E v v>0 vv

where f

v

- N

v/e v>q N„ is the normalized vibrational population distribution and E v is the vibrational energy of CO at the vth level with the zero-point energy excluded. The data presented in figure 1 give rise to E v = 3.7 +0.34 kcal/mole, which indicates that only

' about 3/o of the total available energy, Etot = hv - aH° + RT - 126 kcal/mole, is carried

the CO as vibrational energy.

wo

148
The results obtained for methyl ketene are listed in the table and are compared with

those obtained for the isomeric compound, acrolein [5], and the chemically activated systems,

0(

3 P) + propadiene and propyne. In the propadiene and propyne reactions, it has been shown,

based upon calculations in terms of simple statistical models, that the reaction energies

lhave been randomized in the activated complexes before dissociation takes place [6].

Randomization of E^ot from the propadiene reaction in an activated cyclopropanone complex

followed by its direct dissociation into CO and ethylene provided good agreerr?nt between

the calculated and the experimental values of <E V

>. In the case of propyne, nowever, an excited methylketene complex was assumed, along with its dissociation into CO and CH 3 CH

•which subsequently isomerized to C 2^ . Thus the heat of isomerization of CH 3 CH, AHf so

among the modes of the complex provided a calculated value for <E

y

> which agreed with the

experimental

.

Table. Average Vibrational Energy of CO Formed in the Decomposition of Various C^H.O

Molecules!

0 t

Reaction C

3

H 40 E tot

<E

v> -==0 _==o* ^126 3.7 + 0.3

=-=0 =-=0* ^131 3.3 + .2

0+== 0

+

A

123 6.8+ .6

0+-= -==0 + 122 2.3 + .3

All energies in kcal/mole.

Also shown in figure 1 are the experimental CO population distribution from the photodissociation of acrolein, and calculated curves for methylketene, Curves I and II. Curve I is the distribution obtained by randomizing the full Etot 1n the statistical model and

clearly predicts too hot a CO vibrational distribution. Curve II assumes dissociation of

the complex into CH 3 CH and C0+ , and thus is based upon randomization of Etot-AHi S0 . It provides better agreement with the experimental values, but indicates that energy randomization is not complete before dissociation takes place. The results obtained from methylketene and acrolein are very similar to each other. At this time, it is not known with

certainty whether this indicates that the photodissociation of both compounds proceeds via

similar complexes, or is merely fortuitous.

References

[ 1] Kistiakowsky, G. B., and Mahan, B. H., J. Am. Chem. Soc, 79, 2412 (1957).

[ 2] Chong, D. P., and Kistiakowsky, G. B., J. Phys. Chem., 68, 1793 (1964).

C 3] Frey, H. M. , J. Chem. Soc, 1962 , 2293.

[ 4] Lin, M. C, and Shortridge, R. G., Chem. Phys. Lett., 29, 42 (1974).

[ 5] Shortridge, R. G., Umstead, M. E., and Lin, M. C, presented at VIII International

Conference on Photochemistry, August 1975.

[ 6] Lin, M..C, Shortridge, R. G., and Umstead, M. E., Chem. Phys. Lett., 37, 279 (1976).

149
THE PRODUCTION AND REACTIONS OF VIBRATIONAL!Y EXCITED 1 ,1 ,2 ,2-TETRACHLOROETHANE

M. H. J. Wijnen

Chemistry Department, Hunter College of the

City University of New York

695 Park Avenue

New York, NY 10021

Decomposition reactions of vibrationally excited haloethanes have been investigated

extensively by Setser and co-workers [I] 1 and by Pritchard [2] and co-workers. In the past

vibrationally excited chloroethanes were produced by the photolysis of ketene in the pre- sence of chlorinated methanes or by photochemical reactions of mixtures of chlorinated

methanes. Both techniques yield two different radicals and thus three vibrationally ex- cited ethanes (the direct combination and the cross-combination products of the radicals

involved)

.

It would, of course, be advantageous to study a system in which only one type of

excited molecule is produced. Such a system is provided by the photolysis of phosgene in the presence of chlorinated methanes. To illustrate this method we have studied the photolysis of phosgene in the presence of CH2CI2 at various pressures and in the presence of

octafluorocyclobutane (OFCB) as an inert deactivator.

The important reactions occurring in this system are given by:

C0CL

2 + hv -V 2 CI + CO

CI + CH

2 C12

-> CHC1

2 t HC1

2 CHC1

2

-> TCE* (1,1,2,2-C

2 H

2 C1*)

TCE* + CH

2 C12

-> TCE + CH

2

C1

2 (la)

TCE* t COCl

2

-> TCE + C0C1

2 (lb)

TCE* t OFCB -> TCE + OFCB (1c)

* TCE -> C

2 HC1

3 + HC1 (2)

As clearly shown in the mechanism only one type of vibrationally excited molecule is produced, thus simplifying the analysis and the interpretation of the results. We have care- fully looked for decomposition products other than HC1 elimination products. We have not observed any evidence for the occurrence of H 2 or Cl 2 eliminations from 1 ,1 ,2,2-tetrachloroethane.

1 Figures in brackets indicate the literature references at the end of this paper.

150
The following experiments were carried out:

A) C0C1 2 was photolysed in the presence of CH 2 C1 2 (ratio C0C1 2 /CH 2 C1 2 = 1 ) at total

pressure between 0.3 and 5.1 torr.

B) C0C1 2 was photolysed at a constant pressure of 0.85 torr, the pressure of CH 2 C1 2 was varied form 0.85 to 4.6 torr.

C) C0C1 2 (part, press. 0.85 torr) was photolysed in the presence of CH 2 C1 2 (0.85 torr),

while the pressure of 0FCB was varied from zero to 6.2 torr.

The data were plotted according to the following equations:

(k

la + "ib^part. RTCE/R

C

2 HC1

3

RTCE/R

C

2 HC13

= k ib(

C0C1 2^k 2 + k -| a

/ k 2 CH

2 C12

RTCE/R

C

2 HC13

= k la + k lb/

k 2

p part.

+ Vk 2< 0FCB >

(A)

(B)

(C)

The plot of RfCE/Rc HC] versus the partial pressures of C0C1 2 (= CH 2 C1 2 ) should give a straight line without intercept. The ratio R"TCE/RC2HCl3 varies linearly with pressure but

a small intercept is oberved, indicating that perhaps some other minor reactions may play

a part in the mechanism. The following quantitative data were obtained:

< k la + k lb

)/k

2

.= 3.8 torr" CEq. A)

Vk 2

= .15 torr" (Eq. B)

k lb/k

2

; = 4.1 torr" (Eq. B)

< k la + k lb

)/k

2

= 5.9 torr" (Eq. 0

k lc/k

2

= 2.2 torr" (Eq. 0

It is clear that the data are quite consistent since three different plots (obtained

from different series of experiments) yield a value of 3.8; 5.6 and 5.9 torr" 1 for the sum

of the deactivation steps la and lb over the decomposition step 2 thus confirming the

simplicity and reliabiltiy of our method. A comparison of our data with those available

in the literature will be made.

References

[ 1] Setser, D. W., and co-workers, J. Chem. Phys. } 76_, 283 (1972) and earlier publications.

[ 2] Prltchard, G. 0., et al , , iT, Am, Chem, §oc, 90, 4782 (1968) and earlier publications.

151
MEASUREMENT OF BRANCHING RATIOS FOR THE 0 + CS 9

-> OCS + S Reaction

Ronald F. Graham and David Gutman

Department of Chemistry

Illinois Institute of Technology

Chicago, IL 60616

The reaction between oxygen atoms and carbon disulfide has been shown to proceed by

all three possible exothermic channels [1,2] 2

,

0 + CS

2

kCS + SO + 31 kcal

OCS + S + 55 kcal

CO + S

2 + 83 kcal

(1)

(2)

(3)

Highly vibrational ly excited CO is also produced in this system by a rapid secondary reaction involving the CS produced in Route (1),

0 + CS * CO* + S (4)

This vibrationally exited CO is capable of producing either pulsed and continuous laser

emission in a variety of experimental arrangements. The kinetics of this system has been

studied to better understand the performance of this chemical laser [3,4]. It has been

shown that the products of Routes (2) and (3) can significantly effect the laser output.

The OCS produced by Route (2} selectively relaxes the lower excited vibrational states of

CO enhancing the laser output from higher levels and decreasing it from lower levels. The

CO produced in Route (31 is itself vibrationally excited and is produced with a different

vibrational population distribution than that produced in Reaction (4) [5].

Quantitative information on all three routes is still needed to better understand

the factors which govern the performance of the chemical laser driven by this reaction. We report here measurements of the branching ratio for Route (2), R 2 = k 2 /k, as a function of

temperature. Slagle, Gilbert, and Gutman have measured R 2 at 302°K and found it to be

0.093 [2], Nielsen and Bauer have used this value in their modeling study of the CS 2 + 0 2 chemical laser and report that the OCS produced by Route (2) has a significant effect on laser output and is in fact the most important relaxer of CO near v = 5 [3], This room temperature value of R 2 was used since there have been no determinations of this branching

ratio near the laser temperatures, 400-600°K, and there is no knowledge of its temperature

dependence. Because of the demonstrated importance of Route (2) in this chemical laser, we have measured R 2 at seven temperatures between 249. and 500°K.

i The authors gratefully acknowledge support for this research from the National Science

Foundation.

2 Figures in brackets indicate the literature references at the end of this paper.

152
Oxygen atoms (produced by the N + NO N 2 + 0) reaction and CS 2 were mixed in a fast flow reactor. The rate of CS 2 loss and OCS production were simultaneously monitored

by mass spectrometric analyses of gas sampled through a 0.033 cm diam hole in the end of

the reactor. The experimental procedure and data analysis were the same as described before.

The branching ratio for Route (2) was obtained from the relation [2].

R

2 = k 2 /k * A[0CS]

t/A[CS2

] t

, (I)

where a[0CS]+ is the concentration of OCS at time t, and A[CS 2 ] t

= [CS 2 ] 0 -[CS 2 L, is the

decrease in [CS 2 ] from its initial value. At each of the seven temperatures between 249

and 500°K, R 2 was measured in 6-9 experiments and found to be independent of [0] 0 , [CS 2 ] 0 , [M], flow velocity, and extent of reaction. In experiments in which [0] was in great excess, [CS 2 ] vs. t profiles were used to also obtain k, the overall rate constant f~r the

0 + CS 2 reaction [2], The results of these experiments are given in Table 1.

Table 1. Results of experiments to measure 0 + rate constants and branching ratios

Temperature R2 x 10

2

k x 10

12

(°K) (k2/k)

3 - 1 -1 (cm molec sec )

249 9.8 ( + 0.4)* 2.9 (+ 0.2)*

273 9.8 (+_ 0.5) 3.6 (+ 0.3)

295 9.6 0.6) 4.1 (+ 0.2)

335 9.4 (+ 0.5) 5.1 (+ 0.6)

376 8.7 (+ 0.5) 6.6 (+ 0.3)

431 8.2 (+ 0.1) 8.5 (+ 0.6)

500 8.1 ( + 0.7) 11.2 (+ 0.8)

*Average of at least 6 experiments, with one standard deviation given in parenthesis.

Estimated accuracy + 20%.

The branching ratio for route (2) decreases only slightly in importance between 249

and 50O°K dropping from 0.098 +0.004 to 0.081 +0.007. The small monatonic decrease in

R2 , although statistically evident, cannot be u?ed for lengthy extrapolations to other

temperatures due to the large uncertainty of each determination (about + 20%). We would

suggest a value of R 2 p 0.Q85 as probably being most appropriate in the 400-600°K range.

The overall rate constants measured between 249 and 500°K are consistent with other

dterminations obtained from flow reactor studies in which [0] was in excess, but lie some- what above those from studies in whtch [CS 2J was in excess and in which a stoichiometric

factor had to be determined and used.

Calculations of "prior" branching ratios predict lower values with virtually no tem- perature dependence.

153
References

[ U Smttb, I. W, M, , Trans. Faraday Soc, 64, 378 Q968). .

r 2] Slagle, I. R. , Gilbert, J. R., and Gutman, D., J. Chem. Phys., 61_, 704 (1974).

[ 3] Nielsen, N., M Development of a Computer Model for the (CS2 + O2) Chemical Laser,"

Department of Chemistry, Cornell University, 1374.

[ 4] Lilenfeld, H. V., Webblnk, R. F, , Jeffers, W. Q. , and Kelly, 0. D., "Modeling of

the CW CO Chemical Laser/' I,E,E,E, f J. Quantum Electr,, in press.

[ 5] Hudgens, 0. W., Gleaves, 0. T., and McDonald, 0. D. , f. Chem. phys., 64, 2528 (1976)

154
PHOTODISSOCIATION OF MOLECULAR BEAMS OF METALLIC IODIDES

M. Kawasaki, H. Litvak, S.-J. Lee and R. Bersohn

Department of Chemistry

Columbia University

New York, NY 10027

photodissociation processes are among the most important in photochemistry, but are difficult to study by conventional absorption methods; molecular beam photofragment spectroscopy, on the other hand, has proven to be a very effective technique for elucidating

photodissociation dynamics. We report here photofragment spectroscopy studies of the

elementary photodissociation processes of Til and Cdl 2 at wavelengths ^300 nm. The photochemical and reaction dynamics of metallic halides have long been subjects of interest,

while photodissociation of iodo-compounds is particularly relevant to the development of

new laser systems.

Our apparatus has been largely described elsewhere [I] 2 and consists basically of

mutually perpendicular molecular beam, intersecting polarized photon beam, and ultrahigh

vacuum (MO'" 3 torr) mass spectrometer photofragment detector. The molecular beam effuses

from a high temperature (y60Q K) oven in a separate vacuum chamber (^10~ 7 torr background

pressure); the photon beam is from either a Cnromatix CMX-4 frequency-doubled tunable dye

laser or a 1000W Hg-Xe lamp with appropriate polarizer and filters (laser polarization is controlled by means of a Fresnel rnomb \f1 wave plate). With this apparatus, we are able to observe 01 the photofragement mass spectrum, (21 the angular distribution of recoiling

fragments with respect to the electric vector of the polarized light, (3) the time-offlight (TOFl distribution of photofragments arriving at the detector following a laser pulse,

and (4) the variation of the above quantitites with photon energy. From the angular distribution fCel one can obtain the symmetry of the dissociative transition by use of the

relation

fCel *

2ff H + Itf cos

2 Ce^e 0

) - J]} ,

where e is the angle between the photon electric vector and the fragment recoil direction

(which is necessarily into the mass spectrometer for detected fragments!), e 0 is an angular

offset produced by the transformation from center-of-mass (cm.) to laboratory (lab)

coordinates, and g is the anisotropy parameter Cin the lab system; the anisotropy in the cm. system is usually nearly the same [2]); for purely parallel transitions, 3 = 2 and

f(e) peaks at (e-e 0 l = 0°, while for purely perpendicular transitions, 6 = -1 and f(e)

peaks at Cs-QqI * 90°. (This is true for direct dissociations, which seem to be the case

in the present study; for indirect transitions, one needs to take into account the lifetime

of the predissoviative state [3];) From the TOF distribution, (also processed via a lab -» cm. transformation), one can obtain the translational energy of the recoiling fragments

and thus, through energy conservation, their internal excitation as well.

"

This work is supported by NSF grant MPS<74r22100.

2 Figures in brackets indicate the literature references at the end of this paper.

155
Results and Discussion

1. Til

The TOF distribution of I fragments from laser dissocation at 300 nm shows that either

or both of the following processes must occur:

Til T1 *( 2p

3/2' 22 kcal mol'Witation) + I(

2 P 3/2 )

Tl ( 2 P 1/2 ) + I*t

2

P 1/2 > 21 kca1 mol

_1

excitation);

the energy difference between these two processes is too small to resolve in our experiments.

(The first process has been observed previously using Tl atomic resonance absorption [4]).

There is no evidence for dissociation into two ground state atoms. The angular distribution

of I fragments at <310 nm obtained with the Hg-Xe lamp is nearly isotropic (b = 0.07 +0.02,

e 0 = -6° + 10°) and is consistent with photodissociation via two separate transitions of

oppostie symmetry or via a single transition of mixed (parallel plus perpendicular) symmetry,

as would be the case if either (or both) the ground or excited states contains even a slight

(^5%) mixing of ionic and covalent character [5,6].

2. Cdl

2

The TOF distribution of I fragments (and to a much lesser extent of Cdl fragments)

from laser dissociation at 300 nm shows two partially resolved peaks having different

angular distributions. The angular distribution of the slower peak has a maximum at

{6-e 0 )-0°, corresponding to a parallel transition, while the translational energy of this

peak corresponds to 33 kcal mol" 1 (=80% of the total available energy) internal excitation

of the fragements. The parallel symmetry of the transition suggests that the photodissociation process is Cdl 2 h^ Cd I (

X

2

Z 1 + I*(2 Pi/2» 21 kcal mol" 1 excitation), the remaining 12 kcal mol" 1 going into Cdl vibration and rotation. The angular distribution of the fast

peak has a maximum at (,9-©q ) -90° „ corresponding to a perpendicular transition, while only

= 44% of the total available energy (18 kcal mol"1

) appears as fragment internal excitation.

This is too little energy for production of I*, and the perpendicular symmetry of the transition suggest the production of ground state I and vibrational ly/rotationally excited (the

full 18 kcal mol'^CdlCX 1 ^). The existence of two transitions of opposite symmetry (and of

unequal probabilities) is consistent with the relatively low anisotropy (e = 0.46 +0.08,

6q = 30° + 3°) observed in the I fragment angular distribution at ^310 nm obtained with the

Hg-Xe lamp (but without TOF discrimination). Additional experiments at different photon

energies are in progress.

References

[ 1J Dzvonik, M. J., and Yang, S. C, Rev, Sei, lustrum, , 45, 750 (1974).

[ 2] Busch, G. E. , and Wilson, K, R., J, Chem. Phys,, 56, 3638 (1972).

[ 3] Dzvonik, M. , Yang, S., and Bersohn, R. , Chem. Phys,, 61_, 4408 (1974).

E 4] Davtdovits, P., and Bellisio, J. A., J. Chem, Phys., 50, 3560 (1969).

I 5] Ormerod, R, C., Powers, T, R, , and Rose, T, L. , J. Chem, Phys,, 60, 5109 (1974).

[ 6] Zare, R, N. , and Herschbach, D. R. , J, Mole, Spectrose., 15, 462 (1965).

156
THE PHOTOCHEMISTRY OF 2-FURALDEHYDE IN THE GAS PHASE

A. Gandini, P. A. Hackett, J. M. Parsons and R. A. Back

Division of Chemistry

National Research Council of Canada

Ottawa, Canada

The photochemistry and photophysics of 2-furaldehyde vapor following excitation in

its first two absorption bands have been studied. Excitation in the ir*«-Tr transition (220-

275 nm) led to photodecomposition but yielded no detectable emission of light. Photolysis

at 253.7 nm at 65° and furaldehyde pressures between 0.2 and 7 Torr gave CO, furan, propyne,

allene and cyclopropene as major products, with acetylene and CO2 in much smaller yields.

A study of the variation in product quantum yields with furaldehyde pressure showed that

<t>QQ approaches 2 at low pressure and decreases towards 0 at high pressure or with added

CO2. The yields of the other major products follow a similar trend but with more complex

pressure dependence, particularly in the case of furan, as shown in the figure. The

mercury-photosensitized decomposition at 253.7 nm gave essentially the same results as the

direct photolysis.

The mechanism invoked to explain these observations at 253.7 nm postulates the rapid

ilnitial sequence,

F * hv + V +

3 F * F#

where *F and 3 F are the tt*«-tt excited singlet and triplet states, and F# is the vibration-*

ally excited ground state of furaldehyde, from which decomposition occurs. The reactions

following this fast double intersystem crossing involve F# and excited furan and C 3 H k intermediates which can decompose, isomerize, polymerize, or be deactivated by collision:

F# C 4

H 40 + CO

F# t C

3 H 4

* + 2C0

F# + N F + M

F# + F resin

W* > C

3 H 4* + C0

C 4H 40* + M furan + M

C.H.O* + F resin

C,H, propyne, cyclopropene

157
C

3 H 4 + M "* allene + M

CoH» + F resin

2 4 6

FURALDEHYDE PRESSURE (TORR)

Fig. Furaldehyde pressure (torr).

158

L
It is suggested that the mercury photosensitized reaction directly populates the 3 F state,

which then undergoes the same reactions as in the direct photolysis. Evidence for the

proposed mechanism is discussed, in particular the lack of light emission and the lack of

efficient quenching by oxygen, both of which point to very short lifetimes for l V and 3 F. A comparison of the present results [l] 1 with the earlier ones obtained by Hiraoka and

Srinivasan [2] show some significant differences which are tentatively explained.

The behaviour of 2-furaldehyde excited in the tt*-hi transition (290-370 nm) is quite

different. Although the products of the photolysis at 313 and 336 nm were the same as the

major ones found at 253.7 nm, quantum yields were much smaller, falling in the range 10~ 2 - 10 3

. Resinification on the cell walls was more important, with a quantum yield of about

0.05, which made quantitative studies of the photolysis virtually impossible, particularly

at the lower pressures. The large difference in the yields of decomposition from the two excited states may probably be attributed to the difference in their energies.

Photophysical experiments showed that excitation in the n*<-n band of furaldehyde

gave no detectable fluorescence, but modest phosphorescence was observed. The dependence

on pressure, temperature and excitation wavelength of both the lifetime and the quantum

yield of this emission from the Tr*«-n triplet state were investigated, having established

that the quantum yield of intersystem crossing is close to unity at pressures above about

0.3 Torr. The results have been interpreted in terms of the following mechanism:

1 3

F + hv —-> T F

F F + hv (phosphorescence)

63 ? T + F —^

2F

where 3 F and X F are now the ir*^n triplet and singlet states of furaldehyde. Values of

k x = 1.1 s" 1

, k 2 = 6.3 x 10 7 M^s" 1 and k 3 = 1.2 x 10 2 s" 1 were obtained at room temperature, and k 2 showed a negative temperature dependence. A long-lived emission from a quenching impurity present in trace amounts in all samples of furaldehyde tested was also

observed. This was minimized by rigorous purification and was taken into account in the

; treatment of the phosphorescence results.

References

[ 1] Gandini, A., Hackett, P. A., Parsons, J. M., and Back, R. A., Can. J. Chem., in

press.

[ 2] Hiraoka, H. s and Srinivasan, R., J. Chem. Phys., 48, 2185 (1968).

i Figures in brackets indicate the literature references at the end of this paper.

159
PHOTOINITIATED DECOMPOSITION OF MONOSILANE

E. R. Austin and F. W. Lampe

The Pennsylvania State University

University Park, PA 16802

A mass-spectrometric study of the hydrogen-atom initiated decomposition of monosilane

has been carried out. Silyl radicals were generated by mercury photosensitization of hydro5 gen-monosilane mixtures which consisted of about 95% hydrogen. Under these conditions more

than 94% of the Hg( 3Pi) atoms that are quenched by collision react with hydrogen to form

hydrogen atoms and all the hydrogen atoms formed react with monosilane via (1) to form silyl

radicals [1.2] 1

. Our attention is centered on the further reactions of the silyl radicals.

H + SiH

4

-> SiH

3 + H

2 (1)

The reactions were carried out in a photolysis cell containing a pin-hole leak leading

into the ionization region of a time-of-f 1 ight mass spectrometer. In the photolysis of such

a flow system the concentrations of all substances will approach steady-states that are reached when the rates of introduction into the cell become equal to rates of loss from the

cell.

0 Illumination of the photolysis cell containing the F^-SiH^ mixture with 2537 A radiation results mainly in the formation of S

i

2 H 6 and depletion of S i H t+ . Small amounts of

Si 3 H 8 and Si\H 10 appear after long illumination times. In addition to the volatile products

a solid film is deposited on the walls and quartz window of the cell.

In figure 1 is shown the time dependence of the concentrations of SiH^ and Si 2 H 6 (both:

relative to the initial concentration of Si

H

4 ) for two cells, one with a pyrex wall and one with a stainless steel wall. For irradiation times greater than 50 seconds the behavior in the two cells is different. However, the initial slopes of both the depletion of SiH^ and

the formation of Si 2 H 6 are independent of the cell wall. When the stainless steel cell was packed with quartz wool, identical results for the initial slopes were obtained. A fourth

cell prepared by presilanation of the pyrex wall with (CH 3 ) 2 SiCl 2 gave completely different

results; the depletion rate of SiH^ was reduced by a factor of 2, while the formation rate

of Si 2 H 6 was decreased by a factor of 7.

These data indicate that in a clean cell, with walls which do not contain adsorbed

silanes, the initial rates of depletion of Si and formation of Si 2 H 6 are independent of

the nature and area of the surface. The ratio of the initial rate of depletion of SiH^ to the initial rate of formation of Si 2 H 6 is 2 as the simple stoichiometry of (2) requires.

2 SiH

4

-> Si

2 H

6 + H

2 (2)

When the cell walls are silanated, most of the SiH^ consumed does not produce Si 2 H 6 or any

other volatile produce but results in an increased formation of film on the walls of the cell

T

Figures in brackets indicate the literature references at the end of this paper.

160
Figure 1. Depletion of SiHA and formation of Si ? H

fi during photolysis.

The protium-deuterium isotopic composition of the disilane formed by the Hg( 3 P x ) photosensitized reaction in a (96:2:2) H 2 -Si H^-SiD^ mixture has been studied. The method used

was to determine the mass spectra of the 56-68 region at a series of ionizing electron energies near the ionization potential of Si 2 H 6

. Ion-currents of all parent ions and of m/e 60

: relative to the ion current at m/e 68 were plotted as a function of nominal electron energy.

1 A linear extrapolation of the i 6 o/ 1

* 68 ratio yields a nominal electron energy of 11.0 eV for

the disappearance of all fragment ions from the spectrum. The relative amounts of the various deuterated disilanes are given by extrapolating each of the plots in figure 2 to 11.0 eV and making isotope corrections. These results are shown below.

Figure 2. Ion current ratios versus nominal electron energy.

161
oo Si-Mass Spectrum of Disilane Products at 11 eV

m/e Ion Relative Abundance

62 Si

2 Hg

+ 90 + 26

63 Si

2

H

5 D+ 43 + 11

64 Si 0 H„D 0+

2 4 2 37 + 15

65 Si

2

H

3 D3

+ 3+13

66 Si

2 H2 D

4

+ 100 + 16

67 Si

2 HD5

+ 66 + 11

68 Si

2 D6

+

74 + 16

It is clear that Si 2 H 3 D 3 is virtually absent. This is in agreement with the proposal

of Strausz et al [3] that association of silyl radicals does not occur and that the formation

of disilane occurs by the sequence (3) and (4). The distribution of disilanes suggests that

the H-atom

SiH

3 + SiH3 > SiH

4 + SiH

2 (3)

SiH

2 + SiH4

-v Si

2 H6 (4)

attacks SiH^ to form SiH 3 radicals about twice as fast as it attacks SiD^ to form Si Di+ to form Si

D

3 radicals. This is in accord with the corresponding rate constant ratio of 2-2.5

as reported by Potzinger et al [4].

References

[ 1] Potzinger, P., Glasgow, L. C, and Reimann, B., Z. Natuvforsch. 3 29a, 493 (1974).

[ 2] Mihelcic, D., Potzinger, P., and Schindler, R. N., Ber. Bunsenqes. Phys. Chem.. 78,

82 (1974). —

[ 3] Pollack, T. L. , Sandhu, H. S., Jodhan, A., and Strausz, 0. P., J. Am. Chem. Sod.. 95,

1017 (1973). —

[ 4] Laupert, R., Potzinger, P., Mihelcic, D. , and Schubert, V., to be published in J. Phys.

\ ^

Chem.

!3 tas

162
EXCITATION OF HNO BY 0

2

( A )

T. Ishiwata, H. Akimoto 1 and I. Tanaka

Department of Chemistry

Tokyo Institute of Technology

Ohokayama, Meguroku, Tokyo, Japan

In the discharge flow experiment, the chemi luminescence of HNO (fig. 1) was observed

when NO and C 2Hit were introduced successively to active oxygen containing 0 atoms and

0 2 (

1 Ag). A further experiment indicated that an addition of 02(^9) to the reaction

Reaction system (I)

Figure 1

.

of H + NO resulted in a remarkable increase of the HNO emission intensity and a change of

the spectral profile (fig. 2). The emission spectra of HNO obtained from the reaction systems of (I) 0 + C 2Hn + NO + 0 z { lbg)/0 z and (II) H + NO + 0 2( l^g)/0 z showed a similar spectral profile, but, had quite different vibrational and rotational distribution features

in the upper state from that observed in the reaction system of H + NO.

T National Institute for Environmental Studies

163
Figure 2

The spectra in the reaction systems of (I) and (II) denoted that the sudden intensity

decrease of the chemi luminescence intensity at 42 kcal mol" 1 on the rotational levels of

K' = 9 at (OOO)-(OOO) and the lower K' levels of (OIO)-(OOO) bands (fig. 3). In the reaction

system (I) the R-head of the K

1 = 8 of (OIO)-(OOO) band corresponding the energy of 45 kcal

mol" 1 was hardly discernible. However, for the same band of HNO a series of band heads was observed up to the R-head of the K' = 10 subband in the reaction system (II).

ENERGY LEVEL OF HNO(V")

50 . T , ,-i kcal-mol

48

46

44

42

40

38

K = I4

13

12

(000)

1 1

10

8

7

6

12

1

1

10

(010)

(001)

Fig. 3

Figure 3 164
It can be noted that 0 2 (

1 A

q

) played a predominant role in producing the HNO in the

upper state and the chemi luminescent intensity dependence on the 0 2 (

1 A ) flow rate was proportional to the power of 1.7 ^ 2.0 and 0.8 ^ 1.3 on the 0 2 ( 1 A

q

) flow rate in the reaction

systems (I) and (II), respectively (figs. 4 and 5).

Thus the excitation mechanism of HNO in the reaction system (I) was proposed to be

HN0(

1 A') + 0

2

(

1 A

g

) -> HN0(

3A") + 0

2 (V)

I HN0(

3A") + 0

?

(

] A ) -> HN0(1 A") + 0

2 (V)

^ y ^ y

The internal relaxation in the triplet state of HNO would be responsible to the intensity alternation at 42 kcal mol" 1 in the chemi luminescent spectra. In the reaction

system (I) the formation of the highest rotational level with K' = 7 of (010) would due to the two step energy transfer processes without relaxation in the triplet state, since two

1 M^g) molecules possess the energy of 45 kcal mol" 1

.

In the reaction system (II), the spectral information and the nearly first order dependency of 0 2 ( 1 hq) differed from the reaction system (I) probably indicates the direct

formation of HNO in the triplet state from the reaction of H + NO and the followed excitation step by 0 2 ( 1 A

g

).

This stepwise mechanism suggests the triplet state energy of HNO at 0.8 eV above the

ground state. Recent theoretical values are presented in table together with our value.

165
Table

calc. (eV) exptl. (eV)

State a b c d V 0.0 0.0 0.0 0.0

3A" 0.73 0.68 0.71 0.8

1 A" 1.45 2.04 1.61

References

[ 1] Salloto, A. W., et al . , J. Chem. Phys., 52_, 2936 (1970).

[ 2] Williams, G. R. , Chem. Phys. Lett., 30, 495 (1975).

[ 3] Wu, A. A., et al., Chem. Phys. Lett., 35, 316 (1975).

[ 4] Our value, Chem. Phys. Lett., 27, 260 (1974).

[ 5] Clement, M. J . Y. , et al . , Can. J. Phys., 39, 205 (1961).

166
SOLAR PHOTODISSOCIATION RATES FOR CIONO2; AN ESTIMATE OF CURRENT LEVELS OF C10N0 o IN THE STRATOSPHERE

J. P. Jesson, L. C. Glasgow and P. Meakin

Central Research and Development Department

E. I. du Pont de Nemours and Company

Experimental Station

Wilmington, DE 19898

Rowland et al [I] 1 have suggested that chlorine nitrate may act as a temporary chlorine sink in the mid-stratosphere in addition to the well established temporary sink hydrochloric acid. Chlorine nitrate is formed in the three body reaction (1)

so that both odd chlorine (CIO) and odd nitrogen (N0 2 ) are converted into a less reactive

form. The rate of (1) has been measured by several workers and is ca. 1.5 x 10~ 31 cm 6 molecule" 2 sec

-1 at 300 °K [2,3,4,5], Birks [2] gives the temperature expression as k 5 = 4.0 x 10~ 33 exp(1066/T). ClONO? could be decomposed by photolysis or by chemical reaction.

Preliminary investigations [6,7] of the rates of reaction of C10N0 2 with 0, OH, 0 3 , NO,

N0 2 , HC1 and H 2 0 indicate that these are too slow to compete with photolysis. We report

here our measurements of the C10N0 2 absorption cross-section and calculated solar photodissociation rates assuming a quantum yield of one. A vertical profile for C10N0 2 is calculated using a one-dimensional model of atmospheric chemistry and transport.

C10N0 2 was prepared by the low temperature reaction of C1 2 0 with N 2 0 5 in an atmosphere of ozonized oxygen [8]. Excess N 2 0 5 was used to insure complete reaction of C1 2 0. Undesired side products Cl 2 and N0 2 were removed by distillations at -119° (ethyl bromide

slush) and -78°C, respectively.

C1 20 was prepared by passing Cl 2 through a column packed with HgO on glass beads [9].

N 2 0 5 was made by dehydration of nitric acid with P 2 0 5 10

.

Samples were transferred into 10 cm quartz cells on a greaseless vacuum system and

spectra recorded on a Cary 14 spectrophotometer.

Figure 1 shows the absorption cross-section of chlorine nitrate as a function of wave

length. Each point is the average of at least three determinations at various pressures.

At wavelengths longer than 350 nm, a correction for traces of N0 2 in the sample was applied

The residual N0 2 was calculated from the absorbance at 500 nm using the data of Blacet and

Hall [11]. No samples completely free of N0 2 were produced although levels of less than

,0.1% N0 2 could be achieved by repetitive distillations at -65° (chloroform slush). Even

C10 + N0

2 + M -> C10N0

2 + M (1)

Experimental

Absorption Spectrum of C10N0

T

Figures in brackets indicate the literature references at the end of this paper.

167
CHLORINE NITRATE- CLONO,

10

-17.

10

-18

e

O

LU

CO

CO

COoeno

10

-20

10"21

10

-22

o

o

o

THIS WORK

SPENCER, MOLINA

AND ROWLAND

180 220 260 300 340 380 420

WAVELENGTH, nm

Figure 1

168
with repetitive distillations and correcting for residual N0 2 , there is an apparent

absorption tail in the 250-450 nm region. Pressures of 0.2-1.0 atm of C10N0 2 were required

to make measurements in this spectral region. The possibility that the tail is due to absorption by a species other than C10N0 2 cannot be excluded.

The absorption cross-section data are in excellent agreement with the revised data

of Rowland et al [12],

Solar Photodissociation Rates

Table 1 shows solar photodissociation rates as a function of altitude (J ) calculated assuming a quantum yield of 1.0 for reaction (2)

C10N0

2 + hv - C10 + N02

(2;

The tables of Gelinas were used to calculate the penetration of solar radiation at each

altitude considering absorption by 0 2 and 0 3 only. The 0 2 profile was obtained from U.S.

Standard Atmosphere 1962 assuming a mixing ratio of 0.2095. The 0 3 profile was obtained

from an experimental mid-latitude model (Report of Task Group V U.S. Committee on Extension

to the Standard Atmosphere). These rates are globally averaged according to the equation:

/^

/2J(0,z)sin 0 d 0 J(z) = -±—r-2 (7)

Zf,c sin e d 0

The inclusion of the tail absorption increases J by 16-50% in the critical 35-20 km region.

For comparison, J

z calculated using the data of Spencer et al [12] is also included.

Table 1. Calculated photodissociation cross sections for CIONO2

Comparison of calculated global ave photodissociation rates (xlO sec) for CI ONOg

Altitude

Original

Rowland

Revised

Rowland This Work

This work wit

X absorpt

10.5 1.62 2.92 2.91 1.77

15.5 1.70 3.00 2.99 1.85

20.5 1.91 3.22 3.20 2.05

25.5 2.45 3.78 3.69 2.55

30.5 3.83 5.26 4.91 3.76

35.5 7.32 8.84 7.89 6.75

40.5 15.6 16.6 14.6 13.4

45.5 30.0 29.3 25.5 24.3

50.5 44.3 41.7 36.2 35.0

169
Table 2 shows the variation in photodissociation rate with solar zenith angle and

altitude. Note that C10N0 2 is strongly photodissociated above 30 km and cannot account

the decreasing mixing ratio of HCT with increasing altitude [13].

for

Table 2. Calculated photodissociation cross sections for C10N0,,

CI 0N0

2 Photodissociation Rates (xlO^ sec)

Zenith Angle

'tude, kms 0 15 30 45 60 75 Global

10.5 7 09 7.03 6 .85 6.54 6.05 5 .26 2.92

15.5 7 32 7.26 7 .07 6.74 6.23 5 .41 3.00

20.0 7 99 7.91 7 .66 7.26 6.68 5 .76 3.22

25.5 10 2 10.1 9 .50 8.67 7.68 6 .50 3.78

30.5 16 5 16.1 14 .8 12.9 10.4 7 .78 5.26

35.5 29 7 28.9 26 ,9 23.2 17.9 11 .3 8.84

40.5 53 9 52.9 49 ,8 44.3 35.6 22 .3 16.6

45.5 83 8 83.0 80 .4 75.1 65.3 46 .1 29.3

50.5 102 5 102.0 100 .1 98.1 92.4 77 .6 41 .7

Estimation of the Present Day Concentration Profile for Chlorine Nitrate in the Atmosphere

Atmospheric concentrations have been estimated for C10N0 2 between 0 and 60 km using

a one-dimensional model of atmospheric transport and photochemistry. The reaction set and

reaction rate coefficients defining the H, C, 0, N chemistry were taken from Wuebbles and

Chang [14] and were supplemented by the appropriate "chlorine" reactions. For the reaction

OH + N0

2 + M > HN0

3 + M (4)

the rate expression of Anastasi et al [15]

2.3 x 10" 13

e 880/T

( t&L ) cm

3 molecule"1 sec"1 2.6 x 10

IB + [M]

was assumed. The rate constant for reaction (5)

OH + H0

2 H2 0 + 02 (5)

was taken as 2 x 10" 11 cm 3 molecule" 1 sec" 1 as recommended by Kaufmann [16].

170
The tables of Gelinas [17] were used to calculate most of the photodissociation rates

as a function of altitude. The rate of photolysis of C10N0 2 was calculated including the

tail absorption between 350 and 450 nm, assuming a quantum yield of 1.

The eddy diffusion model of Hunten [18] multiplied by 1.5 at all altitudes was used

to simulate vertical transport in the model. Additional details concerning the model will

be published at a future date [19].

CALCULATED PRESENT DAY ODD CHLORINE

MIXING RATIO PROFILES (WITH C10N02)

60.00

10.00 -

0.00

Figure 2.

-14.00 -12.00 -10.00

LOG (10) MIXING RATIO

171

-8.00
The calculated present day mixing ratio profiles for the members of the odd chlorine

family are shown in figure 2. This calculation includes only the contributions from CF2 C1 2 , CFCI3, CC1 1+ , CH 3 C1 and CH3CCI3. Accurate time dependent fluxes claculated from release data

[20] were used as the lower boundary conditions for CFC1 3 and CF2 C1 2

. For CH3CCI3 the lower

boundary flux was claculated from an estimate of total world release based on U.S. production data [21]. Both CC1 1+ and CH3 CI were assumed to be at steady state with constant

mixing ratios of 100 ppt and 750 ppt, respectively, at the lower boundary [22,23].

A maximum C10N0 2 concentration of ^ 4 - 5 x 10 8 _molecules cm" 3 is calculated near 25

km with a total vertical column density of 5 - 6 x 10~ 11+ molecules cm" 2

. Globally averaged

photodissociation rates were used in this calculation. Since the photochemical lifetime

of C10N0 2 is of the order of a few hours, it is clear that diurnal calculations will be

required if meaningful comparisons are to be made between observed and calculated C10N0 2 concentrations at a particular time and place. However, we do not expect that the results

of a diurnal calculation will vary substantially from those given above.

References

Rowland, F. S., Spencer, J. E., and Molina, M. J., presented at Meeting of NASA

Advisory Committee on Stratospheric Research, JPL, Pasadena, March 1976.

Birks, J., presented at 12th Informal Conf. on Photochemistry, Gaithersburg , MD, 1976.

Kaufman, presented at 12th Informal Conf. on Photochemistry, Gaithersburg, MD, 1976.

Leu, M., and DeMore, W. B., personal communication, 1976.

Young, R. A., and Glasgow, L. C, unpublished results, 1976.

Ravishankara , A. R., Smith, G., Tesi, G., and Davis, D. D., presented at 12 Informal

Conference on Photochemistry, Gaithersburg, MD, 1976.

Birks, J., personal communication.

Schmeisser, M., Inorganic Synthesis, Volume IX, p. 127 (1967).

Cady, G. H., Inorganic Synthesis, Vol. V, p. 156 (1957).

Gruenhut, N. S., Goldfrank, M., Cushing, M. L., and Caesar, G. V., Inorganic Synthesis,

Vol. Ill, p. 78 0950).

Hall, T. C, and Blacet, F. E. , J. Chem. Phys., 2JD, 1745 (1952).

Spencer, J. E., Molina, M. J., and Rowland, F. S., presented at 12th Informal Conf. on Photochemistry, Gaithersburg, MD, 1976.

Lazrus, A. L., Gandrud, B. W., Woodward, R. N., and Sedlacek, W. A., presented at NASA Advisory Committee Meeting, JPL, Pasadena, 1976.

Wuebbles, D. J., and Chang, J. S., Lawrence Livermore Laboratory Preprint UCRL-75602

(Rev. 1).

Anastasi, C, Bemand, P. P., and Smith, I. W. M., Chem. Phys. Lett., 3_7, 370 (1976).

Kaufman, F., in "Atmospheres of the Earth and the Planets," B. M. McCormac, ed.,

p. 219-232, D. Reidel Publishing Co., Dordrecht, 1975.

Gelinas, R. J., Lawrence Livermore Laboratory Preprint, ucrl-74944 (Change 4), 1974.

Johnston, H. S., Kattenhom, D., and Whitten, G., J. Geophys. Res., 81_, 368 (1976).

Meakin, P. Z., Franks, R. G, E., Hirsch, R. G., Glasgow, L. C, and Jesson, J. P., to be published.

172
Bower, F. A., McCarthy, R. L., and Jesson, J. P., preprint, submited to Atmos. Env.,

(1976).

McCarthy, R. L., presented by F. A. Bower at the Fall Annual AGU Meeting, San

Francisco, 1974.

Cox, R. A., Derwent, R. G., Eggleton, A. E. J., and Lovelock, J. E. , Atmos. Env. 3 ]0_,

305 (1976).

Lovelock, J. E. s Nature, 252, 292 (1974).

173
THE TROPOSPHERIC LIFETIME OF CFC1

3

J. P. Jesson, P. Meakin and L. C. Glasgow

Central Research and Development Department

E. I. du Pont de Nemours and Company

Experimental Station

Wilmington, DE 19898

Current models used to predict the effects of CFC1 3 and CF 2 C1 2 on stratospheric ozone assume an infinite lifetime for these compounds in the troposphere [1.2] 1

. However, an em- pirical comparison of the known [3] production and release of CFC1 3 with expected atmospheric concentrations calculated for a range of assumed tropospheric lifetimes indicates

that the most probable lifetime for slow eddy diffusion models is 15-20 years.

Most CFC1 3 is produced and released in the northern mid-latitudes. Hence, a critical

step in converting measured atmospheric concentrations into true global averages involves

correcting for the fact that the troposphere is not "well mixed" with respect to CFC1 3

. Lovelock [4] has made an extensive set of CFC1 3 measurements on a cruise from 65°N to 60°S

latitude and back in 1971-1972. These data can be used to derive a set of scaling factors

for converting a measurement made at a particular latitude into a global average. The scale

factor is simply the ratio of Lovelock's measured CFC1 3 concentration at a given latitude

to the calculated global average (49.8 ppt, obtained by taking the cos 0 weighted average of

Locklock's data, where 0 is the latitude). It should be noted that over 50 data sets are involved and that, since the correction involves a ratio, it is not sensitive to detector

cal ibration.

Figure 1 shows the time dependence of the tropospheric concentration of CFC1 3 as a function of tropospheric lifetime calculated using a one-dimensional model [5] for transport and removal processes, assuming Hunten'S [6] eddy diffusion model. The Points (a) in figure 4 represent the latitude corrected results of continuous CC1 3 F monitoring at Bowerchalk and Adgrigole for a period of five years [7], The round circles (0) are southern

hemisphere data [7] scaled with the appropriate factor. The two lines of experimental data are in remarkable agreement and indicate a tropospheric lifetime of 15-20 years. All of

the data analyzed were obtained with the same electron capture GC detector and are considered to be both consistent and free from systematic error [7]. The possibility of an absolute calibration error will be easy to assess at a later date its absolute standards are established. It would then be a simple matter to revise the overall conclusions reached

in the present paper. However, since the data obtained by other groups (Washington State

University and Stanford Research Institute) leads to similar conclusions, significant

revision due to calibration changes are unlikely.

Faster diffusion profiles would lead to longer calculated tropospheric lifetimes.

Thus, the Crutzen II profile [8] gives a tropospheric lifetime of about 50 years.

If values of CF 2 C1 2 concentrations of 207 ppt for Washington in the fall of 1975 are correct [9], the lifetime for CF 2 C1 2 must be similar to that of CFC1 3

.

This analysis does not attempt to distinguish specific sink mechanisms for halocarbons. Possibilities for such removal mechanisms include: trapping at the poles, photolysis at wavelengths longer than 290 nm, heterogeneous processes, ion molecule reactions

and destruction by atmospheric discharges.

T Figures in brackets indicate the literature references at the end of this paper.

174
HI 1966-1975 EDDY 3 (HUNTEN) 85% RELEASE

1966

' 1967' 1968' 1969' 1970 ' 1971' 1972' 1973' 1974' 1975

'

r igure 1. A comparison of CC1 3 mixing ratios in the troposphere calculated for tropospheric

lifetimes of °°, 50, 30, 20, 15, 10, and 7 years with the measurements of Lovelock.

a Data from exponential least squares fit to all Bowerchalke and Adrigole data (51°N)

I adjusted for latitude, o Southern hemisphere data adjusted for latitude.

References

Molina, M . J ., and Rowland, F. S., Nature, 249, 810 (1974).

Wofsy, S. C. , McElroy, M. B., and Sze, N. D., Science, 187, 535 (1975).

: 3] McCarthy, R. L., Bower, F. A., and Jesson, J. P., to be published.

: 4] Lovelock, J. E. , Maggs, R. J., and Wade, R. J., Nature, 241_, 194 (1973).

: 5] Jesson, J . P ., Gumerman, P. S., Glasgow, L. C, and Meakin, P., to be published.

: 6] Johnston, H. S., Kattenhorn, D. , and Whitten, G., J. Geophys. Res., 81_, 368 (1976)

: 7] Pack, D. H., Lovelock, J. E., Cotton, G., and Curthoys, C, to be published.

8] Crutzen, P. J., Geophys. Res. Letters, 1_, 205 (1974).

: 9] Grimsrud, E. P., and Rasmussen, R. A., Atmospheric Environment, 9_, 1010 (1975).

175
THE GAS KINETIC AND PHOTOCHEMICAL DEGRADATION OF CHLORINE NITRATE

A. R. Ravishankara, G. Smith, G. Tesi , and D. D. Davis

University of Maryland

Department of Chemistry

College Park, MD 20742

Rowland and co-workers have recently suggested that chlorine nitrate (C10N02 ) is of

possible significance as a temporary sink for both chlorine and nitrogen oxides in the midstratosphere. If this hypothesis is borne out to be true, the projected ozone depletion

rate will have to be scaled down significantly. The effectiveness of this new sink in

altering the ozone depletion rate would depend on the rates of both formation and depletion

of C10N0 2 in the stratosphere. There are two possible modes of destruction of C10N0 2 ; Viz,

photolysis by sunlight and removal by chemical reactions, Most likely, such a reaction

would be with 0( 3 P), the most abundant radical in the stratosphere. Hence, we measured the

rate constant for the reaction

at 245 K using the technique of flash photolysis resonance-fluorescence. _ The_bimolecular

rate constant at 245 K for reaction 1 is (2.1 + .2) x 10" 13 cc molecular^sec" 1. This re- sult indicates that the extent of C10N0 2 destruction through photolysis is ^10 times more

than through reaction 1.

In the light of the above results, it is important to identify the photodissociation

products of C10N0 2 . Based on bond energy data, it is generally assumed that C10N0 2 gives

CIO and N0 2 upon photolysis. However, we found that at least below 1900 A and above 1050 8

(Li F cut off), the photolysis products include 0 and CI atoms.

The technique of flash photolysis - resonance fluorescence was employed for this investigation. 0( 3 P). atoms were formed by a N 2 -flash lamp photolysis of 0 2 in Ar atmosphere.

The 0( 3 P) atoms were detected using a discharge flow resonance lamp and photon counting of

the 0-atom fluorescence using an EMR vacuum UV photomultipl ier tube. The pseudo-first order rate constant ^(IClONOal » [0( 3 P)]) for the disappearance of 0( 3 P) was measured as a function of C10N0 2 concentration to obtain the bimolecular rate constant for reaction 1.

The above mentioned 0-atom detection technique was used to detect the formation of

0( 3 P) from C10N0 2 upon photolysis with light of wavelength between 1900 and 1050 %. A similar technique of using a chlorine atom discharge-flow resonance lamp was employed in detecting CI atoms. Our results show that CI and 0 atoms are amongst the photodissociation

products of CI ONO2 when the wavelength of photolyslng light is less than 1900 A\ Under the same conditions, our attempts to detect C10 through the indirect method of titration of

C10 with NO to get CI was unsuccessful; however, this result is not conclusive.

0(

3 P) + C10N0

2

-* Products (1)

1 On Sabbatical from NSF.

176
PHOTOGALVANIC CELLS

M. D. Archer and M. I. C. Ferreira

The Royal Institution

21 Albemarle Street

London W1X 4BS United Kingdom

and

W. J. Albery and W. R. Bowen

Physical Chemistry Laboratory

South Parks Road

Oxford 0X1 3QZ United Kingdom

We consider the performance of photogal vanic cells of the type shown in figure 1 and

cheme 1, with particular reference to the much studied iron - thionine cell [I]*. The

lectrodes are identical, and the device is a concentration cell, working by virtue of the

ifferent chemical compositions of the dark and photostationary states. Cell performance s characterized by three parameters: open circuit potential, short circuit current and

aximum power.

Scheme 1

In dark solution

Photochemistry

At electrodes

A + Z < B + Y

A + Z -> B + Y

A + e + B

Y + e - Z

Light

Dark Illuminated

solution solution

Figure 1. The photogal vanic cell

The open circuit potential change on illumination, from a thermodynamic dark potential

a mixed potential, depends upon the electrode kinetics of A, B and Y, Z [2]. If both

luples are highly reversible, then the electrode restores thermodynamic equilibrium at its

jrface, and there is no potential change on illumination, and now power can be obtained

'om such a cell. If one couple is more reversible than the other, then the electrode

itential does change on illumination, to an extent which depends on the balance between

ie electrode kinetics, and the rates of back reaction in solution and of transport to the

ectrode. Maximum potential shifts are obtained if one couple (A, B say) is completely

'iversible and the other (Y, Z) completely irreversible. In this case, the electrode

•tential follows the Nernst expression for the A, B couple.

+ In+the iron-thionine cell, although both couples, thionine/leucothionine (T/L) and

i 3 /Fe 2 are separately reversible at clean metallic electrodes, the iron couple is rather

:rongly repressed by the presence of thionine in solution, as illustrated by figures 2 and

and so the potential shift on illumination is negative. Full Nernstian behaviour with

ispect to the T, L couple is not observed on platinum, as shown in figure 4. Chronocoulo-

:tric measurements can reveal the extent to which thionine is adsorbed on the electrode

rface.

igures in brackets indicate the literature references at the end of this paper.

177
0.1 V s

0.5 E(NHE)

Figure 2. Cyclic vol tammograms on Pt spherical microelectrode. (a) 10~ 2 M FeS0 4 in 0.5M H 2 SO u . (a 1 ) Same + 3 x 10" 5 M thionine.

CM

I 2

i

1

0)u

U -1 o

-2 U

-3

- v/

13-3 Hz

L _ 1 1.0 0.8 0.6 0.4 E(Hi

Figure 3. DC vol tammograms on platinum rotating disc electrode.

(b) 10" 2 M FeSC\ in 0.1 M K2SO4.

(b 1 ) Same + 3 x 10" 5M thionine.

(c) 3.5 x 10" 3 M Fe 2 (S0 lt ) 3 in 0.1M

I^SO^. (c

1 ) Same + 3 x 10" 5M thionine

When the photogal vanic cell is short circuited, then the current obtained is a maximun

if one couple is completely reversible at the electrode and the other completely irreversible [3]. (The iron-thionine system on platinum is therefore not ideal.) In this case, the

electrode processes are the reduction of A at the dark electrode, and the oxidation of B at the illuminated electrode. If the kinetic lengths of A and B (the distances over which

they diffuse before they react with Z and Y respectively) are shorter than the diffusion

layer thickness at the electrode surface, then there may be an important catalytic contribution to the short circuit current from the following sequence of reactions:

Solution: Y + B A + Z

Dark

Side

Illuminated

Side

Electrode: e + A

Solution: Z + A. -» B + V

Electrode: B > A + e

The homogeneous reactions regenerate the species that is reacting at the electrode. Thus

by appropriate manipulation of the thermal and photochemical reaction rates in bulk solution, it may be possible to increase the present low efficiencies of photogal vanic cells

considerably.

178
-pa

-pO

ft

(D -Pa

-p

Q) -PCO

0.6 . Composition of dark solution

3 x 10-5M thionine, 10-2M FeSCty,

C.1M K2 S04 , pH 2.5

C None 1

/A 5><10-5m i Fe2 (S04 ) 3 1x10~4M

0.2

E = E„ + RT In [TJ (Wernst equation

' 2F [Lj for T,L couple)

0.2 0.4 0.6 0.8 1.0

[L]/(LT] + [L] ) in photostationary state

gure 4. Steady state potential of illuminated electrode as a function of the composition

of the photostationary state.

References

1] Archer, M. D., J. Appl. Eleotroahem. 5_, 1975, 1 7 gives pre-1975 references. See

also R. Gomer, Electroohim. Acta 20, 13 (1975) and W. D. K. Clark and J. A. Eckert

uolur Energy ]]_, 147 (1975).

2] Albery, W. J., and Archer, M. D., Eleotroohim. Acta, in press.

13] Albery, W. J., and Archer, M. D., submitted to J. Eleotroahem. Soo.

179
THE IMPORTANCE OF INTERMEDIATE PARTITIONING IN ENERGY STORING PHOTOREACTIONS

G. Jones, II, W. R. Bergmark and M. Santhanam

Department of Chemistry

Boston University

Boston, MA 02215

The problem which is central to the observation of significant photoreactivity using

low energy (single) photons (practically speaking, the visible portion of solar radiation)

is illustrated in the figure below. The objective of one energy conversion scheme is to drive a starting material A to photoproduct B, which is energy-rich, reasonably kineticall\

stable, and revertible to A. Quantum efficiency will depend largely on the extent to which A* avoids a sizeable ground state potential energy barrier. In selecting materials

which absorb increasingly toward the red, a point is reached at which A* has insufficient

energy to mount the thermal surface, and photoreactivity will cease unless special mechanisms are employed {e.g., significant thermal activation of A*, tunneling). We have

examined

P.E.

Rx Coordinate

several model systems for which the excitation energy (A - A*) exceeds the thermal barrier

(A - T) by no more than 20 kcal/mol. The data permit the generalizations that kineticall^

identifiable intermediates are important for endoergic photoisomerizations to stable products, and that it is the partitioning of these intermediates (and not competitive decays

of the initial excited species) which determine quantum efficiencies.

The case for intermediate partitioning is illustrated for dienone \. Quenching

studies show that the lifetime of reactive triplet 1 is extremely short and temperature

dependent Ck<i ecay

- 1.1 - 3.5 x 101Q sec'" 1

, 4 - 5rc) [I] 1

. Normal (unreactive) radiation

less decay for cyclic enones is no faster than 10 8 sec" 1

. A fast reactive decay channel

for triple \

appears responsible for the short lifetime, but direct formation of 2 without competition is ruled out by the observed quantum efficiency (0 = 0.36, 320 - 380

nm). The required mechanism involves one or more intermediates, formation of which from

triplet \ is lifetime limiting and partitioning of which (to \ and to £) determines the

quantum yield.

i Figures in brackets indicate the literature references at the end of this paper.

180
3 4

X Y v cyc fir Yflr (nsec)

C H

2 CH

2

H 0.26 0.16 1.7

CH2CH2 CH

3 0.05 0.14 2.8

CH

2

H 0.15 0.06 <0.5

CHOH H 0.29 0.02 <0.5

CH2CH2 CH2CH2 0.36 <0.001 (^0.01)

A similar pattern of kinetics and quantum yields is found for linked anthracenes

3 (see table). The linking of anthracenes leads to enhanced quantum efficiencies for photocyloaddition (3 * 4). (The concentration dependent quantum yield for 9-methyl anthracene

, r( photodimerization reaches a maximum of 0.14 [2]). Fluorescence quantum yields are low and

j| fluorescence lifetimes short in a way highly dependent on structure (note table divisions)

."; and not related to quantum efficiency for photoisomerization. Neither emission nor radia-

$! tionless deactivation expected for anthracenes (normally, intersystem crossing, k ^

10 8 sec

-1

, followed by triplet decay) account significantly for the rapid decay of excited

singlets The results are consistent with efficient formation of an intermediate which

partitions to 3 and 4.

)„.[ The photocycloaddition of biacetyl (5) and olefins has been investigated. The

I interaction of excited biacetyl and cis-1 ,2-dimethoxyethene (6) in benzene leads to quenching; k (fir) = 2.5 x 10 9 M

_1 sec-1

. The quantum yield for photocycloaddition (5 + 6

-»

3 7J is 8.05 +0.01 at 0.001 - 0.01 M 6 (a concentration range in which virtually alt bip acetyl triples, but negligible singlets, are quenched). This limiting value of the quantum

yield indicates that triplet quenching encounters not only lead to product [J) but also

provide a path for decay back to 5 and 6. A clue to the nature of an intermediate species

which would account for the results (buf, not exclusively) is found in the reaction stereochemistry [i.e., 5 + 6 * trans oxetane as shown).

181
yOMe

MeO OMe

5 S 7

Should this pattern of intermediate partitioning be even more general? Current

theory regarding reactive radiation! ess decay affirms the trend [3]. The systems selected,

for which kinetic stability and thermodynamic instability in photoproduct are compatible,

exhibit weakly avoided surface crossings (normally S 0 and S 2 ). At some near mid-point

along the reaction coordinate, excited and ground surfaces will be close lying for facile

radiationless transition and minima in Si and Ti will be provided. From this diradical or biradicaloid geometry (corresponding, in the cycloadditions presented, to structures with

one bond made and the other left incomplete) excited molecules funnel to product and back

to reactant. Anticipation of electronic factors which control partitioning should be

valuable in designing energy storing systems which can be drive with high quantum efficiency.

This work was supported by the Office of Naval Research, the Advanced Research Pro- jects Agency, and the National Science Foundation (through a fellowship to W.R.B.)

[ 1] Jones, G., II and Ramachandran, B. R., J. Photoohem. } in press.

[ 2] Vember, T. M. , Veselova, T. V., Obyknovennaya, I. E., Cherkasov, A. S., and

Shirokov, V. I., Izvestiya Akademii Nauk SSR. Seriya Fiziaheskaya 3_7, 837 (1973).

[ 3] Michl, J., Fortsahritte der Chem. Forsehung 46, 1 (1974).

References

182
OPTIMIZATION OF THE IRON-THIONINE PHOTOGALVANIC CELL; PHOTOCHEMICAL ASPECTS

P. D. Wildes, N. N. Lichtin and M. Z. Hoffman

Department of Chemistry

Boston University

Boston, MA 02159

The iron-thionine photogal vanic cell was first investigated by E. Rabinowitch [l] 1 and has since been the subject of a number of investigations [2-6]. Studies of this cell

can be regarded as prototypical efforts directed towards ultimate realization of a photogalvanic device which is a practical and economically significant means of converting the

solar flux to electrical power. One requirement which must be met to achieve this goal is conversion of incident solar power ('vlOO mW cm" 2 at ground level) to electrical power with

an efficiency of ^5% (i.e., a 5% engineering efficiency). If the quantum efficiency of

the iron-thionine photogal vanic cell for conversion of absorbed photons to electrons

delivered to an external circuit were 100%, its engineering efficiency could be at best

only <3%. This follows because no more than 15% of solar power falls within the absorption band of thionine and the potential of the cell is no more than 20% of the potential

of the incident photons. Sensitization to the blue with a quantum efficiency of 100%

would afford the possibility of increasing maximum engineering efficiency to i>9%. Optimization of photochemical and photophysical processes is thus a necessary (but not sufficient)

condition for achievement of a practical sunlight engineering efficiency. Photochemical

aspects with which we have

+been concerned include efficiency of photooxidation of Fe(II)

by triplet thionine ( 3 TH 2 2

) [7], sensitization to the blue and composition as well as kinetics of decay of the photochemical steady state at high concentrations of thionine.

Table 1 summarizes information on the dependence of rate constants upon solvent and

anions for thionine and methylene blue.

The rate constant for the first order decay of 3 Td 2 2+ and 3 MBH

2+ is not significantly

tffected by a change in anion from CF 3 S0 3 " to HSO^'/SO^ or by a change in solvent from

wter+

to 50 v/v per cent aqueous acetonitrile. The rate of reaction of 3 TH 2

2 and

! MBH 2 with Fe(II) is, however, strongly affected by changes in the anion present and in

;ome cases by changes in solvent. The faster rate of reaction of the triplet excited dyes

n the presence of sulfate anions compared to solutions iwth CF 3 S0 3 " anions may result

Tom reduction in electrostatic repulsion by ion pairing. In any case, the efficiency of

'hotooxidation of Fe(H) by 3 TH 2

2 is higher in the presence of sulfate anions than with

F 3 S0 3 " anions.

c igures in brackets indicate the literature references at the end of this paper.

183
Table 1. Kinetics of excited states of thionine and methylene blue first order decay,

k, , and reduction by Fe(II), k ?

, of triplet states

DYE SOLVENT

, b PH ANION

IONIC

STRENGTH

M

ki 10 5 sec

-1 k,

10 5 M

T se(

TH+ H^O 2.0 HS0„"/S0„= 1 .3 600

TH+ III' H 0 1 .7 F CSO

"

r 3 3

11 • 5-J 65

i nt Jw V/ V lo

aq. MeCN

lip flu Hsn ~/sn = \ • C

TH+ 50 v/v % aq. MeCN

"1.7" F-CSO," 1.2 55

MB+ H

2 0 1 HS04 "/S04

= — 2.4 38

MB+ H

2 0 1 F

3 CS03

" 2.2 9

MB+ H

2 0 2.0 hso4 "/so4

= 0.2 2.2 55

MB+ H

2 0 2.1 HS04"/S04

= 1 .0 2.2 80

MB+ H

2 0 3.1 hso4 7so4

= 1.0 2.2 80

MB+ 50 v/v % aq. MeCN

I 2M hso

4 7so4

= 0.2 2.0 700

MB+ 50 v/v % aq. MeCN

ii 2ii F3CS03- 0.2 2.1 10

aAt the pH of these measurements, triplet TH

2 + and MBH

+

.

^Values in quotation marks are pH values of solutions of identical composition in H

?0.

We are also studying factors which affect the rates of other reactions occurring in

the cell solutions. The maximum current obtained from the cell is related to the concentra

tion of reduced thionine present at the anode when the cell is irradiated with sunlight.

Under constant illumination, the cell solution attains a photostationary state composition

in which some of the dye has been reduced. The photostationary state composition of the

solution depends on several parameters including the initial concentratrations of dye,

Fe(II) and Fe(III), the intensity of light and the length of the light path through the

cell solution. Table 2 summarizes some data showing the dependence of photostationary

state solution composition on reactant concentrations and cell thickness. The details of

the solution dynamics which result in the photostationary state compositions shown and the

implications of these values in maximizing the cell engineering efficiency are currently

being investigated.

184
Table 2. Photoreduction 3 of thionine in 50 v/v% aqueous acetonitrile solution

containing 0.01N H

2 S04

% Reduction of

[Thionine]'

3 M x 10_l+ CFe(II)]^

M EFe(III)tfM x 10 -I+ Cell Thickness

cm x If) -1*

i 1 1 l U 1 1 l n kz at Photostationary

7.5 0.01 0.9 80 4D

7.5 0.025 2.25 80 414 1

7.5 0.05 4 5 OU

7.5 O 01 n q j 1

7.5 n o?^ CD DO

7.5 0.05 4 5 4R

3.75 0.01 (J 9 80

3.75 0.025 2.25 80 74

3.75 0.05 4.5 80 42

3.75- 0.01 0.9 25 73

3.75 0.025 2.25 25 66

3.75 0.05 4.5 25 47

a 2 Under illumination with ^50mW/cm of light from a Xenon lamp.

^Initial dark concentrations.

c Probable errors are +10%.

This work was supported by NSF RANN Grant SE/AER/72-03597. We wish to acknowledge

valuable conversations with Dr. E. Berman (Boston University), J. A. Eckert (Exxon Re- search and Engineering Co.) and Professor H. Linschitz (Brandeis University) and the use

laser flash equipment in the laboratories of Professor Linschitz and Dr. E. Hayon (U.S.

Army Natick Laboratories).

References

[ 1] Rabinowitch, E., J. Chem. Phys. 8, 560 (1940).

[ 2] Miller, L. J., U.S. Dept. Commerce, Off. tech. Serv., Report AD 282, 878 (1962).

[ 3] Potter, A. E., and Thaler, L. H., Sol. Energy 3, 1 (1957).

[ 4] Ma thai, K. 6. , and Rabinowitch, E., J. Phys. Chem. 66, 663 (1962).

I 5] Kamiya, N. , and Okawara, M, , Deriki Kagaku 38, 273 (1970).

'. 6] Clark, W. D. K., and Eckert, J. A., Sol. Energy V7, 147 (1975).

. 7] Faure, J., Bonneau, R., and Ooussot-Dubien, J., Photochem. Photobiol. 6_, 331 (1967).

185
A BIOMIMETIC APPROACH TO SOLAR ENERGY CONVERSION

Thomas R. Janson and Joseph J. Katz Chemistry Division

Argonne National Laboratory

Argonne, IL 60439

A generation ago, R. Emerson and W. Arnold developed the concept of the photosyntheti

unit as the basic device used by green plants in photosynthesis for light energy conversion

Chlorophyll (chlorophylls a_ and b_ in green plants, bacterio-chlorophyll in photosynthetic

bacteria) acts as the primary photoacceptor in this photosynthetic unit.

A large number of chlorophyll molecules (300 hundred or more) act cooperatively in this light conversion process with the great majority of the chlorophyll molecules in the

photosynthetic unit passive and acting as antennas to collect light quanta. This light

energy is then funneled to a few special chlorophyll molecules in a photo-reaction center

where energy conversion occurs. Antenna and photo-reactive chlorophyll, together with

electron transfer proteins that serve as conduits, constitute the photosynthetic unit in which light energy is made available for chemical purposes.

It has long been known that the visible absorption spectra of both antenna and

photoreaction center chlorophyll are red-shifted relative to a solution of chlorophyll in,

for example, ether or acetone solution prepared in the laboratory. It has also long been

established that a characteristic free radical signal is produced in photo-reaction center

chlorophyll in the process of light conversion. Laboratory investigations, mainly by

infrared and magnetic resonance spectroscopy, now make it possible to provide plausible

interpretations on the molecular level for the anomalous visible absorption spectra of in

vivo chlorophyl 1 , and for the unusual features of the free radical signal. Thus, new

model s for both antenna and reaction center chlorophyll have emerged and these have become

the basis for a serious effort to produce new devices for solar energy conversion.

The information so far developed on plant photosynthesis can be used in a biomimetic

approach to solar energy conversion. Essentially, solar energy conversion in a biomimetic

context is an attempt to mimic those aspects of photosynthesis whereby plants convert

absorbed solar energy into useful chemical energy with a high degree of efficiency.

First steps have been taken in the direction of such a device. The device studied

consists of a cell composed of two glass compartments, each with its own platinum electrode. External leads are gold soldered to each platinum electrode and connected to an electrometer to monitor current, emf, and resistance changes.

i Work performed under the auspices of USERDA.

186
When assembled the cell compartments are separated by a membrane (a plastic film

or metal foil) carrying a photoactive chlorophyll -water adduct. In the case of metal

ifoils the photo-active chlorophyll species is deposited on one side (generally the side

ifacing the electron donor) by evaporation of an octane suspension of a chlorophyll-water

adduct. This chlorophyll -water adduct is used because of its similar photoactive behavior

jas compared to reaction-center chlorophyll. One compartment is then filled (under nitrogen)

Wth 1-2 ml of a suitable degassed aqueous solution of an electron donor such as sodium

ascorbate. The other compartment is filled with a similar solution of an electron acceptor

such as potassium ferricyanide, dichlorophenol indophenol (DCPIP) or the sodium salt of

anthraquinone sulfonic acid. After thermal equilibration in the dark the supported photoactive chlorophyll film is irradiated with visible (or red) light and the electrical con- sequences observed.

A change in potential (emf) across the separating membrane upon irradiation with

[Visible light was the most commonly measured electrical parameter. Under our experimental

cnditions, we observed increases in emf, decreases in resistance, increases in conductivity,

and increases n current flow. In our best cell to date, using an aluminum support film,

a potential difference of 422 mV was recorded associated with a current flow of 2.36 x

[10 5 amp. A typical photo-response in the cell is illustrated in figure 1. A rough

estimate of the efficiency of the photo-activity of this cell is 0.0024%, comparable to

the best organic photo-voltaic devices in the literature. The magnitude of the photolinduced effects has been shown to depend on the type of film support used, the nature of

the donor and acceptor, and on the light flux. The dependence on metal film support is seen in table 1 where results can be correlated with the photo-electric work functions of

the particular metal involved.

Table 1. Dependence on metal foil support

Metal Work Function Emf (mV)

(eV)

Ag 3.67 181

Pt 4.09 125

Au 4.82 94.5

Finally, although white light was generally used in these experiments, it should

)e noted that red light (^740 nm, which corresponds to the absorption maximum of the

Chlorophyll -water adduct film) is equally as effective in producing photo-responses in the

pell. Action spectra confirm this result indicating that the chlorophyll -water adduct is

indeed the primary photo-active species in the cell and initiates the observed photoresponses.

187
>

Ll

Figure 1

188
LIGHT ENERGY CONVERSION VIA PHOTOREDOX PROCESSES IN MICELLAR SYSTEMS

Michael Gratzel

Hahn-Meitner-Institut fur Kernforschung

Berlin GmbH Bereich Strahlenchemie

D 1000 Berlin 39

This paper describes electron transfer reactions involving excited states of organic

molecules by which exploitation of solar light energy may be achieved. Three different

types of processes are discussed:

1) Photoejection of electrons from a photoactive probe P into a polar liquid such

as alcohol or water:

p^p+

+ e;

o , v „,

where P+ stands for a cation radical and e~olv

for the solvated electron

2) Electron transfer from the triplet state of the molecule (P ) to various

acceptors:

PT

+ A P + + A" (2)

3) Electron transfer from various donors to an acceptor triplet a"*"

A

T

+ D D++ A" (3)

These reactions are examined by conventional photolysis and pulsed laser techniques. A frequency doubled, Q-switched ruby laser (pulse-width 15 ns, 347.1 nm) is used to excite

the photoactive species. Transitory species are identified and their kinetics monitored

by fast kinetic spectroscopy or conductivity technique.

A prominent feature of this work is the comparison of the nature of photoreactions

1-3 and subsequent dark reactions in homogeneous polar media such as alcohol or water and

aqueous micellar solutions. It is attempted to illustrate the important role surfactant

micelles could play in the search for photochemical systems suitable for conversion of

light energy into chemical or electrical energy.

Reaction (1) has attracted attention as a possible pathway for photochemical production of hydrogen from water since hydrated electrons produced in the photoionisation event

are known to undergo the diffusion controlled reaction

e~ + e" H 9 + 2 OH" (3) aq aq c

1

The two major problems which heterofore have prevented successful application of such a system are: (a) The majority of organic sensitizers with ionization potentials low enough

to permit electron ejection by visible or n_ear u.v. light are sparsely soluble or insoluble

in water; (b) Rapid recombination of e^g/P ion pairs leads to annihilation of electrons

before hydrogen can be formed. These obstacles may be overcome by usage of anionic micellar

solutions. A schematic illustration of a photoionization event in such a system is given

in figure 1

.

189
Figure 1. Schematic presentation of photoionization and electron transfer processes

in solutions of surfactant micelles con- taining a solubilized photoactive probe P. The electron acceptor M is located in the

Stern layer of the micelle.

The sensitizer P is solubilized in the hydrophobic interior of the micellar aggregate

whose negative surface^ charge prevents reentry of the hydrated electron and recombination

with the parent ion P . These phenomena are illustrated by laser photolysis results

obtained from solutions of N, N, N' ,N'-tetramethylbenzidine (TMB) in sodiumlaurylsulfate

micelles. Comparison of photoionization cross sections in alcoholic and micellar solution

also reveals the favourable result that the micelles enhance strongly the yield of electrons

This effect is discussed in terms of an electron tunneling.

The triplet states of phenothiazine (PTH^) and TMB were found to have strongly

reducing properties and readily transfer electrons to acceptors such as methyl viologen,

duroquinone, europium(III) and chromium(III) ions. Some of these reactions have particular

importance for the photo reduction of water as the reduced form of the acceptor is capable

of evolving hydrogen from water according to

A" + H

2 0 y 1/2 H2 + A + OH" (4)

This reaction has to compete with back transfer of electrons from A" to P+ which in homogeneous solution occurs at a diffusion controlled rate. Included in figure 1 is an illustration of a forward electron transfer process in a micellar system. These intramicellar processes occur at a very fast rate. The back reaction, however, may be strongly

inhibited by the micellar systems.

The last type of photo redox processes discussed in this paper are electron abstraction reactions of triplet duroquinone (DQ ). In mixtures of_water/_ethanol DQ abstracts

electrons from a variety of substrates such as Fe 2

, Fe(CN)£~, C0§ , diphenylamin and

trimethoxy benzene. The rate constants for these redox processes are close to the diffusior

controlled limit Cl

0

9 -1

0

1 °M~ 1 s~ 1

) except for COf where k equals 7 x lC^NPs" 1

) •+ Anionic,,

micelles were found to have a pronounced catalytic effect on the reaction of Fe 2 with DQ . ! A schematic illustration of such an intramicellar electron transfer process is given in

figure 2. Converseley cationic micelles drastically enhance the rate of electron transfer

Figure 2. Schematic illustration of an intramicellar electron transfer process

from Fe 2 to DQ 1 solubilized in anionic

micelles..

from C0§ to DQ. The latter process is of particular interest as CO3 is known to form

oxygen in its subsequent reactions. Hence 0 2 evolution from water by visible light be- comes feasible via excited qui none redox reactions.

190
PHOTOELECTROLYSIS OF WATER BY SOLAR ENERGY

D. I. Tchernev

Lincoln Laboratory

Massachusetts Institute of Technology

Lexington, MA 02173

Although the free energy required for the decomposition of water into gaseous H 2 and

0 2 is only 1.23 eV, while the peak of the solar spectrum occurs at a photon energy of

about 2.4 eV, solar energy cannot be utilized for the production of H 2 fuel by the direct

photodecomposition of water, because the threshold energy or ths direct reaction is about

6.5 eV. Fujishima and Honda have recently shown, however, that this threshold can be

greatly reduced if decomposition is accomplished by means of photoelectrolysis, a process

in which a semiconductor is used as a catalyst. This is a promising process for the

large-scale utilization of solar energy to produce H 2 fuel. We have been investigating

the physics and electrochemistry of photoelectrolysis by experiments on cells with poly-

! crystalline as well as single-crystal Ti02 anodes. This work has led to studies with

SrTi0 3 anodes, and we report some of these results as well.

The basic photoelectrolysis cell consists of an n-type semiconducting working electrode, an electrolyte, and a Pt counterelectrode. Because of the difference in work

functions between the semiconductor and the electrolyte, the energy bands of the semiconductor are bent at the surface, so that the analog of a Schottky barrier exists at the

semiconductor-electrolyte interface. The semiconductor surface is irradiated by photons

of energy at least equal to its energy gap. The photon-generated hole-electron pairs do

not recombine but are separated by the electric field of the barrier; the electrons move

away from the surface into the bulk of the semiconductor and_ then through the external

circuit to the Pt cathode, where they discharge H 2 (2e" + 2H > H 2 ), while the holes remain

at the surface of the semiconductor anode, where they can interact with the electrolyte to produce 0 2 (2p + H 2 0 + 1/2 0 2 + 2H ). The overall chemical reaction is

2hv + H

2 0 -> 1/2 02 + H

2 (1)

provided the semiconductor is chemically inert, serving only to absorb the light and to produce the holes and electrons that make the reaction possible. Two conditions are necessary for efficient photoelectrolysis: CD The energy bands at the semiconductorelectrolyte interface must be bent in order to separate the holes and electrons excited by

the light and (2) the relevant electronic levels must line up, i.e. J+the hole states of

the anode with e(H 2 0/02 ) and the Fermi level of the cathode with e(H /H 2 ).

The anode material used in most of our experiments was the rutile form of Ti0 2. Some Ti0 2 anodes were fabricated from single crystals, but most were polycrystal 1 ine disks

prepared by hot-pressing powdered rutile at 750°C under a pressure of 10,000 psi. To make

the disks conducting (p ~ 10 3 ft-cm) , they were heated in vacuum for three hours at ^

600°C.

This work was sponsored by the Department of the Air Force.

191
In initial experiments the external quantum efficiency, n> defined as the ratio of

the number of electrons flowing in the external circuit (N ) to the nubmer of photons incident on the cell (N ), was measured as a function of photon energy hv. The maximum values

of n» 82% for the single crystal and 60% for the polycrystall ine disk, occur at hv ^ 4 eV. Quantum efficiency measurements were also made with Ti0 2 films on Ti foil that had been

thermally oxidized. The efficiencies were comparable to those obtained with single crystal

anodes.

When the maximum measured n values are corrected by taking account of estimated

reflection and absorption losses, we find that the internal quantum efficiency was close

to 100% for both single-crystal and polycrystall ine Ti0 2 anodes. This high quantum

efficiency shows that the band bending in Ti0 2 was sufficient to separate the electron-hole

pairs generated by photon absorption and in addition that the hole states of the anodes

lined up with the (H 2 0/0 2 ) level of the elctrolyte. As stated above, however, for efficient

photoelectrolysis it is also necessary for |he Fermi level of the cathode,

+

eF

, to line up with the electrochemical potential of the H /H 2 level in the solution, e(H /H 2 ).

The quantum efficiency experiments just described were performed with the photoelectrolysis cell open to air, so that the electrolyte contained dissolved oxygen. In this case the energetically favorable process at the cathode is the transfer of electrons from

the cathode (at ep) to the H 2 0/0 2 causing the reduction of oxygen:

1/2 0

2 + H2 0 + 2e" - 20H" . (2)

The cell now functions in the galvanic mode, and no H 2 gas is evolved. Under these con- ditions, we have observed that the 0 2 bubbles formed at the anode migrate to the cathode,

where they disappear.

This obstacle to the production of H 2 gas can be eliminated by removing the dissolved

oxygen from the cathode compartment of the cell. However, the cell current then decreases

considerably, so that n becomes only 1-2%, even for the best Ti0 2 anodes. This occurs because the band bending under photoelectrolysis conditions is very small for Ti0 2

. However, better results are obtained for SrTi03, for which the band bending is ^ 0.2 V larger

because of its lower electron affinity. This increase in band bending results in a quantum

efficiency for photoelectrolysis of 10% at hv = 3.8 eV, about one order of magnitude larger

than that for Ti0 2

. The SrTi03 experiments were performed with single-crystal and polycrystalline anodes. When dissolved 0 2 was removed from the solution, the amounts of H 2 evolved at the cathode and 0 2 evolved at the anode were in good agreement with the predictions of eq. (1), showing that photoelectrolysis was the only reaction occurring.

Ultimately, the efficiency with which solar energy can be used to generate H 2 by

photoelectrolysis will depend on the semiconductors that are used as electrode materials.

Only about 10% of the solar energy reaching the earth is supplied by photons with energies

higher than 3.0 eV, the energy gap of Ti0 2 or SrTi0 3

. Therefore the efficiency of solar

utilization will be limited to a few percent unless it is possible to find a suitable

electrode material with a significantly lower energy gap.

192
RECENT WORK IN EXCIPLEX PHOTOPHYSICS

Desmond V. O'Connor and William R. Ware

Photochemistry Unit

Department of Chemistry

University of Western Ontario

London, Ontario, Canada

1. Introduction

While there have been numerous studies reporetd in the literature aimed at revealing

the photophysical and photochemical properties of exciplexes, there have been relatively

few that attempted to obtain simultaneously (a) a test of mechanism (b) all the pertinent

rate constants (c) the temperature coefficients of these rate constants and thus the activation parameters as well as the thermodynamic properties of the exciplex equilibrium. In fact, what is needed are such studies carried out with a number of donor-acceptor pairs of

different properties in a number of solvents ranging from non-polar to polar. The results

of such studies cannot fail to enhance our knowledge of the exciplex. Such a program is currently underway and this paper constitutes a preliminary report.

2. Experimental

Exciplex photophysics have been investigated by observing the fluorescence decay of

both the monomer and exciplex, by observing the relative yields of monomer and exciplex

emission as a function of quencher concentration and by measuring the steady-state monomer

quenching. The fluorescence decay measurements were done with the single photon technique

and deconvolution accomplished by iterative convolution.

3. Results

The principal system to be discussed is a-cyanonaphthalene quenched by dimethyl - cyclopentene. This has been investigated in hexane and diethyl ether (DEE) over a wide

temperature range.

The experimental observations are in qualitative agreement with the well-known

excimer or exciplex mechanism. That is

Ut)

A —- >. A*

A* + Q

\ k„

A + hvp A

(AQ)' prod.

A + Q + hvp

193
The appropriate equations are

I M(t) = + (1)

I E (t) - C

3 (e"

X i t

- e"

x 2t)

(2)

\ 1>2 = 1/2[k

1 + k 2 + k 3 [Q] + k 4 + k p + {(k

] + k 2 + k 3 [Q] - k4

- k p

) 2 + 4k

3

k4 [Q]}

1/2

] (3)

k

p = k 5 + k 6

(4)

One also has

X

1 + x 2

= k

] + k £ + k3 [Q] + k 4 + k p (5)

A

1 A 2 = (k1 + k 2 )(k4 + k p

) + k 3 K

p [Q] (6)

Another useful equation is the indicial equation used to derive eq. (1) and (2).

This is

X 2

- [k

1 + k 2 + k 3 [Q] + (k 4

+ k p )]A + (k

1 + k 2 )(k

4 + k p

) + k 3

k p [Q] = 0 (7)

Finally, the two steady-state equations are

KSV

= (k7 "k^ T-y =

" 1} IQT (8)

$

E k, k, —= {ir} { k 4 r} [Q] (9)

$M

K

l K 4

K p

These equations were utilized to calculate individual rate constants on the assumption that the mechanism is valid. It will be shown that quantitative aspects of the ex- perimental results are consistent with this model.

The results are summarized in the table. The following conclusions appear con- sistent with the data.

(a) The forward rate of quencihing is diffusion controlled.

(b) The low-temperature results in hexane are consistent with the high-temperature

results in hexane where the rapid equilibrium approximation is valid (see J. Am. Chem. Soc. 96, 7853 (1974)). Thus the kinetics are now resolved over a ranae of 85°.

(c) The quenching is inefficient and this appears entirely due to the feedback step

(M.

(d) The thermodynamic parameters and activation parameters are similar to those

obtained for other exciplexes and the exciplex binding energy is consistent with

electrochemical data.

194
Ce) The steady-state studies confirm the fluorescence decay measurement.

Cf) Changing from hexane to DEE has a profound effect on k 4 but leaves ae£ and AH unchanged. While k 4 and k p decrease as one goes to DEE, K$v increases!

(g) In hexane k 5 f f (T)

.

Diethyl ether

303 293 283 273

1.23 1.21 1.08 0.93

k.xlO"

8

-1 sec 1.14 0 49 0 20 0 10 0 040 0 55 0 .25 0 17 0 .054

kxlO"8

v -1 sec 0.98 0 77 0 62 0 52 0 46 0 79 0 65 0 54 0 44

log

k 3/,

K4

1 .94 2 25 2 58 2 82 3 18 2 35 2 69 2 81 3 24

AS./

3 -4.78 -4 68 -4 58 -4 58 -4 44 -4 42 -4 18 -4 11 -4 10

AS/

4 18.97 18 94 19 00 19 25 19 18 14 27 13 97 14 56 13 75

AS 0 -23.15 -23 62 -23 58 -23 83 -23 62 -18 68 -18 15 -18 67 -17 85

AG° -2.42 -2 70 -2 97 -3 13 -3 39 -3 26 -3 60 -3 64 -4 05

2.36 2 31 2 25 2 23 2 17 2 56 2 46 2 42 2 42

AG/ 4.78 5 .02 5 23 5 36 5 55 5 81 6 06 6 06 6 44

Hexane

T°K

273 262 251 242 233

koXlO"

10 0.98 0.88 0.78 0.67 0.60

-1 -1 M sec

195
CHARGE TRANSFER AND HYDROGEN ATOM TRANSFER REACTIONS OF EXCITED

AROMATIC HYDROCARBON - AMINE SYSTEMS

N. Mataga, T. Okada and T. Mori

Department of Chemistry

Faculty of Engineering Science

Osaka University

Toyonaka, Osaka 560, Japan

It is well known that excited aromatic hydrocarbon forms exciplexes with N,N-dimethylaniline (DMA) or N,N-diethylanil ine (DEA). Contrary to this, when amines with free N-H groups such as aniline and N-methylaniline are used as electron donors for the exciplex

formation with excited aromatic hydrocarbons in solution, no exciplex fluorescence can be

observed although the fluorescence of aromatic hydrocarbon is quenched [l] 1 .. On the basis

of preliminary studies upon pyrene-anil ine system by means of laser photolysis as well as the measurements of the effects of solvent polarity upon the fluorescence quenching rate,

hydrogen atom transfer assisted by charge transfer interaction in the encounter complex has

been proposed as a mechanism of the quenching of exciplex fluorescence [2]. Since this is an important problem in the exciplex chemistry, we have made a more detailed and extensive

studies by means of fluorescence and transient absorption measurements with laser photolysis as well as conventional flash photolysis methods.

The bimolecular rate constants (kq) of the quenching of pyrene fluorescence in n- hexane by various primary and secondary amines were measured. The kq values- in the case of

aliphatic amines are considerably smaller than those of the aromatic amines, which seems to indicate that an additional activation energy over that for the aromatic amine system

appears necessary for the quenching by aliphatic amines. The difference of the quenching

rate constant between aromatic and aliphatic amines was also observed in the case of the

fluorescent exciplex formation of pyrene-DMA and pyrene-tri butyl amine (TBA) systems [3].

It may be possible to assume similar structural change in case of the present systems.

It has been confirmed that the kq values increase remarkably with decrease of the

ionization potential (I p

) of the amines. This result suggests the fluorescence quenching

mechanism of the charge transfer in the encounter complex. Although the values of k as well as i for N-methylaniline (NMA) or N-ethylanil ine (NEA) are rather close to tho§e of DMA or DEA, respectively, no exciplex fluorescence can be observed in the case of the

formers. This result suggests the following mechanism of the quenching.

k k k A* + HD -9-* (A"....HD

+

) —B— (AH....D) AH + D (1)

k-j

j kj IkV ^ A + HD^ A + HD and other reaction

product

Assuming that the. rate constants of the radiative and radiationless transitions (k' and

k^) of (A HD ) are equal to those of pyrene-DMA or pyrene-DEA exciplex in hexane, re- spectively, (k + k!) * k has been estimated to be larger than 2 x 10 10

s _1 pip 3 .

figures in brackets indicate the literature references at the end of this paper.

196
If AH and D radicals are formed by the porcess k,, their absorption spectra may be

observed by means of flash photolysis. Transient absorption bands were observed in the

380 * 430 nm region for many pyrene-primary and secondary amine systems in ht xane. The

observed spectra can be explained as a superposition of absorption bands of pyrene triplet

state and those of some other transient species. At the delay time of 50ys, the T-T absorption band becomes much weaker while the absorption band of the other transient re- mains fairly strong. In order to identify the long lived transient, 1 ,2-dihydropyrene

(AH 2 ) in methylcyclohexane-isopentane (3:1) mixture at 77K was irradiated with a low pressure mercury lamp, and absorption spectra of photoproduct were measured. It seems probable

that the radical AH is formed by irradiation. The obtained spectrum of the photoproduct

is rather close to that of the long lived transient species observed by the flash photolysis method. Furthermore, the spectrum of AH was measured by means of. pulse radiolysis of

solutions of pyrene in ethanol containing 1 vol.% sulfuric acid, where AH may be produced

by the following reactions

C

2 H5 0H *W—> e

^ o1v , H, C^OH^, C^O,

and other neutral radicals

e solv

+ H+ —

H + A AH

(2)

The spectrum shows peaks at 405 nm and 395 nm and is essentially identical with the spectrum obtained by subtracting the T-T absorption band of pyrene from the observed transient

spectra in pyrene-primary amine and secondary amine systems.

The confirmation of the formation of D by means of the transient spectram measurement is rather difficult since the absorption spectra of anil i no radicals are observed

in the wavelength region of about 400 nm and their intensities are rather weak. However,

since the absorption spectrum of diphenyl amino radical shows maximum at ca. 770 nm, we

have examined pyrene-di phenyl amine (DPA) system in hexane solution and have observed

clearly the absorption band of diphenyl amioo radical in addition to the absorption band

of AH. Thus, the formation of both AH and D has been confirmed.

Although it is assumed in eq. O ) that the radicals are formed from the singlet

| excited state, it might be possible that they are formed by the reaction in the triplet

state. We have examined this problem by using a dilute solution of anthracene-DPA

system in hexane, and confirmed that no reaction occurs in the triplet state since the

decay process of anthracene triplet state is not affected by the addition of DPA.

If the charge transfer mechanism of the fluorescence quenching is valid, k will

depend upon the solvent polarity. Actually, we have confirmed in serveral cases

qwhere

the additional activation energy over that for the diffusional motion is necessary for

the charge transfer, that the k value decreases with increase of the solvent polarity.

'This result indicates that not 8nly the I value but also the solvent polarity affects

the additional activation energy and it is decreased with increasing solvent polarity.

For example, in the case of pyrene-anil ine system, k is 5.7 x 10 8 M

1 s" 1 in hexane,

2.6 x 1

0

9M

_

1

s

1 in isopropanol, and 5.3 x 10 9 M" 1 s" 1 ?n acetonitrile, and for the pyrenedibutylamine system, it is 3.0 x 10 7 M" 1 s" 1 in hexane, 2.0 x 1

0

8 M

-1

s -1 in ethylether,

8.4 x 10 8 M

-1

s""i in acetone, and 8.7 x 10 8 M

_1

s -1 in acetonitrile.

Roughly speaking, approximately linear relation can be observed between logk and

the reciprocal of the solvent dielectric constant e. This result can be explainedq satisfactorily on the basis of the reasoning given elsewhere [2]. Namely, we assume that

the activation energy for the charge transfer is determined by the Horiuchi-Polanyi '

s

relation. The free energy of the final state of the charge transfer (the solvated ion

pairs) may be written as,

197
F

i = (A/e) + B (3)

where A and B are constants irrelevant of e.

The change of with e (from e° to e) is,

AF

i = A(l/e° - 1/e) (4)

Then, the change of activation energy aE is given by,

AE = aAF^, a : constant (5)

according to Horiuchi-Polanyi 's relation. The ratio of k 's in solvents with dielectric

constant e and e° can be written as,

q

k q (e)/k

q

( e °) = exp(aAF

i /RT) (6)

Thus, logk (e) is inversely proportional to e. Of course, in sufficiently polar solvents

suc_h as acetone and acetonitrile, ionic dissociation into solvated radical ions, A" and

HD , will complete with the proton transfer in the encounter complex. Actually, transient

spectra observed by the laser photolysis of pyrene-anil ine system in acetonitrile show

strong absorption bands of solvated radical ions.

References

[ 1] Mataga, N. , Okada, T., and Oohari , H., Bull. Chem. Soa. Japan 39, 2563 (1966).

[ 2] Mataga, N., in "Exciplex," ed. by M. Gordon and W. R. Ware, Academic Press, New

York, 1975, p. 113.

[ 3] Nakashima, N., Mataga, N., and Yamanaka, C, Int. J. Chem. Kinetics 5_, 833 (1973).

I

198
PROTOLYSIS EQUILIBRIA OF LUMICHROME (6,7-DIMETHYL-ALLOXAZINE) IN THE LOWEST EXCITED SINGLET STATE

Bertil Holmstrom

Sture Bergstrom and Hans-Bertil Larson

Department of Physical Chemistry

Chalmers Univ. of Technology and the Univ. of Gothenburg

Gothenburg, Sweden

Fluorescence is a standard method of assay for riboflavin (Vitamin B 2 ). The substance

is very unstable to light, two major decomposition products being lumiflavin (6,7,9-trimethyliso-alloxazine) and lumichrome (6,7-dimethyl-al loxazine) . A thorough study of the fluorescence properties of these compounds may, therefore, be of some practical importance.

Lumiflavin has a fluorescence spectrum very similar to that of the parent compound (an

emission peak at 520 nm); the emission disappears in acid medium (pH = 1.5) and in basic

medium (pH = 10)

.

O

In contrast to this, the pH variation of 1 umi chrome fluorescence presents a very complicated

picture containing several feautres of more general interest. In the acidity/basicity range

of this study, from concentrated sulphuric acid to strong sodium hydroxide solution, at

least three different protolysis forms are evident in emission and four in absorption (cf

Fig. 1 and 2). The various pK* values were estimated by two principally different methods:

(i) direct observation of the change in fluorescence spectrum with pH, and (ii) from observed

absorption and emission spectra of the "pure" protolytic forms using the Forster cycle.

Additional information on the dynamics of the system in weakly acid and weakly basic media

was obtained by studying the change in emission spectrum with buffer concentration, and also

by measuring fluorescence lifetimes (following pulse excitation).

1. CHANGES IN EMISSION SPECTRA WITH PH

In the acid region, the change in emission spectrum occurs at a slightly lower pH than

the change in absorption spectrum, indicating that the exciting form of the neutral dye is slightly stronger as a base than the ground state molecule (pK^ = -1, pj<* = 0). In strong

sulphuric acid medium, there is another change in absorption spectrum (pj< 0 = -8) without any

corresponding change in the emission spectrum. Both absorption and emission spectra exhibit

marked red shifts when going to stronger acid. In basic medium, there is a change in the

absorption spectrum corresponding to pj<2 =12.

2. THE FORSTER CYCLE

If the entropy change on protonation 1s assumed to the the same in the excited state as

in the ground state e_ aS°* = aS°), the difference between the two states in standard free

energy change on protonation (aG°* - A(5 0

) will be the same as the difference between the two 'states 1n the enthalpy change on protonation (aH°* - A^0

). If the latter can be determined

199
from the observed absorption and emission spectra, it is possible to estimate the difference

in pj< between the two proto lysis forms (cf Fig. 3):

pK* - pK = (he A J In 10) (v" - v') =21.0 (v" - v^/ym"1

300 400 500

Figure 1. Absorption spectra (broken lines; Bausch & Lomb Spectronic

505) and emission spectra (full lines; Aminco Bowman, uncorrected).

(1) Strong sulphuric acid; (2) Weakly acid solution; (3) Basic

solution.

where h c v' and he v" represent the energy differences between the first excited singlet

state and the ground state (the energy of the 0-0 band) in the two protolysis forms. In the

ideal case of mirror image symmetry between absorption and emission spectra, the energy of

the 0-0 transition is quite well defined. The best thing that can be done in the lumichrome

case is to take the mean of the wave number of the absorption maximum, va

, and the emission

maximum, ve

. This leads to the prediction pj(* = pK ± 0.0 0 and pK£ =

pJ<2 - 6.7 « 5, in

rather good agreement with the values obtained by the previous method.

3. CHANGES IN EMISSION SPECTRA WITH BUFFER CONCENTRATION

In "neutral" solutions (5 <_ pH < 9), the shape of the emission spectrum (cf Fig. 4) is strongly dependent on the buffer concentration [B]. At low [B], the emission peak is at 470

nm (as in more strongly acid solution); at high [B], the peak is at 520 nm (as in alkaline

solution). The peak ratio, p = F (520)/F(470) , varies from u . = 0.43 to u = 5.5. — — mm max

200
" H3 Lc

+

1 1

Ht Lc HLc" . © ©

©

©

- © .

1

pH

1

Figure 2. Summary of absorption and fluorescence data for lumi chrome, taken

from curves like those shown in Fig. 1. The ordinate gives the absorption

for (uncorrected) emission maxima. Abscissa vlaues below 0 refer to the

Hammet function.

0 ©

ah"/l

HLc

(hcvri ©

Figure 3. The Forster cycle. I = Avogadro's constant.

201
F(M

-i 1 r

-

\\ /

®

\ \ \ V/V8 V\ \ \ VOO® \\\ vV/v®

j i 1_ A/nm

Figure 4. Fluorescence spectra of weakly acidic 1 umichrome solutions pH = 5.5,

with different concentrations of acetate: 1: 4.50M, 2: 2.25M, 3: 1.12M,

4: 0.56M, 5: 0.02M.

This is interpreted as a competition between the direct fluorescence from H 2 Lc* and the

establishment of a new protolytic equilibrium, according to "Scheme I":

) f

hc\£

H2 LC:

H,Lc

©

A

hcvl

A

- HA

(hcv£)

HLc"

HLc

- J

©

In the region in question, [0H~] is low and reaction (3) is dependent on a diffusional

encounter between H 2 Lc* and B (the base component of the buffer system).

202
One would then expect proportionality between the buffer concentration [B] and the

ratio of transformed fluorescence to direct fluorescence, r\+/rj, which in turn can be shown

to be proportional to the experimental quantity

A - {v '

VJmin ) (vmax " v)

~ l

This prediction is in excellent agreement with experiment, as shown in Fig. 5. From the

slope K of the line in Fig. 5, together with proper absorption and emission parameters for

the two protolytic forms, it is possible to evaluate t

1 J< 3

. At pH « 8, Jk 3 appears to be

slightly lower than kj iffJ at pH « 6 still a factor of 2 lower.

Figure 5. The parameter A (see text) vs. concentration of basic

component of buffer system. • acetate, pH = 5.9; o phosphate,

pH = 8.0.

203
PHOTOCHEMISTRY OF PHEOPHYTINS AND PORPHYRINS 1

D. C. Brune, J. Fajer and S. P. Van Division of Molecular Sciences

Department of Applied Science

Brookhaven National Laboratory

Upton, NY 11973

Recent pulsed laser spectroscopic results have revealed two short lived intermediates

involved in the primary photochemistry of photosynthetic bacteria. The first of these,

which appears in less than ten picoseconds is postulated to consist of a dimeric bacteriochlorophyll cation radical and of an anion radical of a demetallated chlorophyll, bacteriopheophytin. When further chemistry is blocked, this primary charge separation is annihilated

(in nanoseconds) by recombination of the radicals to yield fluorescence and a triplet state

[1-5] 2. We describe here the in vitro photochemical generation as well as the optical and

magnetic characteristization of the anion radicals of bacteriopheophytins a and b, the

pigments found in Rhodopseudomonas spheroides and viridis, respectively. The radicals

undergo isotopic substitution at the proton positions which carry high unpaired spin densities.

The occurrence of chlorophyll-derived anion and cation radicals in photosynthesis has

logically led to attempts at mimicking the natural act with artificial porphyrin systems.

One of these is a heterogeneous system consisting of multiple monolayers of porphyrins

deposited on an aluminum electrode which is immersed in a ferro-ferricyanide aqueous solution. This photoelectrochemical cell, originally described by Wang [6] was reported to generate ^-1 V versus a platinum electrode (open circuit) on illumination of the porphyrincoated electrode. Attempts to reproduce this cell using a number of porphyrins which

exhibit a range of oxidation potentials have been unsuccessful, the photovoltages actually

observed are an order of magnitude less than previously reported, not withstanding efforts

to remove or introduce impurities or charge carriers.

A. Chlorophyll radicals

Photolysis of bacteriopheophytin (BPh) a and b in pyridine containing lOmM Na 2 S and 1 M H 2 0 yields radicals which, on the basis of their optical (Fig. 1) and electron spin resonance spectra (Fig. 2) are similar to the one electron reduction products obtained by

electrolysis in aprotic solvents [5]. The reduction can be induced by irradiation in each

of the absorption bands of the pheophytins. For BPh a (5 x 10" 5 M, room temperature) the

quantum yield at 750 nm is 5 x 10~2

.

Analysis of the esr spectra is facilitated by electron-nuclear double resonance which

reveals some of the photon interactions predicted [5] by self-consistent field molecular

orbital calculations and by selective deuteration experiments. Examples of the latter are provided in Figs. 3-5 which display the esr spectra of dueterated BPh~ a in deuterated

pyridine and water, and computer simulations of the experimental spectra using the hyperfine

splitting constants shown. If photolysis is performed in the presence of H 2 0 instead of

D 2 0, exchange of hydrogen for deuterium occurs first at the a, B, 6 methine positions (Fig.

4), and eventually the methyl groups 1 and/or 5 undergo exchange (Fig. 5). The assignments

have been verified by nuclear magnetic resonance. BPh b~ reacts similarly (Fig. 6).

x This work was performed under the auspices of the U.S. Energy Research and Development

Administration.

2 Figures in brackets indicate literature references at the end of this paper.

204

1
400 600 800 1000

X , n m

Figure 1. Optical changes, in C 5 H 5 N/H 2 0 observed on photoreduction of BPh a to

its anion radical. (Deviations from the isobestic points are due to traces

of a "phase test" impurity [5].)

205
•o

COoo

TJ • us —

s_ E c I

o oc o<o

+-> cu ra sz

+-> T>

01 4- C O ••- HJ - 4-> ooj O

u*)-o o cuc m to

i. CU

+-> Q. U

cu o

Q.-M

S- fO

171 (1) CU CL

CD a) > .c

•i- 4->

+-> (O (/> > 1- Z 3=

CU <

T3

to

OS- o

LiOCO

CNJ T3C

CU ro

S_3 in

206
o

CO Q

to Q

10 O

•I o

J=

Q_

GO

O o

CD CO • rO Hcu- 6 6

ii ii

n z o c o o o c Q Q

<M rO CD

CD

"O

-oc

(M

>1 11 fOO

•i

+-> >• 1 o

+-> O

-C

Cl

CU

n3 cz3

Q> o

<u oo

CJ>

(/) co £Z

°(— n3 c: +-> fO tAC

rO oo

-C

Q_ cri CQ CI "1— *o +-> CD +> •r— n3 1 s_ CL

cu i/> +J3 CU CU .c

T3 +J

s_0) 00 CL CUE

0) -C l/l 4-> l/l <a «4- O -Co E

-EI s- S

+Jo c

d) o

QCO

03 S-

</> 3

CU E

(1) (/) >

-i-> CU

4-i > 3

Q_

S_ E

CU o

T3 <_> o <o co -o u c

OJ ra oo oCM Q n

z.

<u in

S- o rj IT!

lZ

207



The exchange sites are those which carry high unpaired spin densities and a significant

isotope kinetic effect is observed as well. Protonated BPh~ a will substitute deuterium

approximately an order of magnitude faster than the deuterated BPh~will exchange with

hydrogen.

B. Photoelectrochemical cells

Cells patterned on the Wang design shown were tested for photo-effects using copper,

zinc, the u-oxo-iron (III) dimer of meso tetraphenyl-porphyrins , zi nc-porphi ne and-phthalocyanine. The cell consisted of 50-70 layers of porphyrin, deposited on a clean aluminum

surface in a Langmuir trough, and immersed in an aqueous solution of potassium ferri-ferrocyanide.

Al

50-70 monolayers

porphyri n

0.1 M K

3 Fe(CN)

6

K4 Fe(CN)

6

Pt

Photovoltages (open circuit) of approximately 100 mv can be induced using white light with

ultraviolet and infrared cut offs or by direct excitation of the porphyrin bands (~400 or

500 nm). Attempts to improve the photovoltages by changing the electrode surface, or by

including free base porphyrins, or a porphyrin cation radical as possible impurities into

the porphyrin coated electrode yielded no significant effects.

References

[1] Rockley, M. G. , Windsor, M. W. , Cogdell , R. J. and Parson, W. W. (1975), Proc. Nat. Acad. Sci. USA 72, 2251.

[2] Kaufman, K. J., Dutton, P. L. , Netzel , T. L. , Leigh, J. S. and Rentzepis, P. M. (1975),

Science 188, 1301.

[3] Dutton, P. L. , Kaufmann, K. J., Chance, B. and Rentzepis, P. M. , (1975) FEBS Lett. 60,

275.

[4] Thurnauer, M. C. , Katz, J. and Norris, J. R. (1975), Proc. Nat. Acad. Sci. USA 72,

3270.

[5] Fajer, J., Brune, D. C. , Davis, M. .S. , Forman, A. and Spaulding, L. D. (1975), Proc.

Nat. Sci. USA 72, 4956; (1976) Brookhaven Symp. Biol. 28, in press.

[6] Wang, J. H. (1969), Proc. Nat. Acad. Sci. USA 62, 653.

211
A KINETIC STUDY OF CH 3 0 2 AND (CH 3 ) 3 C0 2 RADICAL REACTIONS BY KINETIC FLASH SPECTROSCOPY

Michael R. Whitbeck, Jan W. Bottenheim, Stuart Z. Levine, and

Jack G. Calvert

Department of Chemistry

The Ohio State University

Columbus, Ohio

The reactions of the hydroperoxy (H0 2 ), alkyl peroxy (R0 2 ), and the acylperoxy

(RC00 2 ) radicals are believed to be essential components of the reaction mechanism which

describes photochemical smog generation. For example, these radicals are believed to be

largely responsible for the oxidation of NO to N02 in the NO -hydrocarbon-polluted atmospheres:

H02 + NO -> HO + N02 (1)

R0 2 + NO * RO + N02 (2)

RC002 + NO * RC02 + N0 2 (3)

It is postulated that HO, RO, and RC0 2 radical products of these reactions react in the

atmosphere with 0 2 , hydrocarbons, aldehydes, CO, etc., to regenerate other H02 , R02 and

RC002 radicals. In theory the NO -» N02 conversion continues to occur by way of these

chain reactions, and the increasing ratio of [N02 ]/[N0] causes the ozone to rise through

the rapid chemical interactions of the sequence (4-6):

N02 + hv -> 0( 3 P) + NO (4)

0( 3 P) + 0 2 + M * 0 3 + M (5)

0 3 + NO * 0 2 + N02 (6)

In view of the great significance attached to the reactions (1-3) by atmospheric scientists

today [l] 1

, it is disappointing to observe that the rate constant for reaction (1) has

been determined only in indirect experiments [2], while those for reactions (2) and (3)

have not been determined experimentally by any method to our knowledge; present estimates were derived from computer simulations of very complex smog chamber experiments and are of

very limited accuracy.

In theory the alkyl peroxy radicals may also be important reactants in the atmospheric

oxidation of S02

:

R02 + S02

-> RO + S03 (7)

H0 2 + S0 2 * HO + S03 (8)

1 Figures in brackets indicate literature references at the end of this paper.

212
Davis and coworkers [2a] have studied reaction (8) for which they estimated k8 = (5.2 ± 1.1)

x 10 5 £• mole 1 sec 1

. This rate constant coupled with the computer simulation of the

complex chemistry of the polluted atmosphere suggest that reaction (8) is a major removal

path for S02 in the polluted atmosphere [lb]. In view of the fact that reaction (7) is about 8 kcal/mole more exothermic than reaction (8), the rate constant for reaction (7)

could in theory be somewhat larger than k8

. If this is the case, then the significance of

reaction (7) in the atmospheric conversion of S0 2 is probable.

In this work we attempted to study the reactions of several alkyl peroxy radicals using

kinetic flash spectroscopy. We generated CH 3 0 2 and (CH 3 ) 3 C02 radicals by flash photolysis

of (CH3 ) 2 N 2 -02 and [(CH 3 ) 3 C] 2 N 2 -0

2 mixtures, respectively. Previous work of Bjellqvist and

Reitberger [3], Parkes and co-workers [4], and Burggraf and Firestone [5], £iave shown that

the alkylperoxy radicals exhibit an ultraviolet absorption band (2400-3000 A) of moderate

intensity which is convenient for kinetic studies of these radicals.

The xenon flash lamp employed in these experiments operated at 20 kJ per flash with

a half-width at half-maximum intensity of about 50 usee. The Vycor tube of the flash lamp

paralleled the quartz reaction cell (220 cm in length, 6.3 cm in internal diameter) but

I was separated fromo it by a Pyrex plate filter (0.5 cm thickness) which transmitted only

light of \ > 2900 A. A white surfaced reflector surrounded the flash lamp and cell. The

'cell housed a multiple reflection White optical system which was set for a path length of

800 cm in most of this work. Actinometric estimates of the extent of azoalkane photolysis,

land hence the number of alkyl (and R0 2 ) radicals formed per flash, were made by two independent methods: (1) measurements of the pressure increase which resulted in the

flash photolysis of the pure azoalkane (1-5 Torr); (2) measurements of the amount of

nitrogen product formed following the flash photolysis using mass spectrometric analysis.

Both methods gave essentially the same results. Since very small amounts of decomposition

of the azoalkane occurred in these experiments (less than 0.48 percent of the initial

amount), and the pressure of added oxygen was high (about 50 Torr), the temperature rise

during an experiment was no more than a few degrees. The ultraviolet and visible spectrum

of the flash products was recorded photographically using a 0.65 m Hilger spectrograph and

a small argon flash lamp which was fired with delay times varied from 0.6 to 2.0 msec

, following the flash. In kinetic runs a continuous 150 w point source xenon arc was

employed together with a 0.5 m Jarre! 1 Ash monochrometer equipped with an RCA 1P-28

photomultiplier-oscilloscope combination. Both sets of data define an absorption band

with a maximum near 2550 ft which we believe results from the R02 radicals. In most kinetic

runs absorbance at 2650 ft was followed to obtain the rate data.

The kinetics of the CH3 0 2 and (CH3 ) 3 C02 radical disappearance was found to be second

order when the appropriate azoalkane was photolyzed in mixtures with pure 0 2. The rate constants for the radical decay are in reasonable accord with those of Parkes and co- workers [4] who used a modulated beam photolysis experiment. However, this agreement may

be somewhat fortuitous in that the absorption spectrum observed by these workers and

\ attributed to the CH 3 0 2 radical is somewhat different in band position and extinction than

Iwe observe. Perhaps interferences from the H0 2 radical absorption may have been important

in the previous modulated beam photolysis studies. The build-up of CH 2 0 product and its

I time dependence observed in the previous study would insure a major source of H0 2 species.

Experiments with flash photolyzed mixtures of azoalkane, 0 2 , and S0 2 were made in an attempt to derive the constant for reaction of CH 3 0 2 with S02

. The addition of S0 2 to this

system at pressures in excess of 0.5 Torr resulted in aerosol formation following the flash,

and so the useful range of S0 2 pressure was very limited. The present data suggest that

k 7 < 2 x 106

SL- mole x sec x. We are continuing further work with the flash system and with

a steady state photochemical system with FTS infrared analysis in an attempt to further

define estimates of k2 and k 7

.

213
References

[1] For a general review of the very extensive literature and a more detailed discussion

of the current ideas concerning the mechanism of photochemical smog formation, see:

a) Demerjian, K. L. , Kerr, J. A. and Calvert, J. G. , Adv. Environ. Sci. Technol., 4,

1 (1974); b) Calvert, J. G. and McQuigg, R. D. , Int. J. Chem. Kinet. , Symp. No. 1,

113 (1975); c) Niki, H. , Daby, E. E. and Weinstock, B. , Chapter 2, in "Photochemical

Smog and Ozone Reactions," Gould, F. F. , Editor, Advances in Chemistry Series 113, Amer. Chem. Soc. , Washington, D. C. , 1972, p. 16; d) Altshuller, A. P. and Bufalini,

J. J., Environ. Sci. Technol., 5, 39 (1971).

[2] a) Payne, W. A., Stief, L. J. and Davis, D. D. , J. Amer. Chem. Soc, 95, 7614 (1973);

b) Simonaitis, R. and Heicklen, J., J. Phys. Chem., 77, 1096 (1973).

[3] Bjellqvist, B. and Reitberger, T. , INIS, 1104 (1971).

[4] Parkes, D. A., Paul, D. M. , Quinn, C. P., and Robson, R. C. , Chem. Phys. Letters, 23,

425 (1973).

[5] Burggraf, L. W. and Firestone, R. F. , J. Phys. Chem., 78, 508 (1974).

214
THE REACTION OF OH RADICALS WITH C 3 H 6 AND C 2 H 4

Ralph Overend and George Paraskevopoul os Division of Chemistry

National Research Council of Canada

Ottawa, Canada K1A 0R9

1. Introduction

There is considerable interest in the mechanism and the rates of reaction of olefins

with radicals of atmospheric interest. In the case of hydroxy! radicals with ethylene

and propylene there are few absolute rate data at atmospheric pressure; the majority of

the data [l] 2 having been obtained at low pressures. The reported rates vary by a factor of 3-5 and in the case of ethylene a systematic effect of pressure has been

identified [la]. In the present paper we report measurements of hydroxyl radical reactions

over the pressure range 20-400 torr and with various third bodies, as measured by the

technique of flash photolysis resonance absorption.

The results for ethylene show an effect due to pressure and nature of the third body

while those of propylene are pressure independent in the range of our measurements.

2. Experimental

The apparatus and technique have been described in detail elsewhere [2]. Hydroxyl

radicals were created by the Vacuum U.V (A>160 nm) photolysis of water in the presence of

the other reactant; their concentration was monitored by following the time resolved

attenuation of the OH resonance radiation (Qf or rotational lines of the (o,o) band of

the A

2 I

+^X2 n transition) produced by a microwave discharge in a low pressure Ar/H2 0 nixture. Signal averaging Was used to improve the signal to noise ratio. The concentration

Df the olefin was such that the measured pseudo-first order rates were between 4 x 10 3 and

3 x 10 4 s" 1

.

To minimize the photolysis of the olefin while still permitting the generation of

sufficient hydroxyl radicals from the water vapor, a filter 8 mm deep of NH3 at 10 torr

vas used to reduce photolysis at wavelengths longer than 190 nm.

II *

3. Results and Discussion

A. Propylene

Initial experiments with the propyl ene/OH system showed that the apparent rate

decreased with repeated flashes on the same mixture and that the decrease was larger with

n'gh («600 J) flash energy and high absorption by OH, (i.e., high hydroxyl concentration).

5as chromatographic analyses of C 3 H 6 /He mixtures corresponding to our experimental range

showed that the direct photolysis of C 3 H 6 at 600 J was approximately 5-10 percent per

flash, while with water present ~ 30 - 40 percent of C 3 H 6 was consumed per flash. All

experimental values reported were obtained in experiments of a single flash < 1 00 J and

Issued as N.R.C.C. No. 15482

9 Figures in brackets indicate the literature references at the end of this paper.

215
low H 2 0 pressure (i.e., absorption by OH of <5 percent). Under these conditions, we calculate that photolysis of C 3 H 6 is insignificant and that even at low C 3 H 6 pressures

the ratio C 3 H 6 /0H is larger than 20 and consumption of olefins should not significantly

affect our measurement. The second order rate constant kjj obtained for C 3 H 6 is,

k TT = 2.5 ± 0.5 x 10 13 cm3 mol -1

s" 1 and was not affected by the pressure or the nature of

tne third body (M = He, 50 torr to M = SF 6 , 400 torr. ) A twenty fold variation of the

ratio C 3 H 6 /0H did not produce a change in the measured rate within the experimental error;

this, in accordance with a numerical simulation [2] would indicate no influence of secondary radical reactions on the rate constant.

B. Ethylene

The conditions for minimum OH concentration described above were also used in the

experiments with C 2 H 4. The reaction was studied over a range of pressures and with He, SF 6 , CF 4 and H 2 0 as third bodies. The rate was found to depend slightly on the pressure

and the nature of the third body; the value of kjj was found to be 5.4 ± 0.5 x 10 12 and

6.5 ± 0.5 x 10 12 cm3 mol -1 s" 1

, for 50 torr of He and 400 torr of SF 6 or CF 4 respectively.

These values are higher than those found in previous studies at low pressures of He and

with low levels of H 2 0 cf. reference [la]. In a series of experiments the kjj was measured at 50 torr of He, constant C 2 H 4 concentration, and decreasing pressure of H 2 0. The results are shown in figure 1. Although the precision of our measurement decreases with

in

o

E

roE

CVJ

IO

X

1=1

t r T r i r i—i—i—

r

He 50 torr

O I x 10" 9

mol cm" 3

C2H4 lo 3xlO" 9 -

D.D. DAVIS

J l_ L J I I L J L 2 3

PRESSURE OF H20 (torr)

Figure 1. Pressure of H 2 0 (torr)

216

w
I

decreasing OH concentration, the values of kjj obtained approach those of Davis et al. [la]

with decreasing water pressure as may be seen in the figure. This could be attributed to

fthe high efficiency of H2 0 as a third body as was found to be the case in the OH + NO combination [3]. It could also be attributed to secondary reaction of OH. It was not possible to change the ratio C 2 H 4 /0H over a very large range, because the pressure of H 2 0

ihad to be kept constant. Nevertheless a variation of the ratio C 2 H 4 /0H by a factor of 4 at

several pressures of H 2 0 did not produce a change in the measured rate within the

experimental error. If we assume then, as is indicated by the above tests, that our

results are free from interference from secondary reactions, the rate must depend on the Dressure and nature of the third body and not on the OH concentration, according to the

following scheme:

3. ^ OH + C 2 H 4 ^ZTC2 H 4 0H

C 2 H 4 0H + M -C 2 H 4 0H + M

in which H 2 0, SF6 and CF 4 are very efficient third bodies while He is inefficient.

Assuming this interpretation to be correct a steady state treatment with respect to

the concentration of the adduct C 2 H 4 0H gives,

^11 ka

k

c [M]

!ind a plot of k~j against [M] _1 for H 2 0 gives

"his value is in good agreement with the measured value of 6.5 ± 0.5 x 10 12 cm3 mol" 1 s" 1

it 400 torr of SF 6 and with the value of 5.2 ± 1 x 10 12 cm3 mol- 1 s1 determined at itmospheric pressure of air by a relative method [4]; on the other hand it is double the

'alue by Palmer [5] (~ 3 x 10 12 cm3 mol -1

s -1

) from a similar treatment of the results

if Davis et al. [la].

Work on the reactions of OH with olefins is continuing with emphasis on tests for

econdary reactions.

References

-la] Davis, D. D. , Fischer, S. , Schiff, R. , Watson, R. T. , and Bolinger, W. , J. Chem.

Phys. 63, 1707 (1975).

lb] Atkinson, R. and Pitts, J. N. , ibid. 63, 3591 (1975).

lc] Pastrana, A. V. and Carr, R. W. , Jr., J. Phys. Chem. 79, 765 (1975) and

references therein.

217
[2] Overend, R. P., Paraskevopoulos , G. , and Cvetanovic, R. J., Can. J. Chem. 53, 3374

(1975).

[3] Overend, R. P., Paraskevopoulos, G. , and Black, C. , J. Chem. Phys. 64, 4149 (1976)

[4] Lloyd, A. C. , Darnall, K. R. , Winer, A. M. , and Pitts, J. N. , Jr., J. Phys. Chem.

80, 789 (1976).

[5] Palmer, H. B. , J. Chem. Phys. 64, 2699 (1976).

218
ON THE TRIPLET STATE(s) OF SULPHUR DIOXIDE

Jan W. Bottenheim, Fu Su, David L. Thorsell, Jack G. Calvert and Edward Damon

The Ohio State University

Columbus, Ohio 43210

1. Introduction

Theoretical work [I] 1 indicates that the first three triplet-states of S02 ( 3 B 2 , A2 , 3 Bi) are very close in energy and can be expected in the wavelength range of

1400-3900 A. High resolution spectroscopic work has shown perturbations in 3 B X bands

nhich give indirect evidence of the proximity of the 3 A2 and 3 B 2 states in this region,

il though the major features of the optical spectrum are undoubtedly due to the 3 B 1 - tate [2].

Phosphorescence-sensitization studies with S02 -biacetyl mixtures form other evidence

or the presence of at least one other triplet state besides the 3 B 1 -state in this

egion [3]. Moreover, photochemical reaction studies seem to imply the participation of

lore than the two optically accessible singlet states (

1 A2 and 1 B 1 ) and the 3 B 1 -state

mly [4]. Direct evidence for the presence of the other triplet states is missing as of

oday, however.

The theoretically calculated zero-pressure lifetime of the emitting 3

B 1 -state of S02 , ased on measurements of the integrated absorption spectrum indicates a reciprocal lifetime

f 79 ± 5 sec-1 [5]. The reported experimental values of this lifetime, based on phosphorscence of lifetime studies, are rather dramatically at odds with this number, ranging from

1 75 to 2000 sec

-1 [6]. If these data are correct a large part of the 3

B 1 -state molecules

Should decay in a unimolecular non-radiative fashion, which is not expected within the

resent theories of radiationl ess transitions, and a modification of this theory might be

ecessary.

We have initiated research to establish more clarity into these matters and will report

ome of the results here. Two different approaches were followed. The direct singletriplet absorption spectrum was probed with a much longer pressure-path than before in an ttempt to identify new absorption-bands that might be described to the other triplettates. The major part of this work concerns phosphorescence lifetime studies, in which

e used several different lasers to excite S02 -molecules at a range of important wavelengths.

2. Experimental

The direct singlet-triplet absorption study was performed with a 2 m long multipleass White-cell. With S0 2 pressures of 400 to 600 Torr a pressure-pathlength of 28,800

o 43,200 Torr-meter was obtained. The absorption-spectrum was probed with a 150 Watt

e-high-pressure lamp with a soft-glass filter in front to minimize output below 3200 8,

nd monitored with a 0.5 m Jarrel-Ash monochromator.

In the phosphorescence experiments the reaction cell was of the 22 SL variety (diameter

5 cm) to allow the measurement of long-living emitting species to low pressure regions

Figures in brackets indicate the literature references at the end of this paper.

219
without having major wall-deacti vation effects to correct for [7]. The cell has two side- arms with fused-on Brewster angle windows (Suprasil) for entrance and exit of the laserbeam, and a third side-arm at 90° with a 2 inch diameter Suprasi 1 -wi ndow to which as close

as possible was positioned an RCA 7265 photomul tip! ier. Between the window and the PM-tube

a Kodak Wrattan 2B filter and an Optics Industries cutoff filter nr. WG-395 were inserted to minimize scattered laserlight and fluorescence of singlet-excited S02 - molecules.

All lasers used in this study were KDP-doubled dye-lasers of moderately high power in

the uv (3 to 10 Kwatt). An echelle grating in the 100 percent mirror in the dye- laser

cavity narrowed all laserlight to a 2.5 A bandwidth. For excitation at 3631 A a Ruby laser

pumped solution of dye nr 16 (Candela) was employed, resulting in an approximate 1 Mwatt

25 nsec laserpulse at 7262 A\ All other wavelengths were obtained by fl ashl amp-excitation

of appropriate dye-solutions in a coaxial f lashlamp-dyeflow cell (Candela, model CL-66),

resulting in approximately 0.150 Mwatt, 150 nsec laser pulses.

The lasers could not be rapidly pulsed, so essentially single-shot experiments were

done. The phosphorescence signals were monitored via a Tektronix 7704 oscilloscope, and

digitized with a Biomation 610 transient digitizer for storage on paper tape. The traces

of several shots at a particular S0 2 pressure were computer averaged and then displayed

in a semi-log form on the CRT-screen of a PDP-7 computer, after which appropriate leastsquare treatment was performed to establish the decay-parameters.

3. Results and Discussion

A. Direct singlet-triplet absorption study

o Several new bands were observed in the absorption spectrum between 3800 and 4100 A. A close analysis shows that all these bands are hot bands of the a

3 B 1 -X 1 A 1 spectrum,

however. Table 1 shows the identification of these bands, whereas Table 2 compares some relative integrated absorption-intensities with calculated relative number densities of

ground state S0 2 , assuming a Boltzmann distribution only. A conclusive test was next performed, where the cell was cooled to -15 °C. A dramatic decrease of intensity of all new bands was observed.

It a

Table 1

Band-nr vcm

-1 Identification Av exp

Av -i cal c

1 24631 (0,0,0)^(1,0,0) 1145 1151

2 24732 (0,0,0>(0,2,0) 1042 1035.4

3 24989 (0,1,0)^(1,0,0) 1149 1151

1st hot band 25258 (0,0,0)^(0,1 ,0) 518 518

4 25466 (0,2,0M0,2,0) 1029 1035.4

0-0 band 25776 (0,0,0)^(0,0,0) (b) (b)

5 25991 (0,2,0X0,1,0) 503 518

(1, 1,0)^(0,2,0) 1043 1035.4

a ~

Av indicate the experimental and calculated distance to the indicated triplet band.

This band was used as reference,

to this band.

All other bands are determined from their distance

220
Table 2 (O.O.OMO.l.O) (O.O.OMO, 2,0) (O.O.OMl.O.O)

Calc 1 0.084 0.048

Exp. 1 0.101 0.056

From these experiments it follows that the integrated extinction coefficient of

transitions to other triplet-states, if present in the studied wavelength-area should

be at least a factor 50 less than to the 3 B 1 -state. It seems therefore unlikely that

such other states will be identifiable at all in absorption.

B. Phosphorescence studies

Our data-analysis was initially directed to detect similar non-exponential decaycurves as seen in fluorescence-studies by Brus and McDonald [8]. Only single-exponential

decay was observed to pressures as low as 2 m Torr. Some curvature below this pressure

oecomes noticeable, but this is readily shown to be due to residual fluorescence under

these conditions. (Only with excitation in the singlet manifold was it possible to study the phosphorescence down to these low pressures.)

Figure 1 shows a Stern-Volmer plot of the measured decay-rates of the phosphorescence

signal as function of pressure for some different wavelengths of excitation. Since all

en

io

£

•H

-P

CD <HP

O) H

Pi •HU

Eh

CM O

C/3

CD

OBu

CD

3046 I

o 3130

• 3235

a 3238

* 3631

."8 a*

J L J I L J L 20 kO 60 80

pS02^mtorr ^ 100 120

Figure 1. Stern-Volmer plot for the phosphorescence of S02

221
data points fall very well within the error limits of the least squares line, we conclude

that no matter how S02 is excited, the same phosphorescing state is monitored. This

applies equally well for excitation at 3631 A, at which wavelength strong perturbations are observed in the absorption-spectrum, tentatively ascribed to the influence of other

triplet-states, and for excitation at 3130 A at which wavelength two singlet states are populated (

X A2 and 1 B 1

) as possible precursors for the triplet-state.

The latter observation is of interest, since it indicates that the very efficient

collisional deactivation of the 1 A2 -state should lead at least in part to tripletpopulation. In fact, simple spin-orbit coupling should favor the 1 A2

- state as the

precursor to the 3 B 1 -state; and for excitations at 3235 A and 3251 A, where we estimate

that the x A2 -state is initially populated to the extent of 24 and 58 percent at 3235 A and 3251 A respectively.

All these results point to the same conclusion that only one triple state is seen in

phosphorescence. Although cascading from another triplet-state has been suggested [10]

is not excluded, it is unlikely in view of the fact that our conclusion holds to pressures

down to at least 10 m Torr, where even complete vibrational relaxation within the 3 B X - state is not complete [11].

Our phosphorescence data permit a new estimate of the zero-pressure lifetime of the

3 B 1 -state. It has been suggested that the much higher experimentally determined reciprocal

zero pressure lifetimes are in error due to too long extrapolation to zero pressure [6].

Our data based on several excitation-wavelengths in the pressure-region of 10 to 100 m Torr, see figure 1, all tend to indicate a value of 130 to 150 sec- 1

. Taking the

inherent experimental errors in our experiments and in the determination of the integrated

absorption-coefficient into account, we feel this value is the same order of magnitude

and therefore suggest with some caution that the true zero pressure quantum yield of

phosphorescence of the 3 B 1 -state may be near unity in agreement with the existing

theory.

Many stimulating discussions with Professor J. D. Meadors during the construction of

the different lasers is gratefully acknowledged. Professor C. Weldon Mathews, Dr. K. J. Chung, Mrs. R. Damon and B. Hessler made valuable contributions to parts of this work,

and computer time was contributed by the ElectroScience Laboratory of The Ohio State

University.

References

[1] Hayes, T. F. and Pfeiffer, G. V., J. Am. Chem. Soc. , 90, 4773 (1968); Hillier,

I. H. and Saunders, V. R. , Molec. Phys. 22, 193 (1971); Chung, K. J., Ph.D Thesis,

Ohio State University, 1974.

[2] Brand, J. C. D. , Jones, V. T. , and diLauro, C. , J. Mol. Spectr. , 45, 404 (1973).

[3] Fatta, A. M. , Mathias, E. , Heicklen, J., Stockburger, III, L. , and Braslavski, S.

,

J. Photochem. , 2, 119 (1973/74).

[4] cf. ref. 3; also Chung, K. J., Calvert, J. G. , and Bottenheim, J. W. , Int. J. Chem. Kinet. , 7, 161 (1975).

[5] Strickler, S. J., Vikesland, J. P., and Bier, H. D. , J. Chem. Phys., 60, 664 (1974).

[6] Briggs, J. P., Caton, R. B. , and Smith, M. J., Can. J. Chem., 53, 2133 (1975).

222
7] Sackett, P. B. and Yardley, J. T. , J. Chem. Phys. , 57, 152 (1972).

8] Brus, L. E. and McDonalds, J. R. , J. Chem. Phys., 61, 97 (1974).

9] Mettee, H. D. , J. Chem. Phys., 49, 1784 (1968).

10] James, F. C. , Kerr, J. A., and Simons, J. P., Chem. Phys. Lett., 25, 431 (1974).

I 11] Canton, R. B. and Gangadharan, A. R. , Can. J. Chem., 52, 2389 (1974).

223
THE 502(36!) PHOTOSENSITIZED ISOMERIZATION OF CIS AND

TRANS-1 ,2-DICHL0R0ETHYLENE

Fred B. Wampler

University of California

Los Alamos Scientific Laboratory

Los Alamos, NM 87545

Jan W. Bottenheim

Chemistry Department

Ohio State University

Columbus, Ohio

Recently, several investigations of irradiated sulfur dioxide and 2-butene systems

have been reported by Cox [I] 1

, Penzhorn and Glisten [2], Penzhorn and Filby [3], Demerjian,

Calvert, and Thorsell [4], and Demerjian and Calvert [5]. These studies indicate that the

chemistry is consistent with the S0 2 photosensitized isomerization of one isomer of 2- butene to the other isomer. ^Demerjian, Calvert, and Thorsell [4] populated Q the S0 2 ( 3 B 1 ) state directly from the S02 (X, 1 A 1 ) ground state using broad band 3500-4100 A excitation

radiation and their results indicated that the S0 2 ( 3 B 1

) state can photosensitize the

isomerization of the 2-butenes. The other studies, references [1-3,5], were consistent

with the role of S0 2 ( 3 B 1

) molecules having the capability to cause isomerization, but their

initial excitation routes did involve population of excited electronic states other than

S0 2 ( 3 B 1

) and consequently their results regarding the role of S0 2 ( 3 B 1

) molecules cannot be as unequivocal as those of Demerjian, Calvert, and Thorsell [4].

The study of S0 2 -olefinic systems other than 2-butene certainly seemed worthwhile to try to better characterize the reactions of excited S0 2 molecules with olefins. It seemed

desirable to restrict the excited populated state to S0 2 ( 3 B],) and to investigate its photochemistry with cis and trans-1 ,2-dichloroethylene. These isomers do permit the olefinic

collision partner to be quite chemically different from 2-butene and thus allow one to see

if the Demerjian, Calvert, and Thorsell mechanism can be extended to other olefins.

Lifetime measurements of S0 2 ( 3 B 1

) molecules in the presence of cis and trans-1, 2- dichloroethyl ene were made using time resolved laser induced phosphorescence. A modified

Candella flashlamp pumped dye laser (Rhodamine B) was Juned to 6260 A, passed through a KDP crystal to double the frequency to yield 3130 ± 1 A, and the filtered 3130 A laser

pulse (10 kwatt peak power, 15 nsec full width at half intensity) entered at the Brewster

angle to the 23 I. reaction vessel which had been designed to allow for proper viewing of

emitted light at right angles to the laser beam. The sweep time base on the oscilloscope

was adjusted to allow for the rapid decay of the fluorescence from excited singlet sulfur

dioxide molecules, and the phosphorescence decay could be distinctly separated with conj fidence from the fluorescence. These decay curves permitted a direct evaluation of the

quenching constants at 22 °C for the removal of S0 2 ( 3 B 1

) molecules by the cis and trans isomers.

Quantum yields for the S0 2 photosensitized isomerization of cis and trans-1 ,2-dichlorc

ethylene were measured. The light source was a 1000 watt Hg-Xe lamp properly positioned ir an Oriel Model 6140 housing. An Oriel Model 6128 Universal Lamp Power Supply was used in conjunction with the lamp. A focusing sleeve, to provide proper illumination, was positioned between the lamp and the entrance slit of a Schoeffel 1/4 meter grating

l Figures in brackets indicate literature references at the end of this paper.

224
ongchromator. A grating blazed at 3000 A and having a reciprocal linear dispersign of

3 A/mm slit width was employed. The iponochromator 1 s output was centered at 3712 A and had

full width at half intensity of 150 A. Relative light intensity measurements were made

ith a 7020 Oriel gadiometer Unit utilizing a photodiode which had a spectral sensitivity

aximum near 3400 A. Absolute intensity measurements were made using either potassium

ierrioxalate actinometry or by using a Scientech Model 36-001 disc calorimeter in con- jnction with a Scientech Model 362 power energy meter. Agreement between the two methods

as found to be ±2 percent, and because of its simplicity of use most absolute intensity

sasurements were made using the disc calorimeter. Product analysis was by gas chromatoaphy.

A kinetic treatment of the initial quantum yield data was consistent with the formation

F a polarized charge- transfer intermediate whenever S0 2 ( 3 B 1

) molecules and one of the 1,2-

ichloroethylene isomers collide which ultimately decays unimol ecularly to the cis isomer

ith a probability of 0.70 ± 0.26 and to the trans isomer with a 0.37 ± 0.16 probability,

lenching rate constants for removal of S0 2 ( 3 B 1

) molecules by cis and trans-1 ,2-dichloro-

:hylene have been estimated from quantum yield data and from laser exc*ted phosphorescence

fetimes. Estimates of the quenching rate constant (units of i. mole^sec -1

) for the cis

i.omer are: (1.63 ± 0.71) x 10 10

, quantum yield data, and (2.44 ± 0.11) x 10 10

, lifetime

ita; and for the trans isomer: (2.59 ± 0.09) x 10 10 , lifetime data, and (2.35 ± 0.89) x 10 10

; nantum yield data. An experimentally determined photostationary composition, [cis-C 2 Cl 2 H 2 ]/ :rans-C 2 Cl 2 H 2

] = 1.8 ± 0.1, was in good agreement with a value of 2.00 ± 1.15 which was edicted from rate constants derived in this study.

The mechanism choice, to be discussed, required that 1/$ be a linear function in the

0 2 ]/[C 2 Cl 2 H 2

] ratio and figure 1 depicts a plot to test this relationship. Representative

0

1 IOO

1 200 300 400

' S0

2 /

;c

2

ci

2

h

2 ;

Figure 1. Plot of 1/$ versus the [S0 2 ]/[C 2 C1 2 H 2 ] ratio at 22 °C and 3712 A.

225
from the time resolved phosphorescence experiments are depicted in fig

Figure 2. Plot of the reciprocal lifetimes of 502(38!) molecules at 22

and in the presence of cis-C 2 Cl 2 H 2 versus P cn . S0 2

References

[1] Cox, R. A., J. Photochem., 2, 1 (1973-4).

[2] Penzhorn, R. D. and Gusten, H., Z. Naturforsch. , 27a, 1401 (1972).

[3] Penzhorn, R. D. and Filby, W. G. , J. Photochem., 4, 91 (1975).

[4]

829

e (1974)

K ' L " Calvert ' J - G " and Thorsell, D. L. , Int. J. Chem. Kinet.

[5] Demerjian, K. L. and Calvert, J. G., Int. J. Chem. Kinet., 7, 45 (1974).

226
GAS PHASE PHOTOCHEMICAL SYNTHESIS OF PEROXYACYL

NITRATES VIA CHLORINE-ALDEHYDE REACTION

Bruce W. Gay, Jr. , Richard C. Noonan,

Joseph J. Bufalini, Philip L. Hanst

Environmental Protection Agency

Environmental Sciences Research Laboratory

Research Triangle Park, North Carolina 27711

Reaction mechanisms of photochemical smog involve the radicals HO, H0 2 , R, RO, R0 2

. se radicals are primarily responsible for hydrocarbon disappearance and serve as chain

tiators for propagating reactions. The reaction steps in a system containing oxides of

rogen and hydrocarbons include:

N0 2 + hv * NO + 0 0+02 +M^03 +M

0 3 + NO » N02 + 0 2 Olefin + 0 3

-> R02 + aldehyde

RH + OH -» R + H 2 0

R + 0 2 » R0 2

NO + R0 2

-» N0 2 + RO

N0 2 + R -> Products (including PAN)

chain terminating step involves the formation of stable compounds. Peroxyacetyl nitrate

N) is one such compound formed from the reaction of nitrogen dioxide with peroxyacetyl

ical

.

PAN, a strong oxidizing compound, is one of the principal nitrogen-containing products

photochemical smog; it and its homologs have been shown to be strong eye irritants and

cause severe plant damage.

The infrared absorption bands of PAN were first observed in laboratory irradiations of

0 exhaust by Stephens et al. in 1956 [l] 1

, and shown present in the Los Angeles atmosphere

1957 [2]. It was first called "Compound X" because of the unidentified IR absorption

d in the spectra of irradiated auto exhaust. In 1961 the formula of "Compound X" was ablished as that of peroxyacetyl nitrate.

A simple preparation of PAN-type compounds that produce high yields in a short time was

ded to prepare samples for research, testing and calibration of PAN detection instruments,

r methods have been employed to prepare PAN-type compounds. These are:

1. Photolysis of nitrogen dioxide in the presence of an olefin.

2. Photolysis of di-acyl compounds and nitrogen dioxide.

3. Photolysis of low concentrations of alkyl nitrite in oxygen.

4. The dark reaction of an aldehyde with nitrogen dioxide and ozone.

igures in brackets indicate the literature references at the end of this paper.

227
A new PAN synthesis suggested itself during a study of reactions of chlorine atoms wit

organic material. Chlorine atoms are highly selective in the abstraction of hydrogen atoms The weakest bonded hydrogen is preferentially abstracted to form HC1. The chlorine atom

abstraction of an aldehydic hydrogen from an aldehyde yields the acyl-type free radicals

that are precursors of PAN-type compounds. In the presence of oxygen and nitrogen dioxide

PAN-type compounds are formed. Any of the PAN homologs can be produced from its parent

aldehyde cleanly and at very high yields. In the case of acetaldehyde the reactions are:

CI + Uv -> 2C1

CI + CH3CHO * HC1 + CH3C6

CH3CC1 + 0 2

-> CH 3 C0(02 )-

CH 3 C0(0 2 )- + N02

-> CH 3 C0(02 )N0 2 (PAN)

The reactions were carried out in a 690 liter glass photochemical reaction cell surro with forty-eight 40 watt black lamps in which molecular chlorine photodi ssociated with a half-life of about 2 minutes. A rapid scan Fourier Transform Infrared Spectrometer was us to analyze reactants and products with an absorption path length of 500 meters.

The formation of PAN is shown in figure 1. The conversion of aldehyde is essentially

complete after twenty minutes. Only traces of such side products as CO, HC00H, HN03 and

CH2 0 were observed.

Figure 1. Spectrum at 500-M path length of peroxyacetyl nitrate formed from

the irradiation of 4 ppm acetaldehyde, 2 ppm N0 2 , 2 ppm Cl 2 in 760 torr air.

When 4 ppm acetaldehyde, and 1.6 ppm N02 were photolyzed in the absence of chlorine,

PAN formed very slowly, only 0.15 ppm PAN formed after 120 minutes of irradiation. With 1|J| addition of 0.8 ppm chlorine to the system, PAN increased from 0.15 to 1.55 ppm after foui 1 minutes of irradiation.

Figure 2 shows the peroxypropionyl nitrate formed from propionaldehyde, and figure 3 II shows the peroxybenzoyl nitrate formed from benzal dehyde.

Peroxyformyl nitrate, the simplest homolog of the series, is apparently formed in th' l irradiation of formaldehyde, N02 and chlorine but is very unstable.

The chlorine-aldehyde-N0 2 method of PAN preparation appears to be general in its

application. High yields of PAN are produced relatively free of organic side products and I gives further confirmation to the generally accepted molecular structure of peroxyacyl , j nitrates.

228
Figure 2. Spectrum at 500-M path length of peroxypropionyl nitrate formed from

the irradiation of 3 ppm propional dehyde , 3.1 ppm N0 2 , 1.6 ppm Cl 2 in 760

torr air.

Figure 3. Spectrum at 500-M path length of peroxybenzoyl nitrate formed from

the irradiation of 3 ppm benzaldehyde, 3.4 ppm N02 , 1.8 ppm C1 2 in 760

torr air.

References

1] Stephens, E. R. , Hanst, P. L. , Doerr, R. C. , and Scott, W. E. , Ind. Eng. Chem. , 48, 1998

(1956).

2] Scott, W. E. , Stephens, E. R. , Hanst, P. L. , and Doerr, R. C. , Proc. Am. Petrol. Inst. (Ill) 171 (1957).

229
CHEMICAL CONTROL OF PHOTOCHEMICAL SMOG

Julian Heicklen

Department of Chemistry and Center for Air Environment Studies

The Pennsylvania State University

University Park, PA 16802

We are testing diethyl hydroxy 1 amine (DEHA) as a possible additive to urban atmospheres

for the purpose of preventing photochemical smog. Photochemical smog is generally associated with the atmospheric condition resulting from the chemical reactions of hydrocarbons

and the oxides of nitrogen in the atmosphere under the influence of sunlight. A principal source of the hydrocarbons and oxides of nitrogen is from automobile exhausts.

The automobile exhaust contains unburned and partially oxidized hydrocarbons, as well

as nitric oxide, NO. Relatively little nitrogen dioxide, N0 2 , is emitted directly from

automobiles. It, as well as 0 3 and other oxidants, are produced in the atmosphere and are secondary pollutants.

The initially emitted gases are non-toxic at their concentrations in the atmosphere.

On the other hand, 0 3 and other oxidants are a danger in the atmosphere.

The photochemical cycle which causes the conversion of NO to N02 is now generally

accepted to be of a free radical nature involving the HO radical and can be summarized as

HO + HC -> R(+ H 2 0)

R + 0 2 » R02

R02 + NO -* R0 + N02

R0(+ 0 2 ) » H02 or HO + aldehydes or ketones

H0 2 + NO -» HO + N0 2

where HC represents the hydrocarbons and R represents the free radical. The HO radical,

which must be produced initially as the result of photolysis of some compound in the atmosph

(presumably aldehydes) is regenerated in the chain and can catalyze the oxidation of many NC molecules.

If the conversion of NO to N02 is effected through a long chain mechanism involving HO radicals, then this conversion could be inhibited by molecules which remove HO radicals in such a manner that the chain is not regenerated. Such molecules would most likely contain

an easily abstractable hydrogen atom. However, the radical that is produced (either initial) or ultimately) when this hydrogen atom is removed must not retain a hydrogen atom on the ex- carbon atom, so that the chain will not be regenerated.

In our laboratory several such free radical scavengers have been tested, and they all inhibit the conversion of NO to N0 2 in irradiated synthetic HC-N0-02 mixtures. As an inhibitor for possible use in urban atmospheres, we have chosen di ethyl hydroxyl amine (DEHA):

230
N-O-H

C 2 H 5 / (DEHA)

for two reasons:

1. It is one of the most effective inhibitors found, only surpassed by N-methylani 1 ine,

of the inhibitors tested.

2. It is a simple non-aromatic molecule composed only of C, N, H, and 0 atoms, and thus it

and its reaction products are more likely to be non-toxic than more complex molecules

at the required atmospheric concentrations.

Studies in our laboratory have shown that with C 2 H 4 as the hydrocarbon and DEHA present

at 25 percent of the C 2 H 4 levels, the removal rate of C 2 H 4 is reduced by a factor of 5 and

i the rate of conversion of NO to N0 2 is reduced by a factor of 20. There are two effects

operating. One of these is the scavenging of free radicals by DEHA to inhibit the photochemical oxidation processes. The second effect is that only 03 produced reacts rapidly

with DEHA (as shown in separate experiments) to produce CH3 CH0, C 2 H 5 0H, and C 2 H 5 N0 2

. Therefore any N02 that is produced photodissociates to convert it back to NO, and the oxygen

atom produced adds 0 2 to produce 03 which is removed by DEHA. As a result photochemical

smog formation not only is delayed but is eliminated as long as DEHA is present.

The C 2 H 5 N02 and C 2 H 5 0N02 are not produced in the initial inhibiting reaction but are

formed only when the DEHA is nearly exhausted from our reaction mixtures and N0 2 and 0 3 are

produced. The C 2 H 5 0N02 and C 2 H 5 N0 2 come from the oxidation of DEHA in the presence of N02 and 0 3 , respectively. Under conditions that would prevail in the atmosphere with DEHA

present, no N02 or 0 3 would be produced and consequently little C 2 H 5 0N0 2 or C 2 H 5 N02 should

be formed. The overall effect of adding DEHA to the atmosphere would be to retard hydrocarbon oxidation and to convert some of the NO to N 2 0. N 2 0 is already present at 0.25 ppm

in the air. It diffuses to the stratosphere where 98 percent of it becomes N2

.

The results appear to be the same in the presence or absence of H 2 0 vapor. Changing

the initial concentrations of reactants by a factor of 15 also appears to have little if any

effect, though all of our mixtures were of the same relative composition, i.e. [olefin]/

[N0]/[DEHA] = 4/2/1. If these results can be extrapolated to urban atmospheres then we can expect that 30-50 ppb of DEHA will have a sufficient retarding effect on the oxidation of NO

to N02 (i.e. about a factor of 3 or more) to effectively prevent photochemical smog formation (i.e. keep oxidant levels below 0.08 ppm). Presumably DEHA would be needed for 6-8

lours a day for 100-200 days/year in Los Angeles and 50-100 days/year in another 10 U. S. pities.

The advantages of adding DEHA, or another free radical scavenger, to urban atmospheres

, jrather than using automobile control devices are numerous:

The chemical control method controls stationary as well as mobile source emissions.

The chemical control method is projected to cost about $200 million annually, which is

about a factor of 100 cheaper than automobile control devices.

The chemical control method can provide the immediate elimination of photochemical

smog, whereas it will take at least 10 years to equip all cars with emission control

devices.

No government regulation of the individual is involved with chemical control, thus

minimizing government interference into the lives of the individual citizen.

231
5. The chemical control method will not introduce H 2 S04 into the air, unlike the projected

catalytic control devices which will introduce significant and harmful amounts of H 2 S0 4 into the atmosphere.

6. In fact not only does chemical control not introduce H 2 S04 into the air, but it should

reduce H 2 S04 concentrations because it will inhibit and free radical oxidation of S02. Furthermore DEHA is a mild base and has the potential for neutralizing H 2 S04

.

One obvious problem that must be faced is how DEHA is to be dispersed into the air.

Since the reaction times in the air are of the order of several hours, mixing does not have

to be rapid. DEHA could be introduced by evaporation (vapor pressure is 3-4 Torr at 25°C)

or spraying from moving or stationery sources on the freeways.

DEHA is a colorless liquid and its vapor does not absorb radiation above 2700A,

it is not photochemical ly reactive (no radiation < 2900A reaches the troposphere).

so that

The odor threshold of DEHA is 0.5 ppm, > 10 times the level that would be used. Thus

it will cause no annoyance to the general population. It has a faint odor at 1.4 ppm and a moderate odor at 7.5 ppm, so that the odor is a built-in safety device in case high concentral

develop in some region.

Detailed toxicological studies on rats have been conducted in our laboratory. The

animals were exposed for over 1700 hours to 9 ± 1 ppm DEHA, 10 ± 1 ppm C 2 H 5 N0 2 , and the

/°

vapor of (C 2 H 5 ) 2 N0S — H (the adduct of DEHA and S0 2 ). Animals have been periodically

0 sacrificed and subjected to hematological and blood chemistry evaluation and complete post

mortem examinations. Only four deviations have occurred between the control and test animals.

1. After 855 hours of exposure one male rat was found to have a malignant tumor in the

skin on a rear leg. No other tumors have appeared.

2. After 1227 hours exposure to the two exposed and one of two control females that were sacrificed had fluid in the uterus. Subsequent matings of 9 other exposed females with

3 exposed males (3 females per male) resulted in normal births with normal offspring.

3. Animals sacrificed after 1227 hours of exposure showed chronic tracheitis and degenerati

thyroid lesions. The degenerative thyroid leisons were not present in the animals

sacrificed after 1688 hours of exposure.

4. Mutagenic tests are in progress. Some activity has been indicated in the urine of

exposed animals. At the present time this problem appears serious.

232
PHOTOCHEMICALLY INDUCED FREE RADICAL REACTIONS

IN NITROGEN DIOXIDE-ACETALDEHYDE MIXTURES 1

E. R. Allen

Atmospheric Sciences Research Center

State University of New York at Albany

Albany, New York 12222

Abstract

Photochemical studies have been made of the room temperature rates of formation of products and decay of reactants during irradiation at 366 nm of binary

mixtures of nitrogen dioxide and acetaldehyde. Preliminary results from computerized numerical modelling, incorporating currently accepted kinetic parameters

of the elementary processes occurring in these systems, has enabled a viable

overall mechanism to be established. In addition, this work provides reliable

information on less well known rate constants for hydrocarbon radical -nitrogen

oxide reactions. Knowledge of the latter is of importance in chemical models

describing the propagation of photochemical air pollution and in estimating urban

concentrations of potentially reactive or environmentally hazardous nitrogeneous

products.

1. Introduction

Static photolysis studies of nitrogen oxide-hydrocarbon mixtures were undertaken at the

< CAR laboratories to complement and augment parallel investigations of related atomic oxygenlydrocarbon reactions utilizing dynamic discharge flow techniques [l] 2

. The latter studies

provided kinetic data on the initial (primary thermal process) 0-atom attack on hydrocarbons,

whereas the former investigations were employed to indicate the mode of 0-atom attack on the

hydrocarbons and allowed us to follow the subsequent reactions (secondary thermal processes)

involving product free radicals and the various molecular species present. In addition, the

;tatic photochemical experiments were designed to provide information concerning relative and

ibsolute rate coefficients for the simultaneous and consecutive secondary processes which,

lue to their competitive nature, are not easily isolated for study by other techniques. The

iasic system employed, which consists of a photochemical reactor used in conjunction with

ieriodic sampling and gas chromatographic analysis of irradiated mixtures, has been described

Hsewhere [1-4]. Since these early reports the system has been substantially upgraded and

'efined to the extent that additional details, concerning modifications and improvements

incorporated more recently, warrant inclusion here.

Early work consisted mainly of preliminary investigations designed to: a) test the

feasibility of using gas chromatographic analysis to follow the course of reaction during

hotolysis, b) isolate and identify the products of reaction, c) calibrate the analytical

ystem for known products and reactants, and d) obtain relative rate coefficients of elemenjiary processes from initial rates of product formation. The latter studies assumed that the

teady state approximation method applied for the experimental conditions selected and that

nitial production curves were linear. A similar experimental approach has been used by

"fork performed at the National Center for Atmospheric Research (NCAR), Boulder, Colorado.

SCAR is sponsored by the National Science Foundation.

Figures in brackets indicate literature references at the end of this paper.

233
other investigators [5-7] to study reactions in the nitrogen oxide-acetaldehyde system.

However, these investigators examined different aspects of initial 0-atom attack and endproduct formation and obtained different results to those presented here due to dissimilar

experimental procedures and priorities. Here we are mainly concerned with defining the

overall mechanism of reaction and assigning rate constants to the elementary processes

involved.

These studies were part of a laboratory program to quantitatively identify selected

chemical transformations occurring in polluted air and to provide kinetic data on elementary

processes suitable for incorporation into atmospheric chemical models, which are being used

to describe the urban and industrial atmospheric environment.

2. Experimental

Experiments were conducted in a conventional high vacuum, mercury-free static photolysi

system. All glass stopcocks were lubricated with Dow Corning silicone grease, which proved

to be more stable to attack by nitrogen oxides and organic vapors than other commercially

available lubricants. Radiation at 366 nm was obtained from the filtered (Corning CS-7-83)

emission of a PEK110 high pressure mercury arc lamp. Intensities incident on the photochemi

cal reactor were measured using a potassium ferrioxalate actinometer [8] and transmitted

radiation monitored by recording the output of an RCA935 phototube coupled to a Pacific

Photometric Photometer. Periodically small aliquots (0.1 cm 3

) of the reaction mixture in th

cell (300 cm 3

) were extracted using a leak- free rotary valve developed from a design reporte

by Alperstein and Bradow [9]. These samples were immediately injected either on to parallel

Poropak Q columns at room temperature and -78 °C for separation and detection of CH^, C0 2 , N 2 0 and N 2 , 0 2 , NO, respectively, using helium ionization detection; or on to a 100' capillary column at 50 °C coated with $, e' oxydipropionitrile for the separation and detection o organic compounds using flame ionization. Gas pressures were monitored using a modified

Wallace and Tiernan absolute gauge and a Statham PA707 TC-5-350 transducer.

Eastman Organic Chemicals acetaldehyde and calibration samples of organic products were purified by trap to trap distillation at reduced temperatures. Matheson nitrogen dioxide

(99.5 percent min.) was oxidized by bubbling oxygen through the liquid at 0 °C, dried with

P 2 0 5 and distilled into a blackened storage reservoir. Permanent gases of high purity were obtained directly from Matheson cylinders.

3. Results and Discussion

The thirteen step mechanism outlined below is proposed to account for all of the products observed under the experimental conditions selected. The irradiated system to be

analyzed both theoretically and experimentally was a 10:1 mixture of acetaldehyde (2.7 x 10 mole a' 1

) and nitrogen dioxide (2.7 x

~\0~ k mole at 23 °C and at a total pressure of 55 torr. Although a variety of other mixtures have been studied, they will not be reported

here. Measured products of reaction over a two hour photolysis period for this particular

mixture of reactants were nitric oxide, oxygen, carbon dioxide, methyl nitrate and methyl

nitrite. Water, nitric and nttrous acid vapors, nitromethane and nitrosomethane were predi

ted products also; however, the first three were not measured and the last two were detectable but were not observed in this study. On the basis of material balance the proposed

mechanism accounts for at least 95 percent of the measured products. For example, the balance in nitrogenous materials at 80 minutes into the photolysis (reaction) is given by the

relationship;

-AlN0 2 ] * A[N0] + A[CH 3 0N0 2 ] + A[CH 3 0N0] + a[CH 3 N0 2 ] + a[CH 3 N0]

I

234
where

-A[N0 2 ] = (2.54 - 1.59) x lO" 4 mole jt 1 = 9.5 x 10" 5 mole jt 1

a[N0] = 7.17 x 10- 5 mole IT 1

A[CH 3 0N0 2 ] = 1.04 x 10- 5 mole i~ l

a[CH 3 0N0] = 0.54 x 10- 5 mole ir 1

a[CH 3 N0 2 ] = 0.28 x 10- 5 mole jt 1 (estimated)

Thus, the balance in nitrogenous materials formed accounts for 95 percent of the nitrogen

.dioxide consumed. The small inconsistency is for the most part due to the inaccuracies

'involved in measuring nitric oxide in the system.

Proposed Reaction Mechanism Initially Selected Rate Coefficients at 296

Mfi x tiw v MA j_ n 1NU2 + nv * inu + u (1

)

I = 3.0 x 10- 9 emstem sec

-1

A 1 MA ft 1 MA U + INU 2 U 2 + INU (2) k 2 = 1.2 x 10 y 1 mole -1 sec

-1

0 + CH 3 CH0 -> OH + CH 3 C0 (3) k 3 = 2.0 x 10 8

e mole" 1 sec

-1

OH + CH 3 CH0 -> H 2 0 + CH 3 C0 (4) k 4 = 9.6 x 10 9 i mole -1 sec" 1

OH + N02 + HN0 3 (5) k 5 = 2.5 x 10 9 1 mole -1 sec

-1

OH + NO $ HN0 2 (6) k 6 = 8.4 x 10 8

1 mole -1 sec

-1

CH3CO + N0 2 CH 3 + C0 2 + NO (7) k 7 = 1 .2 x 10 10 1 mole- 1 sec- 1

CH 3 C0 + CH 3 + CO (8) k 8 = 1.0 x 10 5 sec" 1

CH 3 + N0 2 + CH 3 N0 2 (9) k 9 = 4.8 x 10 9 mole -1 sec" 1

CH 3 + N02 > CH 3 0 + NO (10) k 10 = 7.8 x 10 9

1 mole" 1 sec

-1

CH 3 0 + N02

-> CH 3 0N0 2 (11) k n = 1.2 x 10 9 £ mole -1 sec" 1

CH 3 0 + NO -y CH 3 0N0 (12) k 12 = 4.0 x 10 8

1 mole-1 sec

-1

CH 3 + NO + CH 3 N0 (13) k 13 = 2.4 x 10 9 £ mole" 1 sec" 1

The "initially selected" rate coefficients, as displayed above with the proposed melanism, for the twelve thermal reactions, (2) through (13) were either taken from the

iterature as being the best available, or were estimated from previous reported relative

ite measurements, or were arbitrarily chosen to be representative of the elementary process

'escribed. This chemical model was then used to simulate the formation of products and

;moval of reactants over a two hour continuous photolysis period. A computer program was

:veloped to numerically solve the sets of differential equations describing product formaon and reactant removal using the Runge-Kutta technique at sequential 5 second intervals

>r the two hour reaction period. The program additionally provided for plotting the instanineous concentrations of reactants and products at 1 minute reaction intervals on a linear

mcentration versus reaction time graph. The initially produced "theoretical" plots, shown

; dashed lines in the figures were then compared with experimental data obtained under

lentical conditions. A successive iteration procedure was used to match the computed

rves with the experimental data as follows. Each time a rate coefficient or some other

rameter was adjusted to obtain the best fit to a set of data points the complete program

s rerun and a new set of concentration- time profiles was generated. It should be noted

235
that the solid lines in the figures are the result of making necessary adjustments to the

kinetic parameters for all reactions and are the final plots rather than plots at intermediate stages in the development of the quantitative model.

The following sequence of operations was adhered to because of the close interdependence

of the consecutive and competitive processes occurring in the proposed mechanism. The

incident light intensity at 366 nm, I 0 , was determined by external actinometry using potas

sium ferrioxalate solution and was confirmed by internal actinometry using the photolysis of

nitrogen dioxide alone. In the initial computation the starting reactant concentrations wen

[N0 2 ] = 2.7 x lO- 4 mole a1 and [CH3CHO] = 2.7 x 10- 3 mole i- 1

. In practice, however, the

initial concentration of nitrogen dioxide was 2.54 x 10 _1+ mole £ -1 (4.7 torr). To correct

for this discrepancy the program was rerun with the actual concentration of N0 2 used. As a result the computed nitrogen dioxide concentration- time profile shifted from the dashed line

shown in Figure 1 to the solid line running through the N0 2 data points (open circles in

Figure 1). Also, according to the theoretical program the rate of formation of oxygen (dash

line in Figure 1) was approximately half that found by experiment (closed circles in Figure

1). Consequently, the rate coefficient k 2 was adjusted to obtain the best fit to the [0 2 ] data points. A value for k 2 = 2.7 x 10 9 1 mole-1 sec

-1 was found to produce the computed

[0 2 ] concentration-time profile, the solid line running through the oxygen data points

(closed circles in Figure 1). This revised value corresponds closely to reported values by

Ford and Endow [10] (2.1 x 10 9£ mole -1 sec

-1

), Stuhl and Niki [11] (2.6 x 10 9 £ mole- 1 sec

and Hampson and Garvin [12] (2.6-5.7 x 10 9 £ mole -1 sec

-1

)- After the two previous adjustments to the computational program it was found that the nitric oxide data points, shown as crosses in Figure 1, coincided quite closely with the computed profile for NO (dashed line

in Figure 1) during the early stages of reaction, but diverged markedly at longer photolysis

times. However, this product is quite sensitive to the free radicals produced and, thus,

the validity of the NO data cannot be checked until all of the necessary adjustments to the rate coefficients of subsequent reactions have been made. Ultimately, when the quantitative

model was finalized the computed nitric oxide concentration-time profile obtained was the

solid line running through the NO data points (crosses in Figure 1).

The value, k 3 = 2.0 x 10 8

£ mole-1 sec

-1

, was arbitrarily selected in view of the large

variation in reported rate coefficients for this reaction by Cvetanovic [13] (1.2 x 10 81 mole -1 sec

-1

), Cadle and Powers [14] (2.7 x 10 8 £ mole" 1 sec

-1

), Cadle and Allen [1] (2.2 x 10 8 £ mole-1 sec

-1

) and Singleton and Cvetanovic [15] (2.6 x 10 8 £ mole-1 sec

-1

). The rate

coefficient, k 4 = 9.6 x 10 9 £ mole-1 sec

-1 was selected on the basis of measurements report)

by Morris, Stedman and Niki [16]. The computed production of water vapor by reaction (4) i: shown as a dashed line in Figure 2, but no measurements were made of this product. Production of nitric and nitrous acids in this system are represented by reactions (5) and (6),

respectively in the mechanism. The rate coefficient, k 5 = 2.5 x 10 9£ mole -1 sec

-1 is that

,

reported by Niki, Daby and Weinstock [17] and subsequently confirmed by Howard and Evenson

[18]; Anderson, Margitan and Kaufman [19]; and Chapman, Harris and Wayne [20] to fall in thi. range (2.5-3.1) x 10 9 £ mole-1 sec

-1

. Similarly, the rate coefficient, k 6 = 8.4 x 10 8 £ mole-1 sec

-1 was reported by Niki, Daby and Weinstock [17] and subsequently confirmed by

other investigators [18-20]. The computed production profiles for nitric and nitrous acids

in this system are shown as dashed lines in Figures 3. It should be noted that for most of

the reaction nitric acid concentrations exceed nitrous acid concentrations by at least an order of magnitude, but [HN0 3 ] is about one to two orders of magnitude less than the concen

j

trations of carbonaceous products. Thus, reactions (5) and (6) account for a very small

fraction of the nitrogen oxides present and they have a negligible effect (<1 percent) on rll overall material balances.

Reaction (7) is assumed to be the only one capable of producing carbon dioxide in the I system. Initially an arbitrary value of 1.2 x 10 10 £ mole -1 sec

-1 was assigned to k 7

. However, a comparison of the computed carbon dioxide production (dashed line in Figure 2)

with experimental carbon dioxide data points (open circles in Figure 2), clearly showed thcl

the value for k 7 was too large. As a result, a value for k 7 = 8.4 x 10 91 mole -1 sec

-1 wa; j adopted in order to obtain the best fit (solid line in Figure 2) to the experimental value;

for [C0 2 ]. This adjustment, together with other adjustments to rate coefficients for reac tions of free radicals with nitrogen oxides had the effect of reducing [NO] during the lat< stages of reaction but did not markedly affect the [N0 2 ] reaction profile. Reaction (8),

unimolecular first order decomposition of acetyl radicals, is the only reaction producing

carbon monoxide in the system. However, this reaction accounts for less than 5 percent of

the total acetyl radicals produced and under the experimental conditions carbon monoxide

production is negligible, even after a two hour photolysis period.

236
237
238
~> sj- ro oj — O

A0lx,_d3in 310!A! 'N0!lVdlN3DN00

239
Nitromethane is produced by reaction (9), the rate of coefficient k

9 = 4.8 x TO 9 i mole" 1 sec" 1 was arbitrarily selected and gave the computed [CH 3 N0 2 ] profile shown as a dashed line in Figure 4. In practice, however, nitromethane was undetectable after two hours

of irradiation, because of the low sensitivity to detection of this compound. Consequently,

a value for k 9 s 1 . 3 x 10 9 & mole" 1 sec" 1 was estimated. It should be noted that this upper

limit is at least an order of magnitude lower than that suggested by Glanzer and Troe [21]

(1.7 x 10 10 i mole" 1 sec" 1 ). Methyl nitrate is produced by the sequence of reactions (10)

and (11). The rate coefficient, k 10 = 7.8 x 10 9 a mole -1 sec" 1 was chosen so that k 9 /k 10 = 0.61, determined from previous investigations [1,3]. Thus, k 10 was adjusted to be < 2.1 x 109 i mole" 1 sec" 1 as a result of the change in k 9

. It is apparent in this case also that

the value for k 10 reported here is substantially less than that proposed for the same elementary process by Glanzer and Troe [21] (2.0 x 10 10 i mole" 1 sec" 1 ). The rate coefficient,

k n = 1.2 x 109 £ mole -1 sec" 1

, was selected on an arbitrary basis. When used in conjunctior

with the "adjusted" rate coefficients reported earlier, the theoretical curve for methyl

nitrates production was generated (shown as dashed line in Figure 4). However, the experimental data for [CH 3 0N0 2 ] (open circles in Figure 4) lie considerably below the "theoretical'

concentration profile. As a result of this discrepancy, the rate coefficient was adjusted tc k n = 6.6 x 10 8 i mole" 1 sec" 1 to obtain the best fit to the experimental data (shown as solid line in Figure 4). This "adjusted" rate coefficient is at least three times larger

than the value estimated by Demerjian, Kerr and Calvert [22] (2.0 x 10 8 i mole" 1 sec" 1 ). Methyl nitrite is produced by reaction (12), the initially selected value, k 12 = 4.0 x 10 8

j,

1 mole" 1 sec" 1 was responsible for the theoretical methyl nitrite production curve (dashed

line) in Figure 4. Experimental data points for methyl nitrite filled circles in Figure 4)

lie considerably above the theoretically computed curve, consequently the rate coefficient

was adjusted to ki 2 = 1.5 x 109

i mole" 1 sec" 1 to obtain the best fit curve (solid line in

Figure 4) to the [CH 3 0N0] experimental data. The adjusted value for k 12 reported here is an order of magnitude larger than that estimated by Demerjian, Kerr and Calvert [22] (1.0 x 10 8 £ mole" 1 sec" 1

) for reaction (12). Finally, nitrosomethane is produced by reaction (13).

The second order rate coefficient for this reaction, k 13 = 2.4 x 10 9 i mole" 1 sec" 1

, is that

measured by Basco, et al . [23] and confirmed later by Laufer and Bass [24]. Although nitrosomethane was detectable it did not reach the minimum level of detectability (10" G mole it" 1') during the two hour photolysis period of this particular nitrogen dioxide-acetaldehyde mixture.

Assuming that the rate coefficients for reactions (3), (4), (5), (6), (8) and (13) are I representative of the reactions taking place in the system under investigation, then the

following rate coefficients of 296 °K are recommended for use in future chemical modelling

f applications; with a 20 percent maximum deviation estimated from the sensitivity of curve

fitting and the accuracy of experimental data.

•I k 2 2.7 (±0.5) X 10 9 i mole -1 sec

-1

8.4 (±1.7) X 10 9 l mole" 1 sec" 1

kg 1.3 (±0.3) X 10 9 % mol e" 1 sec" 1

2.1 (±0.4) X 10 9 I mole" 1 sec" 1

kn 6.6 (±1.3) X 10 8 a mole" 1 sec" 1

k 12 1.5 (±0.3) X 10 9 a mol

e" 1 sec" 1

These values will be checked and refined further in the near future by performing similar

analyses of different nitrogen dioxide-acetaldehyde mixtures.

Acknowl edgement

The author is greatly indebted to T. R. Englert who performed the large number of

necessary experiments, and M. Sutterlin, who developed and ran the computer program.

240
ro oj —

90i*,-aa±n 3ion 'nollvhinbonoo

241
References

[1] Cadle, R. D. and Allen, E. R. , "Reactions of 0( 3 P) Atoms with Aldehydes in Photochemical Smog." In "Chemical Reactions in Urban Atmospheres," C. S. Tuesday (ed.), Elsevier

Publishing Co., pp. 63-87 (1971).

[2] Allen, E. R. and Bagley, K. W., Berichte Bunseges. Physik. Chemie., 72, 227 (1968).

[3] Allen, E. R., "Recent Studies of the Reaction of Radicals with the Oxides of Nitrogen."

Paper presented at 8th Conference on Photochemistry, Ottawa, Ontario, Canada, June

1968.

[4] Watkins, K. W., Ogren, P. J. and Allen, E. R., "The Effects of Collisional Moderator

Gases on Photochemically- Induced Reactions in Nitrogen Oxide-Propylene Mixtures."

Paper presented at 8th International Conference on Photochemistry, Edmonton, Alberta,

Canada, August 1975. fitei

lit i [5] Avery, H. E. and Cvetanovic, R. J., J. Chem. Phys., 43, 3727 (1965).

[6] Collins, B. M. and Christie, M. I., Nature, 218, 1245 (1968).

[7] Avery, H. E., Hayes, D. M. and Phillips, L., J. Phys. Chem., 73, 3498 (1969).

[8] Hatchard, C. G. and Parker, D. A., Proc. Roy. Soc. (London), A235 , 518 (1956)

[9] Alperstein, M. and Bradow, R. L., Anal. Chem., 38, 366 (1966).

[10

[11

[12

[13

[14

[15

[16

[17

[18

[19

[20

[21

[22

[23

[24

Ford, H. W. and Endow, N., J. Chem. Phys., 27, 1156, 1277 (1957).

Stuhl, F. and Niki, H., Chem. Phys. Lett., 7, 197 (1970).

Hampson, R. F. Jr. and Garvin, D., "Chemical Kinetic and Photochemical Data for Modelling Atmospheric Chemistry," NBS Tech. Note 866, U. S. Dept. of Commerce, Washington,

D. C, June 1975, p. 13-14.

Cvetanovic, R. J., Can. J. Chem., 34, 775 (1956).

Cadle, R. D. and Powers, J. W., J. Phys. Chem., 71_, 1702 (1967).

Singleton, D. L. and Cvetanovic, R. J., Private Communication, July, 1976.

Morris, E. D. Jr., Stedman, D. H. and Niki, H., J. Amer. Chem. Soc, 93, 3570 (1971).

llfjl

n offat

i

iiri

Niki, H., Daby, E. E. and Weinstock, B., "Mechanisms of Smog Reactions." Paper presented at the Symposium on Photochemical Smog, Division of Physical Chemistry, 161st

National Meeting of the American Chemical Society, Los Angeles, California, March 1971

Howard, C. J. and Evenson, K. M., J. Chem. Phys., 61_, 1943 (1974).

Anderson, J. G., Margitan, J. J. and Kaufman, F., J. Chem. Phys., 60, 3310 (1974).

Chapman, C. J., Harris, G. W. and Wayne, R. P., J. Chem. Soc. Faraday I, 71_, 610 (1975]

Glanzer, K. and Troe, J., in Reference 12, p. 44.

Demerjian, K. L., Kerr, J. A. and Calvert, J. G., Advances Environ. Sci . Technol . , 4,

(1974).

Basco, N. , James, D. G. L. and Suart, R. D., Int. J. Chem. Kinet., 2, 215 (1970).

Laufer, A. H. and Bass, A. M., in Reference 12, p. 39.

242
PHOTO-OXIDATION OF T0LUENE-N0 2 -02 -N 2 SYSTEM IN GAS PHASE

H. Akimoto, M. Hoshino, G. Inoue, M. Okuda and N. Washida

Division of Atmospheric Environment

National Institute for Environmental Studies

P. 0. Yatabe, Ibaraki 300-21, Japan

1. Introduction

Although a few studies [1-3] 1 on the photo-oxidation of aromatic hydrocarbon-N0 2 -air

/stem have been made as a model reaction of photochemical air pollution in urban atmosphere,

jps't of the studies are concerned with the so called "photochemical reactivity," and studies

fi the reaction products characteristic of aromatic hydrocarbons have rarely been carried

jt.

The purpose of this study is the elucidation of the reaction mechanism of photoxidation of the to! uene-N0 2 -0 2 -N 2 system in the concentration range of 34 ppm of toluene,

3 - 200 ppm of N0 2 in N 2 and/or 0 2 at 1 atm.

2. Experimental

The reaction chamber was a Pyrex cylinder with the dimensions of 240 mm i.d., 1660 mm

ong and 67 1 volume, and was evacuable to less than 1 x 10" 6 torr. Each end of the

jaction chamber was sealed with a Pyrex window of 20 mm thick, through which photolyzing

ight beam was transmitted into the gas sample. Light source was a 500 W xenon short arc

imp and a parallel light beam of 200 mm <$> was obtained with an elliptic mirror, a lens and

M off-axis parabolic mirror.

The reaction mixture was first sampled into a constant volume glass sampling bulb

about 700 cc) and then concentrated in a GC sampling tube. The sampling tube was a Pyrex

piral with a 2 mm i.d. and about 4 m extended length. The concentration of the reaction

ixture was achieved by evacuating the air and N0 2 while cooling the tube with ethanol - |iquid N 2 at -60 ^ - 80 °C to 200 °C. After the concentration, the sampling tube was heated

ith hot water and the sample was fed into a gas chromatograph directly. This procedure

is found to minimize thermal reaction of N02 with toluene and cresols in the sampling tube.

Product identification was performed with a gas chromatograph mass spectrometer (JE0L,

1S-D100) using a 3 m GC column of Se-30 on Shimalite W. The GC oven temperature was raised

*om 80 °C to 200 °C at a rate of 4 °C/min. Quantitative GC analysis was carried out using

, ie same column as above.

3. Results and Discussion

In the present study, the wavelength of the irradiation light is limited to about 350

i and longer. In this spectral region, the photo-oxidation of toluene-N0 2 -02 -N 2 system is

litiated by the photo-dissociation of N0 2

.

N0 2 + hv NO + 0 (1

)

igures in brackets indicate literature references at the end of this paper.

243
The radiation intensity gave a photo-dissociation rate coefficient for N02 of 1.1 x 10-3 sec

-1

.

In the reaction system of toluene (34 ppm) -N0 2 (50 % 310 ppm) -N 2 (1 atm), oxygen

atoms produced in reaction (1) react with toluene competing mainly with the reaction with

N0 2

. Products observed in the above mentioned 0 2 free system are o-cresol (other isomers

are less than a few percent of o-cresol), a-nitrotol uene and m-nitrotoluene. The relative

product yields are shown in table 1. The relative yield of a-nitrotol uene to cresol increased with the increase of the initial concentration of N0 2 , whereas that of m-nitrotoluene

decreased. The production of a-nitrotol uene strongly suggests the presence of benzyl radical and would imply that the hydrogen abstraction from the methyl group of toluene by oxygen

atom does occur, although Jones and Cvetanovic [4], Grovenstein and Mosher [5] and Gaffney

et al . , [6] did not detect a product originated from primary hydrogen abstraction under

their experimental conditions.

Table 1. Relative product yields in the photo-oxidation of toluene-N0 2 -0 2 -N 2 system as normalized to cresol plus nitrocresol.

N02 (ppm)

HN0 2 (ppm)

Atmosphere

(latm.)

C>-CH0 ^^-CH2 0N0 2 <^-CH2 N02 0-CH3 \I0 2

180 air 0.18 0.057 0.025

180 N 2 0.14 0.029

50 air 0.17 0.044 0.056

50 N 2 0.081 0.039

10 air 0.19 0.019 0.071

2 6 air 0.65 0.30

In the presence of initial 02 , for example toluene (34 ppm) -N0 2 (11 ^ 207 ppm) -air

system, benzaldehyde, benzylnitrate and nitrocresols are formed in addition to cresols and

m-nitrotoluene. The production of a-nitrotol uene is completely suppressed. The relative

yield of benzylnitrate and nitrocresols increased with the increase of the initial con- centration of N0 2 , whereas that of m-nitrotoluene decreased as in the case of 02 free system

The reaction mechanism of toluene-N0 2 -02 /N 2 system under our experimental condition is

most likely,

CH 2 0N0 2 N0 2 ^

(2)

of

CH0

244
Formation of m-nitrotoluene both in the 0 2 free and 0 2 present system would be explained

>y the reaction initiated by OH radical as follows:

ihe competition between the reaction (3) and reactions

OH + NO + M -HN02 + M (4)

OH + N0 2 + M -HNO3 + M (5)

ould explain the decrease of the relative yield of m-nitrotoluene at higher N02 concenrations. This assumption was supported by the experiment of the HN0 2 -N0 -toluene-air system,

here OH was formed in the photolysis of HN0 2

. x

HN0 2 + hv -NO + OH (6)

\$ shown in the last row of table 1, m-nitrotoluene is one of the main products in addition

0 cresols and benzal dehyde.

According to the above reaction scheme the formation ratio of (a-nitrotol uene + Snzylnitrate + benzaldehyde) to (cresols + nitrocresol s ) should be constant in the toluene0 2 -O 2 /N 2 system and give the rate constant ratio l^aAab* Figure 1 shows the ratio in the

scperiment under various initial concentrations. Thus, the branching ratio of abstraction

m addition of the reaction of oxygen atom with toluene is determined to be 0.22 ± 0.02.

bis value should be taken as the upper limit since the contribution of OH radical reaction

iy not be negligible.

1

J

References

] Altshuller, A. P. and Leach, P. W., Int. J. Air Water Poll., 8, 37 (1964).

'] Heuss, J. M. and Glasson, W. A., Environ. Sci. Tech., 2, 1109 (1968).

i] Kopczynsky, S. L., Int. J. Air Water Poll., 8, 107 (1964).

•J Jones, G. R. H. and Cvetanovic, R. J., Can. J. Chem. 39, 2444 (1961).

>] Grovenstein, E., Jr. and Mosher, A. J., J. Amer. Chem. Soc, 92, 3810 (1970).

-] Gaffney, J. S., Atkinson, R., and Pitts, J. N., J. Amer. Chem. Soc, 98, 1828 (1976).

2U5
800

Fig. 1. The formation ratio of a-nitrotoluene (a-N.T.) + benzylnitrate (B.N.) + bezaldehyde (B.A.) to cresol (Cre.) + nitrocresol (N. Cre.). N0 2 /0 2 = 2.2 x 10" 5

(x, ), 2.4 x 10- 5 (o), 3.3 x 10- 5 (o), 30.6 x 10" 5 (a) and « ( ).

246
EFFECTS OF SOLAR ENERGY DISTRIBUTION ON PHOTOCHEMICAL SMOG FORMATION

Gary Z. Whitten, James P. Kill us and Henry Hogo

Systems Applications, Incorporated

San Rafael, California

Marcia C. Dodge

Environmental Protection Agency

Research Triangle Park, North Carolina

1. Introduction

The most documented photochemical reaction leading to ozone formation is N0 2 photolysis by ultraviolet light. Thus, the intensity of ultraviolet light as monitored in urban areas is often related to or even quantified by the rate of N0 2 photolysis (Holmes

et a!., 1 973) [3] 1

. Similarly, the light source in smog chambers is often calibrated by

measuring N02 photolysis (Wu and Niki, 1975) [7,8]. The time to the N02 peak, i.e., the

time to highest N0 2 concentration produced by an irradiated mixture of hydrocarbons and

nitrogen oxides, is widely used to indicate the reactivity of the mixture (Altshuller and

'Cohen, 1963) [1].

Even though N02 photolysis is central to photochemical oxidant formation, more

information regarding the UV spectrum of the light source may be necessary to properly

estimate the oxidant severity. Indeed, Jaffe et al., (1974) [4] observed significant

differences in oxidant formation from light sources that caused similar N02 photolysis

rates but had different UV spectra. Aldehyde photolysis has recently been shown to be

important in smog chemistry (Hecht et al., 1973) [2], and the ultraviolet wavelengths

that photolyze aldehydes to form radicals are different from those that photolyze N02

. This

paper discusses the implications of this wavelength difference on the chemistry of smog

formation.

The quantum yield for N0 2 photolysis is unity at wavelengths below 400 nm, and

rapidly decreases to zero at longer wavelengths. N0 2 photolysis in smog formation occurs nainly by absorption of UV light between 300 and 400 nm (Jones and Bayes, 1973) [5].

Formaldehyde is a major fraction of the total aldehyde found in photochemical smog, but

flits photolysis reactions are not as well understood as N0 2 photolysis. Of its two photolysis reactions,

H 2 C0 + hv * H- + HC0- (1)

H 2 C0 + hv * H 2 + CO (2)

the former is much more important in smog chemistry because it produces two radicals. The

quantum yields for reactions (1) and (2) are known to vary with wavelength. The quantum

yield is nearly unity at short UV wavelengths for reaction (1) and at long UV wavelengths

for reaction (2). The quantum yields equal 0.5 for both reactions at about 312 nm. Thus,

the aldehyde photolysis reaction important in smog photochemistry occurs by absorption of

JV light near the short-wavelength edge of the solar spectrum.

jrFigures in brackets indicate literature references at the end of this paper.

247
2. Results

Personnel at the University of California at Riverside Statewide Air Pollution Research

Center (SAPRC) performed two smog chamber experiments with very similar initial concen- trations and the same light source 4-1/2 months apart (Table 1). During the time between

these two experiments the light source was found to have deteriorated by about 8.5 percent

as measured by the N0 2 photolysis rate. However, spectral distribution measurements were

not obtained for these two experiments.

chamber experiments.

Experiment 2*

1.040

1.11

0. 15

0.0

2.0

Table 1. Initial conditions of smog

Experiment 1

Propylene (ppm) 1.036

NO (ppm) 1.12

N0 2 (ppm) 0.16

HCH0 (ppb) 16.0

CH 3 CH0 (ppb) 7.0

* Experiment 2 was carried out in the same smog chamber with the same light source 4-1/2 months after Experiment 1.

The time to the N0 2 peak differed by a factor of two between these two experiments.

In current mechanistic schemes, an 8.5 percent decrease in N02 photolysis cannot explain

such a drastic difference in time to the N0 2 peak. We were interested to see whether a decrease in the intensity of short UV emitted by the light source could account for this

decrease in reactivity.

Using a chemical kinetic mechanism for propylene/NO systems developed with SAPRC

data by Systems Applications, Incorporated, we performed computer simulations in which

aldehyde photolysis rate constants were varied to obtain best fits to the experimental

data for Experiments 1 and 2. We have recently modified our mechanism to include the

formation of stable ozonides from the ozone-propylene reaction. We found that about

thirty percent ozonide and the remainder to radical formation gave improved fits to observed data and seemed consistent with the findings of Niki et al. (1976) [8]. Moreover, by cutting back on one source of radicals (the ozone-olefin reactions) the dependence on aldehyde photolysis for radicals to sustain the formation of smog has increased

markedly from that reported by Hecht et al. (1974) [2]. The values of these rate constants

that produced the best fits are shown in Table 2.

Thus, large differences in aldehyde photolysis rates can explain the major difference

in reactivity between these two experiments, assuming that our chemical kinetic mechanism

for propylene is basically correct.

Some corroboration for the changes in aldehyde rate constants shown in Table 2 is provided by recent measurements of spectra during the replacement of the light source in the SAPRC chamber. It was found that the used light source emitted less UV light at wavelengths than the new source, and that the difference in intensity was largest at the

short wavelengths.

248
Table 2. Rate constants for best fits of computer simulations

to experimental results.

Experiment 1 Experiment 2

N0 2 photolysis rate

constant (min-1

) 0.223 0.204

HCHO photolysis rate

constant* (min -1

) 4.5 x 10-4 1.4 x 10 -4

CH3 CH0 photolysis rate

rate constant! (min-1

) 3.5 x 10" 4 0.8 x 10" 4

Time to N02 peak

Observed 105 210

Calculated 110 210

* By Reaction (1), HCHO + hu -> H- + HC0-.

t By the reaction CH3 CH0 + hu -> CH 3 + HC0-.

3. Conclusions

Two smog chamber experiments at SAPRC with similar initial conditions showed a difference of a factor of two in reactivity, as measured by the time to the N0 2 peak. To determine whether this difference could be explained by a deterioration of the chamber

light source, we simulated these experiments with a computer model. Our results indicate

that a decrease in the intensity of short UV emitted by the light source could have

caused the observed difference in reactivity.

Calculations and measurements of UV in the real atmosphere show changes in intensity,

as measured by N0 2 photolysis, and in spectral distribution. Recent measurements of

erythemal radiation have been made and the erythemal radiation spectrum resembles the

absorption spectra of aldehydes (Machta et al . , 1975) [6]. Hence the technology apparently exists to easily determine the ratio of light flux in the aldehyde-absorbing region

to the flux for N02 absorption in the atmosphere. The combination of this measurement

iwith the flux for N02 absorption should provide more reliable data for relating the con- centration of photochemical oxidant to the light intensity.

References

[1] Altshullter, A. P. and Cohen, I. R. , Int. J. Air Water Pollut. , Vol. 7, p. 787,

(1963).

£2] Hecht, T. A. , Liu, M. K. , and Whitney, D. C. , Mathematical Simulations of Smog

Chamber Photochemical Experiments, EPA-650/4-74-040 , U.S. Environmental Protection

Agency, Research Triangle Park, North Carolina, (1974).

249
[3] Holmes, J. R. , O'Brien, R. J., Crabtree, J. H. , Hecht, T. A., and Seinfeld, J. H.

,

Measurement of Ultraviolet Radiation Intensity in Photochemical Smog Studies,

Environ. Sci. Tech., Vol. 7, p. 519, (1973).

[4] Jaffe, R. J., Smith, F. C. , Jr., and Last, K. W. , Study of Factors Affecting Reactions

in Environmental Chambers, EPA-640/3-74-004a , U.S. Environmental Protection Agency,

Research Triangle Park, North Carolina, (1974).

[5] Jones, I. T. N. and Bayes, K. D. , Photolysis of Nitrogen Dioxide, J. Chem. Phys.

,

Vol. 59, p. 4836, (1973).

[6] Machta, L. , Cotton, G. , Hass, W. , and Komhyr, W. , Erythemal Ultraviolet Solar

Radiation and Environmental Factors, CIAP Report (February 28, 1975).

[7] Wu, C. H. and Niki, M. , Methods for Measuring N02 Photodissociation Rate, Environ.

Sci. Tech. , Vol. 9, p. 46, (1975).

[8] Niki, H. , Maker, P., Savage, C. , and Breitenbach, L. , Fourier Transform Spectroscopic

Studies of Gas Phase Ozone-Olefin Reactions, 171st American Chemical Society National

Meeting, (1976).

250
PHOTOPHYSICS OF BOUND AND DISSOCIATIVE STATES OF SMALL MOLECULES

IN CONDENSED PHASES

L. E. Brus and V. E. Bondybey

Bell Laboratories

Murray Hill, NJ 07974

The cage effect on dissociating small molecules in rare gas solids has been investigated

via the techniques of time, wavelength, and polarization resolved fluorescence. Strongly

predissociated molecules, such as IC1 , and directly dissociating molecules, such as Cl 2 and CH3 I, have been studied. In the solid the host prevents permanent dissociation, and

the fragments recombine within a few vibrational periods into the various bound electronic

states. In the solid there is no quantized vibrational structure at excitation energies

corresponding to gas phase dissociation. This strong cage effect occurs in the impulsive

limit with respect to the bulk compressibility relaxation time of the rare gas host.

However, bound-bound spectra, involving zero phonon lines of levels very close to the

dissociation limit, imply only a weak van der Waals cage influence on the molecular potential.

This result implies only minor cage effect for processes occurring in the adiabatic limit

: with respect to host compression. Polarized excitation ("photo-selection") studies allow

one to unravel the various radiationless transition pathways in the common case where the

absorption spectrum contains several overlapping continua. The predissociated B 0 state

of IC1 has an effective double minimum shape in the solid. Excitation below the dissociation

barrier produces B (v=0) fluorescence with near unity quantum yield, while excitation

above the barrier produces principally A

3n(v=0) emission via radiationless relaxation

through the outer minimum of B. In CH3 I, it is observed that the "repulsive" 0 excited

state, which is employed in gas phase photodissociation I* 2 P-|/2 "* 2 ^3/2 lasers » 15 actually

slightly (< 2000 cm

-1

) chemically bound. In Cl 2

, the observed emitting bound state is tentatively assigned as the "forbidden" lowest multiplet component 3n2

.

Vibrational relaxation rates of high frequency (> 1000 cm" 1 ) modes in small molecules

are slow because a large number of 50 cm

-1 lattice phonons must be simultaneously produced.

An energy gap law has been predicted, such that the rate should decrease exponentially with

the quantum size. This prediction is observed to be violated in the vibrational relaxation

of OH (A 2 I) and NH (A 3

T1), where the hydrides actually relax faster than the deuterides.

The electronic spectra show these molecules to be undergoing slightly perturbed free

rotation at 4.2°K; and the relaxation data are thought to reflect the involvement of this

localized motion. When energy relaxation is slow, long range energy transfer processes

have time to develop, and may dominate the excited state population evolution. Three near

resonant vibrational energy transfer processes from ND (A 3n) to 12 C0 and 13 C0 have been

studied. The transfer quantum yield, and the non-exponential time resolved donor population,

both follow Forster (dipole-dipole) kinetics as a function of the acceptor concentration.

A strong energy gap (transfer exothermicity) law is found of the form k a exp [-AE/(28 cm

-1 )]

where AE = h" (w , - ui ). Transfer from NH (A 3

fl) to CO is anomalously fast in view of

d a the size of AE, again indicating the hydride dissipates energy into the matrix especially

.efficiently.

A number of spin and symmetry forbidden internal radiationless transitions have been

observed in the first row diatomics C 2 and C 2

. For example, vibrational relaxation in C 2 (B 2 I+) occurs by sequential intersystem crossing, to and from, the nearby 4 I+ state.

251
Similarly in C 2 excitation into A

X U produces with high quantum yield the lowest triplet

a

3n

u

. Population in v=0-3 of a

3

P. u subsequently undergoes the spin and symmetry forbidden intersystem crossing into X

1 Ig on a 10~ 5 sec time scale. An extremely strong

energy gap law prevails over the low intramolecular matrix elements in these processes.

The photodi ssociation cage effect allows one to study transient species that have

very weakly bound ground electronic states. For example, several bound-free and boundbound fluorescences have recently been observed in the diatomics XeO and XeF. The polarized excitation spectra in solid Ar allow one to unravel the symmetries and energies of

the various excited states. In XeO, both the X I and x n states correlating with 0( 1 D) + Xe have been observed in fluorescence from the X I state which correlates with 0( X S) + Xe. The ground X

1 ! state of XeF isolated in solid Ne exhibits iu = 247 cm

-1 a and uj x = 10.7

e e e cm" 1

, yielding an extrapolated Morse oscillator D = 1500 cm

-1

.

252
SPECTROSCOPY AND PHOTOCHEMISTRY OF MATRIX ISOLATED METAL HAXAFLUORIDES

William F. Coleman and Robert T. Paine

Department of Chemistry

University of New Mexico

Albuquerque, NM 87131

and

R. Burton Lewis, Robin S. McDowell,

Lewellyn H. Jones and Larned B. Asprey

Los Alamos Scientific Laboratory

University of California

Los Alamos, NM 87544

Although the uranium hexafluoride (UF 6 ) molecule has been of great interest to

lemists and physicists for more than 35 years, it is only recently that reliable spectro-

:opic data have been available. The photochemistry of UF 6 , an area of great potential

iplicability in an isotope separation program, has not been reported prior to this work.

; survey here recent work from our laboratories on the vibrational, electronic absorption,

n'ssion and excitation spectra and the ultraviolet photochemistry of low temperature thin

lm and matrix isolated UF6

.

1. Vibrational Spectra

The six fundamental frequencies of UF6 have been recently determined in dilute Ar

itrices at 14 °K [1]. In addition to the IR and Raman active modes sufficient combination

inds were observed to allow assignment of all fundamentals. Shifts of up to 6 cm

-1 from

ie most reliable gas phase data [2] were obtained in the solid and some evidence exists

>r the formation of low concentrations of UF 6 aggregates in the matrices. Strong perturbalions are observed in different matrix hosts, the general trend being that stretching

"equencies are more susceptible to the matrix than bending frequencies. Table 1 gives

ne values of v

1

-> v 6 in the Ar matrix.

Table 1

Fundamental Frequencies of Matrix-Isolated UF 6 (cm x )

(A x g stretch) 666

v 2 (E stretch) 530

v 3 (F

1(j stretch) 619. 3

"4 (F lu bend) 183. 5

L'5 (F 2 bend)

y 200

v6 (F 2u bend) 143. 2

Figures in brackets indicate literature references at the end of this paper.

253
2. Electronic Spectra

The general features of the UF 6 ultraviolet spectrum have been known for

sometime [3]. The spectrum consists of two major regions of absorption a) a weak absorptio!

envelope from 340-410 nm (the A band) and b) a strong manifold from 330 nm to higher

energies (the B band). The weak band has previously been reported to exhibit some structur

A recent study has revealed a significant amount of structure in both the A and B bands

for low temperatures matrices [4]. The A band has been assigned as arising from a forbidden

charge transfer transition to two excited electronic states, one predominately singlet in

character and the second (the lower energy state) predominately triplet in character. The

j vibronic transitions have been assigned with (singlet, electronic) = 25,270 cm" 1

, (triple

electronic) = 24,510 cm"1

, v

1 = 580 cm

-1

, ^ 3 = 480 cm

-1 and v

4 = 185 cm

-1

. The B band

region has been assigned as arising from both allowed charge-transfer transitions and a weak forbidden transition. Comparison of the observed spectra with several recent

calculations [5,6] on the electronic structure lead to the conclusion that the spin-orbit

coupling in UF 6 is intermediate and the best fit is obtained by applying spin-orbit couplinj

to the Xa calculations [5].

3. Emission Spectra

The luminescence of solid UF 6 was first discussed in detail by Sheremetev in 1957 [7] ! and has been observed several times since then. Our matrix luminescence spectra represents']

the first instance in which significant vibronic structure is present. At 14 °K in 200/1

Ar/UF 6 matrix more than 200 lines are observed in the emission spectrum. Progressions in

ve, v e ~ n v s (n = 1-16) and several other combinations are observed. Phonons, apparently

arising from the matrix gas, are observed with differing frequencies in the different

matrices. The quantum efficiency is quite temperature and concentration dependent and the

position of the 0-0 band is strongly concentration dependent. The 0-0 band is much more

intense in a UF 6 thin film supporting a structure change from matrix to thin film. The

excitation spectrum verifies the presence of two electronic states in the A band region,

the second having an internal conversion efficiency to the emitting stste of less than

unity. The internal conversion from the B band to the emitting state is a very inefficient

process and is compatible with the observed photochemistry (vide infra).

4. Photochemistry

Photolysis of matrix isolated UF 6 in dilute Ar matrices leads to significant and

reversible changes in the vibrational and emission spectra. New bands attributed to isolated UF 5 appear in the IR [1] and a weak near IR luminescence is detected. These

spectral features disappear rapidly following exclusion of the UV radiation and the spectra

of UF6 are regenerated. In more concentrated matrices (20/1 Ar/UF 6 ) the emission intensity

does not return to the initial value indicative of irreversible formation of polymeric

UF 5

. In CO matrices, which should be efficient fluorine-atom scavengers, the process is

irreversible and IR bands appear which can be assigned to FC0, (FC0) 2 and F 2 C0.

The photolysis is much more efficient for irradiation in the B band than for the A band and a wavelength dependence of the photolysis quantum yield is observed within the A band.

The discussion will focus primarily on recent results in the emission and photochemica

studies.

References

[1] Paine, R. T. , McDowell, R. S. , Asprey, L. B. , and Jones, L. H. , J. Chem. Phys. (Communication), 64,0000 (1976).

[2] McDowell, R. S. , Asprey, L. B. , and Paine, R. T. , J. Chem. Phys.,

61, 3571 (1974).

[3] Lipkin, D. , and Weissman, S. I. , "Photochemistry of Systems Containing

Uranium Compounds," University of California, Radiation Laboratory

Report A-520, 1942.

254
4] Lewis, W. B. , Asprey, L. B. , Jones, L. H. , McDowell, R. S.

,

Rabideau, S. W. , Zeltmann, A. H. , and Paine, R. T. , J. Chem. Phys.

64, 0000 (1976).

15] Boring, M. , J. Chem. Phys., (to be published).

6] Ellis, D. E. , and Koelling, D. D. , unpublished calculations.

7] Sheremetev, Optica i Spektroskopiya, 2, 99 (1957).

255
MERCURY PHOTOSENSITIZED PRODUCTION OF TRAPPED

RADICALS IN ORGANIC GLASSES AT <77 K[1P

Bremer, B. J. Brown, G. H. Morine, and J. E. Willard

Department of Chemistry, University of Wisconsin

Madison, Wisconsin 53706

1. Introduction

Solutions of mercury in liquid hydrocarbons (>5 x 10" 6M at 25 °C)[2] may be stabilized

in the glassy state by quench cooling in liquid nitrogen at 77 K (fig. 1)[3]. When a glassy solution in 3-methyl pentane (3MP) is exposed to 254 nm radiation absorbed by the

Hg, the intensity of the Hg absorption band diminishes while a broad absorption from 200

nm to 300 nm (fig. 2) attributable, in part, to 3-methyl pentyl radicals appears in parallel

1.10

100

0.10

\

Up— \

A\

\ -

1 1 1 1

1 240 250 260 270 WAVELENGTH, nm

Fig. 1. Spectra of Hg in 3MP glass at 77 K (5-cm pathlength in two 2.5-cm 2 quartz cells):

(A) spectrum of cell walls, 3MP is transparent

in this region; (B) 3MP saturated with Hg at

25 °C and quenched to 77 K; (C) 3MP saturated

with Hg at ^55 °C and quenched to 77 K.

BEFORE

PHOTOLYSIS

JMIN

PHOTOLYSIS

220 240 260 200

WAVELENGTH, nm

300

Fig. 2. Spectra of 3MP-Hg glass at 77 K different times of illumination with 200

300 nm medium pressure Hg lamp.

with growth in the known ESR signal of the radicals. The rate of growth of the radical

concentration approaches zero with continued monochromatic 254 nm illumination (fig. 3),

but increases upon exposure to broad band 200 nm-300 nm light. Exposure of a fresh 3MP-Hg

glass sample to the broad band light at 77 K causes continuous growth in the radical

concentration with no plateau, many radicals being produced for each Hg atom present (fig.

4). These observations indicate that a species formed in a primary or secondary act

Figures in brackets indicate literature references at the end of this paper,

if

Vi

i

r

256
'ig. 3 Concentration of trapped radicals, Fig. 4. Concentration of trapped radicals,

leasured by ESR, as a function of time of as a function of time of illumination of

llumination of 3MP-Hg glass at 77 K with 3MP-Hg glass at 77 K, 58 K, 23 K and 5 K

|:54 nm light from Vycor low pressure Hg lamp. with 200 nm-300 nm light from medium pressure

lurve A, 4.1 x 10 5 ergs cm~ 2 sec -:L

; Curve B, Hg lamp.

1.7 x 10 4 ergs cm" 2 sec

-1

.

following photoacti vation of the Hg by 254 nm light (e.g. HgH, HgR or HHgR) absorbs in the

road band region and is decomposed, liberating Hg which again absorbs 254 nm photons and

'roduces more radicals. No ESR signal from the intermediate has been found, indicating

hat it is not paramagnetic or that the spectrum is too broad to detect.

The initial rate of radical production by illumination of a fresh 3MP-Hg sample is irectly proportional to the light intensity. At plateaus of the type of figure 3, the

ratio of radicals to Hg atoms which have disappeared may be greater than unity and it ncreases with the light intensity, but to a power less than first order. This seems to mply that the initially formed intermediate absorbs 254 nm to regenerate Hg (e.g. HgH tyj, » g + H) in competition with a reaction which forms a species with relatively low 54 xtinction coefficient for 254 nm (e.g., HgH + R- -> HHgR), which is decomposed at other

avelengths of the broad band light (e.g. HHgR il^Hg + RH).

Illuminations with the broad band light at 5 K do not produce continuing radical

rowth as at 77 K, but rather lead to a steady state concentration at ^0.2 radicals per Hg

torn (fig. 4). This may be attributed to concerted formation within the low temperature

parent cage of a compound with the properties attributed above to HHgR, coupled with an

(bility of the low temperature matrix to deactivate the HHgR without decomposition when it

s activated by a photon from the broad band source. At 23 K the plateau is higher than

t 5 K, while at 58 K continued growth occurs, though at a lower rate than at 77 K.

hernial decay of the radicals produced by the photosensitization is only a few percent per

jour at 77 K and is undetectable over hours at the lower temperatures studied.

These studies indicate: 1) energy transfer from Hg(6 3 P 1

) atoms to molecules in a ard organic glass inducing bond rupture as in the gas [4] and liquid phases [4]; 2) a new

ethod of producing trapped radicals for study in glassy matrices; 3) a means of trapping

ntermediates from Hg photosensitization reactions which may in further investigations

issist in identifying them and their reactions; 4) a potential source of information on

he kinetics of radical -radical reactions in radical clusters in solids; 5) the possibility

f false interpretation of photochemical results in condensed systems containing unsuspected

g as a contaminant.

j' All experiments were made with purified degassed reagents; optical measurements used

5 cm light path; radical concentrations were determined from the double integral of the

irst derivative ESR spectrum.

257
References

[1] This work has been supported in part by the U.S. Energy Research and Development

Administration under Contract No. AT ( T

1 - 1 )-l 71 5 and by the W. F. Vilas Trust of the

University of Wisconsin.

[2] (a) Spencer, J. N. and Voigt, A. F. , J. Phys. Chem., 72, 464 (1968); (b) Kuntz, R. R. and Mains, G. J., ibid., 68, 408 (1964).

[3] Bremer, N., Brown, B. J., Morine, G. H. , and Willard, J. E., J. Phys. Chem., 79, 2187

(1975).

[4] For reviews of Hg-photosensi ti zation reactions and references, refer to (a) J. G. Calvert and J. N. Pitts, "Photochemistry," Wiley, New York, N.Y., 1966; (b) R. J. Cvetanovic, Prog. React. Kinet., 2, 39 (1964); (c) A. B. Callear and P. M. Wood, J. Chem. Soc, Faraday Trans. 2, 68, 302 (1972); (d) A. C. Vikis and D. J. LeRoy, Can.

J. Chem., 51, 1207 (1973).

258
MAGNETIC FIELD EFFECTS ON TRIPLET-TRIPLET

ANNIHILATION IN CRYSTALS*

S. H. Tedder and S. E. Webber

Department of Chemistry

University of Texas at Austin

Austin, Texas 78712

It has been known since 1967 [I] 2 that external magnetic fields influenced the intensity of delayed fluorescence in molecular crystals. The theory of this effect was first

presented by Merrifield [2], and has been refined by Merrifield [3] and others. Excellent

reviews are available [4]. The essential idea of the original explanation of Merrifield

may be represented by the "reaction"

MA

M

B T + T

'a 'b

k+1

k-1

(TA

...T

B

)

MA

+ M

B

k S SA

+ G

B (1)

kTT

A +GB

Tb) M

A

+M

B is an "intermediate" with

||ttotal M quantum number of MA+MB (spin relaxation is ignored), and Sa and G

B are excited

where T is a triplet exciton in sublevel M, (T^

and ground state singlets, respectively. The SA state is responsible for the delayed

M/\,Mg>) will not be degenerate

Hamiltonian. For certain

fluorescence. At high fields triplet exciton pair states ( llin general because of zero-field coupling terms in the total

orientations of the magnetic field with respect to the crystal lographic axis the levels

will become degenerate and the states involved in reaction (1) are mixed with a concomitant

change in the rate of singlet formation (usually a decrease). Hence the level crossing

resonance is observed as a change in the intensity of delayed fluorescence. An example of

this type of data for anthracene doped phenanthrene crystals for different orientations of

the crystal in the plane of the magnetic field is given in figure 1(a). For this case the

annihilation is heterogeneous, i.e., between a phenanthrene triplet exciton and an anthracene

trapped triplet molecule. To a good approximation the minimum of the delayed fluorescence

corresponds to the angle at which the high field states | ± 1^, Tip >, | O^.Op > (A = anthracene, P = phenanthrene) become degenerate. Similar results have been observed for

the delayed fluorescence of pure naphthalene or naphthalene crystals lightly doped with

anthracene. Heavier doping with anthracene (but still less than 10-6 M/M) totally changes

the pattern of resonances for naphthalene crystals in a way that is not presently understood.

As can be seen from figure 1(a) the character of the level crossing resonance changes

rather drastically with the orientation of the crystal. Simple application of previous

theory does not account for this effect and we have been led to postulate that the triplet

exci ton-trapped triplet interact with each other during this annihilation via the internolecular spin-spin interaction. H ss depends on the orientation of the crystal lographic

axis with respect to the magnetic field, if the spin states are expressed in the high

field representation. If the spin states are mixed by the H §s , and the quantum yield of

singlet formation is calculated using a kinetic scheme like (1) and (2), we obtain results

which have a qualitative resemblance to our experimental data, see figure 1(b) [5]. It is our intention to extend these calculations to the naphthalene system.

Supported by the Robert A. Welch Foundation.

Figures in brackets indicate literature references at the end of this paper.

259
Fig. 1(a). The intensity of anthracene

delayed fluorescence as a function of the

angle (j> H of the applied 8 kG magnetic field.

The values of a are -90° (H//ac*, b up); 30°

(H//bc*. a down), and 90° (H//ac*, b down).

The traces are not all to the same scale.

Fig. 1(b). A model calculation of y using

eq. (2) and considering interexcition spinspin interaction. For these calculations

k_i = 0.05, k s = 1.0 and kj = 0.5 (k+1 does

not affect the relative changes of y and only

the relative values of the other constants ar important)

.

A second theoretical model that has been studied is the use of level avoided crossing

(LAC) theory, which has previously been applied to magnetic field effects on prompt

fluorescence of small molecules in the gas phase [6]. This model is also similar, but not identical to, an approach given by Johnson and Merrifield [3]. In this model it is assumed

that each quantum mechanical pair state evolves in time coherently, and interference

effects between states play a significant role in the observed line shape. This theoretical model is much more difficult to apply than the kinetic model, but preliminary

calculations have demonstrated qualitative agreement with experimental results. Our

current goals are: (1) to establish if one or the other (or either) of these models is to be preferred, and (2) to establish if these experiments elucidate the nature of the

interaction between the triplet state molecules during annihilation.

260
References

1] Johnson, R. C. , Merrifield, R. E., Avakian, P. and Flippen, R. B., Phys. Rev. Lett.

19, 285 (1967).

2] Merrifield, R. E., J. Chem. Phys. 48, 4318 (1968).

3] Johnson, R. C. and Merrifield, R. E., Phys. Rev. B ]_, 896 (1970).

4] Swenberg, C. E. and Geacintov, N. E., Organic Molecular Photophysics Volume 1 (ed.

Birks, J. E., John Wiley and Sons, New York, 1973), Chapter 10; ibid, Volume 2, p. 395.

5] Tedder, S. H. and Webber, S. E., Chem. Phys. 12, 253 (1976).

6] Levy, D., Adv. Magn. Resonance 6>, 1 (1973).

261
iti at

THE CONTRIBUTION OF THE PHYSICAL AND CHEMICAL DEFECTS TO THE PHOTOCHEMISTRY OF CRYSTALLINE DURENE AT VERY LOW TEMPERATURES

A. Despres, V. Lejeune, and E. Migirdicyan

Laboratoire de Photophysique Moleculaire du C.N.R.S.

Universite de Paris-Sud

91405 Orsay, France

1. Introduction

It has been known [l] 1 for some time that line defects (dislocations) and point defects

(impurities) may play an important role in photochemical reactions developed in single

crystals. The disposition of the molecules at crystalline defects is generally different

from that of molecules in the regular lattice. These structural imperfections can affect

the migration of excitons through the crystal and/or may facilitate the formation of a transition complex leading to a chemical reaction.

The aim of the present work is to study the contribution of the physical and chemical

defects to the photochemistry at very low temperatures (10-20 K) of durene single crystals

doped with aldehydic guests in substitutional position.

The solid state photolysis of durene doped with orthomethyl substituted benzaldehydes

like 2,4,5-trimethylbenzaldehyde (TMB) , 2,4- and 2,5-dimethylbenzaldehydes (2,4-DMB and

2,5-DMB) gives rise, between 200 and 300 K, to a photoenol ization reaction [2] (hydrogen

transfer from the methyl to the carbonyl group in ortho position) as well as to the production of duryl (2,4,5-trimethylbenzyl ) radicals [3]. These radicals are formed by the

detachment of a hydrogen from a durene methyl group and are identified by their characteristic green fluorescence [4]. Neither photoenol i zation reaction nor duryl radical production

are however detected between 200 and 300 K, under similar photolysis conditions, in durene

doped with 3,4-dimethylbenzaldehyde (3,4-DMB) or with 3,4-dimethylacetophenone (3,4-DMA),

both guests having no methyl substituent in ortho position with respect to the carbonyl

group. These results suggest that in durene mixed crystals, the guest intramolecular and th<[ guest-host i ntermol ecul ar hydrogen abstraction reactions are in some way related to each

other in this temperature range, since duryl radicals are produced only with ortho-methyl atei benzaldehyde guests where a photoenol ization reaction is also generated. Consequently, the

intermolecular hydrogen abstraction leading to duryl radical production in the solid state photolysis of TMB, 2,4-DMB and 2,5-DMB in durene between 200 and 300 K is not a primary

photochemical process as this is the case for the ketyl radical production during the liquid

state photolysis of benzophenone in hydrogen donor solvents.

In contrast, when the UV photolysis is carried out at 10-20 K, duryl radicals are alway

produced in durene doped with aromatic carbonyl compounds which are substituted (TMB, 2,4-DM I

and 2,5-DMB) or not (3,4-DMB and 3,4-DMA) in ortho position with methyl groups.

At these very low temperatures, the photoenol ization reaction was found to be negligible, in agreement with our previous results [2] which indicate that this process requires an activation energy. These observations suggest that the mechanism of duryl radical formation

is different in the high (200-300 K) and in the low (10-20 K) temperature range.

Furthermore, in highly purified durene single crystals where no impurity emission is detected at these very low temperatures, duryl radicals are not produced during a massive UV irradiation at 10-20 K. This indicates that the presence of the aldehydic guest is necessar

|

:;

: Pi

it is

"ircjr

glass, w

iliot

1 Figures in brackets indicate literature references at the end of this paper.

262
:o induce photochemical activity and consequently, the dissociation of the host molecules is ihotosensitized by the aldehydic guest.

The information concerning the photochemistry of durene mixed crystals at very low

;emperatures are based on the following spectroscopic observations.

Figure 1.

In the figure are presented the emission spectra of durene single crystals doped with 2,5-DMB

it 16 K before (A) and after (B,C,D) photolysis with the full UV emission of a high pressure

«ercury lamp (Osram HBO 200) where the visible light was filtered by means of a Schott UG11

dllass. The bands present on curve A are the weak phosphorescence bands of the aldehydic

luest in this wavelength range. The photolysis of0 the sample at 16 K reveals new emission

,iands shown in spectrum B: a weak band at 4864,5 A and a predominent band at 4884,5 &

'ol lowed by two relatively strong bands at respectively, 430 and 735 cm

-1 from the latter,

'hen the sample is annealed in the dark by a warming up to 60 K and a freezing down to 16 K ycle, no great changes are observed on the spectrum C except for the disappearance of the

6 /eak band at 4864,5 A and the slight increase in intensity of the band at 4898 A\ However,

jirhen the sample is annealed up to 70 K, the entire structure of the emission subsequently

ecorded at 16 K is dramatically changed: the band at 4884,5 & as well as its two companion

.'ands at 430 and 735 cm

-1 have completely disappeared and are replaced by a shifted spectrum

•Starting at 4898 A with two intense bands again at 430 and 735 cm

-1 from the origin. This

,[iew spectrum is identical to that recorded in this very low temperature range for duryl

•adicals prepared at room temperature either by photolysis of durene mixed crystals or by

adiolysis of purified durene. Consequently, the spectrum starting at 4884,5 A with a simiI ar vibrational structure as that starting at 4898 A, can reasonably be attributed to duryl

/adicals which are trapped in a different crystalline environment when prepared at 10-20 K. imilar results are obtained when samples of durene doped with 2,4-DMB, TMB, 3,4-DMB or 3,4-

MA are photolyzed at 10-20 K.

All these observations can therefore be rationalized in the following way: the bands at

„-864,5 & and at 4884,5 ft are the fluorescence origins of duryl radicals produced photochemi-

;ally in transient defects created because of the stress experienced by durene single crytals during the cooling at 10-20 K. These transient species first formed in crystalline

lefects are then reoriented for temperatures higher than 60 K, so as to give more stable

luryl radicals whose fluorescence origin is shifted to 4898 A.

263
EMISSION SPECTRA AT 16 K "<o.ok898a

OF 2.5 -D MB IN DURENE SINGLE

CRYSTAL BEFORE (A) AND AFTER

(BjCD) PHOTOLYSIS AT 16 K

II (0,0)-430crrf1

IK0,0)-525cm-1

(C) =(B) Warmed up to 60 K and refrozen at 16

K

(D) =(C) Warmed up to 70 K and refrozen at 16

K

Figure 2

264
In a recent paper published while this work was in progress, Udagawa and Hanson [5] have

ilso observed that the duryl radicals produced photochemical ly in a durene host either at

,2-15 K or at temperatures o higher than 80 K have different fluorescence origins respectively

ocated at 4885 & and 4898 A, in good agreement with our spectroscopic data. From their

tudy of the Stark effect on the lowest excited state of these species, they have concluded

hat the radicals prepared in the low or higher temperature range have different orientations

hich result from the recoil of the duryl radical in the photodissociation process.

2. The Dependence on Photolyzing Light Intensity of Duryl Radical Production at 16 K

In order to get information on the mechanism responsible for duryl radical production

n transient defects at very low temperatures, the dependence on photolyzing light intensity

f this reaction rate was studied at 16 K.

The relative rates of radical production were measured by monitoring the intensity I f the fluorescence band at 4884,5 A during the UV irradiation. The plot of I versus time

f irradiation with constant light intensity presents an initial linear region starting at

She coordinates origin. This indicates that the reaction takes place either without any

hotochemical precursor or through a precursor having a lifetime of the order of the minute

jr less. The relative rates were determined from the initial linear regions. However, the

changes level off at high radiation doses because of secondary reactions and a possible

nner filter effect of the radical produced.

The relative light intensity was modified by introducing wire screens between the light

ource and the sample. The screens were placed at sufficient distance from the sample to nsure uniform illumination. The use of wire screens which reduce the photolyzing light

ntensity by factors of 3,6 or of 7,7 decreases the rate of radical production by factors of

,5 and 7,6, respectively. This indicates that the duryl radical production at 16 K depends

inearly on photolyzing light intensity. According to the overall kinetic scheme established

or similar processes [6], such a result suggests that the reaction is either monophotonic or iphotonic with the guest triplet state as intermediate. In particular, when the decay

rocess from the guest triplet state is negligible compared with absorption of the second

loton from that state, then the reaction rate can depend linearly on photolyzing light

itensity even for a biphotonic reaction. From our previous investigations, the triplet

ifetime of 2,5-DMB in durene single crystal at 16 K is known to be 15 m sec. This short

ifetime together with an approximate evaluation, for our experimental device, of the incibnt light intensity for second photon absorption, indicates that, in our conditions, the

jacay from the guest triplet state is not negligible compared with absorption of the second

"toton from that state. Consequently, the duryl radical production at 16 K is indeed a )nophotonic process.

A consideration of the singlet and triplet electronic state energies of the aldehydic

juests and the durene host allow us to discard the dissociation of the host via guest-host

lergy transfer in a monophotonic process.

One remaining possibility is the direct hydrogen abstraction of the aldehydic guest

.'.cited in the triplet state from the durene molecule in the ground state. This reaction was

scarded in the high temperature range (200-300 K) on the basis of our experimental results,

pwever, it is very well possible that, in the crystalline defects created in durene single

['ystals at very low temperatures, the steric conditions required by this reaction are

:hieved and lead to the duryl radical production.

References

'] Thomas, J. M. , and Williams, J. 0., Progr. Solid State Chem. 6, 119 (1971) and references therein.

] Migirdicyan, E. , Despres, A., Lejeune, V., and Leach, S. , J. Photochem. , 3, 383

(1974/75).

265
[3] Migirdicyan, E. , J. Chem. Phys. 55, 1861 (1971).

[4] Leach, S and Migirdicyan, E. , Chem. Phys. Lett. 21 (1967).

[5] Udagawa, Y. , and Hanson, D. M. , J. Chem. Phys. 64, 3753 (1976).

[6] Stevens, B. , and Walker, M. S. , Chem. Commun. 2> 8 (1965).

[7] Despres, A., Migirdicyan, E. , and Blanco, L. , Chem. Phys. 14, 229 (1976).

266
PHOTOCHEMISTRY OF ELECTRONS TRAPPED IN ORGANIC GLASSES [l] 1

G. C. Dismukes, S. L. Hager, D. P. Lin, G. H. Morine and J. E. Willard

Department of Chemistry

University of Wisconsin

Madison, Wisconsin 53706

It has been known for some time that mobile electrons generated in organic glasses by

y radiolysis or photoionization of solutes at <77 K become physically trapped with significant yields [2]. These el give broad absorption spectra (>3eV) with maxima in the

/isible or near infrared. The spectra have bandwidths at half-height ranging from ^0.5eV to

>leV, long tails on the higher energy side of \ mv, shorter tails on the low energy side,

and extinction coefficients at Xraax of lOMO4

. Trapped electrons can be removed from their

traps by photons and migrate to encounter solutes with which they can react in competition

vith retrapping. Products may include molecular anions produced by reaction with molecules

with a positive electron affinity (e.g., biphenyl), carbanions (by reaction with trapped

"adicals) free radicals (by dissociative electron capture), and neutral molecules (by

taction with cations). This paper reviews some of the photochemistry of such trapped

electrons observed in recent and continuing work in our laboratory.

Until relatively recently it was not known whether the broad spectra of e", such as illustrated in figure 1, are representative of electrons bound in uniform traps or are

envelopes of the spectra of electrons in trapping sites with a continuum of energies. Data

;uch as those of figures 2 and 3 give strong support to the conclusion that the spectra

'epresent continua of trapping energies [2]. Figure 2 [3] indicates that exposure to

ight in the 700 nm-1000 nm region of the spectrum of e~ in 3-methyl pentane (A ma x

= 1600 nm) ncreases the absorption above 1900 nm and decreases it below, the process being reversed

In the dark. This can be explained if the detrapped electrons are retrapped in weaker traps

'hi ch they cause to deepen by coulombic interaction with bond of dipoles of the surrounding

lolecules. Figure 3 [4] illustrates energy-selective detrapping of e" in methyl tetralydrofuran (MTHF) at 25 K by 1338 nm light, and relatively uniform bleaching by 1064 light,

'igure 4 is a formalized model to suggest the type of continuum of e~ spectra which might

»e responsible for the effects observed. Evidence that the detrapping of e" is monophotonic

,

ather than occurring by a biphotonic mechanism involving promotion to an excited state of

he trap followed by absorption of a second photon, is given by the first order dependence of

hotoconductivity (fig. 5) and photobleaching on the intensity of monochromatic light used

lor the detrapping [5]. When the trapped electrons are produced by y-irradiation of the

rganic glass, the matrix also contains trapped radicals which compete with the trapped

ations for capture of detrapped electrons. The photoionization thresholds of the carlianions formed lie in the region of 600 nm (2 eV) [6]. When a y-irradiated hydrocarbon

'lass is exposed alternately to infrared and ultraviolet light, electrons may be repeatedly

inverted from the trapped state to the carbanion state and back with 5 percent still

emaining after 7 cycles [7]. The heat released by electrons on reaction with cations plus

adicals in y-irradiated glasses following photodetrapping has been measured by differential

hernial analysis at 77 K [8]. It decreases with dose, consistent with an increasing ratio

f radicals to cations (fig. 6). The extrapolated value at zero dose sets a limit on

he heat of solvation of the cation in 3MP. Current work in our laboratory is using

leaching with monochromatic light to further resolve the contributions of varying trap

epths to the broad eT spectra.

Figures in brackets indicate literature references at the end of this paper.

267
References

[1] This work has been supported in part by the U.S. Energy Research and Development

Administration under Contract No. AT(n-l)-1715 and by the W. F. Vilas Trust of

the University of Wisconsin.

[2] For a review and references see Willard, J. E., J. Phys. Chem., 79, 2966 (1975).

[3] Hager, S. L. and Willard, J. E., Chem. Phys. Lett., 24, 102 (1974).

[4] Hager, S. L. and Willard, J. E., J. Chem. Phys. 6J_, 3244 (1974).

[5] Dismukes, G. C, Hager, S. L. , Morine, G. H. s and Willard, J. E., J. Chem. Phys.

61_» 426 (1974).

[6] Dismukes, G. C, Ph.D. Thesis, University of Wisconsin-Madison, 1975.

[7] Lin, D. P. and Willard, J. E., J. Phys. Chem., 78, 1135 (1974).

[8] Hager, S. L. and Willard, J. E., J. Chem. Phys., 63, 942 (1975).

268
ig. 1. Spectra of trapped electrons in

MP-d 14 at 23 K: (A) electrons produced by

hotoionization of TMPD at 72 K and cooled

o 23 K; (B) after partial photobleaching of

at 23 K with 700-1000 nm light.

Fig. 2. Optical density changes resulting

from illumination of trapped electron

populations and of TMPD in 3MP glass. Onset

of illumination indicated by (+) and ter- mination by (-). Analyzing wavelength shown

on each plot, (a-e), 700-1000 nm illumination

at 72 °K of electrons generated in 3MP-d 11+ by UV photoionization of TMPD at 27 °K. (f), same as (a-e) except illumination at

23 °K after TMPD ionization at 72 °K. (g),

700-1000 nm illumination of electrons

generated by y-irradiation, T = 72 °K. (h),

optical density at 1700 nm during and after

generation of trapped electrons by UV photoionization of TMPD in 3MP-d 1[+ ; (-) mark

signifies end of 2 min. UV illumination.

06

WAVELENGTH, NM

]ig. 3. Effect of bleaching with 1338 and

064 nm light on the spectrum of e~ in MTHF.

olid lines, before bleaching. Upper three

ines, 1338 nm experiment, e. produced and

leached at 20 K. Lower three lines, 1064 nm xperiment, 25 K. Intensities ^30 mW cm

-2

.

Fig. 4. Schematic representation of hypothetical spectra of individual electrons in different trap depths (lower lines), the

summation of which gives the envelope (upper

line) typical of the observed absorption by

e~ in alkanes at 77 K.

269
1 100 <

20

P«!mW WITHOUT FILTER T=75° K

FILTER REMOVED

0.5 NEUTRAL

LIGHT DENSITY (ON FILTER

DARK CURRENT

LIGHT ^

0FF FILTER REMOVED.

LIGHT ON 0.5 ND

| FILTER

2.0 0

TIME, MINI. 20

HEAT OF REACTION OF TRAPPED ELECTRONS IN 3MP

050 0.75 1.00 DOSE, «V q-1 1 10-20

Fig. 5. Effect of intensity of 1820 nm laser

light on photoconductivity of 3-methyl hexane

glass at 77 K. y-Dose 2.6 x 10 19 eV g _1

; maximum light intensity of 1.0 mW cm" 2

.

Fig. 6. Heat of reaction of photodetrapped

electrons in y-irradiated 3MP as a function

of y-dose.

270
SPIN LABELS AND THE MECHANISM OF THE S x ~7 T x NONRADIATIVE PROCESS

IN DURALDEHYDE; POSSIBLE MANIFESTATION OF PSEUDO JAHN-TELLER

FORCES ON NONRADIATIVE PROCESSES

Alan Campion and M. A. El-Sayed

Department of Chemistry

University of California

Los Angeles, California 90024

Recently, the new technique of Zeeman PMDR [1-4] 1 has been successfully applied to

le elucidation of the mechanism by which molecules initially excited to the lowest singlet

;ate (S x ) intersystem cross (ISC) to the lowest triplet (T x ). The experiments use losphorescence-microwave double resonance [5] (PMDR) techniques to monitor changes in the

:eady-state populations of the three magnetic sublevels of the 1 1 state, as they are

xed by a weak (< 1 kG) external magnetic field. The results of the experiments to date

live confirmed the importance of a selection rule [6] which predicts that ISC between

:ates arising from different types of electronic promotion (e.g., n,n*

^

n,n*) should

| at least an order of magnitude more important than ISC between states of the same type

r electronic promotion (e.g., n,n* i^n.n* or 7t,n*). This has been convincingly demon-

. rated for the nitrogen heterocyclics where it was shown [3,4] that intermediate triplet

.ates play an important role in the ISC process if they are of different orbital con- guration than S x

. In benzophenone , however, the mechanism is found [2] to involve the

nglet and triplet n,n* states. This was accounted for by a proposed efficient ISC tween these states, in molecules where the carbonyl and aromatic moieties are not planar [7].

In order to examine the n,n* n,n* selection rules in planar carbonyls, we have

lected duraldehyde (2,4,5-trimethylbenzaldehyde). The lowest singlet state [8] is

,7i*, T x is 3

7t,7T* and T2 , which lies [9] ca. 400 cm

-1 higher in energy than T x

, is an

,7t* state. In addition, the principal axes of the zerofield tensor of the Tj state have

en found [10] for this molecule in a durene host single crystal and the dynamic paraters that characterize the J 1 state in this host at 1.5 K have been reported [11].

In the Zeeman-PMDR experiment, a weak, external magnetic field mixes the zerofield

in states to give

= I C. .t. (1)

ere the subscripts label principal axes of the zerofield tensor and the mixing coefcients Cjj depend upon the projection of the field on these axes as well as the zero eld energy splitting AE

ven by

The populating (K) and decay (k) rates in the field are then

K?(kJ) = 1

j

c.

.

1J

2 9$ (2)

the ISC is direct S x ^j Tx . By calculating the field-dependent rates from this equation,

e may predict both the magnitude and sign of the PMDR signal as a function of field in ese low fields for a direct ISC process. Then, by comparison of the experimental results

Figures in brackets indicate literature references at the end of this paper.

271
with these calculations, it is possible to immediately determine whether the ISC is direct or not. If the experiments show that it is not direct, then \$ is modified to

2 K

'm

o (3)

where the primed coefficients refer to the mixing in a second triplet state and the sum over k expresses the projection of the net spin polarization from the higher triplet to

Tj. The expression for k^ remains as that given in eq. (2). These equations, along with

the experimental data, may be used to arrive at a model for the zero-field energies and

populating rates of the higher triplet.

We have followed the change of the 9.0 and 11.4 GHz PMDR signals in duraldehyde with

fields up to 1 kG directed along the durene a. or c_' crystal axes. With the field parallel

to a., there occurred a rapid diminution of tTTe 9.ff GHz signal until the external field was MOfT G, where the signal changed from negative to positive. The calculations, based upon

a direct mechanism, showed that the signal should remain negative while gradually decreasing as the field increases to 1 kG. Similar, though less dramatic, deviations from the

calculated behavior occurred for both the |D - E| and |D + E| transitions with field along

a or c

1 .

These four experiments have led us to conclude that the ISC in duraldehyde is not completely direct. By adopting a model for the second triplet state that is similar to the formaldehyde n,n* triplet, we are able to fit all our observations by setting the x z

1 T2/T1 relative ISC rates to 0.2. Thus, most of the ISC in duraldehyde appears to be

direct, but a non-negligible fraction involves the T2 state.

These observations may be interpreted to give a measure of the degree of vibronic

mixing of pure n,7t* and n,7t* triplets. If we use a pure n,7t* state and pure n,n* state as zero-order descriptions for T2 and T x respectively, and assume zero probability for the

ISC between T 2 and T x respectively, and assume zero probability for the ISC between the

Sn,u* and the zero order T

n 7r * states, then the Zeeman experiments yield the vibronic

mixing coefficient directly; T2 contains ca. 20 percent n,n* character and vice versa. This number is quite reasonable, considering the close proximity (AE^. _ « 400 cm" 1

) of

the lowest two triplet states in this molecule. 1_ 2

The authors wish to acknowledge the financial support of the Energy Research and

Development Administration. AC would like to acknowledge IBM for a predoctoral research

fellowship. Computing time was made available by the UCLA Campus Computing Network, to whom we are grateful.

[1] Leyerle, R. W. , Ph.D. Dissertation, University of California, Los Angeles, CA, 1974.

[2] El-Sayed, M. A. and Leyerle, R. W. , J. Chem. Phys. 62, 1579 (1975).

[3] Zinsli, P. E. and El-Sayed, M. A., Chem. Phys. Lett. 34, 304 (1975).

[4] Zinsli, P. E. and El-Sayed, M. A., Chem. Phys. Lett. 36, 290 (1975).

[5] El-Sayed, M. A., Ann. Rev. Phys. Chem. 26, 235 (1975).

[6] El-Sayed, M. A., J. Chem. Phys. 38, 2834 (1963).

[7] El-Sayed, M. A. and Zinsli, P., J. of Luminescence, in press.

References

272
[8] Fischer, G. , Mol. Cryst. and Liq. Cryst. 85 (1970)

[9] Migirdicyan, E. , Chem. Phys. Lett. 12, 473 (1972).

[10] Sharnoff, M. , Mol. Cryst. 5, 297 (1969).

[11] Cheng, T. H. and Hirota, N. , Mol. Phys. 27, 281 (1974)

273
PHOTODESORPTION FROM METALS: MEASURED DESORPTION RATES

IN COMPARISON WITH A MO-TREATMENT

N. Trappen

Inst. f. phys. Chemie, Universitat

des Saarlandes, Saarbrlicken

Carbon monoxide adsorbed on clean metal surfaces can be desorbed by irradiation with

ultraviolet light [I] 1

. Simultaneously, the formation of carbon dioxide has been observed

This can be explained by excitation of the adsorbed CO due to irradiation followed by

desorption or dissociation with reaction to C0 2. The photo-reaction was measured by

application of a quadrupole mass analyzer in an ultra high vacuum system, the metal sur- faces being ultra pure MARZ-grade polycrystal 1 ine folies. The different metals were

tantal, tungsten, rhenium and molybdenum.

From the experiment follows, that the quantity of the desorbed material depends

strongly both from the wavelength of the inserted light and from the nature of the used

metal. The desorption rate decreases rapidly in the range between 200 to 300 nm being the

most effective using tantal as adsorbens and decreasing via tungsten and rhenium to molybdenum.

A quantum chemical treatment was executed in order to correlate calculated values

with the experimental data. Starting the calculation of a statement of Tompkins [2] was

used. Following these arguments the properties of adsorption complexes are influenced

mainly by the atomic properties of the adsorbing material. Consequently, it should be

possible to compare a system like Me-CO with the adsorption complexes within this experiment.

A modified closed-shell SCF-LCA0-MO program with configuration interaction according

to Bloor and Gilson [3] was used to calculate bonding strengths and excitation energies of CO adsorbed on the different metals. Hereby the parameters for d-orbitals were taken

according to Hartrec-Fock calculations by Maly and Hussenois [4].

It can be shown, that the bonding strength of C=0 in the Me-CO complex decreases fron

MO over Rh and W to Ta. The excitation energy of the first electronic excited state

decreases in the same way. The shape of the calculated values is consistent with the

experimental results.

The dependency of the excitation energy from the kind of metal in the adsorbation

complex explains the corresponding energy acception of the CO molecule with either the

possibility to desorb or to dissociate.

The decreasing bonding strength explains the higher probability of the dissociation

of CO on the metal surface resulting in formation of C02

.

So it seems to be possible to predict the behavior of CO on other metals with the ai< of these calculations. The experimental part of this work was sponsored by the Deutsche

Forschungsgemeinschaft.

References

[1] Moesta, H. and Trappen, N., Naturw. 57, 38 (1970).

figures in brackets indicate literature references at the end of this paper.

274
Culver, R. , Pritchard, J. and Tomphins, F. C, Zeitschr. f. Elektrochem. 63^, 741 (1959).

QCPE Program No. 71 by Bloor, J. E. and Gilson, B. R.

Maly, J. and Hussenois, M. , Theoret. Chim. Acta (Berl.) 31_, 137 (1973).

275
MATRIX ISOLATION SPECTROSCOPIC STUDIES OF FREE RADICALS AND MOLECULAR IONS PRODUCED BY COLLISIONS OF MOLECULES WITH EXCITED ARGON ATOMS

Marilyn E. Jacox

National Bureau of Standards

Washington, DC 20234

In early 1974, a series of experiments was initiated in this laboratory in which the

products of the interaction of simple molecules with the periphery of a microwave discharg

excited in a beam of pure argon were rapidly frozen onto a substrate maintained at 14 °K,

permitting the trapping of free radical and ion products in concentration sufficient for

direct infrared and and ultraviolet spectroscopic detection. The discharge configuration

used for these experiments is shown in Fig. 1, which illustrates the relative orientation

Ar + XY

i

A r >

: -Discharge

.

;

: : yy\;

* :-V/ —:—:———:

• * < • i

ibee

Figure 1

of the discharge tube, the sample inlet, and the cryogenically cooled surface inside the

temperature cell. The discharge is adjusted so that it extrudes slightly beyond the pinht

in the Pyrex discharge tube. The molecule XY, mixed with at least a hundredfold excess of argon, can collide with excited argon atoms in the discharge region between the pinhole ai the cooled surface, which are separated by approximately 3 cm. The pinhole, which has an area of 1 to 2 mm2

, retards backstreaming of molecules into the body of the discharge and

filters most of the vacuum ultraviolet radiation produced in the discharge from the sampli

deposit.

276
In the first experiments using this discharge configuration [l] 1

, the intensity of the

1848 cm

-1 infrared absorption of HC 2 , produced by the interaction of C 2 H 2 with the periphery

of the discharge, compared favorably with that obtained in the best photolysis experiments

utilizing 121.6 nm radiation [2]. The discharge study also led to the assignment of the

C-H stretching fundamental of HC 2 at 3612 cm

-1

, suggesting that the OH bond of this species

is unusually strong. In a study of the visible-ultraviolet spectrum of a similar sample,

shown in Fig. 2, the Mul liken band of C 2 , which appears in an argon matrix at 238 nm, was

J L J L J L 520 500 480 460 440 240 230 nm

Ar:C

2

H

2 (60% Dj = 200 + Discharged Ar. 14°K

Figure 2

nly moderately intense, whereas a prominent progression assigned to C 2 [3] was observed

etween 521 and 430 nm. These results may be contrasted with those typical of 121.6 nm phoolysis of matrix-isolated C 2 H 2 , in which the Mulliken band was extremely prominent and the

2 bands were somewhat less intense.

The emission spectrum of the discharge showed prominent lines between 470 and 415 nm,

11 of which are readily assigned to argon atom emissions between upper-state levels 14.4-

4.6 eV above the ground state and lower-state levels 11.5-11.8 eV above the ground state,

wo of these lower-state levels involve metastable argon atoms in the 3 Pq and 3

P 2 states,

ecently reported [4] to have collisional lifetimes on the order of 1 sec. This observaion suggested a series of studies of the interaction of HCC1 3 with the discharge; preious studies [5] had established that the principal primary product of the photodecomosition of matrix-isolated HCC1 3 by 121.6 nm radiation was CC1 3 , whereas prominent HCC1 2 osorptions and very little CC1 3 resulted when 106.7 nm (11.6 eV) photolyzing radiation

as used. Typical spectra obtained for HCC1 3 and DCC1 3 samples are shown in Figs. 3 and

, respectively. Although some CC1 3 and CC1 3 were present in the discharge experiments,

!hte yields of HCC1 2 +

and DCC1 2+

, which have an appearance potential of 11.49 eV in gasiase studies of the photoionization of chloroform [6], were several times as great as in

le best 106.7 nm photolysis experiments. Only a small concentration of HCC1 2 or of DCC1 2 as observed. There was very little evidence for secondary decomposition products such

; CC1, HCC1 , or CC1 2

. These experiments suggest that collisions of HCC1 3 with metastable

/gon atoms play an important role in the observed product formation, leading to direct

Reduction Q f tne cafj 0 n. The HCC1 2 and DCC1 2 anions, which provide the requisite

£ 'erall charge neutrality of the matrix deposit, have been demonstrated to result from

ssociative electron capture by chloroform. Studies of the photodecomposition of the ion

igures in brackets indicate literature references at the end of this paper.

277
Figure 3

278
products upon subsequent filtered mercury-arc irradiation of the matrix deposit have

yielded evidence for a photoinduced proton transfer from HCC1 2+ to the argon lattice [7].

The corresponding discharge experiments on HCC1 2 F have led to the stabilization of the

C1 2 CF free radical and of the HCC1F and C1 2 C--*HF molecular anions in sufficient concentration for direct infrared observation. Several relatively weak absorptions may be contributed

by a hydrogen-containing cation. Although a value of 12.4 ± 0.2 eV has been reported for the

ionization potential of HCC1 2 F [8], Sieck [9] has detected a weak ion signal due to HCC1 2 F+

Ion exposure of gas-phase HCC1 2

F to 11.6 and 11.8 eV argon resonance radiation.

Although the reported ionization potential of HCC1F2 , 12.59 ± 0.15 eV [8], is somewhat

higher than that of HCC1 2 F and although no ion signal was detected in the gas-phase study

using argon resonance radiation [9], very prominent anion absorptions, assigned to HCF 2 and to (ClCF)-'-HF , were observed in the discharge experiments on HCC1F 2 , with little evidence for infrared absorptions attributable to the cations which must be present to preserve

overall charge neutrality of the sample. Processes which may account for ion production in

Ithis system include chemi ionization on collision of argon atoms excited above the first

Rydberg state with HCC1F2 and charge transfer between electronically excited argon atoms

and HCC1F2 , resulting in the formation of molecular anions and Ar+

, which in turn may form

cluster ions in the argon matrix. The yield of ion products may be enhanced above that

possible for gas-phase collision processes alone by energy transfer between excited argon

atoms and the surface of the matrix; the diffusion length for the "free" n = 1 exciton

state in solid argon has been estimated [10] to be on the order of 100 A.

Discharge experiments on CC1 4 , CFC1 3) CF2 C1 2 , and CF 3 C1 have also yielded evidence for

the stabilization of ionic species in the matrix. Of especial interest are the studies on

"F 3 C1 , recently found to have an ionization potential of 12.5 eV [11]. No ions were detected

in gas-phase observations using argon resonance radiation [9]. Processes similar to those

Postulated for the discharge experiments on HCC1F 2 may account for the observed ion produc-

:ion in the CF3 C1 studies.

References

1] Jacox, M. E. , Chem. Phys. , 7, 424 (1975).

2] Milligan, D. E. , Jacox, M. E. and Abouaf-Marguin, L. , J. Chem. Phys., 46, 4562 (1967).

3] Milligan, D. E. and Jacox, M. E. , J. Chem. Phys., 5J, 1952 (1969).

4] Van Dyke, R. S. Jr., Johnson, C. E. and Shugart, H. A., Phys. Rev., A5, 991 (1972).

5] Jacox, M. E. and Milligan, D. E. , J. Chem. Phys., 54, 3935 (1971).

Werner, A. S. , Tsai , B. P. and Baer, T. J., Chem. Phys., 60, 3650 (1974).

7] Jacox, M. E. , Chem. Phys., 12, 51 (1976).

8] Hobrock, D. L. and Kiser, R. W. , J. Phys. Chem., 68, 575 (1964).

Sieck, L. W. , private communication.

10]

j 11] Ajello, J. M. , Huntress, W. T. and Rayermann, P., private communication

Ophir, Z. , Schwentner, N. , Raz, B. , Skibowski, M. and Jortner, J., J. Chem. Phys., 63,

1072 (1975).

279
ATMOSPHERIC CHEMISTRY OF CHLOROFLUOROCARBONS

Mario J. Molina

Department of Chemistry

University of California

Irvine, California 92717

The chlorofluorocarbon-ozone depletion theory has been the subject of close examination by several research groups. Many facets of the fluorocarbon problem are currently

under investigation; we are concerned here with its chemical and photochemical aspects. The

theory assumes a lack of reactivity for the chlorofluorocarbons in the lower atmosphere, and an appreciable reactivity in the upper stratosphere for their photodecomposition products,

which include chlorine atoms. This chemical reactivity—described in detail elsewhere [l] 1 consists mainly of the following reactions:

HC1 OH

CH,

0 CI CIO

0,N0

The various assumptions on tropospheric and stratospheric chemistry may be tested by two different procedures. First, additional chemical and photochemical reactions can be

studied in the laboratory and evaluated for their possible significance. Second, measure- ments of atmospheric concentrations of the various species may be compared to the values

predicted by theory.

We have recently carried out some laboratory studies on the species C10N0 2 (chlorine

nitrate) [2]. Our current results indicate that an appreciable fraction of the chlorine

atoms in the mid-stratosphere may be present as C10N02

.

The formation of this compound involves the following three-body process:

CIO + NO, + M C10N0, + M

The rate of this reaction has now been measured in the laboratory by at least three

research groups; the reaction is about 15 times slower than the similar reaction between

N0 2 and OH radicals. In the stratosphere, chlorine nitrate will be destroyed mainly by

photolysis. We have measured the optical absorption cross sections in the LIV of purified

samples of gaseous C1QN0 2 [2]. These cross sections, when combined with values of solar

intensities, provide estimates for the photodissociation rates at various altitudes: the

photochemical lifetime is about 3 hours at 30 km and about 1 hour at 40 km (for overhead

sun). We have also studied the reaction between ground state oxygen atoms and chlorine

nitrate [3]:

0 + C10N0, products

6 x 10" 12 exp (^jP) cm: sec"

Figures in brackets indicate literature references at the end of this paper.

280
This rate constant value is small enough to make this reaction unimportant compared to photolysis in the stratosphere.

The possible importance of C10N0 2 for stratospheric chemistry can be assessed by cal-

;;ulating the distribution of chlorine among the species C10N0 2 , HC1 , CIO, and CI, that is,

by considering the formation and destruction of C10N0 2 in addition to the reactions dis-

:ussed earlier [1]. The amount of C10N0 2 present at mid-stratospheric altitudes might be

arge enough for actual in situ detection to be feasible, perhaps by infrared spectroscopy.

r urthermore, due to the nature of the photodissociation process, diurnal variability in ;10N0 2 concentrations is to be expected.

However, due to the relatively large photodissociation rates, our calculations—which

ncorporate this diurnal effect—indicate that the amount of C10N0 2 should be smaller than

.he amount of HC1 at all altitudes, and the ozone depletion predictions are diminished only

iy about 30 percent as compared to earlier calculations.

As mentioned above, another aspect of the theory—namely, the stability of chloro1 uorocarbons in the lower atmosphere—can also be tested by two separate procedures. First,

e can evaluate the various proposed mechanisms or "sinks" which lead to chlorofl uorocarbon

emoval (e.g., reaction with ions, dissolution in the ocean, trapping in Antarctic snow, tc). To date, no such specific sinks have been found to be of importance when compared to tratospheric photolysis. Second, the amount of chlorofluoromethanes now found in the

tmosphere can be compared with the cumulative amount already manufactured and released to

he environment, taking into account that some stratospheric photolysis has already occurred,

e have conducted a careful evaluation of this comparison for CC1 3 F ( fl uorocarbon 11) [4].

lie conclude that a tropospheric residence time of less than 30 years is not compatible with

xperimental observations. The results are certainly consistent with the assumption that

tratospheric photolysis is the only sink, but the existence of an additional tropospheric

ink of similar magnitude cannot be ruled out on the basis of these comparisons.

One of the largest uncertainties in this evaluation results from difficulties in the

bsolute calibration of the analytic instrument used to measure CC1 3 F levels in ambient air.

e have carried out absolute calibrations in our laboratories by successive expansions and

ilutions of a known initial amount of CC1 3 F. Our results are in good agreement with those

f several other research groups employing similar methods.

References

•

I] Rowland, F. S. and Molina, M. J., Rev. Geophys. Space Phys., 13, 1 (1975).

2] Rowland, F. S., Spencer, J. E. and Molina, M. J., The Stratospheric Formation and

Photolysis of Chlorine Nitrate, C10N0 2 , (to be published).

3] Molina, L. T. , Spencer, J. E. and Molina, M. J., Rate Constant for the Reaction

0 + C1N0 3

, (to be published).

|] Rowland, F. S. and Molina, M. J., An Experimental Estimate of the Importance of

Tropospheric Sinks for CC1 3 F (Fluorocarbon-11 ) , to be published in J. Chem. Phys.

281
ULTRAVIOLET PHOTOABSORPTION CROSS-SECTIONS OF CF 2 C1 2 AND CFCL 3 AS A FUNCTION OF TEMPERATURE

J

3!

Arnold M . Bass and Albert E. Ledford, Jr.

< le National Bureau of Standards

Washington, DC 20234

Because of recent interest [1 a , b] 1 in the effect of commonly used halocarbons upon the

stability of the earth's stratospheric ozone layer, we have undertaken a remeasurement of the

photoabsorption cross-sections of fl uorocarbons 11 and 12 (CFC1 3 and CF 2 C1 2 ). Although

some measurements of this absorption have been reported previously [lb,2a,b], the recent

report of a temperature dependence of the absorption [3] motivated us to measure this property of these substances in greater detail.

Measurements were made over the wavelength region 185-230 nm, the wavelength region over which photodissociation occurs in the stratosphere. In order to determine the effect

of the reported temperature dependence upon the estimated dissociation rates of these substances in the stratosphere, absorption measurements were made at two temperatures, 296 K and

223 K.

The measurements were made with a 0.75 m Fastie-Ebert monochromator equipped with a 2400 groove mm grating. Absorption measurements were made at intervals of 0.2 nm with a spectral resolution of 0.05 nm. The absorption cell in which the gas samples were placed was made of stainless steel. It was 50 cm long, and by using the multiple pass design of

White [4] and Bernstein and Herzberg [5] path lengths up to a maximum of 10 meters could be used. Temperature control of the gas sample was obtained by circulation of a refrigerated

fluid, usually methanol, through the outer jacket of the cell. The gas temperature was measured by means of calibrated chrome! -constantan thermocouples inside the cell. At a cell

temperature of 220 K the temperature variation of the sample over the length of the cell was approximately 1 °C.

The absorption cell was placed in the exit beam of the monochromator. Immediately in front of the cell a sapphire plate was used to split the light beam and a portion of the

signal illuminated a photomul ti pi ier tube and served to monitor the variation of the light

source. A broad band interference filter which passed radiation of wavelength between 175

and 250 nm was inserted between the hydrogen continuum source and the entrance slit. This

filter effectively removed all radiation outside of this range, and eliminated all scattered

radiation. Data acquisition was automated by photon counting in conjunction with a stepping

motor control for the monochromator wavelength drive, as described previously [6].

Dichlorodifluoromethane CCF 2 C1 2 ) was purified by trapping with liquid nitrogen the exit 1 stream from a gas chromatograph equipped with a squalane column and a thermal conductivity

detector. The major impurity, which was CF 2 C1H, could be removed entirely by this method. I

After purification the CF2 C1 2 was better than 99.99 mole percent pure with respect to detectable organic impurities. The fluorotrichloromethane (CFC1 3

) was better than 99.99 mole

percent pure with respect to detectable organic impurities, as received. Both components were used after low-temperature degassing.

figures in brackets indicate literature references at the end of this paper.

282
The results of these measurements are shown in Figure 1, and in Table I (a complete

table of measured cross-section is available upon request). The measurements at room tem- perature are essentially in agreement with those reported by Rowland and Molina [lb], but are approximately 15-20 percent lower than the values reported by Huebner, et al . [2b]. The only

measurements reported at low temperature are those by Rebbert and Ausloos [3] at the wave- length of the zinc resonance line, 213.9 nm. The temperature effect on the absorption

cross-section that we have observed between 296 K and 223 K is not as large as they reported,

being approximately a factor of two reduction for CF 2 C1 2

, but only 30 percent in the case of

CFC1 3

.

IM lit lit WRVELENGTHl NM

)

Figure 1

Table I. Absorption Cross-Sections for CF 2 C1 2 and CFC1 3

Cross -Section x 10+2 ° cm 2 mol ec

_

1

/elength/nm CF 2 C1 2 CFC1 3 296 K 223 K 296 K 223 K

230.0 0.34 0.15

225.0 0.94 0.51

220.0 0.063 0.026 2.45 1 .38 215.0 0.22 0.095 6.48 4.26

210.0 0.74 0.38 15.3 12.1

205.0 2.60 1 .50 33.4 29.3

200.0 8.56 5.70 62.6 55.6

195.0 25.2 19.5 106.2 98.4

190.0 61 .3 55.0 169.9 163.9

185.0 110.7 111.9 221 .9 236.7

Cross-section (a) = V

760

273

in 1°

I

283
Photodissociation rate constants for CFC1 3 and CF 2 C1 2 have been calculated for two altitudes using the absorption coefficients applicable at 296 K and 222 K. These are summarized

in Table II. The ratio of the two rates for each condition (same altitude and zenith angle)

provides a measure of the systematic error that can be introduced into a model by ignoring

the change in absorption coefficient with temperature. For CFC1 3 the error is about 15

percent and for CF2 C1 2 it is about 25 percent. These systematic errors probably are maximum

estimates because, when the lower temperature absorption cross sections are used, higher

concentrations will be estimated for higher altitudes where the photodissociation rate con- stant is greater.

The calculations were made using the absorption cross sections averaged over one nano- meter wavelength intervals and direct solar fluxes applicable at 30 and 35 km. Photoabsorption rate constants were calculated for each one nanometer interval from 185 to 230 nm and

summed to give the results in Table X. We are indebted to Dr. John DeLuisi, Environmental

Research Laboratories, NOAA, Boulder Colorado for the solar flux tables used. These are for

a moderately active sun [7], compositions from the U.S. Standard atmosphere for mid latitude

[8], and an average mid latitude ozone model [9]. Absorption by both oxygen [10] and ozone

[11] are included.

REFERENCES

[I] a) M. J. Molina and F. S. Rowland, Nature 249, 810 (1974).

b) F. S. Rowland and M. J. Molina, Rev. Goephys. Space Phys. 1_3, 1 (1 975).

[2] a) J. Doucet, P. Sauvageau, and C. Sandorfy, J. Chem. Phys. 58, 3708 (1973).

b) R. H. Huebner, D. L. Bushnell , Jr., R. H. Celotta, S. R. Mielczarek, and C. E. Kuyatt,

Nature 257, 376 (1975).

[3] R. E. Rebbert and P. J. Ausloos, J. Photochem. 4, 419 (1975).

[4] J. U. White, J. Opt. Soc. Am. 32, 285 (1942).

[5] H. J. Bernstein and G. Herzberg, J. Chem. Phys. 16., 30 (1948).

[6] A. M. Bass and A. H. Laufer, J. Photochem. 2, 465 (1974).

[7] Donnelly and Pope, NOAA-TR-ERL 276-SEL 25.

[8] U.S. Committee on Extension to the Standard Atmosphere, U.S. Standard Atmosphere

Supplements, 1966 (Government Printing Office, Washington, D.C. 1967).

[9] Krueger, NASA-TR-X-91 2-74-291

.

[10] Craig, R. et al . in "The Natural Stratosphere of 1974", Reiter, E., (ed.) (Climatic

Impact Assessment Program Monograph 1, DOT-TST-75-51 (1975)). Chapter 4, Table 4.2

which quotes from the following source: Ackerman, M. in "Mesospheric Models and Relate

Experiments," Fiocco, G. , (ed.), (p. Reidel Publishing Co., Dordrecht, Netherlands

(1971)), pp. 149-1 59.

[II] Ackerman, M., Loc. Cit. Vigroux, E., Ann. Phys. 8, 709-62 (1953).

284
PHOTODISSOCIATION OF CC1 4

R. E. Rebbert and P. Ausloos

National Bureau of Standards

Washington, DC 20234

In several recent investigations [1-3] 1 of the photolysis of CC1 4 it was suggested

that the elimination of a chlorine molecule from an excited state of CC1 4 may occur under

certain conditions.

CC1 4 + hv -> CC1 2 + Cl 2 (1)

The CC1 2 formed in this process was thought [2,3] to insert readily into CC1 4 to form two

CC1 3 radicals

CC1 2 + CC1 4 » C 2 C1 6 * * 2CC1 3 (2)

In order to assess the importance of process 1 relative to that of the Cl-atom

producing fragmentation steps

CC1 4 + hv * CCI3 + CI (3)

* CC1 2 + 2C1 (4)

CC1 4 was photolyzed at 300 °K in the presence of various selective reagent gases such as Br2 , HBr, HC1 , C 2 H 6 and 0 2

. Ethane served mainly as an efficient Cl-atom interceptor,

while HC1 and HBr were found to scavenge CC1 2 species via the insertion reaction

CC1 2 + HX * HCC1 2 X

(where X = CI or Br). No evidence could be found for the insertion of reaction 2. In the

; photolysis of CC1 4 -C 2 H 6 mixtures, CC1 2 radicals react by the recombination reaction

CC1 2 + CH3 CH2 » CH3 CH2 CC1 2 * * CH3 + CH2 CC1 2

. A detailed analysis of the experimental

findings leads to the following quantum yields

1 Figures in brackets indicate literature references at the end of this paper.

285
Table I

Quantum Yields of Primary Radicals Formed in the Photolysis of

CC1 4 and 213.9 and 163.3 nm

213.9 nm 143.3 nm

<|>(C1) 0. 9±0. 15 1.9±0.2

<KCC1 3 ) 0.9±0.15 0.2±0.05

(J)(CC1 2 ) <0.05 0.8±0.1

The primary fragmentation processes 3 and 4 can account for these derived quantum

yields.

In contrast with a previous report [3], no evidence could be found for the occurrence

of process 1 at 213.9 nm independent of pressure.

References

[1] Currie, J., Sidebottom, H. , and Tedder, J., Int. J. Chem. Kin. 6, 481 (1974).

[2] Davis, D. D. , Schmidt, J. F. , Neeley, C. M. , and Hanrahan, R. J., J. Phys. Chem.

79, 11 (1975).

[3] Jayanty, R. K. M. , Simonaitis, R. , and Heicklen, J., J. Photochem. 4, 203 (1975).

286
THE PHOTOLYSIS OF C£0 2

S. Jaffe

California State College

Los Angeles, California

and

W. B. DeMore

Jet Propulsion Laboratory

Pasadena, California

1. Introduction

In a previous study of the photolysis of C£

? - 0 3 mixtures, (C. L. Lin, S. Jaffe, and

W. B. DeMore, presented at Philadelphia ACS meeting, 1975), a mechanism was proposed to account for catalytic destruction of 0 3 which involved the reaction

According to this mechanism, the quantum yields of 0 3 destruction depended mainly on the

I rate of reaction (1) relative to the competing process,

However, the observed quantum yields at room temperature and 1 atm pressure required a ratio

ki/k 2 of approximately unity, this being in disagreement with precious work which indicated

that raction (2) is the dominant path at pressures higher than about 75 torr. (For a review

of earlier work, see R. T. Watson, "Rate Constants of C2.0 of Atmospheric Interest," NBSIR

74-516, June, 1974.)

x

It was therefore considered desirable to obtain independent evidence on the question of

Ci atom regeneration in the Cx.0 disproportionate at high pressures. For this purpose the

photolysis of C£02 was chosen, because the quantum yields of Ci0 2 destruction should be

dependent on the ratio ki/k 2 , as can be seen from the following mechanism:

Cj>0 + CiQ —^C£ + CaOO. (1)

Cs>0 + C£0 —- C£ 2 + 02. (2)

C£0 2 + hv

"ClO + 0 (3)

0 + C£0 2—^C£0 + 0 2 (4)

(4')

C£0 + C£0

-Ca + CaOO —^C£2 + 0 2

(1)

(2)

C£00 + M —-C£ + 0 2 + M (5)

C£ + C£0 2 —-2 C£0. (6)

287
Thus, for k 2 >>k l5 and assuming <|> = 1 , the maximum quantum yield for C&0 2 photolysis would be two. However, for k 2 ^ k : as suggested by the Ci 2 - 0 3 experiments, very high quantum

yields would result because each C£ atom produced can lead to the formation of two C&O radicals by reaction (6). It was therefore considered that quantum yield measurements in

the Ci0 2 photolysis would be a good test for the occurrence of reaction (1) at high pressures

The photolysis was studied as a function of several variables, including pressure,

temperature, and surface.

2. Experimental Methods

^ost of the photolyses were carried out at wavelengths near the maximum C«,0 2 absorption

(3515A), using light from an Oriel medium pressure Hg lamp which was passed through a Bausch

and Lomb monochromator (Cat. No. 33-86-45). Light intensities were measured by means of an Epply gauge, and the fraction of light absorbed was measured by comparison of Epply readings

with the empty cell and the filled cell. Concentrations of Cs,0 2 were determined by means of

a Cagy spectrophotometer, using molar absorption coefficient (decadic) of 3000 M^cm- 1 at

351 5A. The experiments were carried out for the most part in standard cylindrical quartz

eel 1 s

.

3. Results

A. Quantum Yields

The apparent quantum yields were quite high, 30 being a typical value. As discussed in

following sections, the exact quantum yields vary considerably and are dependent on a number

of experimental variables, including the surface.

B. Effect of Pressure

As shown in Figure 1, pressurization with N 2 or 0 2 decreased the rate of photolysis.

Initial rates were faster in the presence of 0 2 than in the presence of N 2

. However, in

experiments with high light intensity the rate with added N 2 increased as the photolysis

proceeded, ultimately exceeding the rate with added 0 2

. No acceleration of rate was observed with added 0 2

.

5

Gm

lO

00

r-HX

c

CD Q

—

i

a

o

+->

Figure 1. Relative Rates of C10 2 Decomposition.

288
C. Effect of Surface

The effect of increased surface area was tested by addition of 0.002 inch diameter

Superbrite glass beads to the bottom of the photolysis cells. Figure 2 shows how the photolytic decomposition rate varied with added surface, with and without added N 2

. It can be seen that the presence of added surface slowed the reaction considerably. This result is consistent with removal of chain carriers at the surface.

Time, Min.

Figure 2. Effect of Surface.

D. Effect of Temperature

The photolysis rate increased by more than an order of magnitude when the temperature

was increased from 25 °C to 45 °C.

4. Discussion

The major conclusions of this work are as follows:

1. The quantum yields of Ca0 2 photolysis at 1 atm total pressure are high, consistent

with k 2 % k 2

. 2. Surface effects are important in determining the actual value of the quantum yield.

It can be shown that this 1s not unexpected.

3. The rate of photolysis decreases upon pressurization with N 2 or 0 2

. This may be due to slight enhancement of reaction (2) relative to (1) by added third body.

4. Increasing temperature increases the quantum yield, consistent with the suggestion

that reaction (1) is enhanced at higher temperature.

.5. The relative effects of added 0 2 and N 2 are complex. However, there is some evidence that 0 2 inhibits reaction (1) relative to (2), based on the observation that acceleration of the photolysis rate was observed with added N 2 but not 0 2

. It is not known why the

initial rate was always faster with 0 2 compared to N 2 , although surface effects may be

Involyed.

289
STRATOSPHERIC REACTIONS OF CHLOROFLUOROMETHANES

R. J. Donovan and H. M. Gillespie

Department of Chemistry

University of Edinburgh

and

J. Wolf rum and K. Kaufmann

Max-Planck-Institut fur

Strbmungsforschung, 34 Gbttingen

1. Introduction

Upper atmosphere reactions involving chlorofl uoromethanes (freons) are of considerable

current interest [1-5] 1. It has been suggested that the diffusion of freons, from the

troposphere into the stratosphere, where photochemical and reaction processes produce free

chlorine atoms, could lead eventually to a depletion of ozone in the upper atmosphere [1,2],

via the C10

x cycle [2,3].

The reaction of 0(2 1 D 2 ) with freons 11-13 (CFC1 3 , CF 2 C1 2 and CF 3 C1) is known to give

rise to CIO radicals [5],

0(iD) + CF

x

Cl y * CIO + CF

x

Cl y

_ 1 (1)

and thus provides a means of direct entry into the C10

x cycle. However, a number of other

channels may contribute to the overall rate of removal of 0( 1 D), viz:

0(iD) + CF

x

Cl y * 0(3P) + CF

x

Cl y

(2)

CF

x

Cl y

_ 1 0 + CI (3)

CF

x_l

C1 y_l

0 + FC1 (° r C1 2) (4)

Little is known about the importance of these other channels, or of the subsequent radical

reactions which can in some cases give rise to further release of CI atoms.

The work to be presented was directed towards obtaining a more detailed understanding

of the primary and secondary processes which take place in the photochemical ly initiated

reaction between 0( ! D) and the chlorof luoromethanes

.

2. Experimental

Two experimental techniques were employed: firstly, nozzle-beam mass spectrometric

sampling from a fast flow system, coupled with flash photolysis (see fig. 1) and secondly,

time resolved optical spectroscopy [5]. 0( 1 D) atoms were produced by photolysis of 0 3 in

l Figures in brackets indicate literature references at the end of this paper.

290
the Hartley Continuun (200-300 nm) and by photolysis of N 2 0 (a^200 nm). In the fast flow

system, reagents were mixed just prior to entry into the flow tube (flow rates of typically

10 m s" 1

, with total pressures in the range 300-400 N m~ 2 and 0 3 :halocarbon:He = 1:10:40, were used). Products of the primary and secondary radical reactions were monitored as a function of time with the mass spectrometer (Extranuclear quadrupole) set to sample a pre- selected mass number. The analogue output from the mass spectrometer was then digitized

and stored in a 200 channel signal averager (Datalab, DL.102A); in most cases it was necessary to average the results of sixteen experiments before adequate signal/noise ratios were observed. Little electronic interference from the flash was observed, however some variation in the interference with mass number was found and was thus checked for all experimental runs. The experimental conditions used for the time resolved optical studies

were similar to those given in reference [5].

3. Nozzle-Beam Mass Spectrometric Sampling Results

Formation of C10 was observed (m/e=51 ) following the photolysis of 0 3 in the presence

of freons 11-13 and CCl^, supporting previous observations using ultraviolet absorption

spectroscopy [5]. No signal at m/e=51 was observed when 0 3 was excluded from the flow

mixtures under otherwise identical conditions. The yield of C10 was found to increase with

increasing partial pressure of 0 3 (over the accessible range, 5-13 N m" 2 ), as expected from

the increase in 0( 1 D) production.

Other products observed immediately following photolysis of mixtures containing 0 3 , were FC1 , CFC10 (both from CFC1 3

) and CF 2 0 from CF 2 C1 2

. Small yields of CC1 2 and CFC1 were

also observed from direct photolysis of CFC1 3 and CF 2 C1 2 , respectively. As these transient

species have the same charge/mass ratio as CFC10 and CF 2 0, a small correction to yields of

the latter species was made.

The formation of FC1 provides direct evidence for the primary process,

O^D) + CFC1 3

-y CC1 2 0 + FC1 (5)

(atomic recombination processes would be too slow to account for our observations on FC1 )

.

4. Optical Spectroscopy Results

Previous work on reaction (1) has been extended using N 2 0 as the source of 0( 1 D). It was shown that the reaction of chlorine atoms with N 2 0, to yield C10, is negligibly slow

under the conditions employed and thus any CI atoms formed in reaction (3) do not contribute

| to C10 formation, (i.e., the C10 observed in the N 2 0 + freon system must result entirely

from reaction (1 ) )

.

In the absence of added freons photolysis of N 2 0 leads to the formation of NO via

the reaction,

0( 1 D) + N 2 0 -)- 2N0 (6)

which provides a convenient means of establishing the initial concentration of 0( 1 D)

produced by photolysis. The absolute concentration of C10 produced when 0( 1 D) reacts with

a freon can then be measured as the extinction coefficient of C10 in the ultraviolet is known [5]. A small correction to the observed C10 concentration is necessary due to removal

'via

C10 + NO - N0 2 + CI (7)

291
By this method the branching ratio into channel (1) was determined as k

1 $-0.4(k 1 +k 2 +k 3 +k lt )

Secondary reactions involving CF

x

Cl _-j radicals and related studies involving bromomethanes will also be discussed. y

"

References

[1] Molina, M. J. and Rowland, F. S., Nature, 249, 810 (1974).

[2] Rowland, F. S. and Molina, M. J., Rev. Geophys. Space Phys., 1_3, 1 (1975).

[3] Stolarski, R. S. and Cicerone, R. J., Can. J. Chem., 52, 1610 (1974).

[4] Pitts, J. N., Sandoval, H. L. and Atkinson, R., Chem. Phys. Lett., 29, 31 (1974).

[5] Gillespie, H. M. and Donovan, R. J., Chem. Phys. Lett., 37, 468 (1976).

292
1 CO o «E

+Jo ai

QJ < CL o Q_

CO CL

=1

m S- s_ CO T3 QJ

ro ro Q_ E CL

cr S S_ rO 4-> QJ S 1 -Q CD- O 1 +-> CD o '

E -£Z N ai CL N +j O l/> c

i/l Q_ -ac CD

ra c CL

reCD

"1

CLE

C5 rO S- L0 ra S. JC CL QJ 00 E 00 cn

ro ra ro i

QJ S_ 4- -O C3 QJ 1 00 > « QJ ro

OJ -O

N +J

=3

N ro +J O c C c =3 cn 3o Cn 00 1

CO

C

4-

0) i_ CO .C OJ ro +J _Q JC

=5

O >> 4- +J ro o 3 => a c o CJ> m o

r- q- -o

+> CL

ro E +J i— 03 ro c U_ 3

QJ 00

00 sz

QJ E oo >

5. QJ ro Q_ +-> ai 00

a

>> >

t/1

U _J CDU.

L

+J cz QJ E oT 3 CL 01 jr E +-> U ro ai Cl CO 00 E E o ro U s_

+-> OJ o oo

QJ

QJ Q_ 3 E oo CL

toPi

293
SOME FUNDAMENTAL AND APPLIED ASPECTS OF THE ATMOSPHERIC

REACTIVITY OF ORGANIC MOLECULES

James N. Pitts, Jr. , Roger Atkinson, Arthur M. Winer, Karen R. Darnall

Alan C. Lloyd and Robert A. Perry

Statewide Air Pollution Research Center

University of California

Riverside, CA 92502

In the last several years, there has been a gratifying increase in the use of fundamental photochemical and kinetic information to obtain a better understanding of atmospheric

chemistry; this has tended to break down somewhat arbitrary distinctions between "basic" and

"applied" research. Thus, for example, it is now widely recognized that the hydroxyl radical is the major chain carrier in the chemistry of both the natural and polluted troposphere

and stratosphere [I] 1

.

A second example involves the current national debate over the degree of control of

photochemically reactive emissions from a variety of industrial and domestic sources. The

significance of this issue has grown with the improvement in the efficiency of control of

emissions from motor vehicles which, of course, increases the relative importance of sta- tionary source emissions.

An important approach to this problem, developed over the last decade, has been the

utilization of photochemical hydrocarbon reactivity scales as the basis for controlling the

formulations of commercial solvents. Such phenomena as rates of NO to N0 2 conversion,

maximum levels, or loss of hydrocarbon have been employed to date to generate such reactivity scales. The vast majority of such data have been obtained by irradiation of N0X -HCair mixtures in smog chambers of a variety of types. However, this use of secondary smog

manifestations, while of great utility, nevertheless has led to some significant inconsistencies when comparisons of various reactivity scales are made.

In this presentation, data will be given for OH radical rate constants determined in our laboratories, together with their use in the development and interpretation of such hydrocarbon reactivity scales. Also, mention will be made of recent experiments concerning the

effects of UV spectral distribution on the rates of product formation in smog chamber experiments with simulated polluted atmospheres.

1 . OH RADICALS REACTION RATES

Rate constants for the reaction of OH radicals with a variety of organic molecules have

been determined in our laboratory by two very different techniques. The first is a flash

photolysis-resonance fluorescence technique in which OH radicals are produced by the pulsed vacuum ultraviolet photolysis ( X>1 05 nm) of H 2 0 and monitored by resonance fluorescence [2].

With this method, absolute rate constants can be determined over a wide range of pressure M 5-650 torr) and temperature (^295-475 °K).

In the second technique, relative rate constants are obtained from the initial rates ofj

disappearance of organics as measured during irradiation of NOx -hydrocarbon-air mixtures in

a 6400 liter, all glass CPyrex) chamber at 1 atmosphere total pressure and at ^305 °K [3-5].

These relative rate constants can be placed on an absolute basis using the literature rate

figures in brackets indicate literature references at the end of this paper.

294
constants for the reaction of OH radicals with either n-butane or isobutene, one or both of

which are included in the NOx -hydrocarbon -air mixture studied. Computer modeling [3,4,6,7]

has shown that in the early stages of such an irradiation the major loss of the hydrocarbon

is by reaction with the hydroxy! radical. For alkanes and aromatics, this is almost the

exclusive reaction loss process, while for alkenes, reaction with 0 3 can become important at later stages of the irradiation when 0 3 is formed. Small corrections for this effect have

been applied wherever necessary.

Tables I and II compare the room temperature rate constants obtained by the flash

photolysis-resonance fluorescence (FP-RF) and all-glass chamber (AGC) techniques with se- lected literature values. It can be seen that in general the agreement between the two techniques is excellent and there is also generally good agreement with literature values,

especially those of Davis and co-workers [8,9] and of Niki and co-workers [10,11]. The

lower value obtained for propene by Morris and Niki [10] using the discharge flow technique

may be due to the reaction being in the fall-off region between second and third order kinetics as has been noted for OH + ethene by Davis and co-workers [9,12] at pressures below

300 torr.

Table 1. Comparison of room temperature rate constants, k, for the reaction

of OH radicals with aromatic hydrocarbons from the present work with

1 iterature values.

Aromatic Hydrocarbon k X 10 12 cm 3 molecule 1 sec

-1

This Work

FP-RF 17 (a)

AGC 3 ik)_

Davis, Bollinger

& Fischer 8 (cj

Benzene 1 24 + 0 12 <3 8 1 .59 ± 0.12

Toluene 5 78 + 0 58 4 2 ± 1 5 6.11 ± 0.40

o-Xyl ene 15 3 + 0 58 12 8 + 3 8

m-Xylene 23 6 + 2 4 23 2 + 1 7

p-Xylene 12 2 + 1 2 12. 3 + 2 5

1 , 2, 3-Trimethyl benzene 26 4 + 2 6 23 + 5

1 ,2,4-Trimethyl benzene 33 5 + 3 4 33 + 5

1 , 3, 5-Trimethyl benzene 47 2 + 4. 8 51 5 + 6 5

Morris I Niki 10 (d)

18.7

a) Total pressure ^50 torr (Ar).

b) Total pressure 1 atmosphere air; rates relative to OH + n-butane placed on an absolute basis using the rate constant for OH + n-butane = 3.0 x 10

"12 cm 3 molecule""1 sec

c) Total pressure 100 torr (He).

d) Total pressure ^1 torr (He), mixture of isomers.

295
Table 2. Comparison of the present room temperature rate constants, k, for the

reaction of OH radicals with propene and the butenes with selected literature

values.

10 11 X k cm 3 molecule -1 sec 1

01 efin This Work

AGC 4 a) FP-RP 18 > 19 Fischer

et al. 9 Morris Wu , Japar.v& Niki 10 & Niki 11 D;

Propene 2.9 ± 0.6 2 51 + 0 25 1 59 ± 0 06

C)

1 .7 2.15

1 -Butene 3 53 + 0 36 2 95 ± 0 28 4.1 2.79

Isobutene 5 07 + 0 51 6.5 4.94

cis-2-Butene 6.5 ± 1.3 5 37 + 0 54 4 26 ± 0 25 6.1 (5.37)

trans-2-Butene 6 99 + 0 70 7.1 6.98

2-Methyl-2-butene 7 8 + 0 9 11 .9

a) Relative to n-butane, placed on an absolute basis using OH + n-butane = 3.0 X 10~12 cm 3 molecule^1 sec

-1

. b) Relative to OH + cis -2-butene, placed on an absolute basis using OH + cis -2-butene = 5.37 X 10"11 cm 3 molecule -1 sec- 1

. c) Davis (private communication, 1976) has obtained a rate constant of 2.6 X 10 -11 cm 3 molecule -1 sec

-1 for propene.

2. APPLICATION OF OH RATE DATA TO DEVELOPMENT OF A HYDROCARBON

REACTIVITY SCALE

Using the OH rate data, we have proposed [13,14] a hydrocarbon reactivity scale based

on the rate of disappearance of hydrocarbons due to reaction with OH, i.e. based on OH rate constants. Previous workers [6,10] have noted correlations between OH reaction rates and

relative rates of NO to N0 2 conversion observed in HQ-N0 X photooxidation studies. We have

derived a five-class scale relative to methane in which the classes differ by factors of 10

The half-lives and reactivity (relative to methane) corresponding to each class in our proposed scale are summarized in Table III.

Table 3. Reactivity scale for hydrocarbons based on rate of

disappearance of the hydrocarbon due to reaction with the

hydroxyl radical 13 ' 14

.

Class Half-Lifea) Reactivity Relative

to Methane (=1)

I >10 days <10

II 1-10 days 10 - 100

III 0.1 - 1 day 100 - 1000

IV 0.01 - 0.1 day 1000 - 10,000

V 0.01 day >10,000

'1/2 0.693/k

0H

[0H],

296
Selected examples of compounds appearing in the various classes are given in Table IV, Our scale re-emphasizes the fact that consideration has to be given to some of the slower

reacting species such as alkanes in order to reduce hydrocarbons to sufficient levels to achieve the Federal air quality standard for photochemical oxidant during episodes of prolonged irradiation (>10-12 hours) [15].

Table 4. Proposed reactivity classification of hydrocarbons based on reaction with the hydroxyl radical 13

.

Class I Class II Class III Class IV* Class V*

Methane Methanol

Acetyl ene

Ethane

n-Butane n-Octane

n-Hexane Xyl enes

Ethene Ethyl toluenes

Benzene Propene

Tol uene 1 -Hexene

Ethyl Acetate Trimethyl

Benzenes

Ethanol

a- & e-Pinene

Isopropyl

Alcohol 1 ,3-Butadiene

Methyl ethyl ketone

Diethyl Ketone

2-Methyl-2-

Butene

Carvomenthene

d-Limonene

*Selected Examples

3. EFFECT OF UV SPECTRAL DISTRIBUTION ON PHOTOCHEMICAL

SMOG MANIFESTATIONS

We have recently observed enhanced photochemical smog manifestations in environmental

chamber irradiations of HC-NOx -air systems under simulated atmospheric conditions, upon

incrementally increasing the short wavelength component of the UV photolyzing radiation

[16]. For example, as shown in Figure 1, a decrease by 15 nm in the short wavelength cut- off (50% transmission point) of filters on the solar simulator caused the ozone maximum in a propene-n-butane-NOx photolysis in the SAPRC evacuable chamber-solar simulator facility to increase from 0.57 to 0.70 ppm and to occur at 2.5 hours instead of 5 to 6.5 hours of irradiation. Similar results were observed for other products such as N0 2 , PAN and formaldehyde. These observations can be shown to be consistent with current chemical mechanisms of

photochemical smog formation. The implications of these results for effects upon urban and

rural smog problems are treated in detail in a forthcoming paper [16].

297
] I ME (min)

Figure 1. Ozone concentration profiles resulting from irradiations of 0.4 ppm propene, 2.0 ppm n-butane and 0.5 ppm N0X with three different Pyrex filters

for solar simulator beam (from reference 16).

298
4. ACKNOWLEDGEMENTS

This work was supported in part by the National Science Foundation (Grants GP-38053X

and MPS73-08638A02) , the National Science Foundation-RANN (Grant No. AEN73-02904-A02) and

the California Air Resources Board (Grant No. 5-067-1 and Contract No. 5-385).

REFERENCES

[I] Pitts, J. N. Jr. and Finlayson, B. J., Angew. Chem., Int. Ed., 1_4, 1 (1 975); Finlayson,

B. J. and Pitts, J. M. Jr., Science, 192, 111 (1976).

[2] Atkinson, R., Hansen, D. A. and Pitts, J. N. Jr., J. Chem. Phys., 62, 3284 (1975);

ibid , 63, 1703 (1975). ~~

[3] Doyle, G. J., Lloyd, A. C, Darnall , K. R., Winer, A. M. and Pitts, J. N. Jr., Environ.

Sci. Technol., 9, 237 (1975).

[4] Lloyd, A. C, Darnall, K. R., Winer, A. M. and Pitts, J. N. Jr., J. Phys. Chem., 80,

789 (1 976). —

[5] Winer, A. M. , Lloyd, A. C, Darnall, K. R. and Pitts, J. N. Jr., J. Phys. Chem., 80,

1 635 (1 976). ~~

[6] Niki, H . , Daby, E. E. and Weinstock, B., Adv. Chem. Ser. No. 113, 16 (1972).

[7] Demerjian, K. L., Kerr, J. A. and Calvert, J. G., Adv. Environ. Sci. Technol., 4, 1 (1 974).

[8] Davis, D. D. , Bollinger, W. and Fischer, S., J. Phys. Chem., 79, 293 (1975).

[9] Fischer, S., Schiff, R., Machado, E., Bollinger, W. and Davis, D. D., "A Kinetic Study

of Several Reactions of the Hydroxyl Radical with Olefinic and Aromatic Compounds,"

169th National ACS Meeting, Philadelphia, PA, April 6-11, 1975.

[10] Morris, E. D. Jr. and Niki, H., J. Phys. Chem., 75, 3640 (1971).

[II] Wu, C. H., Japar, S. M. and Niki, H., J. Environ. Sci. and Health-Environ. Sci. Eng.,

All , 1 91 (1 976).

[12] Davis, D. D., Fischer, S., Schiff, R. , Watson, R. T. and Bollinger, W., J. Chem. Phys.,

63, 1707 (1975).

[13] Darnall, K. R., Lloyd, A. C, Winer, A. M. and Pitts, J. N. Jr., Environ. Sci. Technol.

10, 692 (1976).

[14] Pitts, J. M. Jr., Lloyd, A. C, Winer, A. M., Darnall, K. R. and Doyle, G. J., "Develop

ment and Application of a Hydrocarbon Reactivity Scale Based on Reaction with the

Hydroxyl Radical," Presented at the 69th Air Pollution Control Association Annual

Meeting, Portland, OR, June 27-July 1, 1976.

[15] Altschuller, A. P., in "Proceedings of the Solvent Reactivity Conference," U.S. Environ

mental Protection Agency, Research Triangle, Park, NC, EPA-650/3-74-01 0, November,

1974, pp. 2-5.

[16] Winer, A. M., Breuer, G. M. , Carter, W. P. L., Darnall, K. R., Lloyd, A. C. and Pitts,

J. N. Jr., "Effects of UV Spectral Distribution on the Photochemistry of Simulated

Polluted Atmospheres," to be submitted to Atmospheric Environment, 1976.

,[17] Hansen, D. A., Atkinson, R., Pitts, J. N. Jr., J. Phys. Chem., 79, 1763 (1975).

[18] Atkinson, R. and Pitts, J. N. Jr., J. Chem. Phys., 63, 3591 (1975).

[19] Atkinson, R., Perry, R. A. and Pitts, J. N. Jr., Chem. Phys. Lett., 38, 607 (1976).

299
IR FOURIER-TRANSFORM SPECTROSCOPIC STUDIES OF ATMOSPHERIC REACTIONS

H. Niki, P. Maker, C. Savage and L. Breitenbach

Research Staff

Ford Motor Company

Dearborn, MI 48121

Current knowledge of atmospheric reactions involving polyatomic radicals is severely

limited due primarily to the lack of adequate analytical methods for monitoring reactants

and product. Fourier-transform IR spectroscopy offers a highly sensitive in situ detection

method. In order to maximize system performance in signal detection and in computer-aided

data processing and analysis, we have assembled a rapid scan, high resolution IR Fouriertransform facility based on an EOCOM Model 7001 interferometer, a PDP 11/40 on-line computer,

and in-house software.

The interferometer is equiped with a liq. N 2 cooled HqCdTe detector and is capable of

monitoring IR signals in the 600 ^ 4000 cm

-1 range with Av >_ 1/16 cm" 1 and scanning speed

<10 sec. Spectra of products are derived from interferograms , ratioed against background,

converted to absorbance, and analyzed using linear desynthesis with reference spectra. The

two photochemical reactor-IR absorption cells used in this study are a Pyrex cylinder

(50-cm long, double pass) and a 70-?. Pyrex cylinder (1-m long, 40 pass). The standard

spectra of reactants and products were recorded typically at 0.1 Torr and/or 1 ppm (=2.5 x 10 13 molecule cm

-3

) in the respective cells. The noise level of the product spectra was comparable to 0.5 percent of the reference spectra.

To illustrate this technique, results obtained on the oxidation of HC0 will be presented. The HC0 was produced by Cl-atom sensitized reaction of HCH0 at ppm level in 1 atm.

air. In this system, CO produced by HC0 + 0 2 * CO + H0 2 accounts for up to 80 percent of

the HCH0 reacted. Hydrogen peroxide is a major hydrogen-containing product as expected. In addition, C0 2 and HC00H were observed as products with a combined yield of about 20 percent.

Thus, the addition reaction producing HC000 appears to occur to a small but significant

degree under these conditions. Weak IR signal probably arising from HC000H was detected.

This compound is known to decay to HC00H rapidly. As an attempt to chemically trap the

peroxy radicals, i.e., H02 and HC000, N0 2 was added to the C1 2 -HCH0 system to produce the

corresponding nitrates. These results are shown in Figs. 1 and 2.

Time-resolved product spectra of C1 2 -HCH0-M0 2 mixture at 20 ppm each (Fig. 1) show the

formation of a "new product." The spectrum of this compound is shown in Fig. 2(B), which

was obtained by desynthesizing the difference spectrum (Fig. 2(A)), i.e., difference between

4 and 0 min. spectra in Fig. 1. This spectrum has well-resolved "nitrate-type" bands plus

an OH band at 3540.0 cm

-1 (not shown here). Notably, the "PAN" (peroxyacetyl nitrate)-type"

C = 0 band at 1840 cm

-1 is not present. Thus this new product is likely to be peroxy nitric

acid (PNA) rather than peroxyformyl nitrate. Further evidence for PNA was obtained from the

photolysis of C1 2 -H 2 -N02 mixture in air. The residual spectrum observed in this system is identical to that shown in Fig. 2(B). This compound is unstable and has a lifetime of less

than 5 min. in the dark. Attempts to characterize the kinetics of PNA formation are under

way.

300
HCHO

t - 2 min

3000 2000 1000 (cm-1

)

Figure 1. Photolysis of HCH0-N0 2 -C£ 2 System, 10 ppm Each

301
(A) Difference Spectrum (At = 4 min)

(B) Spectrum of PNA (H00N0

2 )(x2)

1303.9

'1728.2

1396.7

802.7

i

2000 1500 1000 (cm' T

Figure 2. Product Spectra of HCH0-N0 2 -C1 2 System, 10 ppm Each

302
PHOTOCHEMICALLY GENERATED OZONE FROM ISOLATED STRONG POINT SOURCES

D. D. Davis and W. Keifer

Chemistry Department

University of Maryland

College Park, Maryland

In spite of increasingly tight controls on a wide range of emission sources, cities up

and down the east coast of the United States continue to experience very high levels of air

pollution. Of particular concern has been the high level of ozone observed both in urban

is well as non-urban areas of this part of the country. In the Nation's Capital, the summer

bf 1975 delivered yet another bumper crop of days having air pollution index readings of

100 and over.

In probing the question as to where this pollution is coming from, various answers can

De had from various authorities in the field. These answers range from the city automobile

oeing the major source to that of the lush natural summer vegetation of the east coast being

the cause. In general, however, there have been far more predictions made based on desk

calculator experiments than on actual field studies.

To be reported in this talk will be the results of an aircraft field sampling program

:arried out in the vicinity of Washington, D. C. to determine if the air quality of the

Nation's Capital is being controlled from inner city sources, from outlying non-urban sources,

br from both. The aircraft employed in the study was an Essna 205 equipped with an overhead

air sampling duct and sampling instrumentation which included: a Monitor Labs N0/N0 2 /N0

detector; a Monitor Labs Ozone detector; a Meloy S02 , H 2 S, CS 2 (Total Sulfur) Analyzer; &nd

Environmental Labs Condensation Nuclei Counter; an Ecolyzer CO Analyzer, and a Beckman

Total Hydrocarbon Analyzer.

The principal geographical region of investigation in this study was an approximate

12,000 square mile corridor south-southwest of Washington, D.C. One of the major reasons

for the selection of this region was an earlier observation that summer winds from the

south-southwest have resulted in the highest air pollution index readings recorded in the

Washington, D.C. area.

A total of 45 to 50 flights have been made in this corridor over the period of October

1973 to June 1976. Much of this flying has been concentrated on detailed studies of power

plants in the area and has taken place during late spring, summer, and early fall, periods

during which Washington, D. C, like other cities, has experienced its highest air pollution

index readings.

As indicated earlier in the text, one of the major questions which we hoped to explore

fin this study was that of why SW or southerly winds typically result in the highest air

pollution index readings in D.C. The correlation between the air quality index and wind

direction is most definitely not a simple one since a very large number of meteorological

factors can be involved (i.e., wind variability in speed and direction, surface winds vs. winds at one to two thousand feet, humidity, cloud cover, and temperature).

The results of the study to date have shown that the high index readings recorded with

SW or southerly winds do not, in general, reflect strong sources being present in the

immediate southern part of the greater Metropolitan Washington, D.C. area. In fact, what

was observed was that typically under conditions of high temperature, high humidity, and

high solar flux, ozone levels 1.5 to 3 times that of ambient air were found blowing toward

or into the Washington, D.C. area from distances going out to anywhere from 50 to 100 miles.

303
During one of the worst air pollution periods in D.C., with SW winds, levels of ozone in Washington, D.C. -Richmond corridor reached into the 200 to 300 ppb range.

The above information is not to be interpreted, we believe, as a direct indication that

all air pollution problems in D.C. originate from out-of-town air masses. The air quality

of any large city is most definitely the result of multidimensional sources. In the case

of the greater Washington, D.C. area, the high density of automobiles is clearly a major

contributing factor. Our new findings, however, would also indicate that the air mass moving into the city from far out into the countryside also appears to be a major contributor of photochemical oxidant.

One of the important questions to be answered concerning the observed high ozone levels

to the southwest and south of D.C, is that of the source. Thus far, our studies have shown

that at least four sources are involved, (1) automobile traffic on Interstate 95 between

Washington and Richmond, (2) two large power plants located at Possum Point, Virginia, on the Potomac River and Morgantown, Maryland, on the Potomac River, (3) a cellophane factory

located at Fredericksburg, Virginia, and, (4) the urban plume from Richmond, Virginia, which

itself is a complex mixture of point sources. A final source which should be mentioned but

cannot be well-defined is the diffuse source still upwind of the corridor which we generally | refer to as ambient air. Significant variations in the level of ozone in this upwind ambient I air obviously also reflect multidimensional sources consisting of highly diluted urban plumes I diluted strong point source plumes, regional photochemistry involving natural constituents

from forest, and to some extent stratospheric injected ozone. Details on each of the sources mentioned in conjunction with the Washington, D.C. -Richmond corridor will be

discussed. Suffice it to say at this point that a quantitative assessment of the con- tribution of ozone from each of these sources will require much further investigation.

A final comment that should be made to maintain some perspective on the problem being

discussed is the following. Although Washington, D.C. has been placed as the focal point

of a site being affected by external pollution sources, this in no way implies that D.C. itself is not a major air pollution source for other cities downwind from it. Data, in

fact, will also be presented which shows this phenomenon. I'.':!

304
EVIDENCE FOR ALKOXY RADICAL ISOMERIZATION IN C h -C 6 ALKANES IN NO

x ~AIR SYSTEMS

William P. L. Carter, Karen R. Darnall, Alan C. Lloyd,

Arthur M. Winer, and James N. Pitts, Jr. Statewide Air Pollution Research Center

University of California

Riverside, California 92502

Most photochemical smog models, whether detailed or lumped, predict that the following

are the most important reactions in the photooxidation of alkanes in the presence of oxides

of nitrogen under ambient conditions:

RH + OH -> R + H 2 0 where R = CH 3 , C 2 H 5 , etc. (1)

R + 0 2 > R02 (2)

R02 + NO * R0 + N02 (3)

R0 * R

1 + R2 CH0 (4)

R

3 CH 2 0 + 0 2 > H0 2 + R

3 CH0 (5)

To obtain insight concerning the adequacy of this simple mechanism, data concerning

other possible reactions of the major radical intermediates is required. In view of the

paucity of data obtained under ambient conditions concerning the mechanism and products of

alkane oxidations, we have examined the extensive gas phase studies of free radical reactions

and hydrocarbon oxidation carried out at temperatures of ^250 °C and above. A type of

reaction commonly encountered at these temperatures is the isomerization of alkyl, alkoxy,

and alkylperoxy radicals via internal hydrogen (H) abstractions. Similar intramolecular

reactions have been postulated and observed in liquid phase studies at temperatures close to ambient. However, to the best of our knowledge, these processes have not been incorporated

in mechanisms proposed for the hydrocarbon oxidations occurring in photochemical air

pollution.

We have estimated Arrhenius parameters in order to permit calculation of approximate

rate constants (at 300 K) for intramolecular hydrogen shift reactions for alkyl, alkoxy, and

alkylperoxy radicals which are expected to be involved in the photooxidation of C^-Ce alkanes.

To evaluate the potential significance of the radical isomerizations under ambient conditions,

jwe have compared their rates as calculated to the expected rates for the competing processes

with 0 2 or NO which are believed to be important in the atmosphere. While isomerizations

i of alkyl, and probably most alkylperoxy radicals, do not appear to be important, we find

that the possibility of 1,5 hydrogen shifts in alkoxy radicals with 6 hydrogens must be

considered.

Figure 1 shows the reaction sequence and products predicted in the photooxidation of

n-pentane with and without the proposed alkoxy radical 1,5 H shift isomerization. If the

OH radical abstracts a hydrogen from the 3 position in n-pentane, no rapid isomerization is possible while rapid isomerization is predicted following abstraction from the 1 or 2 position.

In order to test these predictions and to ascertain the importance of intramolecular

alkoxy radical isomerization in photochemical air pollution, we have performed a series

of experiments under simulated atmospheric conditions in two environmental chambers.

305
Figure 1. Suggested sequence of major reactions following abstraction from ji-pentane by

hydroxyl radicals in r^-CsH^-NOv-air mixtures. The relative percentages for the OH reactions are derived from the data of Greiner (1970).

Irradiations of alkane Xn_ butane, in-pentane or r^-hexane)-N0x systems in air were carried

out in a 5800 liter evacuable, Teflon-coated environmental chamber using a 25,000 watt solar

simulator. Analogous studies were carried out in a 6400 liter all-glass environmental

chamber equipped with fluorescent black lights. Detailed results from the experiments in both chambers will be presented.

Computer kinetic model calculations with and without the 1,5 H shift reaction of

alkoxy radicals were carried out simulating the experimental systems studied. Results of

calculations of the yields of the pentanone isomers for a selected n-pentane experiment

(Run 164) are shown in table 1. The pentanone yields predicted when alkoxy radical isomerization is not included reflect the relative rates of OH abstraction shown in figure 1. On the other hand, the calculated yields of 1- and 2-pentanone are significantly lower when

i somen" zation of their precursors is assumed.

The experimental results for four n-pentane-N0x -air irradiations are also shown in table 1. It is clear from the results of Run 164 (glass chamber) that both the relative

and absolute yields of the pentanone isomers are consistent with the predicted importance

of 1,5 hydrogen shift isomeri zation of the pentoxy radicals. Results from the evacuable

(Teflon-lined) chamber for the same initial n-pentane concentration are in excellent agree- ment. Data for both higher and lower ji-pentane concentrations give the same relative

yields of the pentanone isomers, although the absolute amounts vary as expected. Additional

support for the occurrence of intramolecular alkoxy radical isomerization is found in the

yields of acetaldehyde and propionaldehyde, both of which are expected decomposition product:

of the 2- and 3-pentoxy radicals under the experimental conditions employed.

Similar results will be presented for studies of the ii-butane and ji-hexane photooxidation systems.

306
Table 1. Calculated and observed absolute yields and percentages of pentanone Isomers from the n-pentane-NOx-air photooxidation under a variety of conditions.

Product Yields In ppb

(X of total pentanones) 1-Pentanone 2-Pentanone 3-Pcntanone

Ho Isomerlzatlon 31 ppb 83 ppb 40 ppb

(202) (54%) (26Z)

With Isomerlzatlon 0.2 9 30

(1%) (23%) (77X)

Initial Run Chamber HC (pp«) NOy

164 Glass

135 Evacuable 4 (Teflon-lined)

165 Glass

166 Glass

35

0.1

EXPERIMENTAL DATA

b)

b)

b)

b)

8(21X)

8

(167.)

32

(172)

<1

(0-175:)

30

(79X)

h2(84X)

160

(83X)

(100-83%)

Using Initial conditions of Run 164. Similar X yields were obtained using initial conditions of other runs and are omitted for clarity.

None observed; detection limit 0.5 ppb.

307
Table 1. Calculated and observed absolute yields and percentages of pentanone isomers

from the n-pentane-NO

x -air photooxidation under a variety of conditions.

Product Yields in ppb

[% of total pentanones)

No Isomerization

With Isomerization

1-Pentanone

MODEL CALCULATIONS

3 ^

31 ppb

(20%)

0.2

(1%)

EXPERIMENTAL DATA

Run Chamber

Initial

HC (ppm) N0V

164 Glass

135 Evacuable

(Teflon-1 ined)

165 Glass

166 Glass

0.9

0.1

35

0.4 0.1

b)

b)

b)

b)

2-Pentanone 3-Pentanone

83 ppb

(54%)

9(23%)

8(21%)

8(16%)

32

(17%)

:1

40 ppb

(26%)

30

(77%)

(0-17%)

30

(79%)

42

(84%)

160

(83%)

(100-83%)

lis

a)

b)

Using initial conditions of Run 164. Similar percent yields were obtained using

initial conditions of other runs and are omitted for clarity.

None observed; detection limit 0.5 ppb.

On the basis of our work, it appears that alkoxy radical isomerizations, which are well

known from elevated temperature free radical and oxidation studies as well as in certain

liquid phase systems around room temperature do, in fact, occur under photochemical smog

conditions at ambient temperatures. The implications of these results for alkane oxidation

under ambient conditions, including the formulation of photochemical smog models which

incorporate higher alkanes {>C k ), will be discussed. Ism

This work was supported in part by the National Science Foundation-RANN (Grant No. AEN73-02904-A02) . The contents do not necessarily reflect the views and policies of the

National Science Foundation-RANN, nor does mention of trade names or commercial products

constitute endorsement or recommendation for use. The authors also gratefully acknowledge

support of the California Air Resources Board (Grant No. 5-067-1 and Contract No. 2-377) for

the construction of the chamber facility employed in this study.

308
COLLISIONAL DESTRUCTION OF ROVIBRONIC LEVELS IN Si GLYOXAL: ELECTRONIC,

VIBRATIONAL AND ROTATIONAL STATE CHANGES

L. G. Anderson, A. E. W. Knight, and C. S. Parmenter

Department of Chemistry

Indiana University

Bloomington, IN 47401

At pressures above TOO mtorr, the decay of Si glyoxal (CHOCHO) is dominated by collisions. In addition to collision-induced vibrational and rotational relaxation within the

Si state, every collision partner so far tried is effective in destroying the S : electronic

state itself. The competition between these channels and some fine details of collisional

state changes can be worked out with unusual clarity because of the open structure in both

JSj-Sq fluorescence and absorption spectra.

'I Early studies identified the collisional destruction of the electronic state as collision-induced intersystem crossing [I] 1

. Subsequent investigation_of fluorescence

lifetimes and quenching from low Si vibrational levels (e^ <_ 2000 cm" 1 ) characterized

this crossing as "small molecule" Si-T mixing [2-4]. This transforms to intermediate mixing as increasingly higher levels are pumped by the exciting light, and finally, statistical decay is observed from very high Sj levels [5].

Quenching cross sections a for destruction of the Si0° level by collision-induced

triplet formation are known for over twenty collision partners [2-4]. Several correlations

of these cross sections have been given [6,7]. We find another and particularly simple

relationship, namely In a a ^qq)

1 /

2

' wnere £qq is the potential well depth between pairs

of quenching molecules (He-He, CO-CO, etc.). This is effective not only for correlation of cross sections for Si glyoxal quenching, but also for correlation of cross sections in numerous other studies spanning vibrational and rotational relaxation as well as collision-induced predissociation. When considering cross sections of a series of quenchers Q

acting on a single excited species S, the parameter Uqq) 1 /

2 is proportional to the well

depth e SQ . Thus the correlation is indicative of interactions using primarily the attractive part of the potential, and consistent with this view, we observe that the correlation

is a poor representation of relative cross sections in cases where collisions are inefficient in bringing about state changes.

We have deduced the absolute cross sections for rotational, vibrational and electronic

state changes after pumping narrow rotational distributions in the 0° or the 7

1 levels of

the Si state (v 7 is the lowest frequency fundamental, 235 cm" 1 ). We observe that the cross sections are all large and competitive with each other. They are given specifically in Table 1 for the collision partners S 0 glyoxal and Ar. They are in the form of the second

order rate constants in units of 10 6 torr" 1 s" 1

. A hard sphere collision has in these

units a rate constant of about 10.

figures in brackets indicate literature references at the end of this paper.

309
Table I. Rate constants (in 10 6 torr~ 1 s~

1 ) for destruction

of J A levels in glyoxal -Hard sphere £ 10

State

change

from

G level 0°Ar

from level 7

1 G Ar

el 2 0.7 2 0.7

vib 3 0.7 13 3 rot 21 12 10 11

The analysis of rotational structure in 12 Si~S 0 absorption bands by Ramsay and co- workers has allowed us to detect a selection rule for rotational relaxation in experiments

which observe fluorescence at ca. 0.5 cm" 1 Resolution after excitation of small groups of

rotational levels with an Av = 0.33 cm" 1 Ar laser line. We observe in glyoxal -glyoxal

collisions that cross sections for aK = + 2 collisions far exceed those of other rotational state changes.

A curious selection rule appears in vibrational relaxation effected by glyoxalglyoxal collisions after pumping the level 7

2

. Fluorescence structure shows that the cross section for Av 7 = -2 is large whereas that for the vibrational change Av 7 = -1 is relatively small. The fluorescence spectra display this in two ways. First, we observe

at pressures where colli si onal effects are first introduced that the 08 band is substantially brighter in fluorescence than the 7} band (these relative intensities represent

the respective 0° and 7

1 level populations). Second, we observe that the "spiky" rotational structure in the 1\ fluorescence band at these low pressures is mirrored by spiky

structure in the developing 08 band, whereas the rotational structure in the 7} band is substantially without these "non-Boltzmann" features. Apparently the cross section for

0° population after 7

2 excitation is competitive with that for rotational relaxation

within the 7

2 level itself. As a result, the rotational disequilibrium produced in 7

2 upon excitation is largely retained during the initial stages of 0° population. On the

other hand, the cross section for 7

1 population is sufficiently small so that appreciable

progress towards rotational equilibration in the 7

2 level occurs before collisions

populate 7

1

. Thus a near Boltzmann rotational distribution is transfered to the level

7

1 during its initial stages of growth.

REFERENCES

[1] L. G. Anderson, C. S. Parmenter, H. M. Poland, J. D. Rau, Chem. Phys. Lett. 8, 232

(1971).

[2] J. T. Yardley, G. W. Holleman, and J. L. Steinfeld, Chem. Phys. Lett. 10, 266 (1971),

[3] L. G. Anderson, C. S. Parmenter, and H. M. Poland, Chem. Phys. 1_, 401 (1973).

[4] R. A. Beyer, P. F. Zittel, and W. C. Lineberger, J. Chem. Phys. 62, 4016, 4024

(1975). —

[5] R. van der Werf, E. Schutten, J. Kommandeur, Chem. Phys. 1J_, 281 (1975).

[6] J. E. Selwin and J. I. Steinfeld, Chem. Phys. Lett. 4, 217 (1969).

[7] C. A. Thayer and J. T. Yardley, J. Chem. Phys. 57, 3992 (1972).

310
THE VIBRONIC DEPENDENCE OF GLYOXAL PHOTODISSOCIATION [l]

1

George H. Atkinson and C. G. Venkatesh

Department of Chemistry

Syracuse University

Syracuse, New York 13210

The selective excitation of single vibronic levels (SVL) in polyatomic molecules has

Iteen established as a very useful method for examining state-to-state relaxation. The

•elaxation mechanism is usually monitored through the detection of radiative decay channels

mostly fluorescence). As a consequence, only the net effect of all nonradiative channels

s usually measured even though several processes may be simultaneously contributing to the

iiechanism (e.g., internal conversion, intersystem crossing, vibrational relaxation and

ihotochemistry) . A significantly more detailed view of the relaxation mechanism can be

ibtained by monitoring only one of these nonradiative channels in conjunction with SVL

(xcitation.

We report in this paper a study of the photodissociation mechanism of glyoxal based

in the direct observation of a dissociation product following the selective excitation of

ingle vibronic levels in the parent molecule. The study was carried out in a low pressure

•egime where the collisional perturbations on excited-state processes can be controlled.

t is intended to establish the relationship between the vibronic character of the initially

iopulated X A level and the net efficiency for dissociation into a specific product, CO. elective exHitation was obtained from a tuned dye laser and the concentrations of CO are

measured by resonance fluorescence. To our knowledge, this is the first reported study

ombining these two experimental techniques for measurements of SVL photodissociation.

Single vibronic levels in glyoxal vapor were selectively populated by a tuned (0.1 A andwidth) nitrogen laser pumped dye laser. The absolute wavelength of the dye laser

ladiation was set by means of a one mgter spectrometer working in the second order of an

i 800 g/mm grating (resolution > 0.08 A). The pulsed laser radiation was detected by a hotomultiplier and the signal analyzed with a box car integrator.

Carbon monoxide was detected by resonance emission from the fourth positive system of

i0 in the vacuum ultraviolet. [2] A xenon lamp, powered by a microwave discharge (2450

jlHz), initiated the resonance emission from CO which was detected by a solar blind

jhotomultiplier (10 6 gain) with a Csl photocathode. The photomul tipl ier signal was mplified and the total signal offset in order to suppress contributions from dark current

nd scattered resonance lamp emission. The resulting signal was measured on a voltmeter.

;he signal was calibrated by using known pressures of high purity CO measured by a apacitance manometer. The error in the linearity of the entire detection technique was

ess than 1%.

The relationships between the <t> co and the SVL initially populated are shown in igurel as a function of glyoxal pressure. Four distinct phenomena are observed: for a oecific SVL, (1) the tj> co increased with pressure below approximately 1 torr and

?) decreases with pressure above approximately 1 torr, (3) the value cj> rn

at a given assure depends on the vibronic level initially populated, and (4) the rate of decrease of

:o with pressure above 1 torr is a function of the SVL initially populated.

o

gures in brackets indicate literature references at the end of this paper.

311
1.4

T 5208 A (TRIPLET)

• 4403 A (8o)

x 4552 A (0°)

• 4528 A (7'0 ) 4362 A (8o7?)

A 4447 A (5q) b 4358 A

0

3 4 5 6 7

GLYOXAL PRESSURE (TORR)

Figure 1. Quantum yield of CO following excitation to SVL in glyoxal versus glyoxal pressure

The change in pressure dependence near 1 torr reflects the presence of two competing

decay channels. Below 1 torr, col 1 isional ly-induced intersystem crossing 1 A to 3 A

u levels appears to control the net efficiency for CO production by virtue of observation 1. Above

1 torr, control of the mechanism shifts to the competive decay of the intermediate

designated as H. The intermediate H was first proposed by Yardley [3] in an analysis of

data obtained by Parmenter. [4] It is formed subsequent to the population of 3 A glyoxal

and undergoes first order decay to yield CO. Coll isional quenching of H occurs in competition with CO formation and accounts for the inverse pressure dependence of 4>pq at higher pressures. Plots of <t> r n

-1 versus the pressure of glyoxal for this pressure

region are linear for all SVL Studied.

The dependence of <}> Cq

on the initial SVL populated obtains from the presence of a non- Boltzmann distribution of energy among the vibrational levels in the ] A state in this pressure region. Fluorescence and collisionally-induced vibrational relaxation (VR) and

intersystem crossing (ISC) tcr ?A all compete to relax the initially pumped SVL. ISC leads

to CO production via the decay ot 3 A levels and the specie H. VR redistributes energy within the *A state among a variety of vibrational levels. This distribution remains

non-Bol tzmann up to pressure of approximately 10 torr. SVL fluorescence spectra have been

obtained which establish the existence of these unique vibronic distributions in this

pressure regime. At a given pressure and for a specific SVL excitation, this unique non- Boltzmann distribution determines the total amont of ISC occuring from 'A to 3 A and

312
thereby controls the <{>~q. Thus, it is from these distributions that the vibronic dependence of <t>f. Q

at a given pressure arises. In a completely analogous way, the rate of

decrease or <j> CQ with glyoxal pressure for excitation of specific SVL also derives from

;hese non-equilibrated vibrational distributions within *A .

REFERENCES

;i] Supported by the U.S. Army Office of Research - Durham under Grant DAHC04-75-G-0104.

"2] T. G. Slanger and G. Black, J. Chem. Phy., 55, 2164 (1971) and references therein.

3] J. T. Yardley, J. Chem. Phys. , 56, 6192 (1972).

4] C. S. Parmenter, J. Chem. Phys., 41_, 658 (1964).

313
BEHAVIOUR OF BENZENE IN LOW VIBRATIONAL LEVELS

S. A. Lee, J. M. White and W. A. Noyes, Jr . Department of Chemistry

University of Texas

Austin, Texas 78712

Fluorescence, triplet state, and isomerization yields from benzene vapor have been

determined at several wavelengths but principally at 266.8 nm where L B2 molecules are formed in the zeroth vibrational level. Fluorescent yields were determined using 0.18

for benzene vapor at 20 torr and 253 nm as a standard. Triplet yields were determined by two methods: 1) The sensitized triplet emission of biacetyl and 2) The sensitized isomerization of the 2-butenes. The former has the advantage of using much smaller amounts of

foreign gas and hence collisional vibrational equilibration is also smaller. When used

under comparable conditions the two methods give comparable results.

It is well known that there are many isomers of benzene, but benzvalene seems to be

the only one which should concern us here. It absorbs strongly from 210 to 230 nm. The

production and destruction of benzvalene was studied using a conventional photolysis

system. In these experiments cis-2-butene was mixed with the benzene prior to irradiation

as a means of removing triplet benzene. Differential absorption spectra were measured

after the photolysis with a Cary-14 spectrometer using a benzene blank. The relative

amount of benzvalene produced was estimated semiquantitatively by measuring the area under

the absorption curve in the region 210-230 nm. In some experiments, a benzvalenebenzene mixture, produced by photolysis at one wavelength were rephotolyzed at a second

wavelength. From the resulting data we are able to draw some conclusions about the wavelength dependence of the probability of benzvalene formation and destruction.

The 1 S 2 molecules in their zeroth vibrational level show no variation of fluorescent

yield with pressure until pressures of 20 torr are reached. At high pressure Q f decreases to a value near 0.18, the same value reached at 253 nm and at 20 torr. This might be interpreted to by the yield from ! B 2 molecules with vibrational energy

equilibrated with the surroundings. Thus at low vibrational levels Q f appears not to increase with increases in vibrational energy. Below about 20 torr and at 266.8 nm, is

constant at 0.22. This result implies that the variation in Q~ with pressure at

253 nm or 259 nm originates with the occupation of different vibronic levels. Presumably

equilibration of vibrational energy in the ! B 2 state does not give the yield of 0.22

because increasing the pressure of inert gases

u causes the yield to be lowered. The

acquisition of vibrational energy at the zero-point energy level is apparently much slower

than the loss of vibrational energy from high levels. To explain the difference between

these two yields, it is suggested that the thermal equilibration of vibrational energy

following absorption at 253 or 259 nm leads to appreciable population of the 237 cm" 1 vibrational level (this is the same mode as the 398 cm" 1 vibrational level in the ground

state and is an out-of-plane motion). Emission from this level to the ground state would

not be symmetry forbidden but should have a very low transition probability due to the

Franck-Condon principle. Population of 237 cm" 1 vibrational level as the result of collisions would make this level important after excitation at shorter wavelength but it would 1 not be so important in the processes occuring after excitation at 266.8 nm.

314
In the isomerization study, it was found there was no benzvalene formation observed

at 266.8 nm, but as shorter wavelengths benzvalene is formed and the steady state concentration is higher at shorter wavelengths. Illumination at 253 nm up to the steady state

followed by irradiation at 266.8 nm caused benzvalene to disappear. Even though

benzvalene is not formed at 266.8 nm, it is destroyed by a benzene sensitized reaction,

since benzvalene quenches 1 B 2u (v=0) molecules presumably energy transfer can occur from

benzvalene to 1 B 2u

molecules and results in such electronic relaxation. Illumination at

i253 nm followed by irradiation at 247.2 nm caused the benzvalene to increase. This

suggests that the photostationary state depends on the excitation wavelength.

It has been assumed that photons absorbed by benzene caused fluorescence, triplet

state formation, and isomerization. A fourth possibility, a radiationless transition

from the 1 B 2u

state (or possibly the 3 B

lu state) to the ground state, has been neglected.

|<\t 266.8 nm the isomerization yield seems to be zero at low pressures and the sum of Q-r and triplet yield is about 0.97 and thus nearly all the input photons can be accounted

for. The energy balance is not found at any shorter wavelength, and is partly due to aenzvalene formation, but since the primary isomerization yields are not known, it is difficult to have complete accountability of the absorbed photons. Benzvalene appears

to quench the 1 B 2u state of benzene so that fluorescent yields tend to decrease with

jincrease in exposure.

i

The authors wish to thank the Robert A. Welch Foundation for financial support of the

work described in this abstract.

315
DUAL LIFETIME FLUORESCENCE FROM PYRIMIDINE

Kenneth G. Spears and Mahmoud El-Manguch

Department of Chemistry

Northwestern University

Evanston, Illinois 60201

The 1,3 diazabenzene (pyrimidine) 1 B 1 (n-nr*) electronic state has a nonradiative

relaxation characterized by a dual lifetime in low pressure gas phase emission. The

molecule is especially interesting as a system with_non-bonded electrons, substantial spinorbit coupling, and a small S-T energy gap (1800 cm" 1 ). A primary goal is to understand

the nature of the S-T coupling with respect to the low triplet state density and non- bonding electrons.

Early work on the molecule has been substantial and a partial list of references

include a critical review of the electronic state spectroscopy of all azabenzenes [l] 1

, the detailed spectroscopy of pyrimidine l Bi [2] and condensed phase analysis of

fluorescence and phosphorescence yields [3]. The recent work of Parmenter and associates [4] has analyzed the fluorescence emission spectroscopy (SVL) and re-assigned

some bands in the absorption spectrum. Recent work by Parmenter and Knight has shown

that the molecule is very sensitive to collisions with self quenching rates higher than

expectations based on hard sphere collision diameters. We have reported our early

measurements on pyrimidine excited to several higher vibrational levels above the 0-0 band

200 cm" 1 ) [5]. We found a dual emission lifetime for these states with lifetimes near

1 nsec and 100-200 nsec at 0.1 Torr. Very recent work of Uchida, Yamazaki and Baba [6]

has also reported dual emission but no precise measurements of longer lifetimes were

possible.

We have extended our results to many absorption bands, including the 0-0 band. We find that the lower bands fit the model of Lahmani, Tramer and Trie [7] for a reversible

intersystem crossing. The data fit the case of a forward crossing rate k. , a reverse

crossing rate k r>

a singlet fluorescence decay rate k^, and a singlet internal conversion

rate k^. For the case of observed dual lifetime emission (k is the fast decay) in the

form I(t) = C e" s t + C, e~'

< L

t we can derive the individual parameters k., k , k^, k. , the

s l

r

i r f vc number of effective states, N, coupling to the singlet state, the average triplet

density of states p.

Our experiments measure C

s

, k

s

, C

L

, k, as well as relative quantum yields for various

bands. We have used a recent absolute fluorescence yield measurement of Parmenter in our calculated results using the case 1 simplifications of the model [1] as well as computer

fits to the equations. Our initial experimental results on the 0-0, 6a 1

, and 12 1 vibronic

bands show that the ratio C

S /C

L is a constant from 0.01 to 0.05 torr. The quenching of

the long lifetime is very efficient with cross c sections of 1.0 X 10 8 torr

_1 sec-1 corresponding to a collision diameter of ^ 16 A 3 times hard sphere). The respective

figures in brackets indicate literature references at the end of this paper.

316
zero pressure long lifetimes and short lifetimes of the 0-0, 6a 1

, and 12 1 bands are (1100,

1.8), (932, 1.6), (643, 1.5) nanoseconds. The following table contains some of the

results of the model

.

state E(cm

_1

) p(cm) N K. x 10" 7 (sec

_1

) ic

Kfx 10" 6 (sec

_1

)

0-0 0 41 6 0.44 2.4

6a 1 613 1100 34 3.5 2.7

12 1 1012 2500 73 11 .0 3.0

The constancy of radiative lifetime is a good check on the quality of the data but an explanation is needed for the rapidly changing internal conversion rate constant. The

number of coupled energy_level s in the triplet, N, are too large to be vibronic levels with

an energy gap of 1800 cm" 1

. The density, p, must be interpreted as rotational level densities. The results indicate that the initially populated singlet state evolves to a triplet

and later in time there is some probability to find molecules back in the singlet where

they can either emit or internally convert to the ground state. Rotational relaxation by

collision is sufficient to remove the molecule from those rotational states that are coupled to the singlet thereby giving an effective wuenching of emission.

The experimental apparatus used in these time-resolved measurements is capable of very

high accuracy for dual lifetimes, especially in the case of very different intensities and

j lifetimes. We have developed an argon-ion laser pumped dye laser in which the dye cavity

is acousto-optically mode-locked and synchronously cavity dumped. For high resolution

studies with etalon line narrowing the cavity dumper will give 20 nsec duration pulses

with frequencies of up to 5 MHz. The mode locked and dumped cavity yields sub-nanosecond

pulses with selected rates as high as 5 MHz. The output of this laser has reasonable

peak power pulses for creation of tunable ultraviolet. The temperature tuned SHG of 5 MHz mode-locked pulse trains depends on laser mode quality and the crystal quality but

0.5 - 3.0 mW average power is typical with 25 - 200 mW average visible input. The lifetime

measurement is by time-correlated photon counting. In this method the time difference

between an excitation pulse and detected photon is stored in a multichannel analyzer to yield number of counts as a function of time. By running an inverted time-to-amplitude

( converter (TAC) we can count every emitted photon and assign its time of emission with

I very low background. In case of time separated spectra we either run complete time

spectra at each wavelength or separate time regions using a discriminators on the TAC

output. The high resolution studies use the latter method with simultaneous absorption

spectra and interferometer measurement of wavelength.

REFERENCES

[]] K. K. Innes, J. P. Byrne and I. G. Ross, J. Mole. Spectr., 22, 125 (1967).

[2] K. K. Innes, H. D. McSwiney, Jr., J. D. Simmons, and S. G. Tilford, J. Mole. Spectr.,

31, 76 (1969).

[3] B. J. Cohen and L. Goodman, J. Chem. Phys., 46, 713 (1967).

'[4] A. E. W. Knight, C. M. Lawburgh and C. S. Parmenter, J. Chem. Phys., 63, 4334 (1975).

i<|[5] K. G. Spears and M. El-Manguch, Paper at Molecular Spectroscopy Symposium, Columbus,

Ohio, June 1975. Paper at VIII International Conference on Photochemistry, Edmonton,

Canada, Aug. 1975.

[6] K. Uchida, I. Yamazaki and H. Baba, Chem. Phys. Letters, 38, 133 (1976).

E'7] F. Lahmani, A. Tramer and C. Trie, J. Chem. Phys., 60, 4431 (1974).

18] C. Parmenter, private communication.

317
TIME RESOLVED EMISSION SPECTRA OF LOW PRESSURE AROMATIC MOLECULES

M. D. Swords , A. W. Sloman, and D. P. Phillips

Department of Chemistry

The University

Southampton S09 5NH, U.K.

An apparatus has been constructed which has enable us to obtain medium resolution

time resolved fluorescence spectra of low pressure gases. The principal experimental

features are:

(i) Mode-locked Ar

+

ion laser with A.D.P. frequency doubling crystal.

o (ii) 1 metre monochromator (8 A/mm dispersion) and fast linear focussed

photomultiplier detection.

(iii) Nanosecond time resolution gating electronics.

(iv) Multichannel scaler

A schematic diagram of the experimental assembly is illustrated below.

Mode locked

Ar Ion Laser

Teletype

Computer

A.D.P.

Crystal

a- 5145 A 2572

Synchronous

Trigger Pulse

Multichannel

r

1

5 A

and

Graph Plotter

Scaler

J

sample

1 ram | Monochromator

Gating

j Electronics

Fast Linear

Focussed P.M. Tube

Figures in brackets indicate literature references at the end of this paper.

318
The principal features to note are:

(i) The laser is a Spectra Physics 166/366 fitted with a combination modelocker and cavity dump. This enables pulse repetition rates from single

shot to 100 MHz to be generated. The integrated average power for the

5145 A line is typically 7-800 mW at 5 MHz for the 10 ns (f.w.h.m.)

gaussian cavity dumped pulses. This decreases to 50 mW average of 500 ps mode-locked pulses at the same repetition rate. Frequency doubling is 2-3% efficient and reduces the pulse width by a factor of / 1/2 in the

normal manner.

(ii) The gating system, based on Emitter Coupled Logic circuitry is of novel

design and has been reported elsewhere [I] 1

. Information can be processed

at the rate of 5 MHz with a minimum operating gate width of the order of

1 ns. Output is digital and is stored in a multichannel scaler (Northern

NS600) but a D.A.C. and chart recorder can also be used.

(iii) Frequency doubling is by a 5 cm temperature tuned A.D.P. crystal; the mono- chromator is a Rank Precision Monospek 1000 and detection is with an E.M.I. 9594 fast linear focussed photomul tipl ier.

Intra and intermolecular processes in polyatomic molecules, particularly aromatic

plecules, has been an active and fruitful field of research in recent years. Theoretical

brk (Freed [2], Jortner [3], Rice [4], and many others) has developed a framework which

xperiment [5-7] has tested and amplified. This is particularly true for the case of

iingle vibronic level (SVL) excitation of low pressure gases.

A logical step from this situation is to the pressure region where collisions

arturb the prepared state, especially the intermediate pressure region where vibrational

slaxation is on the same time scale as fluorescence.

Freed [8], following an earlier technique of Montrall and Shuler [9], developed a )del describing the relaxing polyatomic molecule which reduced the molecule to one

Pfective oscillator. Assuming the processes occurring on collision were n = 0 +_ 1 vibrational quantum" change, he derived a generating function which allowed, from

lput data on fluorescence lifetime versus aE, pressure and collision cross-section,

best fit to an energy loss per collision.

This model gives good fit for experimental data [10]. However, allowing for the

^oss nature of the approximations involved, the results are of limited analytical

ilue. A more detailed picture of the time dependence in both absorption [11] and

n'ssion [12] has been developed from this model.

We report experimental data on the time resolved fluorescence of benzene and substiited benzenes. Using this data a more detailed analysis of the collisional deactivation

a prepared level can be obtained.

] A. W. Sloman and M. D. Swords, J. Phys. (E), submitted.

] K. F. Freed, J. Chem. Phys., 52, 1345 (1970).

] J. Jortner, S. A. Rice and R. M. Hochstrasser , Adv. Photochem., 7_, 149 (1969).

] W. M. Gelbart, S. A. Rice and K. F. Freed, J. Chem. Phys., 52, 5718 (1970).

REFERENCES

f

] M.

K.

G

G Spears and S. A. Rice, J. Chem. Phys., 55, 5561 (1971).

Rockley and D. Phillips, J. Phys. Chem., 78, 7 (1974).

319
[7] A. S. Abramson, K. G. Spears and S. A. Rice, J. Chem. Phys., 56, 2291 (1972).

[8] K. F. Freed and D. F. Heller, J. Chem. Phys., 61_, 3942 (1974).

[9] E. W. Montroll and K. E. Shuler, J. Chem. Phys., 26, 454 (1957).

[10] G. S. Beddard, G. R. Fleming, 0. L. Gijzeman and G. Porter, Proc. Roy. Soc. (A),

340 519 (1974).

[11] G. R. Fleming, 0. L. Gijzeman and S. H. Lin, J. C. S. Faraday II, 70, 1074 (1974)

[12] G. R. Fleming, 0. L. Gijzeman, K. F. Freed and S. H. Lin, J. C. S. Faraday II,

21, 773 (1974).

320
STUDIES IN THE MECHANISM OF RADIATIONLESS CONVERSION OF ELECTRONIC ENERGY

T. A. Gregory and Sanford Lipsky

Department of Chemistry

University of Minnesota

Minneapolis MN 55455

For most polyatomic molecules in condensed phases, excitation to an upper singlet

tate S results ultimately in the production of the lowest excited singlet state Sj by ome noR-radiative or succession of non-radiative processes. The S > S! conversion must e extremely rapid, at least as compared to S emission, since it is usually observed

hat the major emission originates from Si even when there is much larger S -> S Q scillator strength [l] 1

. In the absence of high concentrations of fluorescence quenchers,

he Sj -> S n fluorescence spectrum in solution is invarient to excitation wavelength.

tn the other hand, under the same conditions, the Sj -»• S Q

fluorescence quantum yield may

xhibit important variations [2]. These are conveniently expressed by the ratio of the

uantum yield when excitation is at some wavelength A to the yield when excitation is

Into the first absorption system. This ratio, after suitable correction [2], is usually

nterpreted to be a measure of the overall efficiency, 3(a), with which the A excitaion to some state S converts to the emitting vibronic levels of the state S l9 and, as xpected from this interpretation, is always observed to be less than or equal to

[pity.

For dilute solutions of benzene or toluene in some suitable transparent saturated

ydrogen solvent (e.g., isooctane), 3 generally declines as the excitation wavelength

ecreases and then at a wavelength close to the maximum of the third absorption system

K 185-190 nm) reverses direction and begins to rise [3-6], For p-xylene and generally

ore complex molecules, g remains about equal to unity over this entire spectral

ange [1,2].

The origin of these variations has never really been satisfactorily explained,

learly there exists for benzene and toluene an inefficiency in the S * Si conversion

rocess but its source is unknown. More importantly still, the overall mechanism of the

inversion procee has never been elucidated. It has generally been conjectured that

1 * Sj conversion proceeds via a cascade involving successive S

n

-> S

n

_-j internal

inversions but there is, in fact, no experimental proof of this for any polyatomic

olecule. Also to be considered is that ionization thresholds in condensed phases [7]

ay be lowered sufficiently from their gas phase values that for many aromatic molecules,

It relatively low excitation energies, the non-radiative conversion to Si could imartantly involve electron-ion pairs as intermediates in the process. Indeed, Laor and

ainreb, [8] have reported for excitation wavelengths within the third absorption systems

F benzene and toluene, a decrease in g in the presence of electron-scavenging additives.

Iso Fuchs ,^Heisel and Voltz [9] have conjectured that the upswing in 6 of benzene and

)luene at ^ 180 nm is somehow due to the onset there of an autoionization.

-igures in brackets indicate literature references at the end of this paper.

321
In two recent publications we reported observation of a very weak S3 •* Sg and S 2 S 0 fluorescence from a number of aromatic molecules when excited into their third

absorption systems at 184.9 nm [10, 11]. With the capability now of monitoring the

emission from the three electronic states S 3 , S 2 and Si rather than just the terminal

state Si, we have begun a series of studies directed towards the elucidation of the

mechanism of S Si radiationless conversion. Our first results for p-xylene are presented below.

The fluorescence quantum yields of p-xylene in isoctane excited at 184.9 nm have

been determined for the transitions S 3 + S 0 ($3), S 2 + S 0 ($ 2 ) and Si * So ($1) as functions of the concentration of p-xylene (c ) and of the concentration of added

quenchers (CCl^, CHC1 3 , c_ - CyF^) and comparid with the Si * S 0 fluorescence quantum

yield {q>i) for excitation at 253.7 nm in the same solutions. Also vapor fluorescence

quantum yields have been re-determined for the S 3 S 0 and S 2 + S 0 transitions. It has

been found that the total non-radiative decay rate constants of S 3 (2. 5 x lO 14 sec

-1

) and

of S 2 (1. 0 x 10 13 sec" 1

) are essentially the same for dilute solution and vapor phases.

Also it is found that in dilute solution, $ 3 is independent of concentration of quencher

whereas $ 2 is strongly quenched and to about the same extent as is the ration 3 =

*i/4>i

.

In more concentrated solutions (c > 3 M), $3 rapidly declines to unobservable levels

whereas $ 2 and e appear to increase-but only slightly. On the basis of these results the

following is concluded for a dilute solution of p-xylene in isooctane excited into S 3 (at 184.9 nm); i) the S 3 state either radiates (k = 3.0 x 10 9 sec" 1

) or makes

internal conversion to S 2 (k = 2. 5 x 10 14 sec" 1 ). rNo autoioni zation occurs nor do any

other decay channels such as S 3-> S 2 or S3 -> S 0 have appreciable probability; ii) The S 2 state either radiates (k = 1.3 x 10£ sec 1 ), makes internal conversion to Si (k =

1. 0 x 10 13 sec 1

) or autoionizes (k > 10 14 sec" 1 ). All other decay channels are of

negligible importance; iii) In the absence of quencher, the ejected electron returns to the parent positive ion to regenerate S 2 and not a lower state. In the presence of

quencher the electron is scavenged and no longer can contribute to any emission. Possible

mechanisms for the disappearance of $ 3 at high c are considered. Preliminary results

for benzene and toluene will also be presented.

* This work was supported by the U. S. Energy Research and Development Administration,

Document No. C00-913-61.

REFERENCES

[I] J. B. Birks, Photophysics of Aromatic Molecules (Wiley-Interscience, London, 1970)

and references cited therein.

[2] C. W. Lawson, F. Hirayama and S. Lipsky, J. Chem. Phys. 51_, 1590 (1969).

[3] C. L. Braun, S. Kato and S. Lipsky, J. Chem. Phys. 39, 1645 (1963).

[4] S. Feinleib, Ph.D. Dissertation, University of Minnesota, 1965.

[5] U. Laor and A. Weinreb, J. Chem. Phys. 43, 1565 (1965).

[6] J. B. Birks, J. C. Conte, and G. Walker, J. Phys. B (Proc. Phys. Soc.) Ser. 2,

[7] R. C. Jarnagin, Accts. Chem. Res. 4_, 420 (1971 ) and references cited therein.

[8] U. Laor and A. Weinreb, J. Chem. Phys. 50, 94 (1969).

[9] C. Fuchs, F. Heisel and R. Voltz, J. Phys. Chem. 75, 3867 (1972).

[10] F. Hirayama, T. A. Gregory and S. Lipsky, J. Chem. Phys. 58, 4696 (1973).

[II] T. A. Gregory, F. Hirayama and S. Lipsky, J. Chem. Phys. 58, 4697 (1973).

322
PHOTOSENSITIZATION OF THE 2-BUTENES BY BENZALDEHYDE IN THE GAS PHASE

A. J. Yarwood

Department of Chemistry

McMaster University

Hamilton, Ontario, Canada

G. R. De Mare and M. Termonia

Laboratoire de Chimie Physique Moleculaire

Faculte des Sciences

Universite Libre de Bruxelles

50 av. F.-D. Roosevelt

B-1050 Brussels, Belgium

The sum of the quantum yields of the benzaldehyde photosensitized (a = 366 nm) cis->- ns and trans->cis isomerizations for both the 1 ,3-pentadienes (independent of pressure

r the range studied: 10 - 400 Torr) and the 1 ,2-dichloroethylenes (for substrate

fissures >_ 100 Torr) is near unity, indicating that the quantum yield for intersystem

ssing in benzaldehyde irradiated at 366 nm is also near unity [1, 2] 1

. Supporting

dence for this high intersystem crossing yield for benzaldehyde in the gas phase was

en by Berger, Goldblatt and Steel [3] who found $(C 5 ) - 0.85 for the benzaldehyde

^itized decomposition of 2,3-diazabicyclo (2.2.1) hept-2-ene.

The fall-out in the quantum yields of isomerization of 1 ,2-dichloroethylene with

reasing pressure below 100 Torr was interpreted as being the consequence of the

oetition of unimolecular deactivation steps of triplet benzaldehyde with the bimolecular

rgy transfer [1]. That benzaldehyde phosphorescence actually competes with the energy

nsfer to 1 ,2-dichloroethylene has now been confirmed with laser and flash excitation

'ces.

Preliminary competition experiments have shown that energy transfer from triplet

zaldehyde to cis-1 ,2-dichloroethylene, requiring an average of 10 k collisions [1],

jpeeds at about 1/40 the rate of energy transfer to trans-1 ,3-pentadiene. This was not Upected because the energy transfer to 1 ,2-dichloroethylene is either slightly endo-

|rmic or thermoneutral while the transfer to 1 ,3-pentadiene is exothermic by over

(<cal/mole. For the 2-butenes the energy transfer from triplet benzaldehyde should be

Ibthermic by about 4 to 6 kcal/mole. Surprisingly, competition experiments showed that

I energy transfer to both cis- and trans-2-butene proceeds at about 1/35 the rate of

Ifgy transfer to trans-1 ,3-pentadiene (it is thus slightly faster than the energy transfer

II ,2-dichloroethylene) . We have therefore undertaken a detailed study of the benzaldehyde

Jitized isomerization of the 2-butenes in the gas phase.

—

lures in brackets indicate literature references at the end of this paper.

323
8 cm

" lamp,

(the

for

Experimental

The experiments were performed using a cylindrical Pyrex cell (11.9 cm long; 3 diameter) which was enclosed in a box fitted with glass windows. The temperature was regulated at 60.00 + 0.05 °C. The light source, a_Philips Philora HPK 125 watt "Wood

supplied an incident intensity of 0.46 + 0.02 x 10" 9 Einstein cm" 2 s" 1 at A = 366 nm mercury lines, 365.0 - 366.3 nm). The absorption coefficient of benzaldehyde vapour

this light is e(decimal) = 4.4 cm" 1 1 mole" 1 [4].

Results and discussion

Benzaldehyde sensitization of the 2-butenes at 366 nm and 60.0 °C leads only to geometric cis trans isomerization. No extraneous peaks were observed when the reaction

mixture was analyzed by gas chromatography with a flame ionization detector. Also, prolonged irradiation causes no observable (~ 0.1 Torr) pressure change in the reaction cell,

The quantum yields of the cis > trans, ($(C->t)), and trans > cis ($(t->c)), isomerizations were determined under initial conditions (less than 2% isomerization) for 2-butene

pressures ranging from 9.9 to 250 Torr and benzaldehyde pressures ranging from 3.3 to 8.2 Torr. *(ot) = 0.533 + 0.009 and *(t+c) = 0.258 + 0.010, independent of the 2-butene

pressure (see Figure). Thus $(c->t)/$(t-K;) = 2.07 + 0.11.

>-3!—

z:a

0.5

0.4

0.3

0.2

0.1

« » « . (0.53310.009)

CIS—TRANS

<0.258±0.010>

TRANS—CIS

100 200

2- BUTENETORR

300

The ratios of the quantum yields or of the initial rates for 2-butene isomerization

are given for a few sensitizers. The increase in the ratio c->t/t-*c indicates that the

branching ratio for triplet 2-butene is different for sensitizers with triplet energy

below 80 kcal/mole or that the sensitization mechanism has changed.

324
Sensitizer ET ,Kcal/mole Ratio c->t/t-K: Ref.

Benzaldehyde 71.9 2.07 This work.

Sulfur dioxide 73.4 2 5 Pyrazine (313 nm) 76 1 .60 6 Benzene 84.4 1.02 7 Cadmium 87.7 1.00 8, 10

Mercury 112.7 1.00 9, 10

Financial aid from the N.R.C. (Canada), the F.N.R.S. (Belgium) and N.A.T.O. is gratefully acknowledged.

REFERENCES

[1] De Mare, Fontaine, Huybrechts and Termonia, J. Photochem. 1 (1972/73) 289.

[2] Termonia, D. Sc. Thesis, U.L.B. 1976, in preparation.

[3] Berger, Goldblatt and Steel, J.A.C.S., 95 (1973) 1717.

[4] De Mare, Fontaine and Huybrechts, Bull. Soc. Chim. Beiges, 8J_ (1972) 171.

[5] Penzhorn and Gusten, Z. Naturforsch. , 27a (1972) 1401; Demerjian and Calvert, Int.

J. Chem. Kinetics, VII (1975) 45.

[6] Jones and Brewer, J.A.C.S., 94 (1972) 6310; Ivanoff, Lahmani , Delouis and Le Gouill,

J. Photochem. , 2 (1973) 199.

[7] Lee, Denschlag and Haninger, J. Chem. Phys., 48 (1968) 4547.

[8] Hunziker, J. Chem. Phys., 50 (1969) 1294.

[9] Cundall, Prog. Reaction Kinetics, 2 (1964) 165; Cundall and Palmer, Trans. Faraday

Soc, 56 (1960) 1211 ; Termonia and De Mare, Chem. Phys. Letters, 25 (1974) 402.

[10] Tsunashima and Sato, Bull. Chem. Soc. Japan, 4J_ (1968) 284.

325
NEAR INFRARED DETECTION OF PEROXYL RADICALS IN MERCURY PHOTOSENSITIZED REACTIONS

Heinrich E. Hunziker

IBM Research Laboratory

5600 Cottle Road

San Jose, California 95193

While peroxyl radicals play a pervasive role in low temperature oxidation and photooxidation processes, our knowledge about these transient species has been relatively

limited due to difficulties in applying most of the powerful free radical monitoring

techniques to them. This situation can be improved by making use of the unique property

of specific electronic absorption bands in the 1 to 2 pm region for detection of peroxyl

radicals. We have found that these bands can be observed by phase sensitive detection

of absorption when a modulated concentration of the radicals is generated in a mercury

photosensitized reaction. This method has made it possible to observe the lowest excited

electronic state of H0 2 with sufficient details of vibrational and rotational structure to estimate its geometry and illuminate its chemical nature.

More recently we have also observed analogous electronic band systems of CH 3 02 , C 2 H 50 2 , etc., and CH 3 «C0-0 2 , in the same frequency region but with characteristic shifts

of the electronic term energies relative to H0 2 and interesting differences of vibronic

structure. Just as for H0 2 these spectra are dominated by a progression of the 0-0

vibration, but additional structure due to internal rotation around C-0 bond is introduced

by changes of the internal rotational barrier between the ground and first excited

electronic states.

Since these band systems occur in a generally empty spectral region and are specific

for particular peroxy radicals, they are very useful for determining radical formation,

reaction pathways and kinetics in photochemical and other types of processes. Applications

of this kind will be illustrated with several examples.

326
REACTIONS OF HYDROGEN ATOMS WITH FLUORINATED KETONES

D. W. Grattan and K. 0. Kutschke

Division of Chemistry

National Research Council of Canada

Ottawa, Canada K1A 0R6

In an unpublished study of the reactions of CF 3 radicals, produced by the near uv photolysis of (CF 3 ) 2 C0 (HFA), with H 2 , the product ratio (CF 3 H + C 2 Fe)/2C0 rose beyond the

expected limit of one for some conditions. A radical exchange reaction between H atoms

and the ketone, which formed CF 3 radicals, was thought to be responsible for this

anomaly.

In order to study such a possible reaction, H atoms were generated in the presence

of HFA by the Hg-sensitized decomposition of H 2 or of C 3 H 8 . Attempts were made to evaluate relative rate constants by competitive methods. Each competition studied yielded

unexpected kinetic results; possible mechanistic explanations are suggested.

The constants for the quenching of Hg( 3 Pj) by C 2 H 4 , C 3 H 8 , n-d+Hjo and HFA were

determined with the nitrous oxide method.

Experimental . A conventional apparatus was employed which contained a quartz reaction xeTT(^~200_

cm 3

, optical path 10 cm) illuminated by a low pressure Hg lamp. The cell was part of a loop consisting of a glass circulating piston pump, U-tube cold trap and a second U-tube thermostatted at 26°C and containing a drop of Hg. The loop was connected

to apparatus for volume measurement, and for chromatographic and/or mass analysis.

Results . (1) Quenching Constants. Those for C 2 Hit , C 3 H 8 and n-C^o were determined

by the N 20 method in the usual way [1,2]*. The method measures the total quenching

constant, k ^

= k ^ + k^ ^, for compound X, and is based on the diminution in the rate

Hg( 3 Pi) + X

a A

> Hg^So) + X* (or other product) [a]

Hg( 3 P 0 ) + X [b]

t of formation of N 2 (r.. ) when an addend is present during the photodecomposi tion of

N 20 [1, 2].

N 2

For HFA, the determination was made in the presence of C 2 H 4 , at three ratios of

[C 2 H it ]/[N 2 0), in order to scavenge the 0 atoms formed. The kinetic analysis yields [c];

results are collected in Table 1.

i Figures in brackets indicate literature references at the end of this paper.

327
k k

T a_ = q,N 2°

+

q ' C

2 H

4 [C

2

H 4

] +

kq,HFA [HFA] [c]

r

N

2

ka,N

2 0

k a,N

2 0

[N

2 0]

ka,N

2 0

[N

2 0]

(2J H + HFA; method I . A method which had been devised by Cvetanovic [3] for the

determination of the rates of addition of H atoms to olefins relative to that for H + C 2 H k was employed. H atoms were generated by the Hg( 3 Pi) decomposition of 600 torr of H 2 and

their addition to a constant amount of C 2 H 4 was in competition with that to other olefins

(here to HFA). The yield of jrC

4 H 2

0

> produced by combination of the C 2 H 5 formed in the

addition H + C 2 H 4 , was measured as a function of [HFA]/[C 2 Hit ] . The standard [3]

mechanism and treatment is shown below.

HgOSo) + hv 253>7

-> Hg( 3 P 1 )

Hg( 3 Pi) + H 2 - 2H + Hg( 1 S 0 )

H + C 2 H 4

-> C 2 H 5 [1]

H + HFA -y CF 3 CH0 + CF 3 [2]

2C 2 H 5

-> jx-C lt H 10 (C 2 H 6 + C 2 H 1+ ) [3a(b)]

2CF 3 - C 2 F 6 [4]

CF 3 + C 2 H 5 - CF 3 C 2 H 5 (CF 3 H + C 2M [5a(b)]

Assuming that all radical-radical interaction rates are equal, and that there is no direct

quenching of Hg

(

3 P X ) by either C 2 H 4 or HFA, then, representing the rate of formation of

Jl-Ci+Hjo in the presence and in the absence of HFA by (rBu) and (rBu ) respectively,

((rBu)J

/2

- (rBu) 1/2 )/(rBu) 1/2= k^HFA]/^ [C^]

.

The results (Figure 1), obtained at four pressures of C 2 H (+ , show that this function depends

on the absolute pressures of C 2 Hu and/or HFA and not solely on their ratio. Presumably

this arises from the neglect of [6].

C 2 H 5 + HFA * C 2 H 5C0CF 3 + CF 3 [6]

Resolving the kinetics including [6] and deleting [2] yields:

(rBu)y 2 ((rBu)y 2

- (rBu) 1/2 )/(rBu) 1/2= ( (l/2)k 6 k 3 1/2/k)[HFA]

where 2 k = 2(k

3a + k 3b ) = k ga + k 5b

= k 4> This plot also is shown in Figure 1 which

shows that the points now all coalesce about a single line. If log k is taken as 13.5U moOs" 1 units), k 6 = 1.0 x 10 8 i mol" 1 s" 1

.

During prolonged photolyses more C 2 H lt and HFA were consumed than is predicted by the

mechanism; this is evidence of some additional reaction(s) between HFA and C 2

Hit.

328
Figure 1. Points refer to [C 2 H 4 ] = 20 (+), 15 ( ), 10 ( ) and 5 (a) torr,

lower abscissa. Open circles refer to upper abscissa.

(3J H + HFA; method II . In an attempt to overcome the disadvantages of the previous

method, H atoms were generated from the quenching of Hg( 3 P X ) by C 3 H 8 [4,5]. The rate of

H 2 formation was studied as a function of [HFA]/[C 3 H 8 ]; the following mechanism was expected to apply.

Hg( 3 Pi) + C 3 H 8 - H + C 3 H 7 + HgOSo) [7]

Hg( 3 Pi) + HFA - HFA* + Hg( 1 S 0 ) [8]

H + C 3 H 8 + H 2 C 3 H 7 [9]

H + HFA + CF 3 CH0 + CF 3 [2]

2C 3 H 7 * CgHi i+ (C 3 H 8 + C 3 H 6 ) [10a(b)]

2CF 3

-> C 2 F 5 [4]

CF 3 + C 3 H 7

-» CF 3 C 3 H 7 (CF 3 H + C 3 H 6 ) [lla(b)]

H

2 /r

H

2

=

(1 + k 2t HFA H/k

g CC3

H 8

])(1 + k

7 [HFA]/k

8

[C

3 H8

]).

rom (1), k

7/kg

is 10.2 and Baldwin [6] gives k g

= 1.27 x 10

5

I mol" 1 s" 1. At low [HFA]/

C 3 H 8 ] the measured k 9/k 2 were reasonably constant and give k 2 = 8.5 x 10 5 I molds' 1

. bove [HFA]/[C 3 H 8 ] = 0.04, the k 9/k 2 so deduced increase very rapidly; presumably addition1 processes become important at higher [HFA]/[C 3 H 8 ] ratios.

(4) Product Analysis . Heavier products of the overall reaction (600 torr H 2 , 10 orr HFA) were studied. Many products were detected, most due to secondary processes;

F 3 CH0 was identified.

Conclusions . The values of the quenching constants agree well_with those of other

orkers. This gives some confidence to the k

UFfl = 13 x 10 10 £ mol" 1 s" 1

; cf the much

q»nrtt

f arger value of 39 x 10 10 i mol" 1 s" 1 for acetone measured by Cvetanovic [1].

329
The result that addition of C 2 H 5 to HFA proceeds at some 1000 times the rate of

that of H atoms also is consistent with other work [7].

Table 1. Quenching Constants, k (a)

Cmpd. This work^ Others^

HFA 13.0 _

C

2 H 4 31.0 30.0

(b)

C

3H8

1.27 1.13^

3.35 3.65< b >

(a) k in 10" 10£ mol" 1 s"

1

. q (b) Based on kq^OM n = ,„ , 14.3, reference [2].

(c) Based on k r u _ , n n q,C

2 H4

= 30.0, reference [2]

(d) Reference [8].

(e) Reference [9].

(f) Reference [10].

Others (c)

1 .28

(a)

3.75^, 3.83^

REFERENCES

[1] R. J. Cvetanovic, J. Chem. Phys. 23, 1208 (1955).

[2] R. J. Cvetanovic in Progress in Reaction Kinetics , ed. G. Porter, vol. 2, p. 39. MacMillan Co., New York, 1964.

[3]' R. J. Cvetanovic and L. C. Doyle, J. Chem. Phys. 50, 4705 (1969).

[4] R. A. Back, Can. J. Chem. 37, 1834 (1959).

[5] R. J. Cvetanovic, W. E. Falconer and K. R. Jennings, J. Chem. Phys. 35, 1225 (1961);

K. R. Jennings and R. J. Cvetanovic, J. Chem. Phys. 35, 1233 (1961).

[6] R. R. Baldwin, Trans. Farad. Soc. 60, 527 (1964).

[7] A. S. Gordon, W. P. Norris, R. H. Knipe and J. H. Johnston, Int. J. Chem. Kinetics

7, 15 (1975).

[8] Y. Rousseau and H. E. Gunning, Can. J. Chem. 41, 465 (1963).

[9] G. London, A. C. Vikis and D. J. Le Roy, Can. J. Chem. 48, 1420 (1970).

[10] J. V. Michael and G. N. Suess, J. Phys. Chem. 78, 5 (1974).

330
THE REACTION OF NH

2 WITH OLEFINS STUDIED BY FLASH PHOTOLYSIS

R. Lesclaux and Pham Van Kh£

Laboratoire de Chimie Physique A Universite de Bordeaux I 33405 Talence (France)

In spite of the large amount of studies on the photochemistry or radiochemistry of

ammonia, little attention has been given to the reactivity of NH 2 radical, even though this

radical is generally an intermediate in the dissociation processes of ammonia. However,

the few available data concerning the NH 2 reactivity, indicate that this radical is veakly reactive compared to the isoelectronic OH radical.

In our first experiments, we have studied the reactions NH 2 + NO [I] 1 and NH 2 + NH 2 +

'ft [2] which are fast processes, by using a conventional flash photolysis apparatus. It is aarticularly important to know the kinetics of the recombination process since it often

competes with other NH 2 slower reactions. The rate constants of this process were

determined in the fall off region (0-1000 Torr of N 2 ), by using different third bodies

'NH 3 , N 2 and Ar) and in the temperature range 300-500 K [2].

The reaction of NH 2 with olefins was studied by flash photolysis, the NH 2 absorption

)eing measured by using a CW dye laser operated in single mode. The reaction being

c airly slow, it was never possible to completely eliminate the radical-radical recombination processes by using the conventional apparatus (white analysing light source + mono-

;hromator) because the NH 2 concentration was too high. The CW dye laser was then used

n order to increase the sensitivity of radical detection by resonance absorption (laser

•pectral width narrower than the absorption line).

The flash photolysis set up is essentially the same as the one previously

lescribed [1], NH 2 radicals are produced by photolysis of ammonia, using two flash lamps

elivering 40-60 joules in 20 ys. An absorption optical path of about 30 m is obtained by

lacing the reaction cell in a multipass system. The temperature can be varied from room

i emperature up to 240°C.

The single mode CW dye laser, Spectra Physics model 580 has a spectral width lower

han 50 MHz. Absorption measurements were performed at 597.73 nm which corresponds to

jme of the strongest NH 2 absorption line.

The signals are detected by using two photodiodes and a differential amplifier on

Ihe oscilloscope. One of the photodiodes measures the absorption signal, the other, the

eference laser intensity. This system reduced considerably the perturbations due to the aser intensity variations and allows good measurements of absorption less than 10%.

An important increase in the sensitivity of NH 2 radical detection was obtained by

omparison to the experiments using an analysing light source. For an equivalent absorpigures in brackets indicate literature references at the end of this paper.

331
tion and signal to noise ratio the NH 2 concentration was about 30 times smaller with the

laser. Moreover, the measured optical density is now a linear function of the radical

concentration.

The initial concentration was evaluated by measuring the second order kinetics of

NH2 recombination which rate constant was determined in various conditions [2]. The NH 2 concentration was found to be from 2.10" 9 to 10" 8 M for 10% of light absorption, according

to the pressure broadening of the absorption line, for pressures up to 700 Torr.

This important increase in the sensitivity of radical detection is essentially due

to the fact that absorption measurements can be made at the maximum of the absorption

line and also to the large number of pass in the cell possible with the laser beam. A new increase in the sensitivity should be obtained by averaging the signals from a large

number of flashes.

The sensitivity of detection obtained in our system is in the same range as that of

most other methods of radical detection with the exception of the laser excited fluorescence which is much more sensitive at low pressure. However, the very efficient

collisional quenching of the HH 2 { 2^\) excited state reduces considerably the fluorescence

intensity at high pressure [3]. The advantage of resonance absorption is that it is not

too sensitive to experimental conditions and can then be applied to various systems

(atmospheric conditions, flames etc ...).

The reaction kinetics of NH 2 with ethylene, propylene and 1-butene were determined in

the temperature range 300-500 K. The pressure of olefins were varied from 10 to 100 Torr.

In all conditions the disappearance kinetics of NH 2 were first order and the reciprocal

lifetimes were a linear function of the olefin pressure (figure 1). Thus no radical

recombination contributed to the NH 2 disappearance. The temperature dependence of the

rate constants (figure 2) is accounted for by the following Arrhenius expressions:

k NH 2 + C 2 H 4 = 1 - 2 - 1 °

8 ex P " 3 - 95 (+ 0.2J/RT M'V1

kNH 2 + C 3 H 6 = 2 - 9 ' 108 exP " 4 - 3 (± °- 2 )/ RT M" s"

k NH 2 + 1-CIV 2 - 8 ' 108 ex P " 4 ' 1 (±0.2)/RT M-'swhich correspond to the following room temperature rate constants. 1.6 x 10 5

, 2.2x 10 5

, 3.0 x 10 5 fPs" 1 for C 2 H 1+ , C 3 H 6 , and l-d+Hs respectively. Activation energies are given

in kcal/mole and the errors on the rate constants are estimated to be smaller than 20%.

The reaction is thus fairly slow at room temperature compared to the equivalent

reaction of the isoelectronic OH radical. The difference is essentially due to the

significant activation energy found in the case of NH 2 radicals whereas there is no activation energy and even a slight negative temperature coefficient in the case of OH radical [4].

No information is given in this work about the products of the reaction. However,

the addition reaction is the most likely to occur since hydrogen abstraction reactions from

hydrocarbons by NH 2 radicals seem to be very slow: < 10 3 M

_1

s

1 [5].

332
P (Torr)

80

C

3H

6

Figure 1. Reciprocal NH

2 lifetime as a function of propylene pressure.

1 Ln k

NH2+C

3H

6

10

3 /T(K l )

Figure 2. Arrhenius plot for the reaction of NH

2 with propylene.

Experiments are still in progress in order to extend this study to a series of un- saturated hydrocarbons.

REFERENCES

[ ] J J- Lesclaux, P. V. Khe\ P. Dezauzier, and J. C. Soulignac, Chem. Phys. Letters 35,

[2] P. V. Khe, J. C. Soulignac, and R. Lesclaux - to be published.

[3] J. B. Halpern, G. Hancock, M. Lenzi, and K. H. Welge, J. Chem. Phys. 63, 4808 (1975)

L"4] R. Atkinson and J. N. Pitts, Jr., J. Chem. Phys. 63, 3591 (1975).

[5] R. Lesclaux and P. V. Khe\ J. Chem. Phys. 70, 119 (1973).

333
DETECTION AND REACTIONS OF NH(xV) RADICALS IN THE VACUUM UV FLASH

PHOTOLYSIS OF NH

3 USING RESONANCE FLUORESCENCE

I. Hansen, K. Hoinghaus, C. Zetzsch and F. Stuhl

Physikal ische Chemie I Ruhr-Uni versitat

4630 Bochum, West Germany

In the course of an investigation of the kinetics of imino radicals we have recently

studied the quenching of metastable NH and ND radicals in their (b x z ) state [I] 1

. In the

present paper we would like to describe a method to study the kinetics of ground state

NH(X 3 z") radicals.

Kinetic data on reactions of NH(X 3 z") radicals are sparse. There are only a few

estimates of rate constants in the literature which are based on experimental work such as those for the reactions with NH 3 , N 2 H 4 , NH, NO, HN 3 , 0 2 , C 2 Hk and C 3 H 6

. None of these

rate constants have been obtained from more than one experiment. In the most direct

studies, previously NH(X 3 i") was generated either by electron bombardment or flash

photolysis of NH 3 or HN 3 and was detected by time resolved absorption spectroscopy. With

these techniques concentrations of NH in the approximate range of 3 x 10 13 cm" 3 to

2 x 10 15 cm" 3 were produced and lifetimes t

1 ' 2

<, 0.2 ms were observed. In the presence

of NH 3 it was concluded that NH is removed efficiently by NH 3 [2] and even more efficiently by NH [3].

In the present study the apparatus consisted of a pulsed light source, a reaction

chamber, an NH emission lamp and a resonance fluorescence detection system. A similar

photolysis apparatus for the study of the kinetics of OH has been described previously [4].

Vacuum uv light pulses (t 1/2 = 1.5 ps) were generated by discharging a capacitor

(5 to 60 J) with a repetition frequency of about 0.5 Hz. NH(X 3 z") radicals were produced

by photolysing mixtures of NH 3

, inert gas and reactive gas through a Li F window (x> 105 nm).

The approximate range of the initial concentration of NH was estimated to be from

2 x 10 11 cm" 3 to 2 x 10 12 cm" 3

.

The NH resonance emission was generated by passing a small flow of a mixture of NH 3 at 0.35 Torr and Ar at 1.6 Torr through a microwave discharge. The NH(A 3 tt+X 3 e") - emission

of the lamp is shown in Figure 1. The dominant feature of this spectrum is the Q-branch

of the (0;0)-band. Furthermore the R-and P-branches of this band and the Q-branch of

the (l;l)-band are clearly observed in this spectrum. Care had to be taken to avoid

generation of N 2 (C 3

tt; v'=0->-B 3

tt; v"=0) emission at 337 nm in the discharge. It should be

noted also that under the present conditions emission from NHCc^-^a) at 325 nm appears

to be absent.

Figures in brackets indicate literature references at the end of this paper.

334
3(0 339 338 337 336 335 334 33 3 332 \mt

t

60 *

40 S

8

30 i

eS

20 S

Figure 1. Emission spectrum of the NH lamp,

ference filter used is included.

The transmission of the interThe resonance fluorescence from NH radicals was monitored at right angles to the

incident flash light and to the NH resonance light. It was detected by a photomul tipl ier

through an interference filter, the transmission of which is included in Figure 1 (dashed

line). The signal of the photomultiplier was fed to a multichannel scaler where up to 64 single runs were accumulated and averaged.

Several experiments were performed to show that the observed fluorescence signal

indeed originates from the presence of NH(X 3 z") radicals. In the present system NH radicals can be observed up to 1 s after the light pulse. In the presence of NH 3 the

removal of NH(X 3 z~) appears to be rather complex. Figure 2 shows two decay curves of the

NH resonance fluorescence under the same experimental conditions but with and without in- ert gas present. The semilogarithmic plot shows that the decay of NH cannot be accounted

for by a single first order reaction. In Figure 3 the inverse of the intensity of the

fluorescence is plotted vs. time for these two runs. Similarly this figure shows that no

Figure 2. Semilogarithmic plot of the resonance fluorescence intensity (arb.

units) vs. time. Experimental conditions: 0 0.15 Torr NH,; 0.15 Torr NH. + 700 Torr He.

335
Figure 3. Plots of the inverse of the resonance fluorescence intensity (arb.

units) vs. time. Experimental conditions: 0 0.15 Torr NH-; 0.15 Torr NH., + 700 Torr He.

J J

single second order process causes the removal of NH(X 3 z~). From decay curves like these,

limits of rate constants can be obtained for the reactions of NH with NH and with NH 3. Furthermore, it seems to be likely that NH can survive collisions with the reactor walls

during its long lifetime.

Upon addition of NO to this system the NH lifetime decreases resulting in a rate

constant for the reaction NH + NO [5] in agreement with a value previously reported [6].

These and other results will be discussed.

REFERENCES

[1] C. Zetzsch, F. Stuhl , Chem. Phys. Letts. 33, 375 (1975).

C2] K. A. Mantei, E. J. Bair, J Chem Phys. 49, 3248 (1968).

[3] G. M. Meaburn, S. Gordon, J Phys Chem. 72, 1592 (1968).

[4] F. Stuhl, H. Niki, J. Chem. Phys. 57, 3671 (1972).

[5] C. Zetzsch, F. Stuhl, Chem. Phys. Letts. , to be published.

[6] s. Gordon, W. Mulac, P. Nangia, J Phys. Chem. 75, 2087 (1971)

336
INTERPRETATION OF THE ARRHENIUS PLOTS FOR REACTIONS

OF OXYGEN ATOMS WITH OLEFINS

R. J. Cvetanovic

Division of Chemistry

National Research Council of Canada

Ottawa, Canada

Relative rates of the ground state oxygen atom, 0( 3 P), reactions with a number of

representative olefins and their variation with temperature were determined in this

laboratory almost two decades ago [l] 1

. Since that time, and in particularly in the last

few years, several techniques have been developed and used to measure the absolute values

of the rate constants of these reactions and of their temperature dependences [2-5]. As a result of these developments, the absolute values of the rate constants are now known with

great accuracy. However, the reactions are very fast and are therefore generally not affected strongly by temperature. As a consequence, determinations of the Arrhenius

parameters are experimentally very demanding. Nevertheless, temperature dependence studies

reported in the literature indicate for some of these reactions two somewhat unexpected

features: 1) negative values of the Arrhenius energy parameters [2a, 2b] and 2) curved

logarithmic Arrhenius plots [3c, 5]. A definitive confirmation of these two observations,

especially of the curvatures of the Arrhenius plots, requires precise determinations of the

reaction rates over relatively wide temperature intervals. The present paper will report

the final results of the recent study [2d] in this laboratory by D. L. Singleton and

R. J. Cvetanovic of the temperature dependence of the rates of oxygen atom reactions with

several olefins. The major part of the oral presentation will be devoted to a discussion

of the observed negative values of the Arrhenius energy parameters and the curvatures

of the Arrhenius plots and of the broader implications of these observations. The background experimental information will be briefly summarized in the following.

The determinations of the rate constants have been made by the phase-shift technique [2]. Study of the temperature dependence of the rates was made possible by a considerable further improvement [2c, 2d] in the precision of the measurements, with the

standard deviation reduced to about 3%. The reactions of the following olefins were

studied: ethylene, propylene, 1-butene, 3-methyl-l-butene, isobutene, cis-2-butene and

tetramethyl ethylene. (The results for tetramethyl ethylene have been published [2c] and

those for the other olefins have been submitted for publication [2d]). When (least

squares) linear logarithmic Arrhenius plots are imposed on the experimental points,

negative values of the Arrhenius activation energy parameter are obtained for tetramethyl ethylene, cis-2-butene and isobutene (-0.77, -0.27 and -0.10 kcal/mole, respectively).

For ethylene, propylene, 1-butene and 3-methyl-l-butene the Arrhenius activation energies

are positive (1.68, 0.72, 0.66 and 0.53 kcal/mole, respectively). For the olefins studied

so far by the phase-shift technique the preexponential factors are all very similar

(1.2xl0 10 for tetramethyl ethylene and close to 7xl0 9

1 .mole" 1 .sec" 1 for the other olefins

studied). These results are in very good agreement with the relative data from competitive

experiments [1] and, with minor exceptions, with the resonance fluorescence results [3].

figures in brackets indicate literature references at the end of this paper.

337
Although for the temperature interval (298 - 480 K) of these experiments [2c, 2d] the

observed temperature dependences can be approximately expressed empirically by the

Arrhenius equation, the data seem to be correlated better by slightly curved logarithmic

Arrhenius plots. This is particularly noticeable for the reactions of 3-methyl-l-butene,

1-butene, propylene and isobutene. A theoretical treatment based on the transition state theory shows that the curvatures can be explained by the temperature dependence of the

entropies of activation and do not therefore require the postulate [3c] that abstraction

of the allylic hydrogen from the olefins competes efficiently with the oxygen atom

addition to the olefinic double bond. Great similarity of the overall behavior and of

the apparent Arrhenius parameters for propylene, 1-butene and 3-methyl-l-butene in spite

of the vastly different lability of the allylic CH bonds rules out an important contribution from the hydrogen abstraction reaction. Product analysis study [2e] for the

reactions of oxygen atoms with 1-butene and 3-methyl-l-butene supports this conclusion.

Several explanations have been suggested in the literature for the observed negative

values of the Arrhenius activation energy parameter. There is at the moment no conclusive evidence in support of one of these suggestions to the exclusion of the alternative

possibilities. One of the potential explanations, suggested originally in connection

with the oxygen atom studies in this laboratory, [1, 2a] postulates formation of intermediate molecular complexes ("charge transfer" complexes) between oxygen and olefins.

It seems to be consistent with all the currently available experimental observations and

will be discussed in some detail.

REFERENCES

[1] R. J. Cvetanovic, J. Chem. Phys. 30, 19 (1959); ibid 33, 1063 (1960); Can. J. Chem.

38, 1678 (1960); Adv. Photochem. 1, 115 (1963).

[2a] R. Atkinson and R. J. Cvetanovic, J. Chem. Phys. 55, 659 (1971); ibid 56, 432 (1972);

b) S. Furuyama, R. Atkinson, A. Colussi and R. J. Cvetanovic, Int. J. Chem. Kinet.

6_, 741 (1974); c) D. L. Singleton, S. Furuyama, R. J. Cvetanovic, and R. S. Irwin,

J. Chem. Phys. 63, 1003 (1975); d) D. L. Singleton and R. J. Cvetanovic (submitted for publication); e) R. J. Cvetanovic and L. C. Doyle (to be published).

[3a] D. D. Davis, R. E. Huie, J. T. Herron, M. J. Kurylo, and W. Braun, J. Chem. Phys.

56, 4868 (1972); b) D. D. Davis, R. E. Huie, and J. T. Herron, J. Chem. Phys.

59, 628 (1973); c) R. E. Huie, J. T. Herron, and D. D. Davis, J. Phys. Chem.,

76, 3311 (1972); d) M. J. Kurylo, Chem. Phys. Lett. ]_4, 117 (1972).

[4] F. Stuhl and H. Niki, J. Chem. Phys. 55, 3954 (1971).

[5] A. A. Westenberg and N. de Haas, Twelfth (International) Symp. Combust., Poitiers,

France, 289 (1969).

338
ABSOLUTE RATE OF THE REACTION OF 0(

3P) WITH HYDROGEN SULPHIDE

D. A. Whytoe

k

[l] 1 and R. B. Timmons

Department of Chemistry

The Catholic University of America

Washington, DC 20017

and

J. H. Lee [2], J. V. Michael [3], W. A. Payne and L. J. Stief

Astrochemistry Branch

NASA/Goddard Space Flight Center

Greenbelt, Maryland 20771

The rate constant for the reaction of 0( 3 P) with H 2 S has been the subject of considerable interest (K. Schofield, J. Phys. Chem. Ref. Data 2, 25 (1973); I. R. Slagle,

F. Biaocchi and D. Gutman, Astracts of the 12th Int. Symp. on Free Radicals, Laguna Beach,

California (1976)), arising in part from the possible importance of this reaction in the

chemistry of combustion processes, in polluted terrestrial atmospheres and in the atmosphere of Venus.

Previous investigations, all employing the discharge-flow technique give room temperature rate constants which vary by more than a factor of 3. The results from the

two variable temperature studies also do not agree. Gutman and co-workers obtained

k = 3.3 x 10" 11 exp(-4000/l .987 T) cm 3 molecule" 1 sec" 1 between 250 and 500 K, whereas

Hollinden, Kurylo and Timmons (J. Phys. Chem. 74, 988 (1970)) reported k = 2.9 x 10" 13 exp(-1500/l .987 T) cm 3 molecule" 1 sec" 1 from 205 to 300 K.

The disagreements noted above, which probably result from inaccurate assessment of

the overall reaction stoichiometry, have motivated the present study using the flash

photolysis-resonance fluorescence technique for the temporal measurement of [0]. This

technique allows very low atom concentrations to be used (< 10 11 cm" 3

) and afford experimental conditions under which the first step in a reaction scheme may be isolated. Thus

uncertain stoichiometric corrections do not complicate the analysis even in systems like

the present where fast secondary reactions can contribute to 0 atom removal.

The apparatus and techniques have been described in detail previously (R. B. Klemm

and L. J. Stief, J. Chem. Phys. 61, 4900 (1974)). In the present study, 3-component

mixtures of H 2 S, 0 2 and argon diluent were flash photolyzed at x < 136 nm to produce

0( 3 P) <_ 10 11 cm" 3

) in an excess of both H 2 S and the source compound, 0 2 . Under the

pseudo-first-order conditions employed here, with [H 2 S] » [0], the decay of 0 atoms

may be expressed experimentally as

£n C0] =

" Observed * + * n[0]

o 0>

^Figures in brackets indicate literature references at the end of this paper.

339
where the observed pseudo-first-order rate constant is given by

"observed

= k bi ^ + K*' (2)

k, . is the bimolecular rate constant which is equal to k 3 in the limit of unit stoichiomeiry

0 + H

2

S -> OH + SH (3)

and K* is a correction term arising from loss of 0 atoms by the reaction

0 + 0 (4)

2 + M 03 + M

and from diffusional loss of 0 out of the reaction viewing zone. The correction term

(K* = k lt [0 2 ][M] + k.) was determined independently by flash photolyzing mixtures of 0 2 and Ar at the various experimental temperatures and pressures employed in this study. K* was typically * 15% of k observed .

The evidence of previous work that secondary reactions are important at low [H 2 S]/[0]

ratios suggested that k 0 h serveH wou ^ d exhibit dependence on flash intensity. Preliminary

experiments showed that over a range of flash energies equation (1) was strictly

followed, with plots of £n[0] vs t exhibiting good linearity. At higher flash energies,

k . , and hence k. . was observed to increase and at the upper limit of flash energy

cTearbending in the nn[0] vs t plots was apparent. The increase in k. . at high flash

energies is direct evidence of the onset of secondary reactions contributing to the 0 atom decays. Whichever reactions cause the rate constant k. . to increase, the low

intensity limiting k. . is obtained under conditions which are unaffected by competing

secondary reactions of 0 atoms and corresponds to k 3 . Intensity-independent values

of k 3 were obtained over a wide range of experimental conditions, a typical example of

the data for 400 K being as follows.

Rate Data for the Reaction 0 + H

2

S at 400 K

P total \

PK

H

2

S Flash Energy Number of k bK 3 torr mTorr mTorr J Experiments -14 3 10 cm molecule

10 1000 100 9.0-81 6 10.6 + 1.0

20 2000 200 9.0-56 5 11 .7 + 0.8

50 500 75 9.0-36 4 12.0 + 0.6

100 1000 150 9.0-20 3 11 .9 + 0.7

200 2000 300 14 -36 3 11 .5 + 0.7

200 667 100 9.0-56 4 11.4 + 1.1 300 1000 150 9.0-20 2 11.5 + 1.1 400 1000 100 8.1-81 7 11.3 + 1.2

11.4 + 1.0

a

a - mean value of k 3 at 400 K. b - error limit in k^ is the standard deviation.

340
Mean k 3 values at other temperatures were as follows: 495 K, (27.4 +2.7) x 10~ 14

; 450 K, (17.1 +1.8) x 10" 14

; 345 K, (5.42 + 0.30) x 10" 14; 298 K, (2.97 + 0.23) x 10

263 K, (1.31 + 0.21) x 10" 11+

. The values of k 3 provide the Arrhenius equation

k 3 = (7.24 + T.07) x 10" 12 exp (-3300 + 100/1.987 T) cm^olecule'^ec" 1

. Comparable

experiments with D 2 S showed that the reaction exhibits a primary isotope effect, in support of a hydrogen abstraction mechanism.

[1] On leave from Department of Chemistry, University of Essex, Colchester.

j[2] NAS/NRC Postdoctoral Research Associate.

[3] NAS/NRC Senior Postdoctoral Research Associate.

REFERENCES

341
KINETICS OF THE REACTION: OH + N0 2 (+M) •> HN0 3 (+M)

OVER A WIDE RANGE OF TEMPERATURE AND PRESSURE

C. Anastasi and I. W. M. Smith

Department of Physical Chemistry

University Chemical Laboratories

Cambridge CB2 1EP, England

and

R. Zellner

Institut fur Physi kal ische Chemie der

Universitat Gottingen

34 Gottingen, West Germany

Because of its importance in atmospheric chemistry, the reaction

OH + N0

2 (+M) HN0 3 (+M) (1

)

has been studied several time just recently [I] 1

. It also attracts interest because the

j changeover between second- and third-order kinetics occurs in a range of [M] that is

easily accessible. The present paper reports measurements of effective second-order rate constants, lq = (-d ln[0H]/dt)/[N0 2 ], through the ranges: 3 x 10 16 < [M] <lix 10 19

molecule cm" 3 and 220 < T < 550 K. The observed 'fall-off behaviour is compared with

predictions made by RRKM theory.

Experimental Method and Results

Most of our measurements have been made using the flash photolysis resonance absorption (FPRA) technique that we have described elsewhere [1 2 2]. At T <_ 358 K, OH radicals were generated by photolysis of ^ 1 x 10 16 molecule cm" 3 of HN0 3

, while at higher temperatures H 2 0 was used. For each combination of T and [M], experiments were carried out for several N0 2 concentrations producing values of k, . = -d ln[0H]/dt

between ^ 10 3 and 10 14 s" 1

. IST

At room temperature and below some rate measurements were also made using the discharge flow resonance fluorescence (DFRF) method [3]. These extended the range of [M]

that has been covered down to 3 x 10 15 molecule cm" 3 and were particularly useful at the

lowest temperatures, since some allowance had to be made in the FPRA experiments for the

dimerisation of N0 2 .

The observed values of kj were neither proportional to nor independent of [M],

showing that reaction (1) was passing through its transition region in the range of [M]

and T covered by our experiments. This behaviour is shown clearly by the data for

296 K which is plotted in figure 1.

Figures in brackets indicate literature references at the end of this paper.

342
I/) V

QJ

JSC O

cno

1

Jo —

.

// //

1

1 16 17 18 19

log([N2 ] / molecule cm-3

)

20

Figure 1. Comparison of experimental and theoretical results. The continuous

curve shows the results of our RRKM calculations and the dashed curve those of

Tsang [7]. The straight lines indicate the limiting behaviour at low and high

CM].

The rate constants in the limit of low [M], k ls and of high [M], Iq, were obtained

y following the empirical procedure proposed by Troe [4]. Both the absolute magnitude

nd the temperature dependence of ki that we find is in good agreement with other

ork [1, 3]. The kinetics at high pressure has been studied much less extensively but

he present value of k~ at 296 K is in fair agreement with Morley and Smith's [5]

arlier estimate.

T/K

220

296

358

450

550

1000

Table 1

10

30 k°/cm6 molecule'

2 s"1

M = No M = He

(FPRA) (DFRF)

2.5 3.0

0.91 1.05

Values of k-j and k-j

"2

(FPRA)

6.3

2.6 C

1.6

? i.o

9 0.61

343

10

11 k^/cm 3 molecule" 1 s"1

expt.

(FPRA)

2.0

1 .6

1 .3

0.38

0.44

0.19 [6]

calc.

1.45

1.6

2.6
RRKM Calculations and Discussion

Including the shock tube experiments [6] on its reverse, reaction (1) has now been

studied over a wide range of T and [M] and can therefore provide a good test of unimolecular reaction rate theories. We have carried out conventional RRKM calculations along the

lines followed previously by Morley and Smith [5] and by Tsang [7]. A model for the

transition state was chosen to yield a partition function given by Q = QouQiun / ( kT/h )

,

where kj was the observed rate constant in the limit of high pressure and 2 the other symbols have their usual meanings. This (based on model B of Morley and Smith)

had frequencies of 3550, 1 650 ,1 325 , 800, 135 and 135 cm" 1

, two active internal rotations

(moments of inertia = 1.5 x 10

-i+0 g cm 2 ), and inactive overall rotations (moments of

inertia = 65, 434, and 499 x 10~ 40

g cm2

). With the collisional efficiency (e) for N 2 assumed to be 0.3, the calculated fall-off curve at 296 K agrees quite well with the

experimental results (see figure 1). The transition state model chosen to fit the fall-of

behaviour at 296 K does not, however, predict the correct variation of k^ with T (see

Table 1). This is not peculiar to this particular model but probably constitutes an example of a general failure of 'conventional' transition state theory when it is applied

to systems where motions in the transition state should not be treated as separable,

small amplitude vibrations.

REFERENCES

[1] C. Anastasi, P. P. Bemand and I. W. M. Smith, Chem. Phys. Letters, 37, 370 (1976),

and references therein.

[2] I. W. M. Smith and R. Zellner, J. C. S. Faraday II, 70, 1045 (1974).

[3] J. G. Anderson, J. J. Margitan and F. Kaufman, J. Chem. Phys . , 60, 3310 (1974)

[4] J. Troe, Ber. Bunsenges, Phys. Chem., 78, 478 (1974).

[5] C. Morley and I. W. M. Smith, J. C. S. Faraday II, 68, 1016 (1972).

[6] K. Glanzer and J. Troe, Ber. Bunsenges. Phys. Chem., 78, 71 (1974).

[7] W. Tsang, Int. J. Chem. Kinetics, 5_, 947 (1973).

344
VACUUM UV FLASH PHOTOLYSIS OF PHOSPHINE: RATE OF THE REACTION H + PH AND IMPLICATIONS FOR THE PHOTOCHEMISTRY OF THE ATMOSPHERE OF JUPITER

J. H. Lee, [l] 1 J. V. Michael, [2] W. A. Payne, D. A. Whytock [3] and

L. J. Stief

Astrochemistry Branch, Laboratory for

Extraterrestrial Physics, NASA/

Goddard Space Flight Center

Greenbelt, Maryland

Phosphine has been detected as a minor constituent of the atmospheres of Jupiter [4]

and Saturn [5]. Prinn and Lewis [6] have proposed that the Great Red Spot on Jupiter may

be due to red phosphorous formed as a final product of the photodissociation of PH~. Notably absent from their model is the abstraction reaction

Their neglect of reaction (1) is based on the work of Norrish and Oldershaw [7] who studied

the flash photolysis of PH 3 and concluded that reaction (1) is important only at high

temperatures. Their arguments are not compelling. Furthermore, there are qualitative

results from three separate studies which are not consistent with reaction (1) being

negligibly slow at room temperature. These include product analysis in a flow system, [8]

Product analysis in gamma radiolysis [9] and an ESR-fast flow study [10]. To our knowledge

there has not been a single determination of the rate constant kj and, although the primary

Processes PH 3 + hv -* PH 2 + H is plausible and consistent with experimental results, [7, 11]

the formation of H is not established by direct detection. We have obtained information

on both of these questions by monitoring Lyman-a resonance fluorescence in the vacuum UV

flash photolysis of phosphine.

The apparatus and techniques have been described previously [12]. Dilute mixtures

of PH 3 in He were flash photolyzed at x > 105 nm to produce H-atom concentrations of the

order 10 11 cm" 3 or less while the PH 3 reactant concentration was ~ 10 13 cm' 3 or larger.

In order to minimize photodecomposition of the reactant and to avoid accumulation of

reaction products, reaction mixtures were flowed through the reaction cell at a rate just

sufficient to replenish the sample between subsequent pulses of vacuum UV radiation.

Under the conditions employed here, with [PH 3 ] » [H], H-atoms detected by the pulse

counting resonance fluorescence technique decayed exponentially with time due to reaction

*nth PH 3 and to diffusion out of the reaction viewing zone:

H + PH

3 * H

2 + PH2

. (1)

(2)

k observed

= MPH 3^ + kd" (3)

Figures in brackets indicate literature references at the end of this paper.

345
The diffusion term k , was determined independently using mixtures of CH^ and He at the

appropriate pressures and temperatures employed in this study. Typically k. was of the

order of 20% of k_K-__u

. H

. observed

It was observed under all conditions employed that equation (2) was obeyed and the

decay of H was first order over at least two decay lifetimes. In addition, plots of kQ

. vs [PH 3 ] exhibited good linearity as required by equation (3) and the intercepts yielded

diffusion rate constants k. which agreed with those obtained independently. Rate data were obtained at seven temperatures in the range 209 to 495 K over a wide range of

experimental conditions, i.e. [PH 3 ], total pressure and flash energy (atom and radical

concentration). An example of the range of experimental conditions employed_is shown

in the table for data at 298 K. Mean values of kj (units cm 3 molecule 1 sec" 1

) at the

other temperatures were: (10.3 + 0.6) x 10" 12 at 495 K, (7.64 + 0.95) x 10" 12 at 420 K (5.93 + 0.41) x 10" 12 at 353 K, (2.21 + 0.25) x 10" 12 at 250 K, (1.77 + 0.19) x 10" 12 at

228 K and (1 .37 + 0.11 ) x 10" 12 at 209 K. A least squares treatment of the data

leads to the_Arrhenius expression k! = (4.52 + 0.39) x 10" 11 exp (-1470 + 50/1.987 T) cm 3 molecule" 1 sec" 1

.

k a

K

l

Rate Data for H + PH

3 at

Flash No. PH

3 He Energy Expt

mTorr

torr

J

0.8 20 20-144 5 1.0 50 183 1 2.0 50 20-144 9 4.0 100 20-183 12 6.0 10,20 110-183 3 8.0 200,400 36-144 7 9.0 15,30 81-183 4

-12 3 -1 -1 10 cm molec sec

2.99 + 0.61

4.37

3.43 + 0.47

3.38 + 0.32

4.14 + 0.20

3.41 + 0.25

3.59 + 0.09

(av). 3.45 + 0.46

a. uncertainty in k^ is the standard deviation

REFERENCES

[1] NAS/NRC Resident Research Associate.

[2] NAS/NRC Senior Resident Research Associate.

[3] Department of Chemistry, University of Essex, Colchester; on leave at Catholic

University of America, Washington, D.C.

[4] S. T. Ridgway, Bull. Am. Astron. Soc, 6, 376 (1974).

[5] J. D. Bregman, D. F. Lester, and D. M. Rank, Astrophys. J., 202, L55 (1975).

[6] R. G. Prinn and J. S. Lewis, Bull. Am. Astron. Soc, 7, 381 (1975); R. G. Prinn and

J. S. Lewis, Science, 190, 274 (1975).

[7] R. G. W. Norrish and G. A. Oldershaw, Proc. Roy. Soc, A262 , 1 (1961 ).

[8] D. M. Wiles and C. A. Winkler, J. Phys. Chem., 61_, 620 (1975).

346
[9] J. W. Buchanan and R. J. Hanrahan, Radiation Research, 42, 244 (1970) ibid, 44, 305

(1970).

[10] R. B. Timmons, Private communication.

[11] D. Kley and K. H. Welge, Z. Naturforschg . 20a, 124 (1965).

[12] R. B. Klemm and L. J. Stief, J. Chem. Phys., 6]_, 4900 (1974); W. A. Payne, Jr. and

L. J. Stief, J. Chem. Phys., 64, 1150 (1976).

347
Energy-Dependent Cross Sections for Quenching of Li( 2 P) and Na( 2 P)

2 John R. Barker, Shen-Maw Lin, and Ralph E. Weston, Jr. Chemistry Department

Brookhaven National Laboratory

Upton, New York 11973

Li( 2 P) and Na( 2 P) atoms were produced with varying amounts of trans lational energy by

photodissociation of Li I with light of wavelengths from 200-230 nm or Nal with wavelengths

of 215-240 nm. In the presence of a quenching gas of concentration [Q], the lifetime tq of

an excited metal atom is related to the bimolecular quenching rate constant by

Tq"

1 = Tq"

1 + kQ [Q], (1)

where x Qis the natural lifetime of the excited atom. The single-photon time-correlation

apparatus used to determine lifetimes is illustrated in figure 1. The light source has a pulse width of ^ 4 nsec (FWHM) and a pulse repetition rate of ^ 10-20 kHz. Each light pulse

generates a start pulse for the TAC, and a stop pulse is nenerated if a fluorescence photon

is detected by photomultiplier P2 before the next start pulse. A voltage proportional to the time elapsed between start and stop pulses is stored in the multichannel analyzer, and

after many pulses have accumulated, the number of counts in each channel is proportional to the population of excited atoms at the corresponding time. Thus, a fluorescence decay curve

is obtained directly. Due to the finite width of the exciting light pulse relative to tq,

numerical deconvolution methods were used to calculate lifetimes.

From the measured values of kQ

, cross sections for quenching (S^) can be calculated

from the relation ^

k 0 =/ S0 (g)P(g)gdg (2

' o

H

where g is the relative collision speed and P(g) is the speed distribution function. The

latter quantity is calculated from Franck-Condon factors for transitions from populated

vibrational levels of the ground state to the upper dissociative state of the alkali iodide.

The Franck-Condon factors are derived from observed fluorescence spectra.

Because eq. (2) cannot be uniquely deconvoluted, a "phenomenological " cross section,

<Sq>, is defined in terms of the velocity g corresponding to the maximum in P(g) by the

expression p

<Sn

> = k n/g . ( Q Q

3mp

For both Li( 2 P) and Na( 2 P), the cross sections were found to be inversely dependent on relative collision energy; this is particularly obvious in the case of Na when our results

are combined with those obtained at lower temperatures in experiments with Na vapor. Using

This research sponsored by the U.S. Energy Research and Development Administration.

Present address: Stanford Research Institute, Menlo Park, CA 94025.

348
;qs. (2) and (3), we compared experimental cross sections with those calculated from two

nodels for two-body collisions: (1) The orbiting collision model for particles subject to

i long-range attractive potential V(R) = -C 6 /R 6

, which leads to a cross section

S Q(E) = w(3tt/2)(2C

6

E)

1/3

(4)

(here w is a quenching efficiency. (2) The absorbing-sphere model, in which it is assumed

hat every incoming trajectory reaching a capture radius R leads to quenching, with a cross Action given by c

SQ(E) = uR/[1 - V(R

c )/E], (5)

;sults of this analysis are given in Table I

Hi! HV

| AMP hfblSC I H T-S [—rD~TsC~~2 fori

Figure 1. Block diagram of single-photon time-correlation apparatus. HV, high

voltage supplies; L, lamp; PI and P2, photomultipliers; M, monochromator ; FURN,

furnace; C, sample cell; LP, light pipe; F, interference filter; AMP, amplifier;

DISCI and DISC2, discriminators; T-S, time-scaler; DL, delay line; TAC, time-toamplitude converter; BA, biased amplifier; MCPHA, multichannel pulse-height

analyzer; TTY, teletype printer and papertape punch; REC, recorder.

The orbiting-trajectory model gives reasonable agreement with our results for Li and

but in the latter case it fails to predict the large cross sections observed by other

kers at low energies. The cross sections calculated from the absorbing sphere formula

a in better agreement over a large energy range. The physical basis for this is the

arpooning" or curve-crossing model, in which the potential curves for the covalen^ states

2 P) + Q and M( 2 S) + Q intersect a strongly attractive curve for the ionic state M Q~.

2 quantities R and V(R ) of eq. (5) can then be identified with the distance at which the

^er covalent and ionic 8urves cross, and the potential energy at this distance. A more

-ai led calculation, based on the Landau-Zener probabilities of non-adiabatic transitions,

in agreement with this.

349
Table 1. Parameters for quenching cross section expressions

Orbiting collision model

3 Absorbing-sphere model

Li (2

2 P) Na(3

2 P) Li (2

2 P) Na(3 2 P)

Gas

6 w

6 w R

c V(R ) c Rc V(R ) c

H

2 0.86 0.55 1.29 0.20 2.33 - 40 1.13 -125

D

2 0.86 0.52 1.29 0.15 2.22 - 43 1.26 - 50

N

2 2.00 0.52 2.95 0.28 2.18 -133 1.54 -173

CO 2.20 0.73 3.26 0.50 2.98 - 50 2.46 - 90

co

2 3.00 0.67 4.46 0.63 2.58 -200 2.52 -200

C

2 H4 4.70 0.74 7.05 0.52 3.66 - 24 3.45 c

- 40

C h

10.00 1.80 15.43 2.2 C 6.56 19 6.6 C

- 98C

Sg given by eq. (4); Cg in units of 10" erg cm . h

° Sq given by eq. (5); R

£ in A, V(R

C

) in meV.

c Ref. 6.

Our results fit qualitatively into the pattern of quenching and reactive cross sectic

obtained for other alkali metal atoms with a variety of quenching qases.

References

[1] Barker, J. R. and Weston, R. E. , Jr., J. Chem. Phys. 65, (1976).

[2] Lin, S.-M. and Weston, R. E. , Jr., J. Chem. Phys. 65, (1976).

[3] Isenberg, I., Dyson, R. D. , and Hanson, R. , Biophys. J. 13, 1090 (1973).

[4] Magee, J. L. and Ri , T., J. Chem. Phys. 9, 638 (1941).

[5] Barker, J. R., Chem. Phys., to be published.

[6] Earl, B. L., Herm, R. R., Lin, S.-M., and Mims, C. A., J. Chem. Phys. 56, 867 (1972)

[7] Earl, B. L. and Herm, R. R. , Chem. Phys. Lett. 22, 95 (1973).

[8] Earl, B. L. and Herm, R. R. , J. Chem. Phys. 60, 4568 (1974).

[9] Brodhead, D. C, Davidovits, P., and Edelstein, S. A., J. Chem. Phys. 51 , 3601 (1969

Edelstein, S. A. and Davidovits, P., ibid., 55, 5164 (1971); Gedeon, A., Edelstein,

Davidovits, P., ibid., 59, 3143 (1973); Maya, J. and Davidovits, P., ibid., 61, 1082' (1974).

350

I
ELECTRON I C-TO- VIBRATIONAL ENERGY TRANSFER REACTIONS:

Na(3 2 P) + CO(X 1 S +

, v=0)

D.S.Y. Hsu 1 and M.C. Lin

Physical Chemistry Branch, Chemistry Division

Naval Research Laboratory

Washington, D. C. 20375

Extensive experimental data exist on the quenching of excited Na atoms by various gases,

ft of these studies deal with overall quenching cross sections [l] 2. To understand the

onanism by which the electronic energy is released during collisions, it would be extremely

ilpful to have detailed microscopic data. Hassler and Polanyi [2] have previously indicated

jat CO vibrational excitation up to v=3 was observed in the Na* + CO reaction, employing

fe infrared chemiluminescence method. However, no detailed vibrational distribution was

yen.

Using an infrared laser resonance absorption method, we have obtained the initial visional population distribution of CO produced from the E->V energy transfer reaction,

Na(3 2 P) + C0(X 1 Z +

, v=0)-> Na(3 2 S) + COU 1 ^, v<8)

AE = - 48.5 kcal/mole

results are found to be consistent with the curve-crossing model [3] by Bauer, Fisher

Gilmore and the impulsive model [4] by Levine and Bernstein.

In order to measure the initial vibrational population distribution of the CO, a

bi 1 l zed cw CO laser (preset at the various vibrational-rotational CO lines) and a omatix CMX-4 dye laser (tuned to one of the Na doublets) were directed colinearly along

axis of a low-carbon stainless steel reaction tube, which contained Na vapor and

tures of CO in Ar. An aluminum block oven is used to maintain a constant temperature,

time-resolved absorption curves, signal-averaged over 1024 shots of the dye laser, were

id to obtain the initial vibrational population distribution. The population distributions

ained from using different pumped Na states and CO concentrations (0.5 percent, 1 percent,

ercent) agree closely with one another. The fact that the distributions differ very

tie regardless of whether the 3

2 P-| / 2 or 3P3/2 state of Na is pumped indicates that both

tes are effectively in equilibrium, due to the large cross sections for doublet

ing [5,6], before appreciable energy transfer to CO has taken place. The lack of

Jndence on CO concentration is evidence that the excited CO is indeed produced in the

iary step, and not by secondary reactions. Figure 1 presents the average of these four

i of experimental vibrational population distributions at 528 °K.

Our results show unequivocally that in the Na* + CO reaction, the E-vV transfer process

lonresonant; a resonant transfer would result in a narrow distribution peaking at v=8.

CO vibrational excitation was observed up to v=8 clearly indicates that CO is vibonally excited up to the limit of the available electronic energy. The nonstatistical

ribution, as revealed by the existence of a maximum at v=2, suggest strongly that the

/NRL Resident Research Associate (September 1 975-present)

.

ures in brackets indicate literature references at the end of this paper.

351
E->-V energy transfer process occurs through an impulsive mechanism. In fact, our experimental distribution can be correlated satisfactorily with the "near-col inear" impulsive

model by Levine and Bernstein [4], using <AE>/hv = 2.83 as the average vibrational quanta

transferred in their Poisson distribution. Our experimental distribution also agrees

surprisingly well with the prediction of the curve-crossing model by Fisher and Smith [3b. It can be concluded that the Na* + CO E+V energy transfer reaction occurs impulsively, wi 1 18.2 kcal/mole of its 48.5 kcal/mole of electronic energy channeling into vibrational and

the remaining into translation and/or rotation.

References

[1] Alkemade, C. Th. J and Zeeger, P. J. Th., in Spectro-Chemical Methods of Analysis,

ed. by Winefordner, J. D., Wiley Interscience, New York, 1971. This review summariz

earlier work on the total quenching cross sections of excited alkali atoms.

352
Hassler, J. C. and Polanyi, J. C, Disc. Faraday Soc. 44, 182 (1967).

(a) Bauer, E., Fisher, E. R. and Gilmore, F. R., J. Chem. Phys. 51, 4173 (1969);

(b) Fisher, E. R. and Smith, G. K. , Appl . Opt. 10, 1803 (1971).

Levine, R. D. and Bernstein, R. B., Chem. Phys. Letters 15, 1 (1972).

(a) Pitre, J., F.S.C. and Krause, L., Can. J. Phys. 46, 125 (1968); (b) 45, 2671

(1967).

Chen, H. L. and Fried, S., IEEE J. Quantum Electron. QE-11, 669 (1975).

353
ENERGY TRANSFER IN THE COLLISION OF METASTABLE EXCITED

Ar

3P

2 ATOMS WITH GROUND STATE H

2 S ATOMS

* Penelope B. Monkhouse , Kyle D. Bayes and Michael A. A. Clyne

Department of Chemistry

Queen Mary College

London El 4NS, U.K.

The interaction of metastable excited argon ( 3 P 2 0 ) atoms witn 9 round state H s

atoms presents a very fundamental problem in energy transfer. Conservation of energy and

momentum show that H should receive almost the full energy defect (1.34 eV). From a study

of the shape and width of the H atom Lyman-a (121.6 nm) line emission profile, information was derived regarding the nature of the collision process, what kinetic energy is

actually present in the H atom, and whether any thermal ization occurs. In addition, we have compared our results for the Ar 3 P 2 + H system with the line profiles of Lyman-a

lines obtained from a microwave discharge plasma.

Metastable argon atoms were produced by flowing purified argon through a low-power

(220 v, 5 mA) hollow cathode discharge which has been described [l] 1

. Total pressures were near 1.3 Torr. H 2 was diluted to the required strength (<_ 10%) in argon and passed

with argon carrier gas through a microwave discharge. The reagent flow was mixed

coaxial ly with the metastable Ar flow downstream of the hollow-cathode discharge, in

front of a Li F observation window.

2 2 The Lyman-a 121.6 nm line, H 2p P - S, was examined using a 1.5 m_Fastie-Ebert

vacuum monochromator (Jarrell-Ash) in the fourth order of a 1200 line mm" 1 grating

blazed at 500 nm and using a slit width of 30 ym. The Lyman-a line from the Ar 3 P 2 o + H interaction was found to be broadened (HBW = 0.013 nm), and to possess a profile close

to a boxcar (rectangular) shape. The edges of this line are steep, as expected from the

instrumental profile (Figure 1). The ^(^P 3

, 2

-

2 Pz>i) emission line at 138.0 nm from

the reaction Ar 3 P 0 2 + Cl 2 was chosen to determine the slit function; it was unreversed, near Gaussian shape,' and thermal (HBW = 0.003 nm). The top of the Lyman-a profile was effectively flat, with only limited self reversal at relatively high atom particle

densities in the order of 10 13 cm" 3

.

The Lyman-a and chlorine line profiles were then digitized manually and after iterative smoothing to remove random noise, were processed computationally through a deconvolution program which utilized an iterative routine [2]. The resulting spectrum (Figure 2B)

confirmed our preliminary estimations of an essentially rectangular profile.

Figures in brackets indicate literature references at the end of this paper.

354
355
The results will be discussed in terms of the model proposed by Biondi and Connor [3]

in studies of non-thermal line shapes of Ne produced by electron-ion recombination. De- convolution of the observed line shape has led to the conclusion that all (_> 90%) of the excess energy (1.34 eV for Ar 3 P 2 ) in the Ar* + H interation is found as kinetic energy

of the H atom, allowing for the very small fraction that is transferred to the Ar 1 S 0 atom. This indicates that negligible thermal izati on had occurred under our conditions.

Figure 2 compares the experimental deconvoluted result (B) with the calculated Biondi

profile for the Ar ( 3 P 2 ) + H system. (A)

The overall rate constant for the process

Ar

3

P

2 0 + H

2 S - Ar] S o + H(2s,2p)

has been measured using the Ar 3 P 2 +_Kr reaction as a standard [4]. The value obtained was (2.8 + 1.5) x 10

_1

° cm 3 molecule" 1 s" 1

, i.e. close to unit collisional efficiency at 298 K.

Lyman-a profiles from microwave plasmas operating in pure argon and helium have also

been investigated. In the case of argon, a very broad profile (Figure 3) similar to that

obtained from the Ar( 3 P 2 ) + H reaction was observed. These results suggest that a major

excitation process for Lyman-a in the microwave discharge in argon is by collision of H with metastable argon atoms. The helium plasmas showed much narrower H atom line profiles

tending towards the limit of the instrumental line width at low [H]. Thus it would seem likely that either the predominant excitation of H(2p) in helium plasmas did not involve

metastables or that H(2s) was formed and then partly thermalized by collisions with He,

before quenching to the 2p state could occur.

On sabbatical leave from University of California at Los Angeles.

REFERENCES

[1] D. W. Setser, D. H. Stedman and J. A. Coxon, J. Chem. Phys. (1970), 5_3, 1004.

[2] Unpublished data of M. Heaven, Queen Mary College.

[3] R. T. Connor and M. A. Biondi, Phys. Rev. (1965), 140A , 778.

[4] L. A. Gundel, D. W. Setser, W. S. Nip, M. A. A. Clyne and J. A. Coxon, J. Chem.

Phys., 64, 4390 (1976).

356
Biondi

Model

Doppler

Model

—i

1 2 —

r

-8 -4

CA)

—r- 12

A X CpnO

CB)

A XCpm)

Figure 2

357

QUENCHING RATE CONSTANTS FOR Ar(

3

P

2

), Kr(

3P

2

) AND Xe(

3

P

2

) BY HALOGEN-CONTAINING MOLECULES, AND BRANCHING RATIOS FOR

XeF AND KrF FORMATION*

J. E. Velazco , H. E. Kolts and D. W. Setser

Department of Chemistry

Kansas State University

Manhattan, Kansas 66506

The discovery [1.2] 1 of rare gas-halogen excimer formation from the reaction of rare

gas metastable atoms with halogen or halogen-containing molecules, has led to the develop1 ment of a new class of ultraviolet lasers [3]. However, scaling-up of these prototype

devices faces some practical as well as theoretical problems: halogen donors must be found

that give excimer formation with high efficiency, do not absorb at the laser frequency and

I are reasonably easy to handle. Modeling of these systems requires the knowledge of the

rate constants (and branching ratios) for the relevant steps. In this work total (thermal)

quenching rate constants for Ar( 3 P 2 ), Kr( 3 P 2 ), and Xe( 3 P 2 ) with a variety of fluorine

donor reagents are reported; also, the branching ratios for the formation of KrF* and

XeF* were measured. Emission from a second excimer state of XeF, XeCl , KrCl and KrF, which

correlated with the ( 2 Pi /2 ) state of the rare gas ion will be reported, as will the

observation of vibrational relaxation of the excimers in the 1-10 torr pressure range.

Total quenching rate constant (kg) were measured using a flowing after-glow apparatus [4] with a linear flow velocity of ^ 80 msec" 1

. The Ar metastable atoms were produced

by passing a flow of purified Ar through a hollow cathode discharge. Total metastable

concentrations ^ 5 x 10 10 cc" 1

, were measured at the first observation point, with Ar( 3 P 2 ) and Ar( 3 P 0

) in a ^ 6:1 ration. The Kr( 3 P 2 ) or Xe( 3 P 2 ) metastables were produced by energy

jl transfer reactions from Ar( 3 P 0 2 ) by adding small quantities (^ 0.3%) of Kr or Xe to the

I I Ar flow before the discharge. 'The decay of the metastables along the tubular reactor as

a function of quencher concentration was followed by atomic absorption using Pen Ray source

lamps: the Ar 811.5 nm, Kr 811.2 nm, Xe 881.9 nm lines are used. Fluorine donors were

diluted in Ar and stored in a passivated stainless vessel connected to the inlet part of

the flow reactor by 1/4" ss tubing. Less reactive reagents were stored and metered

through glass lines. The quenching rate constants, K

n

, from these experiments are listed

in Table I. The reproducibility of k Q

for the highly^corrosi ve F-donors is + 20% due mainly to difficulties in purification of^sarnples. For chlorine and fluorides that do not

attack glass, the reproducibility is + 10%.

Figures in brackets indicate literature references at the end of this paper.

359
TABLE I. Rate Constant Summary

k n (10 cm molec" sec" ) Relative K, Reagent

Ar( P

2

) Kr( P

2

) Xe( P

2

) KrCl XeCl

ci

2

71 73 72 100

a 100

a

F

2 75 72 75 100 100

Br

2 65 60

C1F

3 62

b 33

b

IC1 61 50

0F

2 57 53 57 82 80

CF3OF 43 42 47 35 25

SF

6 16 28 % 0 % 0

SeF

6

71 65 0 0

TeF

6 58 63 0 0

S0

2

F

2 42 0 0

NF

3 14 16 9 7 14

N

2

F 431 33 22 27

NOF 36 47 30 < 5 40;(43)

d

N0C1 48 51

HL 1 "37 61 56

HBr 57 61

HI 75

CF3I 52

5 0;(33)

e 0;(23)e

a

,

Cl

2 was used as the reference reaction for assigning k^.

b. Both chloride and fluoride excimer emission was observed.

c. NO (y) and (e) emissions were the main product,

on an estimate of the KrF emission intensity.

This value is based

d. NO (y) bands were also observed. This value includes the

of the Y-bands.

contribution

e. MBr rather than MF was the product.

360
A comparison technique was developed to measure the branching ratio for excimer

formation, k^/kg, which eliminates the need of a direct and absolute measurement of kMX

.

In the absence of quenching or non-radiative processes, the excimer emission intensity

is given by (1 ).

RX

I MX

= kMX

[M*] [RX] (1)

Hence, if k^ is assigned for a reference reaction, and if experiments are done for the

same [M*], we can obtain other k MX from (2),

I^/CR'X]

k

' = K k M (2) MX

K MX T RX /rDvn '

K

' I MX /LRXJ

by simply comparing the relative emission intensities for known RX and RX' concentrations.

In order to perform these intensity comparisons, a system was built that consists of

two gas handling sections (one grease-free but of glass and the other stainless steel

and the flow reactor section. Pressures and flow rates were monitored with a pressure

transducer. Emission from the reaction of Kr( 3 P 2 ) or Xe( 3 P 2 ) (produced a described

above) with added halogen donor, was detected by a computer controlled 0.75 m Jarrell-Ash

monochromator equipped with a cooled photomultiplier tube and SSR photon counter. For

o low pressure spectra, the intensity (counts/sec) was recorded at 1 A intervals; whereas,

for high pressure spectra shorter intervals were used. The data were stored in magnetic

tapes and subsequently corrected for spectral response. Integrated emission intensities

were obtained by simple addition of the counts in each interval.

Molecular chlorine was selected as the reference donor (RX) because the absolute

values of k«

rQi

and k^-j could be estimated, and also because Cl 2 can be handled in a relatively simple and reliable way. Columns 4 and 5 of Table I list the rate constants for

excimer formation relative to Cl 2 (see equation (2)). The absolute value of k^^ from the

Kr* + Cl 2 reaction was calculated by using the absolute rate constant for the formation of

the Cl 2 * 257. nm continum from Ar* + Cl 2 (1.7 x 10~ u cc molec" 1 sec" 1 ). Comparing the

emission intensities of the Cl 2 * 257 nm continuum and the KrCl excimer emission gives

(3).

! C1* k

Cl" 2 [Cl

2 ] [Ar(

3

P 0>2 )]

htf k KrCl

£C1

2 ] tKr(

3

P

2 )]

(3)

The ratio of Ar and Kr metastable concentrations was determined and the results for

equal [Cl 2 ] was k^-] = 52 x 10" 11 ml molec" 1 sec" 1

. This value gives a branching ratio of

0.7 for excimer formation from Kr* + Cl 2

. This value however, may have an uncertainty of

as much as + 50% because the intensity comparisons required to determine k r1 * are at the extreme ends of our calibrated spectral response curved. Furthermore, it is likely that

some KrCl* emission extends into the v.u.v. and, therefore, is not included in l u - * KrCl

Careful recal ibration experiments now under way will decrease the uncertainty of this

branching ration. The lack of reliable oscillator strengths for transitions in the 6s-6p

manifold precludes a determination of kXeC1 by the method outlined above. However,

evidence will be presented that supports the assignment of unity branching ratio for

excimer formation from Xe* + Cl 2

.

361
Extensive vibrational relaxation of the excimers occur in the 1-40 Torr pressure

range as shown in Figure 1 for XeF from Xe* + ONF. Similar results were obtained for

XeCl*, KrCl* and KrF*. Since significant relaxation is evident even at < 5 Torr, simple

consideration of the gas-kinetic collision frequency at these pressure 4.3 x 10 7 at

5 Torr) suggest a lifetime > 50 ns for the rate gas-halide excimers. Figure 1 also shows

the XeF emission at ^ 265 nm, originating from an upper excimer excited state ( 2 iriy2) tnat correlated with the ( 2 Pi^2 ) state of the rate gas ion [5]. The state responsible for the

main excimer emission correlates with the 2 P3,2 i°n state. Emission from the upper state

was also identified for XeCl*, KrF* and KrCl*( The separation between the maximum of the

primary emission and that due to this new state is 9422 cm" 1 (1.17 eV) for XeF, 9857 cm" 1 (1.22 eV) for XeCl, 5050 cm" 1 (0.63 eV) for KrF and 5380 cm" 1 (0.67 eV) for KrCl . The

dynamics of these reactions as deduced from the emission spectra of the excimers will be

briefly discussed in terms of analogy to alkali metal atom reactions.

362
REFERENCES

This work was supported by U. S. Army Research Office (DAHC04-75-60018) and by

U. S. Energy Research and Development Administration ( E ( 1 1 - 1 -2807 )

.

a. J. E. Velazco and D. W. Setser 4th Conference on Chemical and Molecular Lasers

October 21-23, 1974; IEEE J. Quant. Electron. QE-11 , 708 (1975).

b. J. E. Velazco and D. W. Setser, J. Chem. Phys. 62, 1990 (1974).

M. F. Golde and B. A. Thrush, Chem. Phys. Lett. 29, 486 (1974).

a. K. Searles and G. A. Hart, Appl . Phys. Lett. 27, 243 (1975).

b. J. J. Ewing and C. A. Brau, Appl. Phys. Lett. 27, 350 (1975).

c. E. R. Ault, R. S. Bradfor and M. L. Bhaumik, Appl. Phys. Lett. 27, 413 (1975).

d. G. C. Tisone, A. K. Hays, and J. M. Hoffman, Opt. Commen. 1_5, 188 (1975).

L. G. Piper, J. E. Velazco and D. W. Setser, J. Chem. Phys. 59, 3323 (1973).

C. A. Brau and J. J. Ewing, J. Chem. Phys. 63, 4640 (1975).

363
V-V ENERGY TRANSFER IN H 2 - ADDITIVE GAS MIXTURES USING A STIMULATED RAMAN EXCITATION TECHNIQUE

Richard G. Miller and J. K. Hancock

Chemistry Division

Naval Research Laboratory

Washington, DC 20375

We have measured gas phase vibrational energy transfer rates for the following inelastic collision process,

H

2 (v=l) + A (v=0) > H2 (v=0) + A (v >0) + AE(cm

_1

)

where A was various heteronuclear diatomics and triatomics. A measurable population in the

H 2 (v=l ) state was produced using a stimulated Raman excitation technique. Ducuing,

Joffrin, and Coffinet [l] 1 demonstrated the utility of this technique in measuring V-T

deactivation rates in H 2 gas. Matsui, Resler, and Bauer [2] have more recently measured

V-V energy transfer rates in H 2 -C0 and D 2 -C0 gas mixtures using a stimulated Raman - IR fluorescence technique. We have extended this excitation scheme to our system utilizing

two cells which has enabled us to observe V-V energy transfer in H 2 -A gas mixtures at pressures much less than an atmosphere.

Experimental

Our experimental apparatus is depicted in the accompanying diagram. The excitation

source was a doubled Nd 3

: YAG laser (International Laser Systems, Inc.) with a TEM^ mode

output of % 85 mjoules/pulse and a 15 nsec pulse duration. The 532.0 nm beam was bent and

focused into the center of Cell 1 which contained 50 psi of H 2

. Stimulated Stokes-shif ted

beams (at 683.2 nm) and anti-Stokes-shifted beams (at 435.6 nm) subsequently propagated

from Cell 1 in both directions coaxially with the laser beam. The Stokes-shifted beam and

the laser beam are then re-focused into the center of the second cell resulting in the

excitation of the H 2 (v=l ) state. (Figure 1).

Energy transfer rates from H 2 (v=l) to the additive molecules A were obtained by

monitoring the IR fluorescence from the IR-active vibrational modes of the A species. The

A species which we have studied include, HC1, DC1, HBr, C0 2 , N 2 0, 12 C0, 13 C0, and NO. The

IR-fl uorescence from these species was monitored with an InSb photoconducti ve detector.

The temporal profile of the fluorescence was processed with a Biomation 8100 transient

digitizer interfaced with a Nicolet 1072 signal averager. The respective V-V and V-T rates were such that the desired V-V rate was contained in the fall time of the fluorescence

from the additive molecule.

Figures in brackets indicate literature references at the end of this paper.

364
DETECTOR

77° K

Figure 1 Results

The H 2 -HC1 gas mixtures were studied extensively. Pressures of 85 to 692 torr and

1.06 to 73 torr of H 2 and HC1 , respectively, were irradiated. The (P T

t

s

) values ranged

.from 20 to 310 sec" 1 torr" 1

. The composite V-V and V-T energy transfer rate constant

(keVV + VT

) for the H 2 to HC1 transfer was evaluated to be 1510 + 210 sec" 1 torr" 1

. Two

other groups [3,4] have obtained H 2 to HC1 k£VV + VT values by exciting a mixture of H 2
and HC1 containing a trace of HF with an HF laser. Bott's [3] value was 1250 + 150 sec" 1 torr" 1 and Pirkle and Cool [4] reported a value of 1730 + 250 sec" 1 torr" 1

. We have

also made some measurements on the HF-H 2 -HC1 system using HF laser excitation and obtained (Pj values that were in good agreement with our results obtained with the stimulated Raman pumping scheme.

The complete results shown in the accompanying table will be compared further with

available results of others. The importance of V-V transfer to the overtone and combination bands of the A species studied will be discussed. Reasonable correlations of our

H 2 -H(D)X results with V-V results on various homonuclear diatomic - H(D)X gas mixtures

can be shown.

REFERENCES

* NRC Postdoctoral Fellow.

[1] J. Ducuing, C. Joffrin, and J. P. Coffinet, Opt. Comm., 2, 6 (1970).

[2] H. Matsui, E. L. Resler, Jr., and S. H. Bauer, J. Chem. Phys., 63, 4171 (1975).

[3] J. F. Bott, preprint.

[4] R. J. Pirkle and T. A. Cool, preprint.

366
-co

Vibrational Energy Transfer Rates in ^^=1) + Additive M + H

2 (v=0) + M(v=l,2) + AE Mixtures at 296°K

. vv + vt

K H

2 -m Additive Energy, _, _, Molecule M Defect cm (sec" torr" ) Probability

HC1 1274 1510 + 210 8.62 x 10" 5 -1508(v=2)

DC1 2069 689 + 30 3.95 10" 5 34(v=2)

HBr 1604 224 + 18 1 .26 10" 5

- 865(v=2)

DBr 2321 212 + 36 1.19 x 10" 5 527(v=2)

C09 1811(001) 497+ 30 2.01 x 10' 5

c

r 551(021)

444(101)

N? 0 1936(001) 462 + 14 1.82 x 10" 5

£ 795(021)

679(101)

12rn 2017 12.3 + 9.5 6.02 x 10" 7

- 100(v=2)

13rn 2069 9.7 + 1.9

C0

- 6(v=2)

N0(

2

H, /9 ) 2284 42 + 17 2.47 x 10" 6 Wd 436(v=2)

NO(

2 n. /9 ) 2159 iU 311 (v=2)

367
EXPERIMENTS CONCERNING THE LASER ENHANCED REACTION BETWEEN 0 3 t AND NO

Kin-Kwok Hui and Terrill A. Cool

School of Applied and Engineering Physics

Cornell University

Ithaca, New York 14853

Several recent studies of the laser-enhanced reaction of nitric oxide with vibrational ly excited ozone

NO + 0 3 t + NOJ^Bj) + 0 2 (la)

and

NO + 0 3 t + N0 2 t( 2A 1

) + 0 2 (lb)

have been reported [1-5] 1

. In these studies the visible chemi luminescence from N0 2 *( 2 B 1

) has been observed to be significantly enhanced in response to the laser-induced vibrational

excitation of ozone, i.e.,

0 3 + hv •> 0 3 t (2)

The initial experiments of Gordon and Lin [1] revealed a lag in the rise of fluo- rescence from N0 2 *( 2 B 1 ) produced in reaction (la). It was suggested that this lag was associated with the v 3

-> v 2 coupling of vibrational modes in 0 3

. However, later experiments [6] showed that the v 3 v 2 coupling time was an order of magnitude longer than that

necessary to explain the fluorescence lag reported by Gordon and Lin.

Other puzzling aspects of the experimental measurements on reactions (la) and (lb) have

been reported [4,7]. Kurylo et al . , [4] measured the rate sum for reactions (la) and (lb) as a function of temperature and observed a complicated temperature dependence which could

not be expressed with a single choice of Arrhenius parameters. Kurylo et al., [4] have

suggested that an understanding of this temperature dependence requires the resolution of

the influences of intermode vibrational transfer in 0 3

, of vibrational deactivation of 0 3 , and the relative temperature dependence of the two reaction channels (la) and (lb).

We have performed experimental measurements with an apparatus designed to yield new information concerning reactions (la) and (lb). A Q-switched C0 2 laser operated on the

P(30) transition of the 9.5 micron band is employed for laser excitation of 0 3. The laser

produces 1 mJ pulses of 1 psec (FWHM) pulse width and 170 Hz repetition rate. A continuous

flow reactor has been employed which was carefully designed for more rapid and uniform gas mixing than had been accomplished in previous work. The apparatus was designed for operation over the range 150-400 °K, and permits observations of vibrational fluorescences from 0 3 and

N0 2 in addition to the electronic fluorescence previously observed. Measurements have been

performed of the temporal variations in fluorescence from the N0 2 *( 2 Bj) and N0 2 t( 2A 1

) states

as a function of gas composition over a wide range of parameters.

Figures in brackets indicate literature references at the end of this paper.

368
The fluorescence from the N0 2 *( 2 B 1

) molecules does not show a delayed rise, of the type

reported by Gordon and Lin, under any experimental conditions we have explored to date.

These include both high and low partial pressures of 0 3 with and without lar : je amounts of

various diluents. Conditions for which all three modes of 0 3 should be equilibrated and

conditions for which only the and v 3 modes participate were examined. We believe the

delayed fluorescence rise reported by Gordon and Lin was not of kinetic origin, but rather

was caused by incomplete gas mixing. The dependence of the decay of N0 2 *( 2 B 1

) on NO pressure

gives a value of (7.5 ± 0.8) x 10 10 cmSmole^sec" 1 for the sum of rate constants for

reactions (la) and (lb) at a temperature of 308 ± 3 °K in good agreement with recent

measurements [4,5].

Observations of vibration fluorescence from the N0 2 t( 2A 1

) state at 3.7 microns show a rise time determined by collisional quenching of N0 2 t( 2A 1

) and a characteristic decay rate

determined by the combined rate of reactions (la) and (lb). A rate constant for these

combined reactions of (8.6 ± 0.8) x 10 10 cm 3mol e^sec -1 at 308 ± 3 °K is obtained from the

N02t( 2A 1

) fluorescence decay, in agreement with the value found from the relaxation of

N02 *( 2 B 1 ). The rise time data for N0 2 t( 2A 1

) fluorescence give values of (1.1 ,± 0.2)

x 10 12 cmSmole^sec -1 for the quenching of N0 2 t( 2A 1 ) by NO and (3.1 ± 0.8) x 10 11 cn^mole-1 sec

-1 for the quenching of N0 2 t( 2A : ) by 0 2

. These values are in good agreement with the

results of previous measurements [8].

Results for the AC/DC fluorescence ratios for both the N0 2 *( 2 B 1) and N0 2 t( 2A 1) states indicate a surprisingly low yield of N0 2 t( 2A 1 ) by reaction (lb) which suggests that products

of the laser enhanced reaction are predominantly excited in the translational and rotational

degrees of freedom rather than in vibration.

Measurements currently in progress devoted toward an understanding of reactions (la)

and (lb) will be discussed.

[1] Gordon, R. J. and Lin, M. C, Chem. Phys. Lett. 22, 262 (1973).

[2] Kurylo, M. j., Braun, W., Kaldor, A., Freund, S. M. and Wayne, R. P., J. Photochem.

3, 71 (1974/1975).

[3] Braun, W., Kurylo, M. j., Kaldor, A. and Wayne, R. P., J. Chem. Phys. 61_, 461 (1974).

' [4] Kurylo, M. J., Braun, W., Xuan, C. N. and Kaldor, A., J. Chem. Phys. 62, 2065 (1975).

[5] Gordon, R. J. and Lin, M. C, J. Chem. Phys. 64, 1058 (1976).

[6] Hui, K. K. , Rosen, D. I. and Cool, T. A., Chem. Phys. Lett. 32, 141 (1975).

[7] Redpath, A. E. and Menzinger, M. , J. Chem. Phys. 62, 1987 (1975).

[8] Golde, M. F. and Kaufman, F., Chem. Phys. Lett. 29, 480 (1974).

References

369
INFRARED LASER ENHANCED REACTIONS: CHEMISTRY OF NO(v=l) WITH 0 3 *

J. C. Stephenson and S. M. Freund

National Bureau of Standards

Washington, D.C. 20234

A variety of recent experiments demonstrates that bimolecular chemical reaction rates may be enhanced by optically exciting the vibrational levels of the reactant molecules with

an infrared laser. The molecular system studied in greatest detail is the reaction of ozone with nitric oxide to give nitrogen dioxide and molecular oxygen, a process of importance in

the atmosphere. The thermal reaction rate constant has been thoroughly investigated. The

enhancement of the reaction rate following excitation of the 0 3 (001) vibrational level by a C0 2 laser was first reported by Gordon and Lin [I] 1

, and then studied in greater detail by

Kurylo et al . , and Freund and Stephenson [2-4].

The present work continues the investigation of the laser-enhanced reaction between NO and 0 3

. Because it is the ozone 0 2 -0 bond which breaks during the reaction, it was expected

and then proven [2-4] that vibrational excitation of the 0 3 asymmetric stretching motion

increases the rate. For the reactions

k R 0 3 + NO S NO^Bi) + 0 2

0 3 + + NO +

B N0 2 ( 2 B!) + 0 2 kA

0 3 + NO 5 N0 2 ( 2Ai) + 0 2 k

'

0 3 f + NO +

A

N0 2 ( 2A!) + 0 2

where 0 3 + represents vibrationally excited ozone, the room temperature rate constant enhancement factors k'g/kB = 4.1 ± 2 and k'/\/k^ = 17.1 ± 4.3 were determined [2]. Since the NO bond does not break in the reaction, it was not obvious that there would be any increase in

rate for the analogous reactions

. k

1 ! N0

T + 0 3

-> N0 2 ( 2 B!) + 0 2 (1 ')

NO + 0 3

-> N0 2 ( 2 B!) + 0 2 (1)

NO + 0 3 * N0 2 ( 2A!) + 0 2 (2')

k 2 NO + 0 3

-> N0 2 ( 2 Ai) + 0 2 (2)

where N0+ denotes N0(v=l). However, in the experiment described below, we excited the

N0(v=l) state with a CO laser, and found k^Ai = 5.7 ± 1.4 and (k' 2 + kNQ-03// k 2 ±22. A

figures in brackets indicate literature references at the end of this paper.

370
necessary part of the experiment was the determination of the rate at which NO is vibrationally deactivated by 0 2

, k N0-0 2 N0

T + 0 2

•* NO + 0 2

for which ^h0_q 2 = 920 ± 80 sec"1 Torr"1

.

All of the experimental measurements were performed on mixtures of gases flowing in a pyrex cell. The reactants were monitored by means of calibrated flow meters and were admitted to the cell through concentric glass tubing which terminated in a "shower head"

configuration. The laser excitation beam traversed the center of the cell perpendicular to

the gas flow and visible and infrared fluorescence was viewed in a direction perpendicular

to both the flow direction and the laser beam. The distance between the fluorescence viewing

window and the mixing jet could be easily adjusted by sliding the glass tubing in or out through an 0-ring connector to the desired position. Polished BaF 2 windows were fastened to

the cell with epoxy at the Brewster angle laser entrance and exit positions and at the

central fluorescence viewing port. The pressure drop aP along the cell was small (less than

5 percent of the total pressure for condtions under which data was taken). The commercially

obtained reactant gases were used directly from the cylinders without further purification.

The ozone was produced in a commercial generator which gave a maximum mole fraction of 0 3 in

0 2 of 0.045.

The 11+ N

16 0 molecules were pumped by radiation from a liquid-nitrogen-cooled cw CO laser

which produced about 3 watts of power on the fundamental (TEMQ0 ) mode of the 1884 cm

-1 line

(P(13) transition of the 9+8 12 C^ 6 0 band). The beam was focused by a 10 cm focal length

BaF 2 lens and was chopped at the focal point by a servo-controlled chopping wheel which

produced a square-wave modulated beam (100 percent modulation) at a frequency of about 100

Hz. It was then collimated by a second BaF 2 lens and directed into the cell by three

front-surface-reflecting mirrors. The beam diameter at the entrance to the reaction cell was approximately the same size as the tubing through which it subsequently passed.

Since the center of this laser line is 732 MHz above the transition in nitric oxide,

N0(v=0, J=3/2) 2 n 3 / 2

-> N0(v=l, J=5/2) 2 n 3 /

2 , the molecular transition must be tuned into

coincidence by applying a magnetic field. The polarization of the laser was such that E (laser) was parallel to B (magnet). The magnetic field required to maximize the absorption

was 760 gauss; this field was measured to be homogeneous to ± 10 gauss over the region of

the cell near the fluorescence port. Measurements of the laser power absorbed in a short

cell containing NO placed at the center of the magnetic coils gave an absorption coefficient

at room temperature of 0.0209 cm

-1 Torr-1 (standard deviation of 0.0003 cm

-1 Torr -1

) for

absorption of the 1884 cm

-1 laser line in the pressure region P < 4 Torr. The laser frequency

was not stabilized by locking to the top of the gain profile or to an external cavity. The

reproducibility of the laser absorption coefficient measurements showed that any drift in

laser frequency was small compared to the Doppler width of the NO transition.

Visible fluorescence from the N0 2 *( 2 Bi) product, hereafter designated N0 2 *, was detected with a photomultiplier tube cooled to -30 °C. The N0(v=l) fluorescence was monitored

with a large area photovoltaic InSb detector. The analog signals which developed across

appropriate load resistors were amplified by low noise amplifiers (DC to 1 MHz band pass),

processed by a transient recorder which digitized the signal in real time (minimum 10 8 sec/channel), and stored in a multichannel analyzer. The electronics were triggered by the

signal from an InSb detector which monitored the laser light transmitted through the cell.

In a typical Al/I experiment, 40,000 cycles of the square wave signal were summed.

References

[1] Gordon, R. J. and Lin, M. C, Chem. Phys. Lett. 22, 262 (1973).

[2] Kurylo, M. J., Braun, W., Kaldor, A., Freund, S. M. and Wayne, R. P., J. Photochem. 3,

71 (1974).

371
[3] Kurylo, M. J. Braun, W., Xuan, C. N. and Kaldor, A., J. Chem. Phys. 62, 2065 (1975).

[4] Freund, S. M. and Stephenson, J. C, "Laser Enhanced Chemical Reaction Between 0 3 (001

)

and NO," Chem. Phys. Letters 41_, 157 (1976).

372
KINETIC ENERGY - AND INTERNAL STATE DEPENDENCE

OF THE NO + 0 3

-> N0 2 * + 0 2 REACTION

Anthony E. Redpath and Michael Mensinger

Department of Chemistry

University of Toronto

Toronto, ON

The numerous degrees of freedom of nuclear and electronic motion of both reactants and

products make the title reaction rich in dynamical detail. Associated with the problem of

Energy Consumption are the questions of influence of (a) reactant kinetic energy E, (b) of

the N0( 2 n 1 / 2 i 3/2) spin-orbit states and (c) of 0 3 (v 1 ,v2,v 3

) vibrational excitation, on the

dynamics and rates of the different product channels, (d) The effects of NO vib-rotational

and 0 3 rotational energy are believed to be small and we have not investigated them. The

problem of Energy Disposal has its spicy sides since (e) at least two electronic N0 2 states

( 2Ai groundstate and an excited state, presumably 2 B 1

) are populated, but primary vibrational

state distribution are unavailable due to the complexity of the electronic N0 2 * chemiluminescence spectrum and to extensive vib-rotational quenching in the 2A : groundstate. (f)

Furthermore, the 02*( 1 Ag, 1 Eg) states are energetically accessible, but Gauthier and Snelling

have shown [I] 1 them to be produced with negligible probability in the thermal reaction,

(g) Given the long radiative lifetimes of N0 2 *(xR* ^ 10 _lt sec) and N0 24(xRt ^ 10" 2 sec?) it

has to be kept in mind that bulk experiments are usually conducted at pressures where secondary collisions dominate the kinetics. In our "single collision" beam/gas experiments, a He

or H 2 seeded supersonic NO beam issues from a heatable nozzle (300-650 K) , is skimmed,

chopped and shot through an 0 3 filled scattering chamber (variable temperature 150-300 K to populate 0 3 vibration) and monitored. Using lock-in techniques chemil uminescence is detected by [1] a bare photomultipl ier [2] a monochromator/photomultipl ier combination [3] a PbS

infrared detector plus filter for electronic N0 2 * emission (1-3 ym) and [4] PbS detector and

another filter for vibrational N0 2 * (3-4 ym) emission. The beam energy is varied via (a)

the seeding ratio and (b) the nozzle temperature. Beams are velocity analyzed by time-offlight.

It is found that NO beams from a heated (600 K) nozzle yield ca. 4x more visible CL than beams of the same nominal velocity issuing from a room temperature nozzle. It is shown

that the former have a higher population of the N0( 2 n 3 / 2 ) spin-orbit component than the

latter. We conclude that N0( 2 n3/ 2 ) + O3 yields primarily electronically excited N0 2 *,

while N0( 2 nj/ 2 ) + 0 3 leads to groundstate N0 2 ( 2 Ai). The only (N0-0 3 collision complex which

correlates with N0 2 *( 2 B 1

) + 0 2 ( 3 Ig) and N0 2 ±( 2 A!) + 0 2 ( 3 £g) rather than with 0 2 ( 1 A gl

1 Zg)

[1] has C 5 symmetry: NO approaches along the 0 bisecting plane and attacks the cen0

x 0

tral rather than an end-standing 0-atom. This "bisecting C 5

" reaction pathway is expected

to be greatly facilitated by the 0 3 bending mode v 2 , [2] which furthers bond formation

between the end-standing oxygen:

figures in brackets indicate literature references at the end of this paper.

373
A cyclic 0 3 isomer of very similar energy as the "classical" bent 0 3 qroundstate, that has

been predicted on theoretical grounds [3,4] supports this view. However, competing channels

in which end-standing 0 may be transferred cannot be ruled out on the basis of the circumstantial evidence presented here. We observe enhancement of the CL crossection through

thermally populating 0 3 internal modes.

The excitation functions for production of visible and for IR (1-3 urn) electronic

emission are almost identical: till ^3 kcal above threshold they are well represented by

o a(E) = C(E - E/ E > E Q

=0 E < E Q

o with C = 7.1 x 10 5 A

2

, E 0

= 3.0 ± .3 kcal/mole and n = 2.3 ± .3. This yields an activation

energy in good agreement with measured values [5].

The CI spectra, recorded at F = 15 ± 1.5 and 12 ± 1.2 kcal/mole and at thermal energies

show a strong blue shift with increasing collision energy: the short wavelength cutoff invariably agrees with the total available energy, demonstrating that in a small (unknown)

fraction of collisions the total energy is funnel ed into internal N0 2 * and subsequently

emitted as light. The energy dependence of the vibrational IR emission is markedly distinct

from electronic emission: no threshold is discernible even at the lowest collision energy

E . =1.1 kcal/mole, and the signal rises less rapidly as E increases.

In the spirit of information theory [6] a^(E) or ratner

kn (E) = vo(E) = p(E'ME)

is decomposed [7] into a calculable (a priori) product phasespace density p(E') and the

dynamical "averaged state to state transition probability" 10(E). It comes somewhat as a surprise that the time-honoured hard sphere/1 ine-of-center model for reaction crossections

[8] gives an excellent representation of w(E) = (1 - E /E). For the penta-atomic title

reaction, the phasespace factor dominates a(E).

We express our thanks to Dr. Akimichi Yokozeki for the information theoretical analysis, and to Professor Tucker Carrington for the correlation diagrams and for numerous chemiluminating discussions in the course of this work. Financial support stems from the National

Research Council of Canada and is gratefully acknowledged.

374
References

[1] Gauthier, M. and Snelling, D. R., Chem. Phys. Lett., 20, 178 (1973).

[2] (a) Gordon, R. J. and Lin, M. C, Chem. Phys. Lett., 22, 262 (1973).

(b) Gordon, R. J. and Lin, M. C, J. Chem. Phys., 64, 1058 (1976).

[3] Wright, J. S., Can. J. Chem., 51_, 139 (1973).

[4] Shih, S., Buenker, R. J. and Peyerimhoff, S. D., Chem. Phys. Lett., 28, 463 (1974).

[5] Clough, P. N. and Thrush, B. A., Trans. Far. Soc, 63, 915 (1967).

[6] Bernstein, R. B. and Levine, R. D.: "Role of Energy in Reactive Molecular Scattering

An Information Theoretic Approach" in Advances in Atomic and Molecular Physics, Vol.

II. Ed. Bates, D. R., Academic Press, 1975.

[7] Pruett, J. G., Grabiner, F. R. and Brooks, P. R., J. Chem. Phys., 63, (75) 1173.

[8] Levine, R. D. and Bernstein, R. B., Molecular Reaction Dynamics, Oxford U.P., 1974.

375
THE EFFECT OF INFRARED LASER EXCITATION ON REACTION DYNAMICS:

0 + C

2 H

4

+ and 0 + OCS

+

Ronald G. Manning , Walter Braun, and Michael J. Kurylo

National Bureau of Standards

Washington, DC 20234

A variety of experiments have been reported in the literature recently regarding the

energetics and dynamics of atom exchange processes (A + BC -> AB + C) . These studies have

utilized the infrared arrested fluorescence technique to analyze the energy distribution

in the diatomic product [l] 1 and thermal [2], microwave [3], chemical [4], and infrared

laser [5] excitation methods to investigate the effect of internal energy in the diatomic

reactant on the reaction rate. Calculations for these systems show that, depending on the nature of the reaction hypersurface, reactant vibrational energy can be very effective or virtually ineffective in influencing the rate of chemical reaction [6]. A number of more complex systems have also been investigated using either IR laser excitation of a reactant molecule [7,8,9] or chemical or flash photolytic production of an excited transient (i.e. 0H(u>0) [10, 11] and CN(u>0)) [12]. Most of these systems

exhibited rate constant enhancements of factors of three to four for reaction of the

vibrational ly excited species when compared with the thermal process. In the molecular

system studied in greatest detail (0 3 + NO), it was found that the temperature coefficient

remained essentially idential to that for the Boltzman reaction [7]. These results

indicate that while there appears to be little coupling between the reactant vibrational

energy and the reaction coordinate in the (0 3 + NO) system a small but significant effect

can be observed on the overall reaction rate.

vibrational energy on the reactions

(1)

(2)

Experimental

The experimental apparatus for the kinetic measurements (depicted schmatically in Figure 1) consists of a flash photolysis resonance fluorescence (FPRF) apparatus modified

l Figures in brackets indicate literature references at the end of this paper.

In this study we have investigated the effect of

of 0( 3 P) atoms with OCS and C^:

k

l 0 + OCS —> CO + so k,t

0 + 0CS

T —!— CO + SO

k 0 + C

2 H4 CH

3 + CHO

0 + C

2 H4

T —— CH

3 + CHO

376
ATOMiC

RESONANCE

LAMP

Z

TO

VACUUM

SYS] EM

REACTION

CELL

PHOTOMUL.T1PL IER

/

LAMP I

A

VACUUM U.V.

PHOTOMULTI PLIE R

TELETYPE

VACUUM

PULSE

COUNTING

MULT I SCALER

TRIGGER

Figure 1. Schematic diagram of the flash photolysis-resonance fluorescence

(FPRP) apparatus modified for IR laser reactant excitation.

to allow passage of IR laser radiation. The basic FPRF assembly consists of a reaction

cell of arbitrary geometry to which are attached a flash lamp, resonance lamp, and

fluorescence detecting photomul tipl ier on perpendicular axes. The sensitivity and

precision of the FPRF method enable one to discern changes in experimental conditions

which cause perturbation of the measured decay rate by several percent. The C0 2 laser

shown in the diagram can be operated cw over numerous lines in the 9-lOp region with a power level of 3-6 watts. Consequently, under conditions which minimize vibrational

deactivation, steady state concentrations of vibrationally excited species can be

achieved such that ([S]u>0/[S]u = 0) > 0.05. By comparing the atom decay rate for laser

on vs. laser off conditions, any signTficant change in the rate constant due to reactant

vibrational excitation can be detected. The limiting value of the laser enhancement that

can be discerned depends ultimately on two factors: 1) the equilibrium concentration of

vibrationally excited reactant that can be experimentally realized (i.e. absorption of

laser flux vs. deactivation losses) and 2) the magnitude of the activation energy for the

thermal reaction. This determines the extent of a purely thermal (heating) effect.

Results

The excited substrate molecules (S ) can be formed by two methods: direct IR absorption and V-V transfer from another molecule which is pumped by the laser. Such V-V

377
transfer is generally much faster than V-T deactivation in the system and this latter

scheme is often successful in achieving high enough concentrations of S

+ to facilitate

+ + measuring the smallest possible change in k /k 13

. In these experiments CH 3 F has been

used as a transfer agent because of its strong absorption of the P20 line, 9.6y band C0 2 laser radiation.

Under these conditions we observed little or no effect of vibrational energy in C 2 ^k on the rate of reaction with 0 atoms. Because of a somewhat higher activation energy,

it was considerably more difficult to measure an effect in the 0 + OCS reaction. Within

the limits of detectability, all observations in this system could be attributed to heating effects. A factor of 1.5 increase in the rate constant for 0 + CaHi^ over that

for 0 + C2H4 and a factor of 3 for 0 + 0CS

+ over 0 + OCS would have been detectable in these experiments. The results seem to indicate that there is little or no coupling of

vibrational energy to the reaction coordinate leading to activated complexes in these

two reaction systems. Experiments are in progress to determine the effect of vibrational

energy on other reaction systems.

REFERENCES

[I] K. G. Anlauf, P. J. Kuntz, D. H. Maylotte, P. D. Pacey, and J. C. Polanyi, Disc.

Faraday Soc. 44, 183 (1967).

[2a] L.J. Kirsch and J. C. Polanyi, J. Chem. Phys. 57, 4498 (1972).

[ b] J. H. Birely, J. V. V. Kasper, F. Hai, and L. A. Darnton, Chem. Phys. Letters

31_, 220 (1975).

[3a] R. F. Heidner and J. V. V. Kasper, Chem. Phys. Letters 15., 179 (1972).

[ b] L. B. Sims, L. R. Dosser, and P. S. Wilson, Chem. Phys. Letters 32, 150 (1975).

[4] D. J. Douglas, J. C. Polanyi, and J. J. Sloan, J. Chem. Phys. 59, 6679 (1973).

[5a] T. J. Odiorne, P. R. Brooks, and J. V. V. Kasper, J. Chem. Phys. 55, 1980 (1971).

[ b] D. Arnoldi, K. Kaufman, and J. Wolfrum, Phys. Rev. Letters 34, 1597 (1975).

[ c] Z. Karny, B. Katz, and A. Szoke, Chem. Phys. Letters 35, 100 (1975).

[ d] S. R. Leone, R. G. Macdonald and C. B. Moore, J. Chem. Phys. 63, 4735 (1975).

[6] J. C. Polanyi, Accounts of Chemical Research 5, 161 (1972).

[7] M. J. Kurylo, W. Braun, C. Nguyen Xuan, and A. Kaldor. J. Chem. Phys. 62, 2065

(1975); 63, 1042 (1975). —

[8] R. J. Gordon and M. C. Lin, J. Chem. Phys. 64, 1058 (1976).

[9] E. N. Chesnokov, V. P. Strunin, N. K. Serdyuk, and V. N. Panfilov, Reaction Kinetics

and Catalysis Letters 3, 131 (1975).

[10] S. D. Worley, R. N. Coltharp, and A. E. Potter, J. Phys. Chem. 76, 1511 (1972).

[II] G. E. Streit and H. S. Johnston, J. Chem. Phys. 64, 95 (1976).

378
[12] V. H. Schacke, K. J. Schmatjko, and J. Wolfrum, Ber. Buns. Phys. Chem. 77, 248

(1973).

[13a] M. J. Kurylo, W. Braun, and A. Kaldor, Chem. Phys. Letters 2_7, 249 (1974).

[ b] W. Braun, M. J. Kurylo, and A. Kaldor, Chem. Phys. Letters 28, 440 (1974).

[ c] D. I. Rosen and T. A. Cool, J. Chem. Phys. 62, 466 (1975).

379
REACTION OF FLASH PHOTOLYTICALLY PRODUCED CN(X

2

Z +

, v) RADICALS WITH 0(

3 P) ATOMS

K. J. Schmatjko and J. Wolfrum

MP I fur Stromungsforschung

D-3400 Gottingen

West Germany

The reaction (1) CN(X

2

z +

, v) + 0(

3 P) > C0(X

] E +

, v) + N( 4

S,

2 D) was studied using a flash-photolysis arrangement combined with a discharge-flow reactor. CN-radicals were

produced by flash-photolysis of C 2 N 2 diluted in He and their concentrations monitored by

visible absorption spectroscopy. The flash discharge (17 kV, 0.2 uF) is switched by a thyratron with a repetition rate up to 5 Hz. 0( 3 P) atoms present in excess with respect

to the CN radicals are continuously generated in a microwave discharge. The concentrations

of the COU 1 e , v) molecules formed in reaction (1) were obtained from time-resolved

infrared laser absorption spectroscopy (see Figure 1).

Figure 1. Schematic of the experimental apparatus for infrared laser resonance absorption-gain measurements

The CO-laser could be tuned by a grating on single rotational lines of the vibrational

transitions of C0(v) between (1, 0), and 26, 25). For transitions below (3, 2) the laser

output had to be etabilized continuously between measurements.

To maintain isothermal conditions and to avoid the rapid exchange of vibrational

energy during the formation of the C0(v) molecules very low concentrations have to be

applied. This was achieved by using an internal multi-pass mirror system. The adjustable

mirrors allowed up to 60 traversals of the analysis laser beam along the flow tube.

From the absorption or gain signals the absolute concentration of C0(v) formed in the

reaction was evaluated starting with the highest vibrational transition of C0(v) where

[C0(v + 1)] = 0 was found. No absorption or gain was observed for lines in the (26, 25) to the (15, 14) bands. From the measured concentration-time profile (see Figure 2a) the

380
REACTION TIME

Figure 2a. Formation of C0(v) in Reaction (1).

distribution of C0(v) molecules formed in reaction (1) could be obtained directly. The

shape of the distribution measured at early times (see Figure 2b) shows only small changes

10"

5-10"

10" 0\

VIBRATIONAL LEVEL OF CO

Figure 2b. Distribution of C0(v) formed in Reaction (1).

I as the reaction time proceeds. The rate constant k x = (1, 1 +0, 3). 10 cm mol" s"

I at 295 K could be obtained from the C0(v,t) profiles. No systematic variation was

found using different C0(v) levels.

The observed energy distribution (see Figure 2b) shows two different parts which

can be understand by considering the correlation diagram of reaction (1). As can be

seen in Figure 3 several pathways exist: one reaction path direct to CC^X 1 z ) and N( 4 S) atoms and two reaction paths to N( 2 D) atoms and C0(X 1 E ) either directly or via the

stable intermediate NC0(X 2 n). To stimulate the dynamics of reaction (1) three-

. dimensional classical trajectory calculations were carried out using two different

empirical adiabatic potential energy surfaces. The trajectory calculations show that on

the path to N( 4 S) atoms the reaction energy is predominantly channeled into vibrational

excitation of the CO molecule.

381
kj/mol

Figure 3. Correlation diagram for Reaction (1).

On the path to N( D) via NC0(X n) the formation of C0(v = 0) predominates and the

reaction energy is mainly converted into electronic excitation of the metastable N( 2 D) atom. If one sums up the measured absolute population of the CO vibrational levels

according to the shapes of the distribution predicted by the trajectory calculations

one obtains.

0(

3 P) + CN -2-* C0(v) + N(

2 D) (0.85+0.05)

C0(v) + N(

4

S) (0.15+0.05)

Observation of the electron state of the nitrogen atoms produced in reaction (3) provides

an independent way to obtain information on the importance of the two pathways (la) and

(lb).

Observation of the nitrogen atoms produced in the reaction was made by time-resolved

resonance absorption in the vacuum UV (see Figure 4).

K-\02 -

MICROWAVE

DISCHARGE LAMP l-jSp—II—I W

vuv

MONOCHROVATi

J AVERAGER

SCOPE

Figure 4. Detection of nitrogen atoms formed in Reaction (1) by resonance absorption.

382
Figure 5 shows the measured time-resolved absorption signals from the N( D) atoms produced

in (1). While the concentration of NC^S) remains nearly constant during the observation

time a decay of the N( 2 D) concentration due to quenching of the metastable N( 2 D) atoms

by the components of the reaction mixture is observed.

0 —

oS 0.1

% 0.2

N(2

D)

200

jjs/div

Figure 5. Oscillogram of the vacuum ultraviolet absorption of N( D)

formed in Reaction (1).

2 4 From the measured absorption signals a ratio of 5:1 for N( D) versus N( S) formation in reaction (1) in good agreement with the result from the C0(v) distribution is obtained.

383
VIBRATIONAL PHOTOCHEMISTRY: THE RELAXATION OF HC1 (v=l ) and

DCl(v=l) BY BROMINE ATOMS

R. D. H. Brown, I. W. M. Smith and S. W. J. Van der Merwe

Department of Physical Chemistry

University Chemical Laboratories

Cambridge CB2 1EP, England

The atom- transfer reaction

Br + HCl(v) -> HBr + CI (1)

is establishing itself as the protoptype of endoergic reactions whose rates are selectively enhanced by vibrational excitation of the bond that is broken in the reaction. Recently the relative rates of (1) for 1 < v <_ 4 have been determined using a ' chemil uminescence

depletion' method [l] 1 and the totaT rates for reactive plus inelastic processes, i.e.

Br + HCl(v) + Br + HCl(v'<v), (2)

have been measured for v=l,2 in laser induced vibrational fluorescence experiments at room temperature [2]. This paper reports measurements by the second technique of (kj + k 2 ) for

HCl(v=l) and DCl(v=l). The experimental results are interpreted in the light of the results

of Monte Carlo trajectory calculations.

Experimental Method and Results

A description of our general experimental method has been given elsewhere [3]. Gas

samples were prepared in a flow-discharge system and some of the HC1 contained in this

mixture was excited to v=l with the output from a pulsed HC1 chemical laser. The decay

of the vibrational fluorescence was measured with different concentrations of Br atoms

present. These were prepared by means of the reactions

0 + Br

2 - BrO + Br (3)

0 + BrO -> 0

2 + Br (4)

that convert 0 atoms quantitatively to Br. The atomic oxygen was produced by partial dissociation of 0 2 in a microwave discharge and its concentration was determined, prior to addition of Br 2 , by titration with N0 2 .

Figures in brackets indicate literature references at the end of this paper.

384
By measuring the energy absorbed from the laser it was shown that 1 - 1.5% of the HC1 was excited to v=l. The degree of excitation was purposely kept this low to avoid problems

that might arise through fast V-V exchange of vibrational energy followed by rapid removal

of HC1 from levels with v > 1.

Table 1 summarizes the results obtained in our experiments.

Table 1. Summary of Observed Rate Constants

Excited T k- + k ck - k Ck molecule - 5a 5b 5b

K 10" 13 cm

3 molecule" 1 s"1 10" 13 cm

3 molecule" 1 s"1

HCl(v=l) 210 2.8 2.6 2.8

2a

HCl(v=l) 295 5.6 4.8 2.7

2b

HC1(V=1) 371 9.5 7.7 2.6 2c

DCl(v=l) 295 9.4 9.4

Chemical reaction between Br and HCl(v=l) is so endoergic that it cannot remove

HCl(v=l) at low temperatures. There remain, however, two energy transfer processes that

may contribute to the overall rate:

Br*(

2

P 1/? ) + HCl(v=0) (5a)

Br(

2 P 3/2

) + HCl(v=l)

1/2'

Br(

2 P 3/2

) + HCl(v=0) (5b)

Process (5a) is 2.3 kcal/mole endoergic but it can procide a route for loss of HCl(v=l) in our experiments as Br* is rapidly quenched by 02

. The rate constant k 5a can be calculated

from k_g

a for the reverse process [4]. Assuming k_^

a to be independent of temperature, the

values of k^ listed in the last column of Table 1 are obtained.

Our value of k^. at 295 K is almost twice that determined in other recent investigations. In these Br atoms were formed by partial dissociation of Br

? in a discharge.

Because Br2 deativates HC 1 ( v= 1 ) efficiently, [2a] the acceleration brought about by the atoms was only ^ 40% as against a threefold increase in our experiments. Nevertheless it

appears that an as yet unidentified systematic error may cause the difference between

these two sets of results.

Theoretical Results and Discussion

To assist with the interpretation of the experimental data a three-dimensional

quasiclassical trajectory study is now being made of the dynamics of Br + HCl(v) and Br + DCl(v) collisions. These allow one to examine the electronically adiabatic routes for

removal of HC1 (v).

The computations are being carried out using procedures that have been described

previously [5] with a single parameter LEPS potential. Calculations have been carried

385
out on collisions between Br and HC1(1 < v < 4) with translational and rotation energy

distributions corresponding to temperatures of 333.3 and 1000 K. The results are summarized in Table 2.

Table 2. Calculated Rate Constants

T 1 2

k,

1 100k,

i

K

V 10 cm molecule s

t 11 ^ „ 3 "i t"TM! — 1 10 cm molecule s k

1 + k 2 1000 4 8.6 5.6 66.7

3 7.1 4.5 63.4

2 2.7 1 .5 56.5

1 * 0.1 * 0.05 50

333.3 4 4.6 2.1 46.7

3 2.3 1.2 51.5

2 0.3

2 0.09 28

1

The relative values of the rate constants for different v are in reasonable agreement with the experimental results of Douglas et. al_ [1]. For v _> 2 processes (1) and (2)

have roughly equal rates with relaxation becoming more effective than reaction as v and T are lowered. Energy transfer occurs in collisions where at some stage in the trajectory

( r

HCl /re,HCl ^ > ^ rHBr^

re,HBr^» 1,e * reaction 'nearly' takes place.

The calculations provide strong evidence that process (5b) does not occur in electronically adiabatic collisions. The absolute magnitude of the observed rate constant, and

the fact that k,^ for DC1 is greater than for HC1, indicate that relaxation proceeds via

the kind of curve-crossing process that has been proposed by Nikitin [6]. The rate of

this process should be approximately proportional to v [7]. Consequently three detailed

mechanisms may make roughly equal contributions to the removal of HCl(v=2) by Br atoms,

although this proposal is at variance with the conclusions reached by Douglas, Polanyi and

Sloan [1].

REFERENCES

[1] (a) D. J. Douglas, J. C. Polanyi and J. J. Sloan, J. Chem. Phys., 59, 6679 (1973)

and Chem. Phys. , 13., 15 (1976).

[2] (a) S. R. Leone, R. G. Macdonald and C. B. Moore, J. Chem. Phys., 63, 4735 (1975);

(b) D. Arnoldi, K. Kaufman and J. Wolfrum, Phys. Rev. Letters, 34, 1597 (1975)*

(c) Z. Karny and B. Katz, Chem. Phys. Letters, 38, 382 (1976).

[3] (a) R. D. H. Brown, G. P. Glass and I. E. M. Smith, Chem. Phys. Letters, 32, 517

(1975) and (b) J. C. S. Faraday II, 7T_, 1963 (1975).

386
S. R. Leone and F. J. Wodarczyk, J. Chem. Phys., 60, 314 (1974).

(a) I. W. M. Smith and P. M. Wood, Mol . Phys., 25, 441 (1973); (b) I. W. M. Smith,

J. C. S. Faraday II, 71_, 1970 (1975).

E. E. Nikitin and S. Ya. Umanski , Faraday Disc. Chem. Soc, 53, 7 (1972).

I. W. M. Smith, Acct. Chem. Res., 9, 161 (1976).

387
VIBRATIONAL RELAXATION OF HF(v = 1,2,3) IN THE PRESENCE

OF H

2

, N

2

, AND C0

2

J. F. Bott

The Aerospace Corporation

P. 0. Box 92957

Los Angeles, California 90009

The study of vibrational relaxation has greatly increased in the last few years be- cause of the interest in chemical laser. In addition, these chemical lasers have provided

the means for performing the experimental studies. Until recently, experiments have been

largely confined to relaxation rate measurements for molecules in the first vibrational

levels because of the difficulties involved in the controlled production of the higher

levels so that specific upper level processes could be isolated. The techniques of direct

laser pumping of the second vibrational level, low-pressure combustion with spectroscopic

diagnostics, reactive flows in medium-pressure flow tubes, and laser-induced fluorescence

by sequential absorption have been used to study upper vibrational level deactivation.

Laser-induced fluorescence was used in this study to measure the relaxation rates of

HF(v = 1,2,3) in H 2 , N 2 , 02 , HC1 , and C0 2

.

A laser-induced fluorescence experiment in which the sequential absorption of photons

produces the higher vibrational levels in relatively easy to perform, and the interpretation of the data is straightforward. The experiments can be performed with a large ratio

of the collision partner to HF, thereby reducing the HF-HF V-V deactivation processes to negligible contributions.

A knowledge of the upper level deactivation rates is important for understanding and

modeling the performance of the HF chemical laser since the F + H 2 pumping reaction directly populates levels v = 1, 2, and 3. The values for HF(v)-C0 2 deactivation obtained in this experiment can be compared with the theoretical calculations of Dillon and Stephenson

and the measured values obtained in more complicated experiments.

The apparatus for this experiment included a HF TEA laser, a fluorescence cell, and

several detectors. The TEA laser operated on SF 6 and H 2 and produced laser pulses of a few millijoules with time durations of approximately 100 to 300 nsec. The laser was operated at a repetition rate of 3 Hz without substantial degradation of the fluores- cence signal. A concave mirrow and a partially transmitting flat formed the laser cavity.

No spectral line selection was necessary, and the laser output contained various lines of

the 3->-2, 2V|, and l->0 vibrational bands. By appropriate adjustments in the pressures of

SF 6 and H 2 , the laser was made to pump the HF to vibrational levels v = 1,2, and 3 or just

v = 1 and 2. The pressure tuning determined whether the 1-+0, 2->-l , and 3-*2 laser transitions

occurred in the proper sequence or overlapped sufficiently to produce upper level pumping

of the HF in the fluorescence cell.

The fluorescence from HF(v = 3) and HF(v = 2) were monitored with a RCA C-31034

GaAs photomultipl ier and a RCA 7102 (S-l) photomul tipl ier , respectively. An InSb detector

was used to monitor the fluorescence from HF(v = 1). Spectral filters were used to pass

only the desired 3-*0, 2-*0, or 1-+0 fluorescence from HF. The signals from the detector or photomul tipl ier were recorded with a Biomation 805 transient recorder and transferred

to a Nicolet Model 1072 signal averager in which 32 to 512 experiments could be stored and

averaged before being displayed on an X-Y recorder.

388
The relaxation rates of HF(v = 3) were measured in gas mixtures containing 0.02% HF,

0 to 99.98% H 2 , and a balance of Ar at a total pressure of 20 Torr. The slope of the rates plotted versus H 2 concentration was interpreted as the sum of the V-V transfer rates from

HF(v = 3) to H 2 and the V-R, T deactivation rate of HF(v = 3) by H 2

. The measured relaxation rates extrapolated to zero H 2 concentration indicated a value that is compatible with

the HF(v = 3) -HF deactivation rate obtained by Osgood and coworkers. Data were also

obtained for HF(v = 1) and HF(v = 2). Similar data were obtained for the deactivation of

HF(v) by N 2 , 0 2 , HC1, and C0 2.

Although this technique is not applicable to the study of all molecules, it has many

advantages for those diatomic molecules capable of laser action on several vibrational,

rotational transitions. This technique can most likely be extended to HF(v > 3) by the

use of such fuels as HBr or HI in the pumping laser.

The rates of vibrational energy transfer from the upper vibrational levels of HF to

N 2 , 0 2 , HC1 , and C0 2 increase faster with v than the predicted rates for harmonic oscillators having no energy defect. Part of this increase can be attributed to the decrease in the energy defect, aE(v), with v so that less energy must be absorbed by the rotational

and translational degrees of freedom for the transfer of a quantum of energy. However,

the dependence of the rates on v can be explained only qualitatively in terms of the

anharmonicity of HF and the resulting energy defect dependence on v.
QUENCHING OF NO(B

2 n

r

) v

, =0 PRODUCED BY THE REACTION OF N(

2 D) with N

2 0

G. Black , R. L. Sharpless, and T. G. Slanger

Stanford Research Institute

Menlo Park, California 94025

Very little information is available on the quenching of N0(B 2 n

r

), largely because

the state has not been observed in NO fluorescence [I] 1

. However, N0(B 2 n

r

) can be made

and its quenching studied in many other ways. We have used the reaction

N(

2 D) + N

2 0 -> N

2 + N0(B

2n

r

) (1)

as the source of N0(B 2 n r

), generating the N( 2 D) by photodissociation of N 2 0 [2]. This

reaction produces most of the N0(B 2 n

r

) in the v'=0 level [3].

The apparatus for these measurements has been fully described [2]. Previously, [2]

O we have shown that over the region 1100-1500 A, the largest quantum yield of N( 2 D) from

o o NoO occurs between 1150 A and 1200 A. Most of the measurements were therefore made at

o o 1175 A. A Wratten No. 18A filter was used to isolate the 3000-4000 A region in which the

strongest (0, v") NO e bands occur. For these measurements, a cooled EMI 9635AM was used (dark count at -20°C, ^ 20 counts/sec). The experiments were performed in a very

similar way to those described previously [2]. Figure 1 shows plots obtained for N 2 = 0 and 17.2 torr.

In all cases, single exponential decays were observed covering at least one decade

change in intensity. Figure 1 shows two effects of adding N 2 —the decay rate increases

and the intercept decreases.

The increasing decay rate with quenching gas addition has previously been used [4-6]

to determine rate coefficients for quenching N(*D). The decay rates shown in Figure 1 give a rate coefficient of 1.96 x 10" 11+ cm 3 molec" 1 sec" 1 for the quenching of N( 2 D) by N 2 , in good agreement with the two most recently reported [6,7] values of (1.5 + 0.1) x 10 11+ and (1.85 + 0.15) x 10~ 11+ cm 3 molec" 1 sec" 1

.

Our main purpose here is to determine rate coefficients for quenching N0(B 2 n ) from

the decrease in the intercept with quenching gas addition (Figure 1). (Recently, we

used similar measurements [8] to study the quenching of Se 2 (B 3 E~)). Absorption by the

added gas was always negligible over the ^ 10 cm from the Li F window to the center of the

region viewed by the photomul tip! ier. Therefore, for plots like those shown in

figures in brackets indicate literature references at the end of this paper.

390
10,000

0 0.1 0.2 0.3 0.4 0.8 0.G 0.7 TIME (msec)

Figure 1. Quenching of N(

2 D) and N0(B2

n

r

) v

, =0 . Ar = 3 torr; N

2 0 = 28 mtorr.

20 min. experiments at 1175 A.

Figure 1, we can write

fo

I

1 +

xk

Q [Q]

1 + xkAr[Ar] + xk

N2() [N

2 0]

(2)

where I is the intercept at time zero (the time the lamp is pulsed with no quenching gas,

I is the intercept with quenching gas addition [Q], t is the radiative lifetime of

N0(B2 n

r

) v ,_Q (3.0 x 10~ 6 sec), [9] and kg, k^

r

, and k^ q are the rate coefficients for

quenching N0(B2 n

r

) v,

_

0 by Q, Ar, and N 2 0, respectively.

Figure 1 gives I Q

/I = 1.91 +0.09 for N

2

= 17.2 torr. This point and values of I Q

/I at serveral other nitrogen pressures are shown plotted as suggested by Equation (2) in

Figure 2. The straight line predicted by Equation (2) is indeed observed. The slope S of this Stern-Volmer plot can be written

Tk

N, S

=

1 xk

Ar

[Ar] xk

N2() [N2 0]

(3)

To determine k we must know k^

r and k^ g. The rate coefficient was determined with

sufficient accuracy for substitution in Equation (3) by making measurements similar to those described above in argon-N 2 0 mixtures containing N 2 0 = 15 mtorr and neglecting

quenching by N 2 0. Below, we described how L, n was determined.

IN 2 U The quenching of the intercept by C0 2 was studied at several N 2 0 pressures from 40 to 760 mtorr. For these experiments, Equation (3) was rearranged:

391
|

kAr[Ar]

| (4)

CO,

Figure 2. Stern-Volmer plot for N 9 quenching of N0(B n ) ,_n. Ar = 3 torr;

N

2 0 = 28 mtorr.

£ r v u

where S" , the reciptrocal slope of the Stern-Volmer plots, is the half-quenching pressure.

The data for the C02 experiments are shown plotted as suggested by Equation (4) in

Figure 3. The slope of this graph gives qAqq = 4.5 + 0.5, and the intercept, after

correcting for the small argon contribution, gives a "true" half-quenching pressure (the

pressure at which the rate of quenching by C0 2 equals the rate of radition) for C0 2 of

1.47 + 0.2 torr. Hence, k CQ

= (7.0 +1.0) x 10" 12 cm 3 molec" 1 sec" 1 (since x = 3.0 x 10" 6 sec) and q

= (3.2 + 0.8) x 10" 11 cm 3 molec" 1 sec" 1

. Table 1 shows true half-quenching pressures and corresponding rate coefficients for several gases, to- gether with the values of Campbell and Thrush [10] for C0 2 , H 2 , and N 2 0, and the value of

Melton and Klemperer [11] for the quenching of N0(B 2 n ) by NO.

392
Table 1

Quenching of N0(B n ) . Q at 298°K

Half-Quenching Pressures and Rate Coefficients

Hal f-Quenching

Pressure Rate Coefficient Other values [10,11]

(torr) (cm 3 molec" 1 sec" 1

) (cm 3 molec

36 + 6 (2.9 + 0.5) X io13

36 + 5 (2.9 + 0.4) X io13

17 + 3 (6.1 +1.1) X 10" 13

1.47 + 0.2 (7.0 + 1 .0) X IO"

12 1.0 x IO"

11

1.0 + 0.25 (1.03 + 0.26) x IO"

11 2.0 x IO"11

0.41 + 0.09 (2.5 + 0.6) X IO"

11

0.36 + 0.04 (2.9 + 0.3) X IO"

11

(3.2 + 0.8) X IO"

11 4.5 x IO"

11

(1.4 + 0.1)

0.064 + 0.013 (1.6 + 0.3) X IO"

10

0.038 + 0.009 (2.7 + 0.6) X IO"

10

0.032 + 0.008 (3.2 + 0.8) X IO'

10

,-10

393
J J J L L _i I L

0 0 1 0.2 0.3 0.', 0.0 0.S 0.7 D.8 0.9

|N 2 0j PRESSURE (lorr|

Figure 3. quenching data. The reciprocal of the slope of Stern-Volmer

graphs vs [N

2 0]. Ar = 3 torr.

REFERENCES

[I] A. B. Callear and I. E. M. Smith, Trans. Faraday Soc. 59, 1720 (1963).

[2] G. Black, R. L. Sharpless, T. G. Slanger, and D. C. Lorents, J. Chem. Phys. 62,

4266 (1975).

[3] K. H. Becker and K. H. Welge, Z. Naturforsch. , 20a, 442 (1965).

[4] G. Black, T. G. Slanger, G. A. St. John, and R. A. Yound, J. Chem. Phys. 5J_, 116 (1969).

[5] T. G. Slanger, B. J. Wood, and G. Black, J. Geophys. Res. 76, 8430 (1971).

[6] T. G. Slanger and G. Black, J. Chem. Phys. 64, xxxx (1976).

[7] D. Husain, S. K. Mitra, and A. N. Yound, J. Chem. Soc. Farad. II 70, 1721 (1974).

[8] G. Black, R. L. Sharpless, and T. G. Slanger, J. Chem. Phys. 64, 3993 (1976).

[9] M. Jeunehomme and A. B. F. Duncan, J. Chem. Phys. 4J_, 1692 (1964).

[10] I. M. Capbell and B. A. Thrush, J. Quant. Spectrosc. Radiat. Transfer 8, 1571 (1968),

[II] L. A. Melton and W. Klemperer, J. Chem. Phys. 59, 1099 (1973).

394
CW INFRARED LASER ISOTOPE SEPARATION:

CI + CH3 Br (v6 )

•* HC1 + CH2 Br

T. J. Manuccia and M. D. Clark

Naval Research Laboratory

Washington, DC

and

E. R. Lory

Cornell University

Ithaca, NY

Using a novel CW IR scheme, either Br 79 or Br81 is enriched by vibrational enhancement of the above reaction whose rate is slow compared to the CH3 Br interisotope VV rate. Data from this first reaction studied indicated excellent electrical efficiencies and

throughputs should be possible using this scheme to separate a variety of isotopes.

Summary

The attractiveness of the infrared for laser induced chemistry (LIC) and laser isotope

separation (LIS) has been stated for some time and stems from the availability of very

efficient, scalable lasers, and considerations such as the number of photons per joule and

reactivity changes upon excitation of reagent vibrational modes. For example, the large

changes in reaction rates associated with shorter wavelength excitation may be of little

use in applications such as some LIS schemes where net enrichment is limited by factors

such as spectroscopic selectivity or interisotope energy energy exchange.

A serious obstacle to IR gas-phase photochemistry has been distinguishing vibrational

enhancements from purely thermal effects. We obviate this problem by observing isotope

separation: the non-absorbing isotope serves as a reference reaction, monitoring any

temperature rise. Unfortunately, excitation of the non-absorbing isotope can also occur

by fast near resonant energy transfer. To overcome this, it is usually stated that the

chemistry of the excited state must be fast compared to the rate of VV transfer. This

very severe constraint is met by very few reactions and has seriously limited single IR photon LIS work.

We circumvent this difficulty by VT deactivation of the excited states of both isotopic

species of the reagent. For the absorbing species, deactivation competes with absorption

of photons, but relative rates can be adjusted to allow the absorbing transition to ensure

saturation and hence maximize the excited state density. For the non-absorbing isotope,

deactivation competes with the rate of resonant energy transfer and can be made to dominate

it by reducing the density of the reagent in the reaction vessel. Thus, in principle, one

can prepare on a CW basis a sample of reagent with one isotope excited and the other

arbitrarily close to thermal equilibrium. With this "competing deactivation" method, we

have avoided the constraint of fast chemistry and allowed vibrational enhancement of quite

slow reactions to be studied. This may be of interest in itself, or if isotope separation

Lis the goal, this technique allows a controlled trade-off between separation factor and

quantum efficiency. In the IR where photons are relatively cheap, this may be economically

interesting.

395
Using this technique, we have studied v 6 mode vibrational enhancement of the reaction

CI + CH 3 Br -> HC1 + CH2 Br in a low pressure discharge/flow reactor intracavity to a line

selected CW C0 2 laser. The radical produced by the above reaction reacts quickly with

excess molecular chlorine to produce the stable product CH 2 BrCl which is then detected by

an on-line mass spectrometer. A boxcar integrator is used to average the output of the mass spectrometer and calculate isotope ratios. The radical chain formed by the above

two reactions is not a priori detrimental to isotopic specificity. It is terminated by

heterogeneous recombination before the reaction mixture leaves the irradiation zone. Argon was chosen as the carrier gas and deactivating agent. The available intracavity

flux of about 100 W/cm2 and published deactivation rates set the maximum argon pressure

in the 1 to 5 torr range to ensure saturation of the absorbing transition. Assuming an interisotope VV rate of 10 collisions, CH3 Br pressures below 5 (j will provide for a large

difference in the degree of excitation between the two isotopic species. Under such

conditions, the power absorbed per unit volume will be low and thermal effects should be

negl igible.

With the laser operating on R(14) of the 10 u C0 2 band, and the reactor at -100 °C, Br 79 in the CH2 BrCl product is enriched by 3.0 percent ±0.5 percent, tuning to P(10) of

the same band enriches the Br81 by 2.0 percent ±0.5 percent. Other lines produce little

effect as expected from small signal absorption measurements we have performed using a spectraphone. The data seem consistent with a single IR photon mechanism. The component

of the rate enhancement which is not isotopically selective is only somewhat larger than

the selective component and may reflect residual thermal heating, VV exchange or chemical

scrambling between enriched products and depleted reagents. The enrichment drops only by

a factor of 2 going from -100 °C to 20 °C, but drops rapidly to zero above room temperature.

With this lab-scale apparatus, throughput of enriched bromine was 122 mg/day with

5.9 MeV of electrical energy (including pumps) spent per product molecule. As the reactor

was optically thin and the path included non-reacting gas, improvements of 10 3 in both

numbers should be possible by simply changing to a folded optical path without making any

other changes.

From the point of view of pressures, flow rates, absorption, coefficients, and in particular, scaling factors, this first reaction studied seems typical of many reactions

one might investigate using the method of competing deactivation. As an isotope separation

scheme, many reactions exhibit considerably larger vibrational enhancements than CI + CH3 Br and promise excellent electrical efficiencies and throughputs. As a scheme for

investigating the reactivity of vibrational ly excited states we have demonstrated sensitivity

to changes of less than 1 percent in reaction rate. This scheme has the further advantage

that it is reasonably state selective within a given molecule. For example, v 6 = 2 of

CH 3 79 Br which could be populated by VV processes is not because of the competing VT deactivation. With the exception of one early experiment whose interpretation has been

questioned, this seems to be the first demonstration of isotope separation using a cw IR photochemical technique.

396
APPENDIX I

AUTHOR INDEX (Underscoring indicates author presentation)

MD ralllOW 1 XL t o . R9DC. | /-\ y» 1/ 1 < IT) D

UOrKUiFI 9 K

.

u J AH amc An A 1*1 ttQalllbUn 9 H • W • b 1

i v, iit7on D uruxzen , r . u

.

rero

A rl rl -l ¥~\ n ~i~ r\r\ Ul huu i ny tun 9 u. w. Pn A uo

ttls. 1 IIIU UU $ n • HRno , KRi\o LUi r an j m. n. FT h

Al kopw Ul 1

i Uc ry J w . o . l 1 uvcLanuvnc9 k. u. FRro, nAlhrprht 1 Ui cLML) r\. A fL, . Q O uaiiiuii 9 c

.

NO Allan F R FTl\/ Uarria 1 1 9 l\ . K . MlIN 1 , Hrlab tab 1 9 L • r / Udv 1 QbOil 9 U . M

Ft; MilUcr bUil 5 U. u. Flr i IJ a V 1 b 9 U . U . COro, Mriucrbon 9 l • u

.

(TT U 1 ue rtnnonQ 9 i v

i . i\

.

bO

ni Cllci ) rim U. T I 1 I Uc rlai c 9 13 • r\ • u / ms prey , l . d . Uclllab 9 u . m . , HLKIilbUilj fith'ncnn Gu. Hn. L/cHU 1 c 9 W . D . MArlH n UK 1 ilbUN 5 r\ . IN 1 Utib Rrcb 9 H .

1 R

A 1 1 c 1 r\r\c D HUb 1 UUb 9 r

no U 1 blllU Kcb 9 a . U . LO

A 1 1 c 1 1 n F R H7n / UUUyc 3 rl . L. . l/Q RarU DaCK M M

9 r 1 . n

.

uunric i i y 9 v . 1 1

.

F?

D 3r |/ D A DdCK) K. H* l-o , no UOnOVarl 5 K . U . no

Qal <Ji.it rt P P Dd i aw i n , b , l . f Q Lo Dwyer, P. uo

Dd 1 Zan 1 9 V

.

bo 1 1 -1'iangucn , n

.

Barker , J . R

.

m ti-bayea, pi. a. 1 7 L / DaPOnaVSKl, H. Ft; to ty 1 c r , u • rs . Ffi to baSS, A. rl. MO

v\c Fajer, J. DdUer, o. n. L4 re i aer , w

.

RlD 1 ayes , i\. u

.

go rerrei ra , <\. i . t

.

1

1

1 1 DergmarK, w. k. T 9 r i nKenuerg , t

.

b / Dergsurom, o. r \ srier 9 r . 0.1 b / Dersonn, k. no ronnjn, h. RlD 1 Rl art G D 1 d C K 2 b . F9 DP rourri i er 9 o . HQ Rnn art n 1 oogan, u. j. D4 r razi er , b . r

.

HO

bo

i

I etta , r

.

bo r reuna , o . 1 1

.

uo

,

Bondybey, V. E. L 1 barney, n. u. b/ Rn + + 1 F b0 zz , 0 . r

.

D7K/ banai m , h . no

R/^4* + ftrtrt£aTm dot, uennci ni , j. K

k"l k"? VA 1 , l\0 , l\t Raw bay R Ul

, D W

.

l\0 Bowman, W. D. A /I A4 ui 1 1 espi e , n . i v

i . no Bowmen, W. R. T 1

1 1 bratze i , n

.

I 0 Braun, W. DO D/I branam, k. t. n't Breitenbach, L. Vic. brarxan, u. w. P9

Bremer, N. Lo Gregory, T. A. Brown, B. J. broLn , w. U 1 Brown, K. u. h. Kb Gutierrez, A. R. r l b 1 Brune, D. C. 1/1 J4 bULiiidn, u. HA Brus , L. t. L 1 HacKeri;, r. h. ucno BuTa urn, J . J

.

I/C l\D nager, o. i_. LO Calvert J G Kl , K3 Hakala, D. C8

Campion, A. LT Hancock, J. K. 05

Carter, W. P. L. N4 Hansen, I. P4

Chameides, W. L. F4 Hanst, P. L. K5 Chien, K. R. C4 Harris, E. W. A4,

Cicerone, R. J. F4 Harvey, R. B. F4

Clark, H. M. C8 Heicklen, J. K6 Clyne, M. A. A. E3, Q4 Henry, M. S. D4

Cody, R. J. E4 Hoffman, M. Z. D4,

Coleman, W. F. L2 Hoggard, P. E. G5

Cool , T. A. Rl Hogo, H. K9

397
Hoinghaus, K. P4 McDermid, I. S. E3

Holmstrbm, B. J3 McDonald, J. R. E5 Hoshino, M. K8 McDowell , R. S. L2

Howard, C. J. F5 Menzinger, M. R3

Hsu, D.S.Y. Q2 Merrill, J. 63

Hui, K-K Rl Michael , J. V. F7, P6, P8

Hunziker, H. E. PI Migirdicyan, E. L5

Inoue, G. K8 Miller, R. G. C2, Q5

Ishiwata, T. H8 Molina, M. J. Ml Jacox, M. E. L9 Monkhouse, P. B. Q3

Jaffe, S. M4 Mori , T .

J2

Janda, K. D5 Morine, G. H. L3, L6

Janson, T. R. 14 Niki, H. N2

Jones, G. 12 Noonan, R. C. K5 Jones, L. H. L2 Noyes, W. A. 03

Juris, A. G6 O'Connor, D. V. Jl Kaduk, B. B3 Okada, T. J2

Katz, J.J. 14 Okuda, M. K8

Kaufman, F. E2 Ors, J. A. D7

Kaufman, K. MB Ott, W. R. A3

Kawasaki , M. H5 Overend, R. K2 Keifer, W. N3 Paine, R. T. L2

Khe, P. V. P3 Paraskevopoulos , G. K2 Killus, J. P. K9 Parmenter, C. S. 01 Kirk, A. D. G5 Parsons, J. M. H6

Klapthor, R. D5 Pasch, N. F. D6

Klose, J. Z. C7 Paulson, J. F. E7 Klosterman, N. E. A2 Payne, W. A. F7, P6, P8

Knight, A. E. W. 01 Perry, R. Nl Koeppe, M. D5 Peterson, S. G4

Kolts, J. H. Q4 Phil en, D. L. F3

Krauss, M. B2 Phillips, D. 05

Kurylo, M. J. B2, R4 Pitts, J. N. Nl , N4

Kutschke, K. 0. P2 Plummer, B. F. D8

Lahmani , F. noD2 Porter, G. B. G5

Lampe, F. W. H7 Rebbert, R. E. M3

Larson, H. B. J3 Redpath, A. E. R3

Ledford, A. E. M2 Reeves, R. R. C8

Lee, E. K. C. C2, El Ritter, J. J. C6

Lee, J. H. P6, P8 Rockley, M. G. G5

Lee, S. A. 03 Rockwood, S. D. C5

Lee, S.-J. H5 Sabety-Dzvonik, M. E4

Lejeune, V. L5 Santhanam, M. 12 Lesclaux, R. P3 Savage, C. N2

Levine, S. Z. Kl Schiff, H. I. F5 Lewis, R. B. L2 Schmatjko, K. J. R5

Lewis, R. S. C2 Schmeltekopf , A. L. F5 Lichtin, N. N. 13 Schwerzel , R. E. A2

Lin, D. P. L6 Scott, L. J. D8

Lin, M. C. H2, Q2 Serpone, N. G6

Lin, S-M Ql Setser, D. W. 04

Lindsay, J. M. A5 Sharpless, R. L. R8

Lipsky, S. 06 Shedd, A. D5 Litvak, H. H5 Shortridge, R. G. H2

Lloyd, A. C. Nl , N4 Simons, J. P. HI Lyman, J. L. C5 Singleton, D. L. F8_ Maestri , M. G6 Slanger, T. G. F2, R8

Maker, P. M2 Smith, I. W. M. P7, R6

Manning, R. G. R4 Spears, K. G. 04

Martin, T. W. G2 Srinivasan, R. D2 Masanet, J. H9 Stedman, D. H. F4

Mataga, N. Stephenson, J. C. DOtic. McAfee, J. F6 Stevens, B. D7 McBride, R. P. AT, G4 Stevens, M. V. G2

398
Stief, L. J. P6, P8

Stratton, K. D5

Streit, G. E. F5 Stuhl, F. P4

Su, F. K3

Swords, M. D. 05

Tanaka, I. H8

Tang, K. Y. El Taylor, D. G. AT

Tchernev, D. I. 16 Tedder, S. H. L4

Termonia, M. Testa, A. C. 07

D3

Thomas, T. F. E7

Thorsell , D. L. K3

Timmons, R. B. P6

Toby, F. S. B3

Toby, S. B3

Trappen, N. L8

Umstead, M. E. H2

Van, S. P. J4

Van der Merwe, S. W. J. R6

Velazco, J. E. 04

Venkatesh, C. G. 02

Verber, C. M. A2

Vermeil, C. H9

Walters, R. T. Gl Wampler, F. B. K4

Ware, W. R. Jl Washida, N. K8

Watson, R. T. F3

Webber, S. E. D6, L4

Welge, K. H. CI Weston, R. E. Si Wettack, F. S. D5

Whitbeck, M. R. Kl White, J. M. 03

Whitten, G. Z. K9

Whytock, D. A. F7, P6, P8

Wijnen, M. H. J. H3

Wildes, P. D. 13 Wilkerson, T. D. A5

Willard, J. E. L3, L6

Willis, C. C3

Windsor, M. W. G5

Winer, A. M. Nl, N4

Wolfrum, J. M5, R5

Wright, R. E. Gl Yardwood, A. J. 07

Zalewski , E. F. Al Zellner, R. P7 Zetzsch, C. P4

399

if U.S. GOVERNMENT PRINTING OFFICE : 1978 O—261-238 (253)
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP-526

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

12th Informal Conference on PhotochemistryProceedings of the 12th Informal Conference on PhotochemistryHeld at the National Bureau of Standards, Gaithersburg, MD

June 28-July 1 , 1326

7. AUTHOR(S)

Michael J. Kurylo and Walter Braun

3. Recipient's Accession No.

5. Publication Date October 1978

6. Performing Organization Code

8. Performing Organ. Report No

f~9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

ttb

iM

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

Department of Transportation

Department of Energy

National Science Foundation

13. Type of Report & Period Covered

tafi

I J!

14. Sponsoring Agency Code

m

mi

lis f

Ml

nan

rams;

:tiiin

tare c

in i

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 78-600105

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The 12th Informal Conference on Photochemistry was held at the National Bureau

of Standards Laboratories in Gaithersburg, Maryland, from June 28-July 1, 1976. Near

300 members of the international scientific community attended the 123 individual

research papers presented. Extended abstracts of these presentations (many revised

after the Conference) comprise the present proceedings. The wide range of highly

specialized topics include Environmental Chemistry (both photochemical Smog and Upper

Atmosphere/Ozone Layer), Laser Photochemistry, Photochemical Isotope Separation,

Photochemical Conversion of Solar Energy, Actinometric and Radiometric Measurements,

Chemiluminescent Processes, Primary Photochemical and Photophysical Processes, Inorgai

Photochemistry, and Elementary Reaction Processes.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Chemical kinetics; energy transfer; fluorescence; lasers; photochemistry;

solar energy; spectra

18. AVAILABILITY Unlimited

I j For Official Distribution. Do Not Release to NTIS

I Y! Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Stock No. SN00^003-

| | Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PA(

411

22. Price

$6.00

USCOMM-DC 290"

nasi.
NBS TECHNICAL PUBLICATIONS

PERIODICALS

joURNAL OF RESEARCH—The Journal of Research

if the National Bureau of Standards reports NBS research

Lnd development in those disciplines of the physical and

ngineering sciences in which the Bureau is active. These

include physics, chemistry, engineering, mathematics, and

omputer sciences. Papers cover a broad range of subjects,

(vith major emphasis on measurement methodology, and

|he basic technology underlying standardization. Also in- cluded from time to time are survey articles on topics closely

jelated to the Bureau's technical and scientific programs. As

}l special service to subscribers each issue contains complete

pitations to all recent NBS publications in NBS and nonpIBS media. Issued six times a year. Annual subscription:

pmestic $17.00; foreign $21.25. Single copy, $3.00 domestic;

j>3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences." t)IMENSIONS/NBS

This monthly magazine is published to inform scientists, engineers, businessmen, industry, teachers, students, and

j:onsumers of the latest advances in science and technology,

Jvith primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

[ibatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties

pf matter and materials, engineering standards and services, instrumentation, and automatic data processing.

, Annual subscription: Domestic, $1 1.00; Foreign $13.75

NONPERIODICALS

VHonographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific

iand technical activities. handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications—Include proceedings of conferences

iponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manpals, and studies of special interest to physicists, engineers,

;;hemists, biologists, mathematicians, computer programmers,

jand others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National

Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical Reference Data (JPC'RD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of

Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest

to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSIR's and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register.

Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domesj tic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly.

Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM.2 1 5

SPECIAL FOURTH-CLASS RATE BOOK

1298



m
I