NBS

PUBLICATIONS

NAT L INST OF STAND & TECH Reference

AlllOb 3 1 A 5

5

NBSIR 86-3313

Technical Activities 1 985

Molecular Spectroscopy Division

A. Weber, Chief

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Chemical Physics

Molecular Spectroscopy Division

Gaithersburg, MD 20899

December 1985

Issued January 1986

Prepared for: U.S. DEPARTMENT OF COMMERCE

~QC ial Bureau of Standards

^qq

irsburg, MD 20899

• U56

86-3313

1986

KBS

RESEARCH INFORMATION CENTER

NBSIR 86-3313

* » TECHNICAL ACTIVITIES 1985

MOLECULAR SPECTROSCOPY DIVISION

A. Weber, Chief

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Chemical Physics

Molecular Spectroscopy Division

Gaithersburg, MD 20899

December 1985

Issued January 1986

Prepared for: U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

ABSTRACT

This report summarizes the technical activities of the NBS Molecular

Spectroscopy Division during the Fiscal Year 1985. The activities span

experimental and theoretical research in high resolution molecular

spectroscopy, quantum chemistry and laser photochemistry, and include the

development of frequency standards, critically evaluated spectral data,

applications of spectroscopy to important scientific and technological

problems, and the advancement of spectroscopic measurement methods and

techniques. A listing is given of publications and talks by the Division

staff.

iii
o

.
FOREWORD

This report is a summary of the technical activities of the NBS

Molecular Spectroscopy Division for the period of October 1, 1984 to

September 30, 1985. The report was prepared as part of the Annual Report of

the Center for Chemical Physics within the National Measurement Laboratory of

NBS. The Molecular Spectroscopy Division is organized into three working

groups comprised of permanent staff, post-doctoral fellows, guest workers,

and support personnel. An organizational chart of the Division is given at

the end of this Forword.

The goals and activities of the Division and the general nature of its

technical programs are described in the Introduction, Section 1. Sections

2-4 present the descriptions of technical work done by the High Resolution

Spectroscopy, Laser Photochemistry, and Quantum Chemistry Groups. Each group

report gives project objectives, details of results obtained during the past

year, and plans for future work. A mini-table of contents precedes each

group report to afford a quick overview of the group's activities.

The technical descriptions are followed by Sections 5 and 6 which list

the publications and talks given during the past year, while Section 7 lists

the seminars hosted by the Division. The Division has had a number of

visiting scientists during the past year; these are identified in Section 3.

This report was prepared during the period July-August, 1985. While

anticipating some of the accomplishments that resulted from the work done

during the remainder of the reporting period, the report clearly can not do

v

justice to the full range of activities and accomplishments that actually did

take place. Seme of these will be presented in the report for the following

year

.

Further information on the activities of the Division can be obtained by

contacting the scientists identified in each project report, cr by writing to

Dr. Alfons Weber, Chief,

5263, National Bureau of

Molecular Spectroscopy Division, Physics Building,

Standards, Gaithersburg, MD 20899.

Vi

Staff of the Molecular Spectroscopy Division

A. Weber, Chief

A. Flynt, Secretary

C. V. Kurtz, Division Technician

Laser Photochemistry Quantum Chemistry

Stephenson, J. C., Group Leader

Elwell, L. B. Jacox, M. E. King, D. S.

Casassa, M. P. (P.D,)

Heilweil , E. J.

Mantell, D. A. (P.D.)

Perry, J. W. (P.D.)

Woodward, A. M. (P.D.)

Mialocq, J. C. (G.W.)

Bodaness, R. (G.W.)

Cavanagh, R. R.*

Burgess, D. (P.D.)*

Krauss, M. , Group Leader

Julienne, P. S.

Mies, F. H.

Stevens, W. J.

Jasien, P. (P.D.)

Basch, H. (G.W.)

Lengsf ield, B. H. (G.W.)

Konowalow, D. (G.W.)

High Resolution Spectroscopy

Maki, A. G. , Group Leader

Hougen, J. T.

Lafferty, W. J.

Lovas, F. J.

Olson, W. B. Pine, A. S.

Rotter, G. L.

Suenram, R. D.

Weber, A.

Thompson, G. A. (P.D.)

Bevan, J. W. (G.W.)

Hollis, J. M. (G.W.)

Howard, B. J. (G.W.)

Jennings, D. E. (G.W.)

Lovejoy, R. W. (G.W.)

Mills, I. M. (G.W.)

Ohashi, N. (G.W.)

Sattler , J. P. (G.W.)

Wofford, 3. A. (G.W.)

P. D. - Post Doc

G. W. - Guest Worker

* - Surface Science Division

Vil

ABLE OF CONTENTS

Abstract iii

Forword v Staff of the Molecular Spectroscopy Division ............ vii

1 . Introduction ......... .... 1

A. Goal 1

B. Organization 2

C*. Division Programs 2

2. High Resolution Spectroscopy Division 5

A. Introduction 7

B. Calibration and Data Center Activities. ..... 8

C. Spectroscopy for Environmental Studies. ........... 12

D. Spectroscopy of Hydrogen-Bonded and van der Waals Complexes ................... 19

E. General Spectroscopy and Theory ............... 30

F. Future Plans. ................... 41

3. Laser Photochemistry Group ................... 45

A. Introduction. ........................ 46

<s B. Condensed Phase Energy Transfer ......... 46

C. Desorption of Molecules from Surfaces ............ 59

D. Photodissociation Dynamics of van der Waals Molecules .............. 61

E. Matrix Isolation Spectroscopy ................ 63

F. Future Plans ......... 66

4. Quantum Chemistry Group. . . ..... 73

A. Introduction 74

B. Electronic Structure. . 74

C. Scattering in the Presence of Radiation 80

D. Multiphoton Effects in Resonance Fluorescence

Spectra 84

E. Future Plans 35

5. Publications 87

6. Talks 93

7. Molecular Spectroscopy Division Seminars 99

8. Visiting Scientists. *01

ix
.
MOLECULAR SPECTROSCOPY DIVISION

A. Weber, Chief

SUMMARY OF ACTIVITIES

Fiscal Year 1985

1 . INTRODUCTION

A. Goal

This Division is a discipline oriented unit and its goal is to be at

the forefront of modern theoretical and experimental spectroscopy in order

to:

• Advance spectroscopic measurement methods and techniques

• Develop models, theoretical concepts and quantum theoretical

methods for predicting molecular properties and interactions

• Obtain reliable reference spectroscopic data for NBS,

government, research community and industrial needs

• Understand and model fundamental chemical processes

• Provide state-of-the-art frequency standards

• Transfer forefront spectroscopic expertise to other NBS

Divisions, other government agencies, industry and the research

community.

Within the framework of these general goals the Division also follows

three specific thrusts which are part of the overall program of the Center

for Chemical Physics - Properties of Weakly Bonded Systems, Biotechnology,

and Energy Transfer. Properties of weakly bonded systems are obtained for

small atomic and molecular complexes held together by van cer Waals and

hydrogen bond forces in order to develop a data base to understand

conformation, thermodynamics, and reactivity of such systems. The

objectives of our thrust in biotechnology are to develop and implement

methods for the study of metal interactions in proteins and nucleic acids

using primarily quantum mechanical techniques, while the major effort of

the energy transfer thrust has been directed to the study cf vibrational

energy transfer.

This summary contains a report; of the activities of each of the three

groups within the Division during the past year and demonstrates bow they

are meeting the above objectives.
B. Organization

To carry out its work the Molecular Spectroscopy Division is

organized into three working groups: High Resolution Spectroscopy (HRS),

Laser Photochemistry (LPC), and Quantum Chemistry (QC). The permanent

Division staff is augmented by that of postdoctoral fellows and guest

workers. During the past year we have had thirteen guest workers and seven postdoctoral fellows engaged in Division projects. In addition, two

members of the Surface Science Division participated in the work of the

LPC group. A list of the guest workers, their permanent affiliations, and

their activities is given in Section 8 of this report.

C. Division Programs

With the technological advances of micro- and main frame computers

and the development of tunable dye and diode lasers that have taken place

during the last eight years, the fields of experimental molecular

spectroscopy and theoretical quantum chemistry have grown rapidly.

To be able to respond to the present and projected needs of NBS, the

Center for Chemical Physics (CCP), and the user community, a significant

fraction of the Division's effort must be directed toward achieving and

maintaining the expertise of its staff in their discipline oriented fields,

and into maintaining its experimental facilities at the state-of-the-art

level. We must remain at this level of expertise in spectroscopic

measurements, theory, and interpretation in order to respond quickly to

provide standards, serve national technical needs, and to insure adequate

response to future, unknown problems. The soundness of this approach is

justified by the past and present applications of experimental and

theoretical spectroscopy by our Division. These applications also provide

much of the direction and motivation for our efforts. Remote sensing for

industrial and atmospheric monitoring, laser isotope separation and

fusion, laser chemistry, and combustion diagnostics are a few of the many

important high-technology areas that depend on modern spectroscopic

techniques.

In order to meet the calibration and standards requirements for the

design and application of tunable infrared lasers and Fourier transform

instrumentation over the next decade, several frequency standards

activities are pursued in collaboration with several groups from within

and outside NBS. A project on spectral frequency standards is undertaken

jointly by scientists from the Time and Frequency Division of NBS/Boulder

and members of the HRS group. Details about our Calibration Standards

work are given in Section 2.B.

There has always been a need for accurate intensity parameters for

molecular transitions. Recently these needs have been emphasized and the

requirements for accuracy enhanced in connection with the determination

and monitoring of trace atmospheric species. Accordingly we are now engaged in the determination of accurate transition moments, line
strenghts, and line broadening parameters of several molecular species

which are believed to play an important role in the chemistry of the

earth's atmosphere (see Section 2.C)

We continue to be involved in the spectroscopy of the environmental

and space sciences. Our work is directed toward the broadening of the

data base upon which spectroscopic measurements of trace atmospheric

constituents by means of ground based, balloon and satellite borne

instruments are performed and evaluated. This, as well as similar efforts

of interest to various branches of NASA and other user groups, is part of our continuing involvement in problems of national concern. Details about

these and other activities may be found in Section 2.C.

Our Division has significant other agency contracts with Air Force

Office of Scientific Research (AFOSR), Department of Energy (DOE),

National Aeronautics and Space Administration (NASA), Chemical

Manufacturers Association (CMA), and the National Foundation for Cancer

Research that help these agencies fulfill their missions. Details are discussed in the group reports. These efforts along with the

consultations, talks, publications, and collaborative activities show our strong coupling with industry, other government agencies, and the research

community.

During the past year efforts of the LPC group have been redirected

and have acquired a stronger focus in the quantum-state specific studies

of molecular dynamics, especially in the role played by energy transfer in

the spectroscopy and kinetics of molecules in the condensed phase.

Investigations of energy transfer from vibrationally excited bonds in

liquids and solids, as well as in molecules bound to solid surfaces are described in Section 3-B. As an outgrowth of the work done in previous

years on infrared multiphoton dissociation experiments, new work has been

started on the dynamics of van der Waals complexes, especially on vibrational predissociation (see Section 3-D). These experiments,

performed in the time domain, measure directly the vibrational lifetimes

and supplement the work done in the frequency domain in which lifetimes

are inferred from spectral line widths (see Section 2.D as well as earlier

descriptions in the FY'83 and '84 Annual Reports).

Work was also carried forward in Matrix isolation spectroscopy

(Section 3-E) and the desorption of molecules from surfaces (Section 3-C).

Thermal desorption of molecules chemisorbed on well characterized surfaces

were studied with a dedicated facility incorporating several techniques

for surface sensitive analysis, a computer controlled data acquisition
system and a laser probe system with sub-Doppler resolution capability.

Since thermal desorption takes place slow enough to be modelled as an equilibrium process, new work has been started on fast desorption brought

about by rapid heating of the surface by non-resonant laser radiation.

The field of Quantum Chemistry has in recent years experienced

significant new developments which extend the range of reliable and useful

calculations to complex polyatomic systems. A new program to attack the

problems of the stabilities, conformations, spectroscopy and reactivity of

complex molecules, clusters, ions, radicals, and transition states, among

others, was successfully conducted for the third year by the Quantum

Chemistry group. The ability of the QC group to attack large scale

computational problems using ab-initio rather than semi-empirical methods

has been significantly enhanced through the acquisition of an IBM-4381

superminicomputer which forms the core of the newly formed Molecular

Structure and Modeling Facility of the Molecular Spectroscopy Division.

Details of the activities of the QC Group are given in the group report.

In the Annual Report for 1982 we described the results of our initial

work on the spectroscopy of molecular complexes in the gas phase. This

initial work, on the rotation-vibration spectrum of the hydrogen-bonded

HF-dimer has been expanded since that time to include the study of a variety of hydrogen-bonded and van der Waals complexes by all three groups

of the Division. Thus, detailed investigations are underway with new instrumentation of microwave and infrared high resolution spectra (see the

reports by the High Resolution Group in Section 2.D). The measurements

for these investigations are made in the frequency domain and knowledge

about the lifetimes of excited states is had only by inference from the

widths of the spectral lines. Direct information about lifetimes and,

especially, pre-dissociation rates is obtained from measurements made in

the time domain; these are performed in the Laser Photochemistry Group and

are reported in Section 3-D. These experimental efforts are often done in

conjunction with work done in the Quantum Chemistry Group (see report

4.8.2). The study of weak chemical interaction as exemplified by the

hydrogen bonding and van der Waals forces is of considerable interest for

the present and the forseeable future (see the Pimentel Report

"Opportunities in Chemistry", National Academy Press, Washington D. C.

,

1985) and our Division is expected to fully participate in this field

during the coming years.

The research described in the following report was performed in a cooperative manner between the permanent staff of the Division, NBS/NRC

postdoctoral research associates, and guest-scientists from the U.S. and

abroad. Close collaboration also exists with members of the Surface

Science Division of the Center for Chemical Physics in areas of picosecond

spectroscopy, spectroscopic studies of surface desorption processes, and

energy transfer.

4
2 . High Resolution Spectroscopy Group

J. W. Bevan, J. T. Hougen, B. J. Howard, D. E. Jennings,

W. J. Lafferty, R. W. Lovejoy, F. J. Lovas, A. G. Maki, I. M. Mills,

N. Ohashi, W. B. Olson, A. S. Pine, G. Rotter

R. D. Suenram, G. A. Thompson, A. Weber, and B. A. Wofford

A. Introduction

B. Calibration and Data Center Activities

1 . Tying the Lower Vibrational Levels of N

2

o to the Cesium Frequency

Standard

2. Infrared Calibration Atlas

3. Revised Recommended Rest Frequency List

Microwave Spectral Tables

5. Supplement to Structural Data Tables for Polyatomic Molecules

6 . Review of S0 2

C. Spectroscopy for Environmental Studies

1 . High Resolution Spectrum of the v

2 and Bands of HOI

2. Measurement of the and vjj Bands of Nitric Acid (HNO

3

) 3. New Intensity and Frequency Measurements on C2,0

4. Self -Broadening in the Fundamental Bands of HF and HCS. 5. Analysis of the Spectrum of H

202

D. Spectroscopy of Hydrogen-Bonded and van der Waals Complexes

1 . Hydrogen-Bond Energies of the HF and HC2, Dimers

2. Rotational Predissociation and Libration in the Infrared

Spectrum of Ar-HCS,

3. Study of Hydrogen-Bonded Complexes in the Gas Phase Using

Fourier Transform Spectroscopy

4. Pulsed Beam Fabry-Perot Cavity Fourier Transform Microwave

Spectrometer

5. Microwave Study of the Ar. . .Acrylonitrile van der Waals Complex

6 . Microwave Study of the Kr...0CS van der Waals Complex

7. A Generalized Internal Axis Method for High 3arrier Tunneling

Problems, as Applied to the Water Dimer

E. General Spectroscopy and Theory

1 . Efficient Sampling Systems for the Fourier Transform Spectrometer

2. Diode Laser Measurements on Unstable Diatomic Molecular Species

3. Fourier Transform Raman Spectroscopy of H

2 and D2 in Flames

4. Infrared Spectrum of Cyanogen

5. Fourier Transform IR Spectra of Monodeuterosilane

6 . Far Infrared Spectrum of the Torsional Band of Hydrazine

7. The Torsional-Wagging-Tunneling Problem and the Torsional-WaggingRotational Problem in Hydrazine
8. The Use of Extended Permutation-Inversion Groups for Constructing

Hyperfine Hamiltonians for Symmetric-Top High-Barrier Internal

Rotor Molecules Like H^C-SiH^

9. Improved Least Squares Determination of Potential Energy Surfaces

from Spectroscopic Data

10. The Proper Choice of the Lagrangian for a Relativistic Particle in

External Fields

11. A Note on the Uniform Asymptotic Expansion of Integrals with

Coalescing End Point and Saddle Points

12. Generalized Stirling Approximation to N!

13. Accurate Emission Spectra from Planar Strong Field Wigglers with

Arbitrary Field Variation

1M. Dynamical Symmetries and Equivalent Lagrangians for

One-Dimensional Systems

15. On the Relationship of Normal Modes to Local Modes

F. Future Plans

6
A. Introduction

The work of this group involves experimental and theoretical applications of molecular spectroscopy for characterizing gas phase molecules,

primarily in the infrared (rotation-vibrational) and microwave (rotational) spectral regions, the development of frequency standards for the

infrared, and the development and application of new experimental techniques for high resolution spectroscopy. The emphasis of these studies is

in the applications of our expertise to problems in structural chemistry,

atmospheric chemistry, chemical analysis, radio astronomy and the

properties of novel molecular species, particularly floppy molecules.

The major activity of the Molecular Spectra Data Center, supported by

the Office of Scientific Reference Data (OSRD) is to provide evaluated

data on molecular rotational spectra in the microwave region and to

develop evaluated infrared spectra to be used as standards in calibrating

diode lasers and infrared laser spectrometers. This work is based on the

needs of the user community. Thus the work on infrared standards is a response to requests from the instrumentation industry, industrial

research laboratories, applied technological user groups, and government

agencies and laboratories. Improvements in the stability of diode lasers

have emphasized the need for accurate calibration standards. The

absorption frequencies of ^0 that involve the lower energy levels are now being tied to the cesium frequency standard. This work has been made

possible through the joining of the unique facilities of the Time and

Frequency Division of NBS/Boulder with our spectroscopic expertise. Other

collaborations, with non-U. S. research groups exist as well in an effort

to develop the most extensive and accurate set of frequency calibration

data.

Molecular radioastronomy continues to demand accurate laboratory

microwave data and spectroscopic expertise and the measurement, critical

evaluation, and dissemination of microwave data of astrophysical interest

is being continued. Our collaboration with radio-astronomers provides the

contacts and experience necessary to insure that these evaluations and

compilations are both practical and useful.

The environmental problem posed by the pollutants of the earth’s

atmosphere, especially the role played by trace constituents in the

chemistry of the upper atmosphere, are of major concern and a portion of

the group’s activities is devoted to providing critically needed data to

help solve some of these problems. In this effort our activities are guided by the needs of the environmental research community, and are conducted with the support of NASA and the Chemical Manufacturers

Association. These efforts will continue to play an important role in our work during the coming years.

Weakly bound molecular complexes, i.e. van der Waals and hydrogen

bonded molecules have received considerable attention in our laboratory

during the past year and will continue to do so in the future. The group

/
is in a unique position of having available personnel and instrumentation

that permits such studies to be performed using any and all of the

facilities from the microwave to the near infrared region of the spectrum

in a concerted attack on the problems posed by these weakly bound systems.

The work conducted during the past year has been aided through the

efforts of a number of guest-scientists from the U.S. and abroad.

Moreover, sustained collaboration with non-NBS groups serves to

disseminate the results of our work in the quickest and most far reaching

way to the user community. Such collaboration also benefits our work

through access to data and facilities not presently available at NBS.

Particularly noteworthy among the non-NBS institutions with which

beneficial collaborations have occurred during the past year are: The

University of Oulu, Oulu, Finland, The University of Illinois, The

University of Alberta, Texas A&M University, Lehigh University, Oxford

University, Oxford, England, The University of Reading, Reading, England,

The National Research Council of Canada, The Institute of Molecular

Science, Okazaki, Japan, Kanazawa University, Kanazawa, Japan, The Harry

Diamond Laboratories of the U.S. Army, The NASA-Greenbelt, NASA-Langley

NASA-Goddard, and NASA-Ames Laboratories, and The National Solar

Observatory at Kitt Peak, AZ.

In the following the group's activities are described according to

the three broad topics of calibration and Data Center activities,

environmental problems, and spectroscopy of weakly bound molecular

complexes. In addition to these programmatic activities there were others

of a more general and varied nature, including several studies in the

field of mathematical physics by one of our guest workers. These are described in the fourth section under General Spectroscopy and Theory.

B. Calibration and Data Center Activities

We collect under this heading the work of the group connected most

closely with the core mission of NBS, namely the production and

dissemination of compilations of accurately measured frequencies or wavenumbers for use as secondary standards, of compilations of critically

evaluated molecular constants for use in other disciplines, or occasionally of spectral fitting programs, for use in reducing data in

other laboratories. The group devotes a significant portion of its effort

each year to this program, and frequently collaborates with workers in

other laboratories. Outputs of the program serve needs in both the

infrared and microwave spectral regions.

1 . Tying the Lower Vibrational Levels of N2O to the Cesium Frequency

Standard

(A. G. Maki and J. W. Wells)

Our continuing collaborative effort with Dr. J. W. Wells (NBS

Boulder) this past year has seen some new frequency measurements that were

the key to tying all the lower vibrational energy levels of N2O to the

cesium frequency standard. These levels previously had been measured using

S
wavelength measurement techniques, but now we have independent frequency

measurements of these levels. One of the most satisfying aspects of this

work was the incorporation of the very accurately measured N

2 o laser

frequencies.

The new measurements that were made in Boulder involved the 00°1-00°0

and 01' 1-01*0 transitions of N

2o which were measured against a CO laser

local oscillator in the 1257 to 1340 cm" 1 region. The CO frequencies were

in turn measured with respect to combinations of C0 2 laser frequencies.

Since the well measured N

20 laser frequencies involved the 10°0-00°1

transition, combining these measurements gave accurate frequency values

for the 10°0-00°0 transitions between 2135 and 2268 cm

-1 (see Fig 2.1).

(a)

0221 0002

0420. 0400

0000

j

observed

Energy Levels for N2O

(b)

Frequency

Measurements

Wavenumbers

(cm-i)

2563 •2475

2462,2355

-LLL

Calibration

Tables

2224

1880

1767

1749

1284

1178

1168

589

2331

2322

1 not yet observed

I

Figure 2.1. Energy level diagram that illustrates on the left (a) the

frequency measurements that have been made on the lower

levels of N

2 0 (solid arrows), or that we hope to make (dashed

arrows). On the right (b) are the resulting calibration

tables that can be prepared based on the frequency

measurements.

9
Also, combining the new 01*1-0110 measurements with earlier ones on the

01’1-00°0 transitions gave us good frequency values for the 01 1 0-00°0

transitions near 590 cm

- ”' . These new frequency measurements

have been combined with older frequency measurements and with other

microwave and infrared data that determined some of the rotational

constants for the various states to obtain the best term values for the

lower vibrational states of ^0, all related to the cesium frequency

standard. These term values are being used to prepare the infrared

calibration atlas for ^0 described in the next section.

In the coming year we expect to be able to measure some of the weaker

hot band lines in the 1285 cm ^ band of N

2 O to obtain term values that

will provide frequency calibration in the 2000-3000 cm

-1 region. Our long

range goals also include the measurement of hot bands accompanying the

laser transitions near 9^0 cm

-1 and the frequency measurement of some

lines due to the 10°0-02°0 transitions near 1060 cm

-1

. Finally, with a little bit of luck it may be possible to obtain a diode with enough power

in a single mode to make a few saturated absorption measurements on the

1285 cm

-

' band of N

2 O. That would give an order of magnitude improvement

in the accuracy of the frequency measurements.

2. Infrared Calibration Atlas

(A. G. Maki)

During the past four or five years we have been developing infrared

spectral frequency tables for calibrating tunable IR laser spectrometers.

We have worked with several groups in obtaining highly accurate heterodyne

and Fourier transform spectra on many bands of the 0CS and N

2 O molecules

which will form the basis of two reviews. Much of the manuscript and

tables for the 0CS review were completed previously. The new results on the v

2 band will be added to the existing tables to complete this review.

During the past year further heterodyne measurements on N 9 o bands up to

2200 cm

-1 have been obtained (see preceeding section) and allow

calibration tables to be produced in the regions of 520 cm

-1

, 930 cm

- "' , 1300 cm

-1

, 1880 cm

-1

, and 2200 cm

-1 with each covering a range of 90-150

cm

-1

. Some additional measurements needed for calibration near 2700 cm

-

"

1 are being sought to complete the analysis and produce a set of tables

which will replace the previous publication on ^0 in the Journal of

Chemical and Physical Reference Data. This work is supported by the 0SRD.

3. Revised Recommended Rest Frequency List

(F. J. Lovas)

In May 1985 the manuscript entitled "Recommended Rest Frequencies for

Observed Interstellar Molecular Microwave Transitions - 198-4 Revision" was submitted to J. Phys. Chem. Ref. Data by F. J. Lovas. This is a complete

revision and update of a 1979 publication in Astrophys. J. Supplement by

F. J. Lovas, L. E. Snyder, and D. R. Johnson. The reported astronomical

observations of new species and new rotational transitions of previously

identified molecules has more than tripled the number of lines reported in

the first publication.

10
Preprint copies of this manuscript have been distributed to the

nearly 300 astronomers and spectroscopists on our mailing list with the

request that they examine it for oversights or errors. A number of useful

responses have already been received. In fact among the responses was a preprint of a survey of Orion A between 247 GHz and 263 GHz which contains

more than 200 new transitions which were added to the table. We have also

filled a request for a magnetic tape copy of the data file to be used on

the new British - Dutch 15-metre submillimeter wave telescope being

constructed on Mauna Kea, Hawaii. The most pleasing aspect of the

responses has been the interest of the astronomy community in the accuracy

of the reported information as well as providing new observations to keep

it as up-to-date as possible. The manuscript has been submitted to J.

Phys. Chem. Ref. Data and the major tables of data have been converted

via a magnetic tape to the Bedford system used by OSRD. This work is

supported by the OSRD.

4. Microwave Spectral Tables

(R. D. Suenram and F. J. Lovas)

We continue to carry out the general literature search for all

published (and unpublished) microwave spectra for this project.

Evaluation and coding new data continues on the diatomic and triatomic

species to keep the computer files from the earlier publications current.

Work on the third publication of the Microwave Spectral Table series

"Hydrocarbons’1 is progressing. The spectral fittings of all the normal

symmetric top and asymmetric top species are complete. The corresponding

data files of the measured and predicted spectra have been produced and

all isotopic and vibrational species have been coded. The analysis of the

species with internal rotation remains to be accomplished since the

programs for these analyses were not fully implemented on the Uni vac. Since we have moved all programs and data files to NBS’s new Cyber

computer, implementation of these programs on the Cyber is in progress.

This work is supported by the OSRD.

5. Supplement to Structural Data Tables for Polyatomic Molecules

(W. J. Lafferty)

Ten years ago members of the Molecular Spectroscopy Division

collaborated with spectroscopists from Japan, Germany and England to

produce a set of structural tables of polyatomic molecules. All

structures obtained for gas phase molecules by electron diffraction and

infrared, Raman, electronic, and microwave spectroscopy were included.

The tables were published by Landolt-BCrnstein in their New Series (Group

II, Volume 8).

This year we have updated these tables. The literature has been

searched from 1974 to the beginning of 1985. All new structures as well

as structures whose parameters have been significantly improved have been

included in the tables. The entry for each molecule includes the

11
structural parameter as well as a schematic drawing of its structure.

This supplement will once again be published by Landolt-BOrnstein . This

work is supported by the OSRD.

6. Review of SCU

(F. J. Lovas)

After several years in the process, the publication of a review on

the spectrum of SO2 is forthcoming in J. Phys. Chem. Ref. Data Vol. 14

(1985). This is part 22 of the series "Microwave Spectra of Molecules of

Astrophysical Interest." This work is supported by the OSRD.

C. Spectroscopy for Environmental Studies

Many small molecules play a significant role in the chemistry of the

earth’s atmosphere. As such, they are of significant interest to agencies

like NASA, DOD and the CMA (Chemical Manufacturers' Association). The

interest of the latter agencies generally focusses on molecular

spectroscopy as a remote sensing tool and for the determination of

concentration profiles. Consequently, the emphasis in this section is not on band analyses per se, but rather on line intensities, line widths, line

shapes, and the effect of pressure on these quantities. As in all

branches of spectroscopy, the very narrow band width of infrared laser

sources now permits quite reliable studies of phenomena involved in

spectroscopic concentration determinations.

1 . High Resolution Spectrum of the and v? Bands of H0C2,

(W. J. Lafferty and W. B. Olson)

Our interest in the spectrum of HOCX, has been motivated by very

practical reasons. Cl atoms produced from the photolysis of freons in the

upper atmosphere react with stratospheric 0^ via the routes:

Cl + 0

3

= C£0 + o 2

and

C£0 + 0 = Cl + o

2

There are a great number of competing reactions in the stratosphere, and

the extent of the damage done by the freons is at this point still

controversial. H0C2, can be produced from CX.0 by the reaction

CX.0 + H0 2

= H0C2. + 0

2

which, if the HOCX, formed is sufficiently stable, serves to withdraw CX, atoms from the chain reaction.

The stability of HOCX, in the upper atmosphere is still somewhat

uncertain. A number of laboratory studies have been carried out on these

and other reactions, but it appears that the most practical way in which

12
to get a handle on the role played by HOC! in stopping or slowing the

ozone destroying chain reactions is to directly observe this species in

the stratosphere and directly monitor its concentration.

A number of unsuccessful attempts to detect HOCS, with both ground

based and balloon spectrometers have been made. One of the problems of

detection of this molecule arises from that fact that even in the

atmospheric windows, the H0C2, absorption bands are overlapped by the lines

of very minor stratospheric constituents which have about the same intensities expected from the lines of H0C1.

In order to positively identify the lines arising from the HOC!

spectrum, it is necessary to have very precisely known line frequencies

since only a few lines of a band may fall in a clear spot in the spectrum.

We, therefore, with the partial support of the Chemical Manufacturers

Association, have recorded and assigned the two lowest frequency bands of

H0C2, with the NBS BOMEM FTS spectrometer with a resolution of 0.004 cm

-1

. Careful attention has been given to frequency calibration; fortunately,

both bands fall in regions where there are very well measured calibration

lines. The v

2 band centered at about 1220 cm

- "' , was calibrated against

N

2 0 while the V

3 band was calibrated against CO 2 absorption lines. The

absolute frequency calibration of the BOMEM spectrometer was found to be

surprisingly good. Corrections of only a few ten thousands of a cm

' 1 were found to be necessary. We conservatively estimate that the absolute

precision is better than +/- 0.0005 cm

-1

; the relative precision is

somewhat better, and is on the order of 0.00002 cm

-1

.

Both v

2 , the bending vibration, and V

3

, the C£,-0 stretching mode,

are a-type bands. Figure 2.2 shows a much compressed scan over the v

2

band.

The assignment is complicated by the presence of two chlorine

isotopes, but line assignment presented no difficulties; a very complete

assignment has been accomplished for both bands of both isotopic species.

The observed lines have been fit using a Watson Hamiltonian, and the

standard deviation of the fittings for all the bands was 0.0002 cm”'.

Currently relative line intensities are being measured for both bands

in order to determine the magnitude of the Herman-Wallis effect on the

intensities. When this is completed, line intensities will be computed,

and a band atlas including line frequencies and line strengths at several

temperatures will be produced. This work is supported by CMA.

2. Measurements on the and 3ands of Nitric Acid (HNOo)

(A. G. Maki)

The and bands of nitric acid were investigated in collaboration

with Prof. A. Fayt of the University of Louvain - la Neuve, Belgium. We

have used the NBS BOMEM interferometric spectrometer to measure the

15
HOCI Vt BAND

K » 0 1 2 3 4 s ho”ci

Pr««a«r« 1 tort

1 199.0 1101.0

U 1 tL.I i 1 t it hi t tLt li.tAi li — 1229.0 124 9.0 1299.0

WAVENUMBER (cm-’)

Path 8.29 nttsn

i .... i . 1299.0 1309.0

Figure 2.2 The \>2 band of H0C2, recorded under full instrumental

resolution of 0.004 cm

-1

. The sub-band centers of the major

isotopic species are labelled at the top.

spectrum of nitric acid (HNO^) in the 1250 to 1 360 cm" 1 region which

includes the band at 1303 cm

-1 and the band at 1326 cm” 1

. In

collaboration with Dr. Andre Fayt transitions for both of these bands

have been assigned and fit to appropriate asymmetric rotor constants.

These bands are used by certain laboratories for atmospheric

measurements since they fall in a spectral region where the atmosphere is

relatively transparent. It is important that the rotational line

structure of these bands be understood so that modeling calculations can

be used to simulate atmospheric spectra both to interpret the atmospheric

distribution of nitric acid, and also to cancel out the nitric acid

features when searching for underlying transitions due to other

atmospheric species, such as chlorine nitrate.

To supplement the BOMEM spectrum we have also measured diode laser

spectra for several small spectral regions in order to resolve some of

the more tightly clustered features such as the Q-branch. Figure 2.3

shows an example of the diode laser spectrum of vjj. Although only the

K

c =0,1 transitions are labelled many other features seen in the figure have

been identified and assigned.

Thus far the spectra have been fit with a standard deviation of about

0.002cm 1

. It is not clear if the fit could be improved by including some interactions with nearby bands. In the coming year we hope to obtain some

laser diode spectra of the band of HNO^ in order to see if some of the

14
ABSORPTION

13020 1303.0 WAVENUMBER (cm'1 ]

Figure 2.3. Diode 1

(HN0

3

).

gennani

aser spectrua cf the vi, Q-branch for

,er trace i;

i - 2 ^ *

um etaion;

he frings pai ism iron i s>

UjO>
frequencies or the intensities of the CIO lines might contribute to the

non-observation of CIO in the Mumma _et_ al_. experiments, we have undertaken

an independent remeasurement of the intensities and frequencies of the CIO

spectrum in the infrared.

Accurate intensity measurements are not easy to make on stable

molecules and are even more difficult for unstable molecules since it is

normally necessary to know how many molecules are present in the

absorption path. The microwave measurements do not have this problem

since the microwave absorption intensity is proportional to the square of

the dipole moment and the dipole moment can be measured quite accurately

from the Stark effect. The several measurements that have been made of

the infrared intensity indicate that it is relatively weak and for that

reason Dr. Chackerian (NASA/Ames) has suggested the possibility that the

Herman-Wallis effect, which relates the infrared band intensity to the

ratio of the intensity of R-branch transitions to the intensity of the

P-branch transitions, can be used to determine the transition moment

(infrared intensity) without measuring the CIO concentration. Since the

measurement of the CIO concentration is quite difficult, we believe that

such a technique that by-passes that one difficult and controversial step

merits further investigation and we have began such a study.

What we plan to do is measure the intensity ratio of P- and R-branch

transitions that arise from the same energy levels (in order to eliminate

any temperature dependence). For maximum accuracy these measurements must

be made under identical operating conditions for both the R-line and the

P-line. We plan to switch back and forth between measurements of the

R-line intensity and the P-line intensity several times in order to verify

that the operating conditions are indeed identical. This work is

supported by NASA.

Table 2.1 gives an example of the intensity ratio expected for the

different band intensities suggested by various other papers.

Table 2.1. R- and P- Branch Intensity Ratio Calculated for CIO for

Different band Intensities.

Band Intensity (cm - atm

- *

1 ) 32.0 1 1 .8 5.0

Intensity Ratio R ( 1

6

•5)/P(1 6.5) 1 .243 1.334 1 .469

Finally, this project also includes a re-measurement and re-analysis

of the infrared frequencies of the CIO absorption lines in order to

improve the reliability of the calculated infrared spectrum of CIO.

(1) A. Parish, R. L. DeZafra, P. M. Solomon, J. W. Barrett, and E. R.

Carlson, Science 21 1 , 1158 (1981).

(2) P. M. Solomon, R. deZafra, A. Parish, and J. W. Barrett, Science 224_,

1210 ( 1 984)

.

16
(3) M. J. Mumma, J. D. Rogers, T. Kostiak, D. Deming, J. J. Hillman, and

D. Zipoy, Science 221 , 268 (1983).

4. Self-Broadening in the Fundamental Bands of HF and HC1

(A. S. Pine and A. Fried (CAC))

In collaboration with Dr. A. Fried of the Center for Analytic

Chemistry, the self-broadened lineshapes have been measured for the

fundamental bands of HF and HC1 with the high-resolution tunable

dif f erence-frequency laser spectrometer in the Molecular Spectroscopy

Division. This is a continuation of a project to improve the data base on

the spectral band parameters (intensities, self- and N

2 - and air-broadened

lineshapes and shifts) in support of the NASA Upper Atmospheric Research

Program (Project HALOE) to monitor ozone depletion caused by reaction with

halogen-containing molecules. Using the narrow-linewidth (-1 MHZ)

linear-scan-controlled tunable laser system, we observed spectral

lineshapes free of instrumental distortion that exhibited deviations from

the usual Voigt profile due to collisional (Dicke) narrowing. In Figure 2.4

1 00

.00

OJo

£0

L.O

in 40 n

<

.20

0 00 3644.3 3644.0 Wavenumber

Fig. 2.4. Self-broadened lineshape of the P(7) line of HF compared to

various collisional models.

we show an example of the P(7) line of HF at 50.5 Torr least-squares fit

by a Voigt function and by the "soft" and "hard" collisional narrowing

models which yield much reduced obs.-calc. residuals. The self-broadening

coefficients and shifts for the v=1<-0 transitions of HF and HC1 are displayed in Figure 2.5. No significant differences were found for the two

chlorine isotopes in the natural abundance sample. Particular care was taken in sample handling for these reactive, readily polymerizable, gases

to ensure accuracy, linearity and reproducibility in the pressure and

broadening measurements. This work is supported by NASA.

17
10 a 6 4 2 0 -2 -4 -6 -0 -10 m

Fig. 2.5. Self-broadening coefficients

T=295K.

H 35 C1 v=1-0

m

and shifts for HF and HC1 at

5. Analysis of the Spectrum of H

2 02

(W. B. Olson)

The pooling of our data for hydrogen peroxide with those of Prof.

Robert Hunt of Florida State University has led to some reassignraents of

the absorption lines in the OH stretching region of the spectra, and to a direct determination of energy differences between ground state rotational

levels and the levels perturbing them, up to J = 25 on the most perturbed

levels (ground state K = 8). The previous limit was J = 15. These

reassignments were made possible by a clarification of the group theory,

and enumeration of the types of perturbations possible in hydrogen

peroxide, provided by the permutation-inversion group theory treatment of

hydrogen peroxide by J. T. Hougen. 1

In addition to extending assignments, it has been possible, using

Hougen’s symmetry treatment, to introduce some reforms in terminology and

notation for hydrogen peroxide energy levels and transitions which we feel

will greatly simplify the understanding of hydrogen peroxide, and greatly

facilitate verbal and written communication. The basis of the new terminology is a two valued overall symmetry level (U for ungerade and G

for gerade), for the two torsional substates for a given principal

torsional quantum number, rather than the old four valued numerical x label

.

18
Selection rules for transitions had been worked out in terms of the t labels, but these had no direct connection with overall symmetries, and

this led to considerable confusion as to the source of perturbations when

these were evident. A preliminary account of this work has been given in a report to NASA.

A coupled asymetric rotor program for simultaneous fitting of

combination differences in the ground state (OG) and perturbing first

excited torsional state (1G) was expected to be available in Sept. 1984,

but was delayed, and is now expected to available in Sept. 1985. Fitting

of our data with this program should lead finally to a definitive set of

ground state rotational constants for H

2 O2

. This work is supported by

NASA.

(1) J. T. Hougen, Can. J. Phys. 62 1392-1402 (1984).

D. Spectroscopy of Hydrogen-bonded and van der Waals Complexes

The program of study of hydrogen-bonded and van der Waals complexes

initiated several years ago now represents about 25? of the group’s total

effort. In addition to studies using a difference frequency laser

spectrometer in the 3 micron region, this year brings forth studies using

the BOMEM Fourier transform spectrometer in both the mid- and farinfrared, studies using the new pulsed Fourier transform microwave

spectrometer (Flygare type), as well as theoretical studies. This program

has benefitted greatly this past year from collaborations with guest

workers. We certainly plan to continue work in this direction for the

forseeable future.

1 . Hydrogen Bond Energies of the HF and HC1 Dimers

(A. S. Pine and B. J. Howard)

The dissociation energies of the hydrogen-bonded complexes, (HF), and

(HCD 2

, have been obtained from high-resolution measurements of absolute

infrared line strengths at a single temperature and pressure under the

assumption of minimally-perturbed local-mode behavior of the outer

hydrogen stretch. The measurements were made wit.h a dif f erence-frequency

laser system on low pressure (P-1-2 Torr) gas samples cooled to near the

condensation point ( T—21 9 K for HF, T-127 K for HC1) in a long path

(L-64-80 m) White cell. The observed absolute intensity pattern of the

r

Q 0 subbranch of the outer or "free H" stretching band, v

1

, of (HF) 2 is

shown in Figure 2.6 along with 'a simulated trace based on the usual

Hdnl-London and Boltzmann factors for the J assignments and tunneling

splittings obtained in our previous work. The transition lineshapes in

the simulated trace are Voigt profiles obtained by convolving the calculated Doppler distribution with a pressure-broadened Lorentzian of

half-width = 34 MHz. The agreement between the observed and calculated

spectra is excellent as can be seen in the subtracted residuals trace cf

the bottom of Figure 2.6 which exhibits only the r R lines and some hot band

structure not included in the model.

19
2.00 HF Dimer Observed

P = 1.89 Torr

T = 213 K

L = 54 m

*-i 3964.50 3964.00 3963.50 3963.00 3962.50

Obs . -Calc

.

3964.50 3964.00 3963.50 3963. 00 3962.50 Wavenumber/cm 1

Fig. 2.6. Observed, calculated and residual spectra of the r

Q

Q subbranch

of the band of the HF dimer.

The spectral intensities can be related to the dimer concentration

n

D , and partition function, QD , and the transition moment, <u-|>b > for

the band component measured using standard formulae. In turn, n

D and QD can be expressed in terms of the monomer concentration, n

M , and partition

function, QM , and the zero-point dissociation energy, D 0 , under thermal

equilibrium by

K

eq

= nD^ nM

)2 = e Do/kT

.

Since nM

is measured from the pressure assuming negligible polymerization

and Qm

can be calculated trivially for a diatomic molecule, the only

unknowns in the problem are D

Q and the transition moment for the dimer.

Recent ab initio calculations have shown that the bond transition moment

for the outer hydrogen is only -10? enhanced over that of the monomer,

which is known quite accurately from our recent measurements of the

20
absolute intensities of the monomers. Thus we need only to project the

value of this moment onto the b-axis of the dimer to relate it to the

<u-|> b observed.

We obtain zero-point dissociation energies for the HF and HC1 dimers

of D

Q

« 1038 (+43,-34) cm

” 1 and 431 (±22) cm

-1 respectively. Estimates of

the zero-point energies of the low frequency intermolecular vibrations

enable us to obtain the well depths and the equilibrium dissociation

energies, D

e

= 1595 (

+

1 03 , -94) cm

” 1 for (HF ) 2 and 794 (± 86 ) cm

” 1 for

(HC1 ) 2 for comparison to ab initio and model calculations for these

complexes.

2. Rotational Predissociation and Libration in the Infrared Spectrum

of Ar-HCl

(B. J. Howard and A. S. Pine)

Fully-resolved rotational structure in the high-frequency vibrational

region of the Ar-HCl van der Waals complex has been recorded under thermal

equilibrium conditions at T=127 K with a tunable dif f erence-frequency

laser. Both the fundamental H-Cl stretch and its combination with the

low-frequency large-amplitude bend (libration) have been observed with

comparable intensities. The fundamental H-Cl stretch gives rise to a parallel-type band in the null gap of HC1 as shown in Figure 2.7. The

assigned Ar-HCl spectrum is indicated in the calculated trace below the

Ar-HCl Observed

2887 2886 2885 2884 2883 2882 2881 2880 2879 2878 2877 2876 2875 2974 Wavenumber/cm 1

Fig. 2.7. Observed and calculated spectra of the \j 1 fundamental H-Cl

stretching band of Ar-HCl.

21
observed spectrum which is unavoidably contaminated by HC1 dimer structure

(known from previous work) and hot bands. Both Cl isotopes contribute in

their natural abundance. The abrupt cutoff of the spectrum at J = 60

arises from rotational predissociation as the rotational energy exceeds

the binding energy and may tunnel through the top of the centrifugal

barrier. The rotational eigenvalues and predissociation have been modeled

with a one-dimensional radial potential of the Maitland-Smith form to give

a zero-point dissociation energy of D 0

=i 14.7 (±1.0) cm

-1 and a van der

Waals stretching frequency of = 31.3 (±1.0) cm

-1

. These values are in

good agreement with empirical models based on precise microwave

measurements near the minimum in the potential well, but disagree

seriously with prior estimates based on the temperature dependence of the

low resolution infrared spectrum. The rotational constants in the ground

and excited H-Cl stretch vibrations yield a measure of the extension of

the center-of-mass separation due to the motion of the proton. The red

shift of the Ar-HCl spectrum from the isolated HC1 monomer implies a stronger van der Waals attraction upon excitation of the H-Cl stretch.

The combination band of the H-Cl stretch, , with the van der Waals

band, \> 2 , is shown in Figure 2.8. It is a perpendicular-type band (n-I) of

anomalously high intensity transferred from the fundamental stretch by the

strong modulation due to the wide librational motion. The blue shift from

the fundamental band is about 3^ cm

-1 corresponding to the frequency of

the bending mode. The rotational constants yield the extreme shortening

of the van der Waals bond length upon bending the molecule, and they give

the ^-doubling Constant qx

* Bq-Br

= + 34.2 (± 0.2) MHz arising from

Coriolis perturbations from nearby states with Z-symmetry. These measurements should lead to improved estimates of the anisotropy of the potential

and its dynamical behavior.

We have also obtained preliminary assignments of hot bands arising

from v

R , the van der Waals stretching mode, in the fundamental band region

of Ar-HCl. This gives us further information about the intermolecular

potential and about interactions between the low frequency modes. Spectra

have also been recorded for other rare gas (Ne, Kr, Xe)-HCl complexes to

obtain the trends of the van der Waals potential with regard to atomic

mass and polarizability.

3. Study of Hydrogen-bonded complexes in the Gas Phase Using Fourier

Transform Spectroscopy

(B. A. Wofford, J. W. Bevan, W. B. Olson, and W. J. Lafferty)

During the past year, we have had a very fruitful collaboration with

Professor John Bevan and B. A. Woffard from the Chemistry Department of

Texas A&M University. From designs developed by W.‘ B. Olson (see Section

3.1), Prof. Bevan and his co-workers have built a long-path absorption

cell which can be cooled to ca. -100° C. At low temperatures , the

hydrogen-bonded complexes are present at higher concentrations. The long

absorption path (up to 64 meters) serves to enhance the instrumental

22
Wavenumber/cm 1

Fig. 2.8. Observed and calculated spectra of the stretch-bend

combination band of Ar-HCl.

sensitivity. This cell has been coupled into the NBS 30MEM spectrometer

and used to study a number of rotation-vibration bands of several

hydrogen-bonded complexes including HCN • •

• HF , HCN**HCN, and HCN-*«HC1. At

this point the work on the HCN-HF complex is most complete. Five of the

absorption bands of this complex have been observed, and many of the

individual line transitions assigned. These bands arise from the C-H

stretching fundamental and its overtone, the CN stretching vibration, the

internal HCN bending vibration, as well as the bending vibration in which

one end of the complex twists with respect to the other end. The latter

vibration is unique to the hydrogen-bonded complex, and is not overlapped

by lines from either HCN or HF. The Q-branch of this band is shown in

Figure 2.9.

The dissociation energy for these weakly bound complexes is, of

course, rather low; e.g., for HCN***HF, Dq is about 1600 cm

-1

. Thus many

of the molecular vibrations lie above the dissociation limit, and high

resolution spectroscopy, in addition to providing information on the

energy levels of hydrogen-bonded molecules, allows one to approximate the

life time of molecules in energy levels above this limit. The width of a

23
Vg HCN--HF

14 13 12 11 10 9 8 P(J) Q(J) 20 15 10. 5

I i l

i l i I

|

i i n |

i 'i rT[TTTTj

Fig. 2.9 A Q-branch of the high-frequency bending vibration, vg

1

, of the

hydrogen bonded complex HCN***HF recorded at a temperature of

-50°C. Despite the unusually high stability of this complex, a path length of 64 meters was required to obtain the spectrum

with a reasonable signal-to-noise level. This vibration is

unique to the hydrogen-bonded complex and there is no overlapping from lines of the parent species.

ro-vibrational line is inversely proportional to the lifetime of the

molecule in the upper state of the transition. The lines of several bands

of the HCN***HF molecule were found to be considerably broader than the

instrumental resolution, and, therefore, lifetimes of several vibrational

states could be estimated. For example, the lines of the C-H stretching

vibration at 3310 cm 1 were not broadened and one can estimate that the

molecular lifetime of this state is at least 10~® sec. The overtone of

24
this vibration at 6517 cm

-1

is slightly broadened, and the lifetime of

this level is on the order of 2.4 ns. The CN group is directly involved

in the hydrogen bond, and the lines of the CN stretching vibration at 2121

cm' 1 are very broad. The lifetime of molecules in this state is about

2.56 ns. Not surprisingly the lower frequency bands, which have

vibrational frequencies below D

Q , have sharp lines, and only an upper

limit estimate of the lifetimes can be made for the upper states of these

vibrations

.

These studies will shortly be extended to the far infrared region to

study the rest of the vibrational transitions unique to the complex.

4. Pulsed Beam Fabry-Perot Cavity Fourier Transform Microwave

Spectrometer

(R. D. Suenram and F. J. Lovas)

The construction of a pulsed beam Fabry-Perot cavity Fourier

transform microwave spectrometer was briefly described in last year's

annual report.

The major emphasis of our efforts in the past year have involved

testing the new spectrometer with a number of previously observed hydrogen

bonded species and van der Waals complexes to obtain experience in the

operation of the instrument and optimization of the detected signals.

Two areas in which the instrument excels are high sensitivity and

high resolution. In order to assess the sensitivity a 2 % mixture of OCS

in argon was prepared and the 1-0 transition of the various isotopes of

OCS studied in natural abundance. Figure 2.10 shows the 1-0 transition of

8 30 _

’•OCS J=1-0

# < 23 _ a : 29 _

’•OCS J=i-0

C.2C%

a. aa a.oe i. 2.70 3.38 4.58

•c- O- !.«• £-2

8 08 L*

a do a. >a 820 a 38 a +* asa 0-M*0G Slip** I>— ’ A3 1 **r*r

Fig. 2.10. Rotational spectrum of the J=1 -G transition of ^CCS. left:

time domain spectrum obtained with 500 nozzle pulses. Right:

frequency domain spectrum; Fourier transform of spectrum shown

on the left. The signals were observed with a 2 % mixture of

OCS in argon.

25
1 ®OCS which occurs in 0.21? natural abundance. The trace is a summation

of signals for 500 nozzle pulses but this signal is easily observable on a single pulse. The weakest signal observable was that of which

occurs in 0.0086? natural abundance. This signal was observed with a summation of 2500 pulses. The 1 ^OCS signal, which is observable on a single pulse, corresponds to approximately 1 0*

1 3 molecules per pulse.

Several improvements of the instrument are planned for the next year which

should easily increase the sensitivity by an order of magnitude.

The high resolution aspect is shown in Figure 2.11 where the 1-0

transition of (H

2 o) 2 is shown. The four individual peaks arise from the

spin-spin hyperfine interaction associated with the H nuclei. The

splittings are

Fig. 2.11. Time domain (left) and frequency domain (right) spectrum of the

J=1-0, E-state transition of the water dimer at v = 12321.0

MHz. The frequency splittings (right side) are approximately

16-20 kHz.

on the order of l6-20KHz. We have found that with certain nozzle/ pressure

configurations we can obtain line widths on the order of 8-1 2kHz (FWHM).

The following two reports (D.5 and D.6) describe new results obtained

for Ar •• ‘acrylonitrile and Kr*‘‘0CS.

5. Microwave Study of the Ar. . .Acrylonitrile van der Waals Complex

(F. J. Lovas and R. D. Suenram)

The van der Waals complex of argon with acrylonitrile ( Ar . .

.

CH 2 CHCN)

has been observed with the pulsed beam instrument. This particular

complex was chosen as the first new molecular species for the new instrument for a variety of reasons. First the dipole moment is large so

26
the spectra should be quite intense. Second, the **N nucleus provi

characteristic quadrupole splittings which should be predictable ar. useful for the assignments of the transitions. Third, the species

large enough so that the transition density is fairly high (a stron

transition every *00 MHz or so) which should enable several transitions to

be found without a great deal of searching, last, there are a variety of

sites where the Ar could bind to acrylonitrile. It will be extremely

interesting to see which conformation is the dominant one.

Two discrete sets of rotational transitions have been observed,

one set, the energy levels lie approximately 0.5 - 1.0 cm

-

;

higher in

energy than the other set. Mere than thirty transitions have been

observed and fitted for the low energy (ground state) and over twenty

or

transitions have been measured and tne nigr.er iergy

(vibrational) state. Each of these transitions is split by the nuclear

quadrupole interaction associated with the ~N nucleus.

From a rotational analysis of the transitions, and analysis of the

nuclear quadrupole hyper f ine structure, the mclecui.e appears to favor one v, p -> n

' of the two possible structures shown in Figure 2.12. Fu

:

deuteri un isotope labeling will have to be done in order

Our

worx witn

.scr imitate

which is the true structure. Our intuitive feeling is that tne 1 r. :r.s: ce structure is probably the correct one, since the "outside" stru r> o requires an Ar...H distance that is shorter than for the normal van der

Waals radii of the atoms. In either case, the Ar atom lies at an angle o: approximately -C° with respect to the plane of the acrylonitrile subunit.

A a - Ar ns i ce Ar 'outs ce

' . c

.

Favored structures for the ar gon-acrylcni

complex.

e van cer «aa»s

V owave Study of

o . Lovas and R

e Kr .

.

D. Sue

a aa_s ex

X)O

-I

tiO

•!
University had provided approximate values for the rotational constants of

both Ne...OCS and Kr...0CS in the ground state and v=1 of the CO

stretching vibration. Accurate values for the rotational constants of

these species in the ground state, obtained by microwave spectroscopy,

could be used in the analysis of the infrared spectra to obtain very

precise values for the upper state constants.

Some time was spent searching for Ne...0CS but no transitions were

found. Transitions of Kr...OCS on the other hand were readily observed.

Subsequent microwave-infrared double resonance work on Ne...OCS in Oxford

indicated that the preliminary rotational constants were much more

uncertain than originally thought; as a result, we never searched the proper

region to detect a transition of Ne...OCS.

For Kr...OCS transitions for four isotopes (®^Kr, 57$; ®%r, 1 7%

;

^ 2 Kr, 11$; and 83Kr , 11$) have been observed and analyzed. The structure

of the complex is planar and T-shaped, similar to Ar...OCS (see Fig 2.13)-

One note of interest is that nuclear hyperfine structure has been observed

O

s

Kr

Fig. 2.13- Structure of the Kr...OCS van der Waals complex

on transitions (see Figure 2.14). The ®^Kr atom has a nuclear

spin of 9/2 and thus, a nuclear electric quadrupole moment. Although the

electron orbitals of the atom are spherically symmetric, these are distorted in the van der Waals complex and cause hyperfine interactions.

Analysis of this hyperfine structure should enable us to study the

distortion of the spherical symmetry upon complexation.

Quite recently we have incorporated a set of Stark plates into the

spectrometer for dipole moment measurraents. These were placed -25 cm apart, so that the cavity Q would not be changed appreciably, and provide

AMj = ± 1 selection rules. Analysis is in progress on the Stark effect of

Kr***OCS. Figure 2.15 shows three Stark components of the

transition.

28
-4 00

0 00 3 S3

1 1 I L 1.30 I • S0 2.00 2.50 3 00 CH- So« E-2

F=7 / 2 -»4 ”KrOCS J=l-0

R 2.10

n i.40 l_ ; F=n/r9 /j

0.25 0.50 3.75 1 00 1 25 F0®7S0S.I!0 flHx CK-3 . 9063**n*

Fig. 2.14. Time domain (left) and frequency domain (right) spectrum of

the J=1-0 rotational transition in ®3Kr...0CS at 7505.11 MHz.

The AF**0 transition is not shown in the figure; it was found

at 7504.6 MHz.

2 96 _ M=3-2

ft I .50

I

M=2-1 KrOCS 2qj E=l55V/cm

M=1HD ! \ / W

S-rd- 1L a as a.i 0 0.i5 a 20 a zs F»»tee0i . 4samc o>-i as3 i^t»

Fig. 2.15. Time domain (left) and frequency domain (right) Stark spectrum

of the J=3"2 line of Kr..0CS obtained with a field of

1 55V/cm.

7. A Generalized Internal Axis Method for High Barrier Tunneling

Problems, as Applied to the Water Dimer

(J. T. Hougen)

When more than one large amplitude vibrational motion is present in a molecule, it is often not possible to define a global internal-axis-method

(IAM) coordinate system and set of basis functions. In the present work,

a method is presented for extending the IAM treatment to tunneling

problems in such cases, using as an illustration a model for the water

dimer with three large amplitude vibrational coordinates. The method

: S0

a 3*

29
involves the construction of two different sets of local IAM-like

coordinate systems. The first of these contains n coordinate systems, one

for the small neighborhood surrounding each of the n equilibrium

frameworks. The second contains on the order of n

2 /2 coordinate systems, one for each feasible tunneling path between each pair of frameworks.

Basis functions written in the second set of local IAM-like coordinates

are used to determine the complex phase factors associated in this method

with tunneling matrix elements of the phenomenological rotational

Hamiltonian in the high barrier limit.

These phase factors govern the way in which the various real

tunneling frequencies in the' molecule constructively and/or destructively

interfere in the Hamiltonian matrix elements and final energy expressions.

Various mathematical approximations are involved in using the local

IAM-like basis sets to obtain matrix elements; the full extent of the

adverse effects of these approximations will not be known until an attempt

to fit experimental data is carried out.

E. General Spectroscopy and Theory

1. Efficient Sampling Systems for the Fourier Transform Spectrometer

(Wm. Bruce Olson)

A transfer optics system has been designed, and constructed for the

coolable White cell made from our design by Prof. J. Bevan at Texas A&M

Univ. Prof. Bevan also had made an evacuable chamber for the transfer

optics. The utility of this system for use in the far infrared has been

demonstrated.

A preliminary optical design using a single concave mirror has also

been worked out for the transfer optics to couple the BOMEM FTIR

spectrometer to an existing heat pipe for use in the far infrared. An

important feature of this and the previous system is that a single concave

mirror is used to accomplish two objectives. An image of the input

aperture of the FTIR is placed at the entrance of the cell, and an image

of the beamsplitter is placed on, and exactly fills, a back mirror of the

White cell, or exactly fills the exit aperture of the heat pipe. Under

these conditions a simple thin window, but no field lens is required at

the input of the sample cell, in order to pass the maximum possible signal

radiation, and the problem of a suitable optical material for a field lens

is eliminated.

2. Diode Laser Measurements on Unstable Diatomic Molecular Species

(A. G. Maki , G. A. Thompson, and A. Weber)

In an effort to study molecular species that are unstable in the gas

phase under ordinary conditions, we have used a high temperature technique

to measure the infrared spectrum of GeO, Lil, and LiD in the gas phase.

These are the first resolved gas phase infrared measurements to be made of

these species although the electronic and microwave spectra have been

studied in the gas phase. The extent of the measurements is indicated in

30
Figure 2.16. which gives the range of vibrational and rotational levels

covered by these measurements for ^Ge^o and All five isotopic

germanium species of GeO have been measured in natural abundance as can be seen in Figure 2.17 which shows a portion of the measured spectrum. So far

only the most abundant isotopic species of Lil and LiD have been observed.

To analyze measurements of this sort we have developed a non-linear

least-squares fitting program to determine a set of Dunham potential

coefficients that define the lower part of the potential well for the

molecules under study. Most other workers use such measurements to

Fig. 2.16. Extent of data base for rotation-vibrational bands of GeO

(left) and Lil (right) determined from infrared diode laser

spectra. In these displays the vibrational states are depicted in strips one unit wide, and centered about the

integral value of the vibrational quantum number v.

determine directly a set of ro-vibrational constants that ignores the

underlying concept of a potential well and therefore may be in conflict

with any reasonable potential function.

We believe that a more useful analysis uses the data to determine the

potential function and then uses the potential function to determine a consistent set of ro-vibrational constants.

The problem with using ro-vibrational constants for calculations that

represent an extrapolation outside the range of observations is that the

constants are usually determined without restraining them to a physically

meaningful model. Our way of determining the ro-vibrational constants

gives them greater reliability for calculating levels and transitions

outside the range of measurements because the underlying concept of a potential function restrains many of the higher order ro-vibrational

constants to physically realistic values.

In the case of GeO all five measured isotopic species have been fit

to a single potential function which can be used to calculate the observed

transitions with an rms deviation of 17 MHz which we believe represents

the accuracy of the measurements.

31
GeO

76

74

U.

72

74

73

76

74

76

1

P o ( 5 7 )

P 2 (4 5 )

70

72

74 70

73 1 O

70

P i ( 5 5 ) P 4 ( 3 0

)

il I . P s ( 2 2

)

P , ( 5 2

)

P o ( 6 0

)

1 P « ( 3 2

)

P 3

( 4 1

)

| P 2 ( 4 8 )

i P 2 (4 3 ) P o ( 5 9 ) * P i ( 5 0

)

910.80 91 1.00 91 1.20 91 1.40

WAVENUMBER (cm- 1 )

Fig. 2.17 Portion of the infrared diode laser absorption spectrum of GeO.

Numbers in the top part identify the Ge-isotope. The

assignments of the individual rotation-vibrations are given in

the lower half; the subscripts label the vibrational state. The

line marked with an open circle is due to an impurity.

The work on LiD is not yet finished. In the coming year we expect to

also observe the spectrum of ^LiD.

We have also been measuring transition frequencies and relative

intensities for CIO. Since those measurements are of interest for

atmospheric studies, they are described in section C on studies related to

atmospheric chemistry.

3. Fourier Transform Raman Spectroscopy of H

2 and D2 in Flames

(A. Weber and D. A. Jennings)

The successful demonstration of the use of a Fourier transform

spectrometer to record Raman spectra of gases under high resolution was reported in last year's Annual Report. Together with Dr. J. W. Brault of

32
the National Solar Observatory located at Kitt Peak, AZ, work on the high

temperature spectra of H

2 and D 2 has been done with the 1 -meter FTS

instrument of the McMath Solar telescope at Kitt Peak.

Laboratory spectra of high rotational transitions in H

2 are important

in astrophysics because they may be observable in hot stellar and

interstellar sources, and have already been seen in quadrupole emission

from the Orion molecular cloud."1 The Orion emission lines are characteristic of an -2000K rotational temperature. Infrared spectra of H

and D

2 at high temperatures are difficult to obtain in the laboratory

because path lenghts of hundreds of meters are necessary to observe the

quadrupole absorption. Vibration-rotation coefficients obtained from room

temperature spectra cannot be extrapolated to the high-J lines observed in

Orion to within the accuracies of the astronomical measurements.

Prompted by these considerations, we have recorded Raman spectra of

high temperature molecular hydrogen and deuterium using a Fourier transform spectrometer. A diffusion flame source burning H

2 or D 2 in air was placed at the focal point of a multi-pass mirror system. 2 Figure 2.18

shows the pure rotation spectrum of D

2 obtained in this manner; the room

temperature spectrum is shown for comparison.

FTS -RAMAN SPECTRA OF D2

v • 0-0

SIS)

500 500 3CC

RAMAN SrtfT (cnT

Fig. 2.18. Pure Rotational Raman Spectra of D

2 for a diffusion flame in

air (top) and a stagnant gas at room temperature (bottom).

As expected the higher temperature spectrum (flame) shifts the

intensity distribution toward higher J-values, with S(1 2) being the

highest transition observed. The noisy and varying bacKground in the high

temperture spectrum is caused by the flame itself. Similar observations

were made on H

2 for which both the v=0-0 and v=1-0 spectra were recorded
to S(7) (v=0-0) and Q ( 5 ) (v=1-0) in the flame spectra. The data bases

for the v=0-0 and v=1-0 transition of H

2 and D 2 have been extended through

the inclusion of the Raman flame data. Improved ground state constants

for D

2 are listed in Table 2.2.

Table 2.2 Ground State Parameters of D

2 (cm "*

)

Constant Flame + Room Temp. Room Temp. Only

B

o 29.91371 (44) 29.91 439(40)

D

0 0.01 1527(22) 0.01 1599(19),

H

o 6.84(40)x10"°

-1 .1 1 (29)x10~8

9.10(24)x10~5 L

0 —

M

o 3.09(69)x10~ n —

AEm AE6 (J) AEij( J)

(1) R. F. Knacke and E. T. Young, Ap. J. (Letters) 249 » L65 ( 1981 ).

(2) R. W. Hill, A. J. Mulac, and C. E. Hackett, Appl. Opt. _1_6_* 2004

(1977).

4. Infrared Spectrum of Cyanogen

(A. Weber, W. J. Lafferty, and W. B. Olson)

Previous work on the high resolution infrared spectrum on C

2H2 was described in last year’s Annual Report. In that work the and

fundamental bands were recorded with the BOMED DA3.002 Fourier transform

spectrometer using only a 20cm absorption path. The band was recorded

at a pressure of 5 torr which caused noticable pressure broadening of the

lines. In the present work a White cell (maximum path length 20 m) was employed to re-record the band system, as well as to obtain spectra of

the v^+vt^, v^+v^ 1

, v^-vc 1

, and v

2 -v^ combination and difference bands at an apodized resolution of 0.004 cm

-1

. Because of the low lying fundamentals vjj"' and vc

1 these bands are accompanied by numerous hot bands which

complicate the assignment process, but which yield precise values for the

energy levels of the lower states. Figure 2.19 shows the region of the

band using a path length of 2.25m and a gas pressure of 1.51 torr. The

0CS is present as an impurity in -the gas sample. The band plus its

hot bands is now present with a good signal to noise ratio. In the

previous spectra only its Q-branch was visible, with very poor signal to

noise. Similarily the and bands of the isotopic species

^C 12CN 2 , present in natural abundance, are observed in the present

spectra. * 1 4 - **— -* 1 —1 Extensive structure was observed in the

1 '1 and v 3+V4

combination bands. The +v^ band contains an heretofore unreported

perturbation. The band system near 61 0cm

-1 was recorded for the

first time under high resolution. This band is weak and, owing to the use

34
MAX . 3 92E *12

Fig. 2.19 Region of the v

3

~band of C 2 N2 recorded with the BOMEM DA3.002

Fourier transofrm spectrometer at a (opodized) resolution of

0.004 cm

-1

. The spectrum is the result of 64 scans taken one a time of 5 h. 20 m.

of the less sensitive HgCdTe detector, a rather high pressure of 39.9 torr

was necessary for its observation. The analyses of the various bands are well under way though the perturbation in the +v^ band remainds to be

±1 il 1 The upper state of the band is split into a

+ J " component, both of which have been identified. This is only

identified,

and a E tie second Known case (the other being C

2h2

) in which a Eg” - n u transition has been observed and identified. 3esides improved values for the

constants of the band systems new results have been

obtained for the

, v-| +v^ ‘ and

system as well as for the ground state of

bands

has also enabled us to obtain more accurate values for the frequencies of

the and v

2 fundamentals. These fundamentals are of species and are

thus only observable in the Raman spectrum. The present results are:

v., =2330.4847(5) and v

2 =845. 5965( 1 5) cm

-1

. They are more accurate than the

Raman results and will serve in the determination of a more accurate

force field of cyanogen.

1

10

J

1 ? C ^CN,. The extensive data base of the combination and difference

5. Fourier Transform IR Spectra of Monodeuterosilane

(Wm. B. Olson and R. W. Lcvejoy)

Assignments in the vibrational bands vg, vo, v^, v

2 and 2v$ have been

greatly extended since last years report. Identification of pertur baticr.

35
allowed transitions with high values of the K quantum number in and vg

has enabled for the first time a direct experimental determination of

D q

k and a smaller error (by a factor of 150) than that of our previous

determiantion 1 of A

Q

.

We were unaware at the time of last year’s report that another group

of workers in Europe had taken the same spectra and had nearly completed

the assignments. This group is headed by Prof. H. BOrger,

Uni versitSt-Gesamthochschule, Wuppertal, West Germany. Since that group

had access to computer software for fitting the three bending

fundamentals, we (by agreement with Prof. Burger) abandoned work on these

and have concentrated on the analysis of the v

2 and 2vg parallel and

perpendicular bands.

The extreme weakness of the perpendicular component of 2vg leads to

no observable transitions in this band except those whose intensities are obtained by perturbations with v

2 and the parallel component of 2vg.

Several perturbations are observed, and about twenty transitions of 2vg

have ben assigned, but we do not yet have enough data for a reasonably

good analysis, and the slow assignment and checking process is still in

progress

.

(1) Olson, W. B. , and Lovejoy, R. W., J. Mol. Spectrosc. 66_, 31 14— 330

(1977).'

6. Far Infrared Spectrum of the Torsional Band of Hydrazine

(N. Ohashi, W. J. Lafferty, and W. B. Olson)

Hydrazine has two large amplitude motions, an NH

2 wagging vibration

(similar to the umbrella motion in NH^), and the torsional vibration.

Only recently have theoretical methods become available to treat the

observed spectra 1

, and they have been successfully used to fit the pure

rotational microwave spectrum. While the ground state has large

splittings arising from the inversion motion, torsional splittings in the

ground state are quite small.

We have studied the torsional vibration of N^Hjj where the torsional

splitting contributions will be much larger than in the ground state

spectrum in order to further test the theory, as well as to attempt to

determine the barrier to internal rotation. The center of this band is at

376 cm1

. It is a b-type band, and since the A rotational constant of the

molecule is large, it covers a large portion of the spectrum from about

300 cm

-1

to 1150 cm

-1

. The spectrum was recorded with the NBS B0MEM FTS

spectrometer at a resolution of about 0.01 cm

-1

.

Although the spectrum is somewhat complex due to the fact that each

rovibrational transition is split into either four or six components

(depending upon whether the lower state K of the transition is even or

36
odd), and also because of overlapping by "hot band” transitions, we have

been able to make a very satisfactory line assignment with the aid of

combination differences calculated from ground state constants obtained in

a recent microwave study. 2

The assigned transitions were combined with the observed microwave

data and fit using Hougen's Hamiltonian. 1 The inversion and torsional

splittings of both the ground and torsional states were obtained. As

expected, the torsional splitting in the vibrational state ( 1 82 -4 Mhz) was found to be raany-fold larger than the torsional splitting in the ground

state (5.8 MHz). However, a surprisingly large vibrational change in the

inversion splitting was found. This splitting was determined to be 7976

MHz in the torsional state as compared to a splitting in the ground state

of 160^0 MHz. The height of the barrier to internal rotation was estimated

to be 2100 ± 50 cm

-1

.

(1) J. T. Hougen, J. Mol. Spectrosc. 89, 296 (1981).

(2) S. Tsunekawa, T. Kojima and J. T. Hougen, J. Mol. Spectrosc. 95, 133

(1982)

.

7. The Torsional-Wagging Tunneling Problem and the

Torsional-Wagging-Rotational Problem in Hydrazine

(N. Ohashi and J. T. Hougen)

Results derived previously for the rotational levels of the

eight-framework and three-large-amplitude vibrational problem in N^H^,

using a tunneling formalism based on a treatment of the vibration-rotation

problem as a whole, were rederived in a much simpler fashion, using a tunneling formalism based on separate treatment of the vibrational and

rotational problems. The present formalism is thus much more akin to the

usual vibration-rotation formalism, and the origins of the various

contributions to the vibration-rotation energy levels can be understood

relatively easily. It is convenient here, as earlier, to make extensive

use of permutation-inversion and extended-group (double-group) ideas, but

it is necessary in the present treatment to consider tunneling between 16

minima in molecular coordinate space, i.e. between a number of minima

which is twice the number of nonsuperimposible molecular frameworks that

can actually be constructed for ^Hjj.

8. The Use of Extended Permutation-Inversion Groups for Constructing

Hyperfine Hamiltonians for Symmetric-Top High-3arrier Internal

Rotor Molecules like HgC-SiHg

(J. T. Hougen)

The m-fold extended group G^g (' m \ corresponding to the

permutation-inversion group G-jg for molecules like HgC-SiH-, has been

obtained. In this extended group, m is the smallest integer for which mo

is also an "integer," where p is the usual ratio of the moment of inertia

of the top about the rotational A axis to the moment of inertia of the

molecule about the A axis. The extended group has 1 3m elements, divided

37
into (9m+3)/2 or (9m+6)/2 classes, for odd and even values of m, respectively. Using this extended group, it is possible to assign

definite symmetry species in an internal-axis-method (IAM) treatment to

laboratory-fixed, molecule-fixed, top-fixed and frame-fixed projections of

vector operators like the rotational angular momentum and the nuclear spin

angular momenta. Thus, it is possible to express the spin-rotation and

spin-spin contributions to the hyperfine interaction operator in terms of

rotational angular momentum components, nuclear spin angular momentum

components, and functions of the torsional angle, all of which have known

symmetry species and selection rules in the IAM basis set. Using a hyperfine Hamiltonian constructed from these considerations, it is

possible to understand the large variations in magnitude of some observed

hyperfine couplings in recent molecular beam studies of symmetric top

internal rotor molecules.

9. Improved Least Squares Determination of Potential Energy Surfaces

from Spectroscopic Data

(C. Leubner and J. T. Hougen)

The value of a knowledge of potential energy surfaces for the

understanding of the overall properties and behavior of a molecule can hardly be overestimated. With the exception of the simplest molecules, it can safely be stated that currently it is the observed spectrum that

contains the most complete - albeit indirect - information on the

molecular force field. As a consequence, much effort has been expended on

the retrieval of potential energy surfaces from spectroscopic data.

Vidal and coworkers have shown in a number of applications that such

a program - termed "Inverted Perturbation Approach (IPA)" and reviewed in

(1) - can indeed be carried quite far in the diatomic case, where a number

of problems are absent that become crucial in the polyatomic case. These

qualitative differences preclude a straightforward, similarly successful,

generalization of the IPA to the determination of potential energy

surfaces. However, the main ingredients of the IPA have been used by the

workers in the field of polyatomic molecules with more moderate results.

Chen et al. (2) recently showed that none of the surfaces determined by

these methods for the water molecule agrees with the observations to

within* spectroscopic accuracy.

However, it is possible to identify three promising areas for the

improvement of the conventional approach. First of all, the variational

character of the IPA can be combined with the necessarily variational

calculation of the eigenvalues of the Schroedinger equation for a polyatomic molecule to give a very compactly formulated optimization

problem, the solution of which would, in principle, immediately give the

optimum potential energy surface corresponding to the measured energy

eigenvalues in a least squares sense.

38
Secondly, this optimization problem involves as a crucial step a sufficiently accurate determination of the ro-vibrational eigenvalues of

the Hamiltonian of the molecule under study. Chen and coworkers (2) nave

reported encouraging results with the conventional expansion of the

eigenstates in products of "rotationless vibrational” wavef unctions and

symmetric top functions. Alternatively, one could invoke so-called

"distributed Gaussian bases", as recently introduced into the nuclear

molecular problem by Hamilton and Light (3), which, after all, have proved

superior to all other basis sets in the electronic molecular problem,

where they are now exclusively used.

Thirdly, one -requires a sufficiently flexible representation of the

potential energy surface. As an improvement over the conventional Taylor

series expansion in suitably chosen coordinates around the equilibrium

point of the surface, we propose to supplement this by a Spline

approximation (4) that adequately incorporates the full symmetry of the

problem. The parameters of this Spline approximation are then to be

optimized in a least squares sense with respect to the observed spectrum.

An investigation that focuses on these three areas of improvement is

currently under way in collaboration with M. Wolfsberg of the Department

of Chemistry, University of California, Irvine, CA.

(1) C. R. Vidal: Accurate determination of potential energy curves

(preprint) 1985.

(2) C.-L. Chen, B. Maessen, and M. Wolfsberg: Variational

calculations of rotational-vibrational energy levels of water

(preprint) 1985.

(3) I. P- Hamilton and J. C. Light: On distributed Gaussian bases

for multi-dimensional vibrational problems (preprint) 1985.

(M) J. H. Ahlberg, E. N. Nilson, and J. L. Walsh: The Theory of

Splines and their applications (Academic: New York 1967).

10. The Proper Choice of the Lagrangian for a Relativistic Particle

in External Fields

(C. Leubner)

It is an unsatisfactory feature of the educational literature that

renowned authors propose apparently conflicting prescriptions for the

construction of an appropriate Lagrangian to describe the interaction cf a relativistic point particle with external fields. The point where

opinions sharply differ is whether, in the case where the independent

parameter is to be the proper time of the particle, the equations of

motion should be derived from Hamilton’s principle by constrained or unconstrained variation. Therefore, it is shown that there exists a unifying point of view, which allows a consistent derivation of all these

seemingly unrelated or even incompatible Lagrangians, which completely

39
clarifies their status and interrelations, and, moreover, provides a sound

basis for discussing coherently a multitude of related issues that have

emerged in the literature in an often obscure and even misconstrued

manner. The results are to appear in the European Journal of Physics.

11. A Note on the Uniform Asymptotic Expansion of Integrals with

Coalescing End Point and Saddle points

(C. Leubner)

Together with H. Ritsch of the University of Innsbruck we have

introduced a new method for the uniform asymptotic expansion' of certain

types of contour integrals, one of whose critical points is an end-point

of the interval of integration. The method, which is alternative to

Bleistein's, has been tested numerically by way of a non-trivial physical

example. The results will be published in Journal of Physics A: Mathematical and General.

12. Generalized Stirling Approximations to N!

(C. Leubner)

Generalized asymptotic approximations to r(x+1), which contain an arbitrary parameter, are derived both from the integral representation of

the Gamma function without assuming a knowledge of the Stirling series,

and through elementary rearrangements of the Stirling series. By

optimizing the arbitrary parameter according to appropriate criteria,

several known Stirling-like approximations are recovered in a unifying

way. These are as compact as but numerically superior to the standard

Stirling approximations, and are meaningful on intervals that even include

parts of the negative x-axis. It is pointed out that these results - arrived at by elementary but generally applicable asymptotic techniques - can be exploited in physics teaching to demonstrate the power and utility

of asymptotic methods in the analysis of a variety of physics problems.

The results are to appear in the European Journal of Physics.

13- Accurate Emission Spectra from Planar Strong Field Wigglers with

Arbitrary Field Variation

(C. Leubner)

In collaborative work with H. Ritsch of the University of Innsbruck

we showed that the spectral and angular distribution of the single

particle wiggler signal can be conveniently evaluated by asymptotic

complex contour integration, if the wiggler magnetic field is an arbitrary

function of only the coordinate along the wiggler axis. The method is

efficient and accurate, especially in the most interesting range of

frequencies around the characteristic frequency of the wiggler, where for

strong field wigglers the conventional procedure encounters serious

40
computational difficulties. The method exhibits promise for extensions in

various directions of experimental interest. The results will appear in

Nuclear Instruments and Methods in Physics Research.

14. Dynamical symmetries and equivalent Lagrangians for

one-dimensional systems

(C. Leubner)

In collaboration with M. A. M. Marte of the University of Innsbruck

we have shown that for one-dimensional Newtonian equations of motion a surjective mapping from the set of dynamical symmetries onto the set of

equivalent first-order Lagrangians can be established, where - up to an integration - the Lagrangian associated with a particular dynamical

symmetry is explicitly given.

15. On the Relationship of Normal Modes to Local Modes

(I. M. Mills)

A simple model for the effective vibrational Hamiltonian of the XH

stretching vibrations in H

2

O, NH

3

and CHjj is considered, based on a Morse

potential function for the bond stretches plus potential and kinetic

energy coupling between pairs of bond oscillators. It is shown that this

model can be set up as a matrix in local mode basis functions, or as a matrix in normal mode basis functions, leading to identical results. The

energy levels obtained exhibit normal mode patterns at low vibrational

excitation, and local mode patterns at high excitation. When the

Hamiltonian is set up in the normal mode basis it is shown that

Darling-Dennison resonances must be included, and simple relations are found to exist between the x^, gtt , and Krrss anharmonic constants (where

the Darling-Dennison coefficients are denoted K) due to their

contributions from Morse anharmonicity in the bond stretches. The

importance of the Darling-Dennison resonances is stressed. The

relationship of the two alternative representations of this local

mode/norraal mode model are investigated, and the potential uses and

limitations of the model are discussed.

F. Future Plans

1. Calibration and Data Center Activities

During the coming year we will devote a major effort to the

completion of "Microwave Spectral Tables III. Hydrocarbons". In

conjunction with this manuscript we will produce a magnetic tape of the

combined measured and calculated spectra which is too extensive to publish

in printed form. Eventually we will combine this data with the existing

files on diatomic and the triatomic species to form the initial spectral

line atlas. We will also begin extracting and evaluating data on a new

41
group of organic species. Hydrocarbons containing the heteroatoms, oxygen

or nitrogen will be selected for the next review in this series after we examine our files of literature references on these compounds.

In addition to the general search of the microwave literature we will

continue searching the astronomy literature for interstellar molecular

observations. Due to the rapid increase in new data during the 5 years

between revision of the Recommended Rest Frequency publication, we plan to

up-date the computer data files on a continuing basis in order to publish

subsequent revisions in a shorter time frame. Publication of revisions

every two years is the current plan.

Further work is planned in collaboration with NBS/Boulder on developing infrared calibration frequency standards. New heterodyne

measurements on hot bands of N

2 O will be performed to extend the present

calibration tables into the 2800 cm

-1 region. Preliminary results from

this work have already been provided to workers at NBS in support of their

work on NO2

. Similar work will also be done on the v^+2v2 band of OCS

near 1900 cm

-1 as well as on the fundamental band of NO.

2. Spectroscopy for Environmental Studies

Work on the spectroscopy of molecules of importance in the

chemistry of the earth’s atmosphere will continue in support of NASA and

CMA programs during the coming year. Intensity and frequency measurements

will be made on CIO. New infrared diode laser measurements on the

spectrum of nitric acid will be made and combined with existing microwave

and infrared data to develop an atlas of transition frequencies and

intensities for the range 1 680—1 7^40 cm~^ . Work will also be done on the

vc and 2vg bands of nitric acid. The analysis of these bands, which span

tne region from 850 to 930 cm

-1

, requires the inclusion of a perturbation

due to a AK=±2 interaction. Similarly, although the and bands of

nitric acid have been assigned, they are found to be perturbed and are still to be satisfactorily analyzed.

The work on the collisional line-shapes of HF and HC1 undertaken in

support of the NASA project HAL0E will be concluded during the coming

year. We will examine the effects of collisions on the electronic fine

structure of NO as well as for overlapping Q-branch transitions, and make

precise frequency measurements on the fundamental band in collaboration

with the NBS/Boulder group.

3. Spectroscopy of Hydrogen-Bonded and van der Waals Complexes

As first reported in the report for FY82 the High Resolution

Group has embarked on a program of investigating the spectra of weakly

bond molecular complexes and the present report indicates the expansion of

these efforts. During the coming year we plan to continue to do tunable

infrared laser spectroscopy of hydrogen bonded and van der Waals

molecules. Using a coolable White cell with the gas under thermal

equilibrium conditions investigations of hydrogen-halide complexes will be

continued with the difference frequency laser spectrometer while the diode

42
laser spectrometer will be used in the study of heavier complexes (e.g.

NO-dimer) as well as for the study of the far-infrared van der Waals modes

of HX complexes.

Infrared investigations will also be performed on molecular-beam

adiabatically-cooled systems. A pulsed molecular beam apparatus is now

under construction and will be used for absorption spectroscopy with the

aid of various tunable lasers. A cw-molecular-beam laser resonance

apparatus is now under development and scheduled for completion during the

coming year for vibrational predissociation measurements. Also under

development is a high resolution tunable F-center laser system for high

power excitation of van der Waals complexes for vibrational

predissociation studies.

The good performance of the initial version of our pulsed-beam

spherical Fabry-Perot cavity microwave spectrometer has prompted us to

improve the sensitivity and efficiency of this instrument. Modifications

of the hardware in the signal detection and digitization systems coupled

with streamlined software will allow more rapid pulsing. Modifications

are also planned to improve the search efficiency of the instrument by

computer control of a number of current manually performed operations.

With these modifications we expect a five-fold improvement in the spectral

search over the present value of 100 MHz/day.

Several techniques are under investigation for producing pulsed beams

of non-volatile materials e.g. thermo-spray and laser ablation methods, to

broaden the range of studies possible with this spectrometer.

In addition to these instrumental improvements planned during the

coming year we will engage in spectral studies of hydrogen-bonded species

as well as van der Waals complexes. Several of these planned microwave

investigations will be carried out in conjunction with the infrared

studies done in the group.

The experimental studies planned for next year will be complemented

by theoretical investigations. In particular work will continue on the

problem of the water dimer.

4. General Spectroscopy

Several projects are planned in the realm of large amplitude

motions such as the study of the torsional band of methyl alcohol and the

spectrum of methyl amine. These projects will serve to further test the

Hamiltonian models for rotation-vibration spectra for the case of large

amplitude internal motions.

43
' u

'
3. LASER PHOTOCHEMISTRY GROUP

D. F. R. Burgess, Jr., M. P. Casassa, R. R. Cavanagh, L. 3. Elwell,

E. J. Heilweil, M. E. Jacox, D. S. King, D. A. Mantell, J. C. Mialocq,

J. Perry, J. C. Stephenson, and A. M. Woodward

Introduction

Condensed Phase Energy Transfer

1 • T

.j (Vibrational Energy Relaxation) times for vibrations

on surfaces

2. T.| times for vibrations in liquids

3. T

1 for vibrations in solids

T

2 (Vibrational dephasing) times for vibrations in

liquids

Desorption of Molecules from Surfaces

Dynamics of van der Waals Molecules

Matrix Isolation Spectroscopy

1 . Fluorine-Atom Reactions

2. Excited Rare-Gas Atom Reaction

3. Vibrational and Electronic Energy Levels

of Transient Molecules

Reaction Intermediates in the Decomposition

of Energetic Materials

Future Plans
A. Introduction

This group primarily does quantum-state specific studies of molecular

dynamics. A common factor in the research described below is the role of

energy transfer in the spectroscopy and kinetics of molecules. Research

on molecular dynamics of molecules on surfaces is done in collaboration

with the Surface Science Division.

The following staff changes have occurred in the year since July

1984. The 1984 annual report mentioned five post doctoral fellows working

in this group. One of them (Woodward) remains at NBS as an NRC post-doc.

Two (Perry, Mantell) left NBS to take permanent jobs in the private

sector. Two ( Heilweil and Casassa) agreed to join the Molecular

Spectroscopy Division staff. Don Burgess (post-doc, Div. 541) came to

NBS in January 1985 to study thermal and laser-induced desorption of

molecules from surfaces. A visiting senior scientist, Dr. Jean-Claude

Mialocq from CNRS (Saclay, France), spent eleven months on sabbatical at

NBS where he did picosecond photochemistry and pulse radiolysis

experiments

.

3. Condensed Phase Energy Transfer

(M. P. Casassa, R. R. Cavanagh, L. B. Elwell, E. J. Heilweil,

J. Perry, J. C. Stephenson, A. M. Woodward)

The major effort in the condensed phase energy transfer program has

been directed at the study of vibrational energy transfer (VET) . During

the past year we have successfully measured the rates of energy transfer

from vibrationally excited chemical bonds in liquids, solids, and in

molecules bound to surfaces.

For simplicity, the experimental results on VET have been organized

in four sections: T^ times for surface vibrations; T-j for vibrations in

liquids; T-, for vibrations in solids; and ?£ times for vibrations in

liquids. This distinction among studies of VET done in various phases of

matter is not fundamental. At least for the non-metallic systems we have

studied so far, VET is caused by short range forces, and the similarity of

results for the same vibration in different phases is striking. This work

is supported by AFOSR

.

46
1. T-, (Vibrational Energy Relaxation) times for vibrations on surfaces

We used an infrared pump/probe method to do time-resolved measurements of the vibrational energy relaxation rate (T^

-1

) for the high

frequency modes of species chemisorbed on a silica (Si0 2 ) surface in a variety of chemical environments. Previously, energy transfer rates have

been measured for the vibrations of many molecules in the gas phase, and

for a few molecules in liquids and low temperature solids. However, our experiments were the first to measure vibrational energy transfer rates

for vibrations on surfaces.

Our initial measurements were on the surface hydroxyls (OH) bound to

dispersions of colloidal fumed silica (Degussa Aerosil 200, 120 A diameter

particles, 200 m

2 /gm surface area). We have since measured T

1 for several

different surface vibrations at the solid-vacuum interface and at the

solid-liquid interface. We explain the method for the OH-stretch (v=3750

cm

-1

) vibration; the same technique was used for all the vibrations we have studied. The surface hydroxyl v=0-M transition at the Si 02 ~vacuum

interface was excited with a strong picosecond IR pulse which transferred

up to 10 percent of the hydroxyls from the v=0 to v=1 level. Due to

anharmonicity, OH (v=1) does not absorb at the pump frequency, so excitation causes a transient increase in sample transmission, T. As

population relaxes, the sample transmission returns to its equilibrium

value, Tq; 2,n(T/T0 ) is proportional to OH (v=1) population. Recovery of

sample transmission was monitored by a weak, time-delayed probe pulse

derived from the saturating pulse and a variable optical delay. At each

time delay (t D ), the probe pulse transmission was measured to 0.3 percent

accuracy with pump excitation and with the pump blocked using a dual

pyroelectric detector energy ratiometer. A typical data run is shown in

Figure 3*1* The population in OH (v=1 ) decreases exponentially with a characteristic decay time of T-j = 216 ps.

The picosecond apparatus used in this investigation was based on a raodelocked Nd+ 3;YAG laser. Single 1.06 pm pulses of -35 mJ and 10 Hz

repetition rate pumped a chain of three LiNbO^ crystals (0PA) to

parametrically generate tunable infrared picosecond radiation, which had a spectral bandwidth of 15 cm

--1 FWHM, duration of 20 ps FWHM, and delivered

about 400 yJ of pump energy to the sample.

The room temperature results for several surface vibrations, at the

solid-vacuum and solid-liquid interface are given in Table 3-1* For all

these systems, the bulk substrate Si0 2 was the same (fundamental substrate

modes are less than 1200 cm

-1

). The T-] lifetime for 0K(v=1) on Si0 2 in

vacuum corresponds to roughly 2 x 10^ vibrational periods. In various

solvents the 0H(v=1) T-j lifetime decreases as the highest solvent

fundamental vibrational mode frequencies approach the 3700 cm

-1

OH^

frequency. This is particularly evident for CH-stretch (-3000 cm

-1

) containing molecules such as CH 2 C1 5 and C

5 H5 where T-; = 102 ± 20 ps and 87

± 30 ps, respectively.

47
t D

(ps)

Fig. 3-1* T

1 decay for isolated surface 0D(v=1) and 0H(v=1) on a dried

evacuated SiC^ disk at T=293K. The exponential recovery of

transmission as a function of probe delay (t D ) yields a lifetime of T

1 = 1 35 ± 1 0 ps for OD(v-l) and T-j = 21 6 ± 20 ps

(1o) for 0H(v=1) from a least squares fit.

In the presence of adsorbed ^0, which possesses a broad, partially

overlapping OH absorption with SiOH, T-| reduces even further to -56 ps.

In this case, direct V-V transfer from the surface OH to adsorbed water

may participate in the deactivation process.

Insight into vibrational energy transfer at surfaces may be gained by

studying other species bound to the silica substrate. In this way the

potential accepting modes of the bulk material remain unchanged while the

adsorbate local modes and adsorbate-substrate coupling are modified. The

simplest substitution for the OH group is isotopic exchange to 0D. In this

case T-j is reduced from 220 ps for SiOH to 150 ps for SiOD (as pressed

disks in vacuum) . Decay of the lower energy 0D quantum into the substrate

was expected to require fewer lattice quantum transitions than for OH

(i.e., Av 2 3 vs. Av > 4) and result in faster decay rate. However, the

observed isotope effect was smaller than anticipated.

The hydroxyl T-j lifetime for BOH modified silica (vQH - 3700 cm

1 ) in vacuum or CCl^ is found to be -70 ps. Since the boron mass and

coordination to the surface oxygens is different from silicon, changes in

the local site bending and stretching frequencies must occur. This

surface modification is found to substantially alter the relaxation

lifetime- of the surface OH species. Both NH-stretching modes of -NH 2 were

found to saturate to the same extent (AT/T - 5%) although time-resolved

measurements produced an instrumental pulsewidth-limited response. While

precise _ T-| values could not be deduced from these results, the two modes
apparently exchange population rapidly and have comparable lifetimes (£20

ps). Attempts to saturate the CH-stretching transitions of -OCH^ were

unsuccessful, however. This may imply very fast vibrational relaxation

(T 1 < 5 ps) for the CH-stretches of chemisorbed methoxy. The above results

for adsorbates on Si0 o suggest that increasing the complexity of adsorbate

vibrational level structure leads to more rapid vibrational energy

relaxation at the silica surface.

We have not yet directly measured where the energy goes when it

leaves the initially excited high frequency vibrations. To obtain

indirect evidence concerning the effective lattice accepting modes, we studied the isotope and temperature dependence of 0H(v=1) relaxation at

the SiC^-vacuum interface. The results are shown in Figure 3.2. The solid

lines show fits to the data using the theory of multiphonon relaxation in

solids. For OH, the observed T dependence is well fit by a n=4 quantum

process, i.e. VqH (3750) -* ^ v

i a ttice* For 0D data are best fit by

n= 3. i.e. vQD (2760) - 3vlattice .

200 400 600 800

Temperature (K)

Fig. 3.2 Decay rates, T-, _1

, for 0H(v=1) and 0D(v-1) on a dried evacuated

Si0 2 disk as a function of temperature T. Rates have been

normalized by dividing the OD rates by 1 / 1 -4 ^ ps = 6.7 x 10" s~ , and the OH rates by 1/220 ps = 4.5 x 10^ s~

1 . Error bars give

the 1o uncertainty in rate. The solid curves give the relative

multiphonon relaxation rates vs. T for an n-phonon process see

text)

.

49
Table 3.1: T-, vibrational decay times at roan temperature for the various

adsorbate/silica systems studied.

System v( cm

-1

) T-, (ps) k ( 1

0

9 s' 1

) k'(109 s

-1

) a Notes

SiOH/vacuum v0H = 3750 220±20 4.9 - Pressed Si0 2

disk

SiOH/CCl 4 voh =3690 1 59±1

6

6.3 1 .4 Dry Si02

dispersion

SiOH/CF2Br2 voh=3690 1 40+30 7.1 2.2 tf

SiOH/CH 2 ci 2

voh=3660 1 02±20 9.8 4.9 ft

SiOH/C 6H6

v0h=3625 87±30 1 1 .0 6.1 ft

SiOH/C6 D 6 v0h=3625 80±30 12.0 7.1 Tt

SiOH/H 2 0/CCl4 voh =3690 56±10 18.0 1 3.0 SiOH Tt

,

-5H 2 o/ 100A2

physisorbed

SiOD/vacuum vod=2760 1 55±1 6 6.5 ‘ OD T-j with

67% OH re- placed by OD

BOH /vacuum

BOH/CCl^

vOH= 3700 -70 -14.3 Early decay

time; long

time com- ponent present

SiNH2 /vacuum

SiNH2 /CCl4

vnh = 346o

3520

<20 >50 Pulsewidth

limited

signal, AT/T

-

5

% for both

stretches

SiOCH^/vacuum v=2860

3000

<5(?) > 2 80 ( ?) No pulse

saturation

observed for

any CHstretching

mode

a ui _ w vacuum

50
Acceptor mode frequencies compatible with this simple theory are also

consistent with the known frequencies of modes spatially near and hence

likely to be coupled to the surface 0H(v=1). For example, the Si-OH

stretch occurs at 970 cm

-1

, the Si-O-H bend at 795 cm

-

', and Si-0

stretching vibrations produce broad maxima at 800 and 1100 cm

- "' in the

vitreous silica phonon spectrum. The temperature dependence of the

multi phonon theory is inconsistent with energy going to the surface Si-OH

torsion vibration at 380 cm

-1 (or to SiOD at uu = 270 cm

-1

). The n=4

process for 0H(v=1) and the n=3 process for 0D(v=1 ) are in fact the lowest

order processes possible, in keeping with the propensity rule that the

most probable relaxation path will require the fewest simultaneous changes

of vibrational quantum numbers.

The observation that 0D on the surface relaxes faster than OH is

qualitatively consistent with the expected increase of T^ with increasing

energy gap between the excited level and the frequency of the accepting

modes. Multiphonon theories give the low T rate, Y(0), as Y(0) = aexp(-bu)) or Y(0) = a(e) n where to is the OH or 0D frequency, n the order

of the process, and the constants a, b, and e (e < 1) depend on molecular

structure. However, it is surprising that the increase is only a factor

of 1.5 (220 ps OH vs. 150 ps OD; e = .68) when the energy difference is

1000 cm

-1

. For comparison, measurements of the spin-orbit relaxation of

ions in crystals give values of e in the range 0.037 to 0.2 (i.e., the

rate for an n phonon decay is 5 to 27 times faster than an n +'1 phonon

decay in the same solid). We used a simple multiphonon relaxation theory

to estimate the relative rates for a 3 phonon (0D) and 4 phonon (OH)

relaxation of SiOp. If the accepting modes (u^ = 930 cm"*

1 ) are the same,

theory predicts Y(0) for 0D should be 13 times Y(0) for OH (e = .077).

Although theoretical estimates of the anharraonic coupling strength, and

hence of e, are uncertain, it seems difficult to reconcile the raultiphonon

theory to both the isotope and temperature dependence.

In addition to the multiphonon theory, we have considered other

mechanisms of VET for these surface vibrations. The damping of

vibrational energy is not due to coupling to an image dipole formed in the

substrate, nor is damping by electron hole pair formation possible for

vibrations on a dielectric like Si0 2

. Ffirster energy transfer (site

hopping) is not important because of the small transition dipole moments

(uqh = *035 D) and sparse adsorbate density (£ four 0H/100A2 surface).

For molecules like CCl^ and CF2 Br 2 » which have only low frequency

vibrations, VET to the solvents is unimportant, although the internal high

frequency modes of solvents like C^Hg may accept some of the energy.

Any theoretical approach to VET for vibrators on these dielectric

surfaces must also take account of the data available for VET in solids and

in liquids. Perhaps molecular dynamics simulations and classical

trajectory studies will provide a consistent method of understanding the

observed T^ values. Certainly a direct measurement of the pathways (i.e.

accepting modes) of VET will be of crucial importance to further the

understanding of this subject (c.f. Future Plans).

51
Spectral linewidths observed in surface vibrational spectroscopy have

sometimes been interpreted as uncertainty broadening. However, the

width of the absorption features on silica surfaces must arise from site

inhomogeneities or other dephasing processes, since the linewidths

corresponding to T-j are 200 to 2000 smaller than observed. Note that

similar remarks pertain to vs spectral linewidths for the solids and

liquids described in the next two sections.

2. T.| times for vibrations in liquids

Although there have been thousands of measurements of VET rate

constants for gas phase molecules where relaxation is caused by

bimolecular collisions, there is very little information available (< 20

measurements) about times in room temperature liquids.

Over the past nine years, picosecond laser techniques have been used

by several research groups (most notably Laubereau, Kaiser, et al., and

Chesnoy and Ricard) to measure vibrational energy relaxation times for

particular vibrational modes in room temperature liquids. Chesnoy and

Ricard measured T

1 for neat liquids of the simple diatomics HC1, DC1, HBr,

and DBr. They found that the liquid phase relaxation rate constants were comparable to gas phase rate constants when corrections for dimer

formation in the liquids were taken into account.

Kaiser and Laubereau (KL) measured T^ for the stretching vibrations

(vCH = 3000 cm

-1

) of eleven different hydrocarbons in solution (mostly

dilute in CCl^). They found T-| varying from < 2 ps (C 2H2 , CCl^H) to 100 ps

(CH^CClo). The dependence of T-j on molecular complexity was surprising

(i.e. why should a simple molecule with only one high frequency mode like

CCl^H relax rapidly while complicated molecules like acetone or trichloroethane relax 50 times slower?).

We have recently measured T-j for the OH and 0D stretching vibrations

of several alcohol and silanol molecules in dilute room temperature CCljj

solution using the same experimental technique as for the surface studies.

In part these experiments were done for comparison to the

results of KL. Of particular significance to our program is the

comparison between T-j for vibrations such as OH, OD, and NH2 bound to the

surface of a dielectric such as Si 02 , and T-| for these same functional

groups attached to a small molecule (dissolved in a liquid like CCl^ which

has vibrational frequencies comparable to those of Si0

2 ). If coupling to

nearest neighbor vibrational motions determines the VET rates, then those

small molecules may exhibit similar lifetimes to the surface systems.

The results we have obtained to date are shown in Table 3»2. T^ times

for the OH (-3690 cm

-1

) and 0D (-2720 cm* 1 ) stretching vibrations bound to

the Si atoms of silanols (in CCl^) are long. The T-| values are similar to

those for OH and 0D bound to the Si atoms on a silica surface either at

52
Table 3.2 Vibrat

al cone ional relaxation t

Is and silanols di

imes for OH(v-l)

lute in CClij at

and 0D(v=1) bonds of

7 = 295K.

Alcohols "l (ps) Silanols T-j (ps)

Me

3 C0H <15 <-

> Me^SiOH 205

Me

3 C0D <15 <—-> Me-jSiOD 2115

Et

3

C0H <15 <——> Zt^SiCH 135

—

t

3 cod <20 < -> Et 3 SiOD 224

60D <15 P^SiOH 206

c

6 f

5 °d <15 o-^SiOD

J 292

CH

3 0H <30 02^1 ( CH ) p a:'J J

cd

3 oh 73

CH -5 CD 52 $2Si(0D ) 21 RU

CD

3 0D 79

'-' 2^5 wH 70

Accuracy ~ r\a sjp

the vacuum interface or interacting with liquid CC1_ . for the surface

species in vacuum, CD relaxes 50% faster ti —1CU1C S 1-c.r.C-S

the OH relaxation rate is systematically slightly faster .US

The relaxation rates for OH ( -3620 cm ') and CD (-2680 cm

-

’) stretches in alcohols are much faster than for the corresponding silan:

For most .oo no Is *1 was too fast for us to measure accurately .7- < ‘5

ps); only for deuterated methanols and ethanol (OH) was 7. long enough tc be determined. For the four instances where we studied both silanols an: the corresponding carbinols (i.e. Si substituted for central 3), 7. is a: least twelve times shorter for the OH or CD groups when bound to 0 than

when bound to Sr. We are presently trying to interpret these results in

terms of coupling to specific adjacent vibrational modes and determine Ll these rates relate to the chemical orooerties of these molecules.

All the 7- results to date have been obtained with the one color I?, pump/ probe method. With the aid of a new low f-n umber Raman speotrometer

(acquired in August 1 985) , we hope to do further experiments on VET m

liquids, using the technique of viorationa_ excita: >n followed ov
spontaneous anti-Stokes Raman scattering (SPARS) probing (with harmonics

of the Nd-YAG) of population in particular vibrational levels of these and

other systems. For example, if we probe the OH, NH, or OD vibrations

initially populated by the IR pulse, then we should obtain the same T-j values deduced from the IR probe experiments. This will be an independent

verification of these results. Of more importance, the SPARS experiments

should permit us to identify the pathways of energy transfer (i.e., what

are the lower frequency accepting modes) during the relaxation. For

instance, using the IR pump/SPARS probe method, KL found that T-j for the

CH-stretch in ethanol is 40 ps, and we found T

1 for the OH stretch is 70

ps. If the CH stretching modes are transiently populated following

excitation and relaxation of the higher frequency OH-stretch, we should be

able to identify the rise and fall times of population in those modes. Of

course, by tuning our OPA to pump CH stretches, we could also

independently do the KL experiments.

3« T-j for vibrations in solids

The IR pump/probe technique described above was also used to

determine T

1 times for hydroxyl stretching vibrations in both amorphous

and crystalline solids. T^ times for hydroxyl groups coordinated to atoms

(ions) in a variety of crystals have been studied in order to assess the

importance of different local environments, including the mass of the

metal atoms (heavy atom effect) on VET. It may be easier to model, e.g.

via molecular dynamics simulations, the relaxation of bonds in these

crystalline materials than for liquids or amorphous solids like fused

silica.

The temperature dependence (T _< 1450K) of the relaxation rate (T^

-1

) of OH ( v=1 ) impurity oscillators inside fused silica is given in Figure 3.3*

Note that the intrinsic vibrational relaxation rate for the bulk OH at room temperature is twice as fast as that for the surface OH on Si02. The

T-dependence is perfectly fit by the multiphonon relaxation theory for an

n = 4 process, i.e. vqh (3690) ^lattice* The 0H impurity concentration in fused silica is so small (<_ 460 ppm) that no dependence of T-j on OH concentration was observed.

The room temperature relaxation times for 0H(v=1) ions inside samples

of crystalline mica have very recently been measured. In these crystals,

OH is part of the unit cell, and the interatomic distances, the vibrational frequencies and the orientation of the OH bonds with respect to the

crystallographic axes are known. In the mineral muscovite, formula [O^q

A l^ Si^ (0H)2 K]2» the OH groups have a frequency of 3630 cm

1 . There are

four equivalent hydroxyls per unit cell, located in a plane such that each

is hexagonally surrounded by 0 atoms, and each OH is coordinated to two A1

ions. For these OH groups, T^ « 1 00 ±20ps. In the mineral biotite, Mg

ions are substituted for some. of the A1 ions in muscovite, and two

54
T

to o

”o

?-

0)

CO occ

o

rex

«

<u cc

Fig. 3.3. Temperature dependence of the vibrational relaxation rate,

1/T.j, of OH ( v= 1 ) impurity oscillators in bulk fused silica.

The solid curves give the relative multiphonon relaxation rates

vs. T for an n-phonon process (see text).

different OH species (v=3660, 3590 cm""*) occur. For the 3660 cm

” 1 vibration, T-j = 210 ± 20 ps. These results are too recent to permit a detailed interpretation at this time. It should be emphasized that the

high frequency lattice vibrations in these crystals are similar to those

of fused Si 02 , and the measured T-j values are similar to the values found

for the surface OH on Si 02 , for the bulk OH in Si 02 , and for OH bound to

Si atoms of silanol molecules in liquid CCI 4

.

4. T

2 (Vibrational dephasing) times for vibrations in liquids

Picosecond coherent anti-Stokes Raman scattering (CARS) determined the

dephasing times (T2

) for vibrations in simple liquids. Dephasing rates

(2/Tp) were determined at T=295K for the symmetric stretch of CS 2 (v-|=65o cm

-1

) as a function of mole fraction in several solvents, and as a function

of temperature in neat CS2

. Also, the dephasing rate of the ring

breathing mode of benzene (v = 991 cm” 1 ) was studied as a function of its

dilution in CS2

.

Since neither the apparatus nor the results have previously been

described in an Annual Report, details of both will be given below. We do

not anticipate any further studies of dephasing rates. However, the

apparatus used for the T 2 measurements, with some modification, will be
used in several future experiments, e.g. measurements of for C0(v=1) on

Pt (111) and measurements of vibrational predissociation times for van der Waals molecules.

The apparatus is shown in Figure 3.4. Two dye lasers of independently

tunable wavelength (via 3 plate intracavity birefringent filters)

Fig. 3- Schematic diagram of apparatus used to measure T

2 for

vibrations in simple liquids by picosecond CARS.

are synchronously pumped by a modelocked Ar + laser operating at 82 MHz.

Tunable dye laser pulses at that repetition rate had about 0.6 nJ energy,

an autocorrelation width (FWHM) of 8 ps, and a spectral bandwidth (FWHM)

of 2.5 cm

- ”' . If the pulses were Gaussian, this would correspond to a AvAt

product of 0.42, very close to the transform limit. The width of the cross correlation was about the same as the autocorrelation. A cw modelocked Nd-Yag laser (scheduled for delivery in November 1985) will

replace the Ar

+ laser in future experiments. Uncompressed dye laser

pulses of 0.5 ps duration have been reported with (doubled) Nd-Yag

pumping.

In our CARS experiment, the frequency difference between the dye

lasers ( 10 = 11^ -u^) was chosen to be resonant with the vibrational mode to be

studied. Pulses at and 0) 2

, vertically polarized and overlapped in time

were incident on the sample at the phase-matching angle. The pair of pump

pulses creates a coherent vibrational excitation in the liquid at

frequency u)-,-<d2

. At a delayed time t D

a probe pulse at frequency o>-j

,

horizontally polarized, is sent through the sample in the correct

propagation direction for phase matching the anti-Stokes frequency

co^=2u)-| — (jl> 2

• A CARS signal (different in both frequency and propagation

direction from the other beams) is generated by this pump pulse. The

intensity (i.e. pulse energy) of the CARS signal decays in time after the

56
pump pulses as S(t

D ) * exp(-2t[)/T2) . The CARS signal, spatially and

spectrally filtered to eliminate scattered dye laser light, is detected by

a cooled photomultiplier operated in the pulse counting mode, and the

count rate is constantly stored in a computer. When necessary, neutral

density filters are inserted in the CARS beam to keep the count rate much

less than 82 mHz.

A typical experimental run to determine T

2 for the symmetric stretch

of liquid CS2 is shown in Figure 3»5. Counts were summed for 10 sec at each

delay stage position t D

.

0.00

• Dephasing rate cs

2 r, 656 cm' 1

b Response Function

<

Z

9 -5.00

c/)

10.00 J I -60.0 0.0 60.0

TIME DELAY (PS)

100.0

Fig. 3-5. Typical experimental run for the determination of the

vibrational dephasing time T

2 for the totally symmetric

stretching vibration v-,=656 cm

-1 of CS 2 in the liquid state.

The CARS signal decays exponentially over at least 10 e-folding

times, with a characteristic decay time T

2 /2 = 10.2 ± 0.2 ps. This result

for neat CS2 agrees with previous work. Also shown in Figure 3.5 is the

measured apparatus response function for this experiment. For t D > io ps,

the third order response function has a characteris ti c exponential decay

time (1/e point) of 1.8 ps. Therefore, if the CARS signal is sufficiently

strong, one may determine values of T

2 /2 almost as short as this response

time, despite the fact that the cross correlation FWHM is 8-9 ps. The

shortest T

2 we report below is 5.2 ps.

57
A summary of the room temperature dephasing rates of CS2(100) in

various liquid mixtures is shown in Figure 3.6. When CS2 is diluted in

CS

2 (% by volume)

Fig. 3-6. Room temperature dephasing rates (2/T2

) of CS 2 in the totally

symmetric vibrational state v

1 = 656 cm

-1 in various liquid

mixtures. The points designated by filled triangles (A) are for

the ring breathing mode of benzene (v = 991 cm

-

') in CS 2

.

CCljj, the dephasing rate decreases compared to the pure CS2 rate. On

dilution in benzene, T

2 for CS2 (100) is constant. Nitrobenzene, which

has a large permanent dipole moment, and ethanol, which can form hydrogen

bonds to other liquids, significantly decrease the dephasing rate of CS 2

(100). This is probably due to the stronger long range forces that these

molecules can exert on CS 2

.

Also shown in Figure 3*6 is the dephasing rate of the ring breathing

mode of benzene (v=991 cm

-

') in CS 2

. Our result for pure benzene (T2

= 5.2 ps) agrees with previous measurements. As benzene becomes more dilute

in CS2

, 2/T2 decreases. At low concentrations, so that each benzene

molecule is surrounded by CS 2

, T2 increases by a factor of 2.5 (T2

= 12

ps). This is completely different behavior from the relaxation of

CS2 (100) for which T

2 was independent of CS2 /benzene ratio.

We are presently working on the interpretation of these data.

Although the data in Figure 3-6 do not give straight lines, it nevertheless

appears that all the rates are consistent with an isolated binary

collision (IBC) expression of the form

2/T

2

- p x k ii g ii (o) + p (1-x) k

1 j g i j(o)

58
where x is the mole fraction of the molecule for which T

2 was determined,

p the liquid density, gi j ( o ) the radial distribution function at

separation o, and k the bimolecular dephasing rate constants

( cm^/molecules/sec)

.

We have also measured the dephasing rate of neat CS

2 as a function of

temperature from its freezing point at T=165K to T=298K. Over the range

21 OK < T < 300K the rate decreases; then as T decreases further, 2/T 2

increases from 8.2 x 10 10 s _1 at 21 OK to 11.2 x 10 10 s" 1 at T = 170K. These

time-domain data may be compared to earlier frequency domain results for

the isotropic Raman linewidth Av of CS 2 (100). In principle, 2 /T2 = Av

(FWHM). The agreement is only fair. In this case the picosecond measurements are probably more accurate, since the linewidths are narrow (hence

difficult to resolve), and since the slope of the line in Figure 3»5 can probably be determined with more accuracy than a Lorentzian can be fit to

the Raman spectral line. We are presently interpreting these temperaturedependent results.

These T

2 experiments have supported the T-j measurements which have

been done or are planned. Both T-j and T2 experiments in liquids (or at

the solid/liquid interface) can make use of similar theoretical treatments

(which we are now attempting). The T

2 experiments have played a crucial

role in developing and testing our ability to use the synch-pumped lasers

which (when amplified) will form the basis for many future T^

measurements.

C. Desorption of Molecules from Surfaces

(D. F. R. Burgess, Jr., R. R. Cavanagh, L. B. Elwell, D. S. King and

D. A. Mantell)

A dedicated experimental facility including surface sensitive analysis

(e.g., mass spectrometer, argon ion sputtering, LEED and Auger electron

spectrometer), computer controlled data acquisition and sub-Doppler (250

MHz) laser resolution is being applied to the study of the thermal

desorption of molecules chemisorbed on well characterized surfaces.

Results in past years have emphasized the N0/Ru(001) system.

In FY’85 measurements of the internal state distributions for NO

desorbing from a saturation coverage of NO on Pt (111) were published as a communication in J. Chem. Phys. Results for an extensive study of the NO

coverage dependence, over a ten-fold range in coverage, and the influence

of co-adsorbates has been submitted to J. Chem. Phys. The results of the

work on the N0/Pt(111) system may be summarized as follows: 1) the

rotational levels and spin-orbit states are characterized by a common

rotational temperature; 2) although the kinetic parameters describing the

desorption process change significantly with coverage and the presence of

co-adsorbed CO, the rotational degree of freedom of the desorbing NO is

always nearly fully accommodated with the surface temperature; 3) there

was no measurable alignment of the desorbing species.

59
The results of a series of thermal desorption experiments following

saturation coverage of NO on Pt (111) are shown in Figure 3.7. The solid

Fig. 3.7. Dependence of rotational temperature of NO desorbed from a saturation covered Pt (111) surface at various substrate

temperatures

.

curve is a thermal desorption spectrum obtained using a mass spectrometer

to measure the total NO desorption as a function of increasing surface

temperature. The data points (with 2 sigma error limits) give the

measured rotational temperatures describing the NO species desorbed during

intervals of 20 K (during a period of 2 sec) during the desorption heating

ramp. There are two distinct desorption features for the NO/Pt (111)

system and from this figure it becomes obvious that the rotational

temperature of the desorbing species follows the instantaneous surface

temperature for both features. All this data fits in the narrow range of

having the rotational temperature being equal to 95 ± 5% of the surface

temperature

.

Despite the fact that these results are consistent with the

expectations from transition state theory, such a theory would lead to the

additional expectation of a desorbing angular flux described by the cosine

law and a Maxwellian kinetic energy distribution at the surface

temperature. Preliminary results on Doppler profile measurements for NO

desorbing from saturation coverages from P t ( 1 1 1 ) indicate that these

expectations are not upheld. These measurements show equivalent Doppler

profiles for NO species desorbing in all final rotational states in the

range of 2.5<J<15.5, with energies from 13<E

r <426 cm

-1

, indicating no dependence of final kinetic energy on rotational state. Both the

60
rotational state distributions and the dependence of kinetic energy on

final rotational state are markedly different for the reactive N0/Ru(001)

versus the non-reactive N0/Pt(111) systems.

Thermal desorption samples processes occurring under "equilibrium"

conditions; this is to say that a heating rate of 1 0K/sec is much slower

than molecular energy transfer processes in condensed phases. We have

initiated a study of direct desorption processes following rapid heating

of the surface using non-resonant laser irradiation. Here it is hoped that

the short time scales for heating, i.e., 1-10 nsec, might allow desorption

to compete favorably with other energy exchange or relaxation processes.

Progress to date includes the assembly of an ultra-high vacuum chamber

designed for laser-heating/final-state resolved measurements. Initial

measurements have focussed on obtaining reproducible laser desorption

signals and achieving an understanding of the dependence of desorption

probability versus laser pulse energy. This work is supported by DOE.

D. Dynamics of van der Waals Molecules

(M. P. Casassa, L. B. Elwell, D. S. King, J. C. Stephenson and A. M.

Woodward)

These experiments represent a new direction of research, in part an outgrowth of infrared multiphoton photodissociation experiments. The

experiments consist of the characterization of the time scales and final

state distributions in the dissociation of van der Waals complexes excited

to well-defined vibrational or electronic states. The systems studied

during FY '85 included the electronic predissociation of NO-Ar, and the

vibrational predissociation of both NO-ethylene and NO-NO. The van der

Waals species were formed in a pulsed free jet expansion. Mass

spectrometric techniques have been used to characterize the cluster

distributions formed in our beam as a function of backing gas composition

and pressure. The dynamics studies consisted of a one-photon laser

excitation to a predissociating state, followed by measurements of the

rotational (J), spin-orbit (0), and lambda doublet (A

+ /A~) distributions

of the NO photoproducts via laser excited fluorescence of the A—

X

transition at 226 nm. The spatial distribution of J-vectors (alignment)

and the velocity distribution (both speed and angular flux) were

determined by polarization and Doppler techniques.

NO-Ar undergoes an electronic excitation to the NO-like 3so A state

at excitation frequencies near 4*1,200 cm

-1

. Although excited IT states of

the NO-Ar species are stable and exhibit discrete electronic absorption

spectra, the A

2

E state is distinctly different. This is a Rydberg state,

with an electronic orbital radius that extends well beyond the NO-Ar

separation in the electronic ground state. Thus, in the Franck-Ccndon

excitation process, the excited state created through the absorption

process is more analogous to N0+ -Ar, which should be more tightly bound

and have a shorter van der Waals radius. The result of this is that there

is a predissociative curve crossing involving the ground state potential

energy surface. The NO fragments formed in this predissociation are predominantly in the electronic ground state (>99?) and exhibit Doppler

61
profiles that indicate that the lifetime of the electronically excited

parent is substantially less than a rotational period — i.e., on the

order of or less than a few periods of the van der Waals vibration. Very

little of the total available energy appears as rotational excitation of

the fragments — only about 0 . 5%. Presumably this reflects a constraint

imposed in the conservation of angular momentum.

There has been substantial recent theoretical interest in the

vibrational predissociation of van der Waals molecules, but very few

experiments that have looked at either the final state distributions or obtained real time measurement of the dissociation lifetimes. Earlier

measurements (by Hoffbauer, Liu, Giese and Gentry and by Bomse, Cross and

Valentini) of the kinetic energy released in the dissociation of the

ethylene dimer excited at 950 cm

-1 to the ethylene vy mode indicated, in

consonance with theoretical expectations, that most of the available

excess energy went into rotational excitation of the two ethylene

fragments. In our experiments on NO-ethylene excited to the same ethylene-like Vy mode by our short pulse CO 2 laser we found very little

rotational excitation in the NO fragment and very little kinetic energy

released. Equal populations were found in the two NO lambda doublets.

This contrasts with the disequilibrium of lambda states found in the

products of the prompt electronic photodissociations of HOH and H0N0 and

in many direct collisional H-atom abstraction reactions. Presumably, most

of the available excess energy (i.e., roughly 50%) went into excitation of

a-axis rotation in the ethylene fragment. Doppler profiles were measured

for various orientations of the electric ' vector of the pump laser and the

directions of propagation of both the pump and probe lasers. Equivalent

Doppler profiles, of 0. 060cm-1

FWHM, were obtained for NO products under

all conditions. This observation requires the distribution of NO

velocities to be spatially isotropic, i.e, not aligned with respect to the

pump laser electric vector. Similar spatial flux isotropy was reported

for the vy predissociation of the ethylene dimer; spatially anisotropic

flux distributions have however been reported for prompt photodissociation

processes. Real time measurements of the appearance of the NO fragment,

in a dual laser pump/delayed laser probe experiment, proved that the

lifetime of the vibrationally excited van der Waals complex was less than

10 ns. The isotropic flux suggests that the vibrationally excited

complexes survive more than one rotational period. At the temperature of our beam, the average rotational period is -35 psec., implying a lower

limit of = 50 psec on the predissociation lifetime of the excited

complexes.

In the vibrational predissociation of the nitric oxide dimer,

following excitation by a frequency doubled short pulse C0 2 laser at 1870

cm

-1

, the total distribution of available energy was determined. The

rotational and spin-orbit states of both NO photofragments (for values of

internal energy <426 cm

-1

), are described by a rotational temperature of

110 K. This corresponds to only 20$ of the available excess energy going

into rotational excitation of the two fragments in dramatic contrast to

the results obtained on the ethylene dimer. The two lambda doublet states

were observed to be statistically populated and the relative populations

62
of the two spin-orbit states were consistent with a spin-orbit temperature

equal to the rotational temperature, indicating equilibration of the

electronic spin- and nuclear rotation angular momenta in this reaction.

This is an unusual instance, where a singlet parent dissociates to form

two doublet state fragments since in previous examples (e.g., including

CHoONO, HONO, and dimethyl methoxy phosphate) apparent selection rules

favored one spin-orbit component. Real time measurements of the fragment

appearance times indicated that the vibrationally excited complexes

dissociate within 10 nsec. We determined the NO fragment flux to be

spatially isotropic by measuring Doppler profiles at various experimental

configurations. This is similar to our results on ethylene-NO and

previous results on ethylene dimer and implies a lower limit on the

predissociation lifetime of - 50 psec.

It is possible for us to probe the photodissociation dynamics of the

NO-ethylene van der Waals complex following excitation to several

different vibrational states. Of particular interest are the results

following excitations at 1870 and 1900 cm

-1

to either the NO stretch or the ethylene (v^+vg) combination band, respectively. Here there are several lower frequency vibrational modes that might act as receptors for

the available excess energy -- on the order of 1500 cm

-1

. These mode

specific experiments are in progress. In addition, experiments are in the

planning stages (see section F) to determine the lifetimes of these

vibrationally excited species using psec lasers for excitation and

probing.

E. Matrix Isolation Spectroscopy

(M. E. Jacox)

The assembly and testing of the vacuum systems and the cryogenic

cell for use with the high resolution Bomem interf erometer have been

completed. Experimental studies using this apparatus are scheduled to

begin in September.

With the help of Dr. Thomas Buckley, Chemical Thermodynamics

Division, and Mr. Ricardo Metz, a summer worker in the Chemical Kinetics

Division, substantial progress has been made in the software development

for use with the Ebert visible-ultraviolet scanning monochromator. As

this is written, it is planned to resume the long-suspended studies of the

ultraviolet absorption spectra of transient molecules in the near future.

The work is supported by AR0.

1 . Fluorine-Atom Rections

A major effort was the preparation of a review paper, entitled

’’Spectroscopy and Photochemistry of Free Radicals Formed by the Reaction

of F Atoms with Small Molecules,” which has recently been published in

Reviews of Chemical Intermediates. This paper provides a comprehensive

review of both gas-phase and matrix isolation studies of the various types

of F-atom reactions (e.g., addition, H-atom abstraction) which have yielded

spectroscopic or photochemical data for the primary free radical products.

63
Because hydrogen-atom abstraction from a generalized molecule, RH, in an argon matrix leads to the formation of hydrogen-bonded R***HF, this review

has included an assessment of the extent of perturbation of the

HF-stretching vibration of R***HF species which results from trapping of

the complex in an argon matrix. Also included is a summary of shifts in

the molecular vibrations, observed in solid argon, upon hydrogen-bonding

of various types of molecules with HF.

Studies of the reaction of F atoms with methyl chloride, bromide, and

iodide were completed. Evidence was obtained for a steric effect on the

mechanism of F-atom reaction with these molecules, with two major reaction

channels for each system. Attack of the F atom at the halogen position

results in the formation of the CH^XF addition product, which has a moderately strong X-F bond and is photochemically stable at wavelengths as short as 250 nm. F-atom reaction with a hydrogen atom of the methyl halide

results in the stabilization of a weakly bound F***HCH

2x complex,

intermediate to the formation of H

2 CX***HF in the solid. For all of the

species studied except CH^Cl, the barrier to the decomposition of this

complex is sufficiently great to require exposure of the solid deposit to

visible light for the production of H

2 CX«**HF. A paper presenting these

results in detail is scheduled to appear in an October issue of Journal of

Chemical Physics.

Studies of the reaction of F atoms with formic acid were initiated,

with the objectives of determining whether the H-C02 structure has a ground-state potential minimum and of obtaining the spectra of the

important combustion intermediates cis- and trans-HOCO in solid argon.

(Previous studies in this laboratory had yielded the infrared spectra of

these two species in solid CO, with which HOCO would be expected to

hydrogen-bond.) Although some of the isotopic substitution studies,

useful for a final normal coordinate analysis, remain to be completed, the

following conclusions are evident:

(1) If the H-C02 structure has a ground-state potential minimum, it

is formed with too much energy for stabilization in this

system.

(2) Evidence for the formation of an HCOOH* **F intermediate when the

F atom attacks the OH bond of HCOOH is provided by the

appearance of a high yield of C0 2 ***HF, with little evidence for

isolated C0 2

, in the initial deposit.

(3) F atoms abstract H directly from the C-H bond of HCOOH, leading

to the stabilization of isolated HOCO, as well as of H0C0***HF.

(4) Trans-HOCO is more stable than the cis-rotamer.

Preliminary studies of the important F + SiH^ reaction system

indicated that extensive reaction occurs.

64
2.

Excited Rare-Gas Atom Reaction

The interaction of excited argon atoms with formic acid was found to

lead to the stabilization of trans-HOCO in solid argon. Studies of this

reaction system were extremely valuable in the detailed assignment of

absorptions in the corresponding F-atom reaction study to isolated HOCO and

to HOCO • • *HF

.

The interaction of excited argon atoms with silane was found to

provide a relatively "clean” yield of SiH 2 , with only weak contributions

of other products to the observed infrared spectrum.

3 . Vibrational and Electronic Energy Levels of Transient Molecules

A paper entitled "Ground-State Vibrational Energy Levels of

Polyatomic Transient Molecules," summarizing both gas-phase and inert

matrix data for almost 500 species containing from 3 to 1

6

atoms, was published in the October 1984 issue of the Journal of Physical and

Chemical Reference Data. In keeping with the rapid pace of new

developments in this field, on May 1 a supplement containing data for 76

molecules which had appeared in the literature since August 15, 1984, was prepared for distribution with the reprints. An up-to-date digital record

of developments in this fast-moving field is being maintained, and in one

or two years it is planned to publish a supplement.

A proposal entitled "Spectroscopic Properties of Excited Electronic

States of Small Polyatomic Transient Molecules" was submitted to the

Office of Standard Reference Data, and beginning on September 1 will be

supported for a period of two years. The output of this project will be a critically evaluated compilation of the experimentally determined

spectroscopic properties of the bound and dissociative excited electronic

states of covalently bonded transient molecules containing from three to

six atoms. Spectroscopic data observed for transient molecules in the gas

phase and 'in dilute solid solution in the rare gases or in non-polar

small-molecule solvents will be included. The extent of deviation of the

transition origins, vibrational fundamental frequencies, and radiative

lifetimes of transient molecules observed in these media from the values

reported for the gas-phase species will be assessed; in order to provide

an estimate of the expected deviations for molecules for which gas-phase

data are not available.

4. Reaction Intermediates in the Decomposition of Energetic Materials

With support from the Army Research Office, studies of the infrared

and ultraviolet spectra of reaction intermediates derived from the

decomposition of benzene and nitrobenzene are being pursued.

Spectroscopic identifications are also being sought for a number of other

small free radicals which play important roles in the decomposition of

explosive or propellant molecules.

65
Preliminary infrared studies have been made of the photodecomposition

of benzene isolated in solid argon by 254-, 122-, and 105-1 07-nm

radiation. There is evidence for the stabilization of at least four

products:' phenyl radical, benzyne (C^H^), and two or more other species, one of which photodecomposes when the sample is exposed to the full light

of the medium-pressure mercury arc.

Preliminary studies were also conducted of the photodecomposition of

nitrobenzene isolated in solid argon, using the highly dilute

Ar: nitrobenzene = 2000 samples which could be prepared in a conventional

sampling system. Photolysis sources were 254-nm mercury-arc radiation and

a beam of excited argon atoms, which provides a source of energy in the

11.5-11.8 eV range. Infrared absorptions of new products did not appear

in either system. Previous studies had indicated that in the 254-nm

spectral region the most important photodecomposition process is 0-atora

detachment, but that some N02 is also produced. No photodecomposition

products were detected in the preliminary experiments. Since 0 atoms can diffuse from the site of their photoproduction in solid argon, failure to

detect nitrosobenzene or 0-atom reaction products suggests that previous

studies may have overestimated the extent of 0-atom photodetachment in the

primary decomposition of nitrobenzene. Phenyl radical and N02 , products

of the other primary photodecomposition process, would be expected to

undergo cage recombination to re-form nitrobenzene or, possibly, to form

the previously unobserved species phenyl nitrite. The negative results

obtained in these preliminary studies are most consistent with the

formation and recombination of phenyl radical and N0

2

. Experiments with

more concentrated nitrobenzene samples will be necessary in order

definitively to exclude the formation of nitrosobenzene and/or phenyl

nitrite under these conditions. Construction of an auxiliary vacuum manifold to permit higher concentration studies of low vapor pressure

materials is in progress.

In a search for infrared absorptions of the H

2 CN free radical, which

may play an important role in the decomposition of RDX and HMX, studies of

the infrared spectra of the products of the H + HCN reaction were

initiated, using a microwave discharge through an Ar:H

2 mixture as the

H-atom source. Rather weak infrared absorptions of CN and of three other

products were detected. Positive identification of these three products,

which differ in their photodecomposition thresholds on exposure of the

sample to filtered mercury-arc radiation, must await the completion of

planned isotopic substitution studies.

F. Future Plans

1. Condensed Phase Energy Transfer

The most significant questions for us to answer in the area of energy

transfer for molecules on surfaces are: (1) What is T-j , for a molecule on

a metal surface? (2) What are the pathways of energy transfer (i.e. where

does the energy go)? ‘(3) How is VET at surfaces related to chamistry?

Experiments are planned to answer these questions.

66
We have measured vibrational relaxation rates for high frequency

vibrations on dielectric surfaces at the solid/vacuum and solid/liquid

interface, in liquids, in amorphous and crystalline dielectric solids. We

do not have one simple theory to interpret all the results in a consistent

fashion. It appears qualitatively that one may interpret all the results

in simple terms of masses connected by springs. However for vibrational

relaxation on a metal surface, it is thought that a qualitatively different energy transfer mechanism, damping by excitation of electron-hole

pairs in the metal, may be dominant. If this is true, then perhaps the

broad spectral linewidths observed for high frequency vibrations on metals

really are due to broadening as has often been assumed. A direct time

resolved measurement of vibrational relaxation for a molecule on a metal

surface is crucial to assessing the importance of this damping mechanism,

and for understanding surface spectroscopy and kinetics.

We have several different strategies for measuring for a molecule

on a metal surface. One approach is to perform our usual one color IR

pump/probe time-resolved bleaching experiment for molecules on supported

metal clusters. Another approach is to use the same technique for ordered

monolayers on large metal single crystals under UHV conditions. A third

approach involves IR excitation of a molecule bound to the surface of a metal (e.g. Au) particle suspended in liquid as a sol; this would be

followed by anti-Stokes Raman scattering (surface enhanced) as a time

dependent probe of vibrational population. These experiments require

infrared laser pulses of suitable energy, 'bandwidth, frequency, and

duration. The present Nd-Yag pumped OPA setup has several important

limitations. Only frequencies vIR > 2600 cm“ 1 can be generated, which

restricts us to high frequency vibrations. However, because of its large

transition dipole moment, and because of the wealth of surface information

available, we want to study CO on metals. This requires = 2000 cm

-1

, not possible with the present setup. Also, the pulse duration of 15-20 ps

with the present system is too long for many experiments (e.g. NH2 on

Si0 2 , and probably CO on metals). The broad spectral bandwidth from the

OPA is also undesirable for many reasons (e.g., for the experiment to work

an extremely large anharmonici ty is required)

.

Therefore, in future T^ experiments we will produce tunable IR

radiation by generating the difference frequency , cj tr = u-]-w?, between

two visible laser pulses in non-linear crystals. For frequencies a, TR > 1800 cm

-1

we will use LilO^ and for lower frequencies 900 < < 1800 we

have tried AgGa

S

2

. We are trying two approaches to generate the necessary

visible pulses to be' used for mixing. At a 20 Hz repetition rate we will

amplify the outputs of both synch-pumped tunable dye lasers so that each

has an energy of 0.5 mJ/pulse (as of 7/85 one of the two necessary

amplifiers was operational). An alternative is to double the Nd^rYAG

light to synchronously pump a dye laser cavity. The dye laser output

should be of significantly shorter duration than the 532 run pumping

pulses. The tunable IR, generated by difference frequency mixing (u

w532 ~ wdye) should have the duration of the input dye pulse. 3oth

67
difference frequency schemes should be capable of generating 1-10 pJ

pulses in the 5 urn region. This should be sufficient to perform

experiments described below.

It is conceptually straightforward to do the one color IR pump/ probe

transmission IR experiment to measure T-, for CO (v=1 , u>= 2000 cm

-1

) on dispersed Pt particles supported on a Si02 disk. The transmission IR

spectra of such samples have been extensively studied over the last 30

years. During the past year we have attempted this type of experiment for

higher frequency vibrations (3000-4000 cm

-

^ ) of molecules chemisorbed on dispersed metals (M-0H, M-NH, M-C2H2 ) without success. Two major problems

- sample damage by the pump pulse and excessive scattering of pump light

into the probe detector - have been solved by improved sample preparation.

However, it is difficult to chemisorb these species in sufficient

concentrations for study. Previous work shows that this should not be a problem for CO on metal surfaces. Also, molecules which absorb in the

high frequency range tend to have relatively low vibrational transition

dipole moments compared to CO chemisorbed on metals. Thus CO/metal

systems offer important advantages. We expect in 1986 to perform the

simple pump/probe experiments to measure T-j for C0(v=1) bound to a variety

of dispersed supported metals.

In principle the IR transient saturation experiment is applicable to

any sample for which ordinary IR absorption spectroscopy is possible,

including monolayers on metal single crystals. For most vibrations on single crystals, infrared reflection-absorption spectroscopy (IRAS)

requires the detection of very small absorptions, typically 2 x 10

-24

. With our present detectors and poorly behaved 0PA pulses, we can only

detect transmission changes greater than 0.3?; to determine the slope of

the In (T/Tq) plot (i.e. T-| ) presently requires a change in T (caused by

the pump) of at least 1.5?. The present apparatus thus has two orders of

magnitudes less sensitivity than that necessary to measure T

1 for typical

surface vibrations. We expect that our sensitivity can be significantly

improved since other laboratories have claimed 1 part in 10^ using pulsed

lasers. In fact, for CO on metals, sensitivity limitations are not as severe since absorptions of 1-2$ are usually reported in IRAS experiments.

We are working toward attempting to measure T^ for CO on P t ( 1 1 1 ) or related metal single crystal surfaces using the IR pump/probe method.

Better laser pulses, better pulsed IR detection and signal averaging, and

significant additional surface preparation and characterization (e.g. high

resolution IRAS capability) equipment will be needed.

A direct measurement of the pathways (accepting modes) involved in

the VET experiments requires a determination of population in different

sample vibrational modes. Time-dependent Raman or IR spectroscopy may be

used. For samples of good optical quality, such as colloidal silica

particles suspended in CCl^, or for the liquid samples of R^Si-OH

molecules (which may be models for the surface vibrations), we will

perform IR pump/SPARS probe experiments (using either OPA/532 nm or IR

difference frequency systems) in 1986. To decrease background signals

from the thermal population in very low energy modes the samples may be

68
cooled (as done for the CS2 T2 experiments). It is likely that other

samples (pressed disks, dispersed supported metals) will scatter too much

visible light to permit detection of the expected weak SPARS signals. For

these samples 2 color IR transmission spectrosocpy (pump one mode, probe

another) may be used. This approach is likely to be successful only for

a few surface species where energy flows from one high frequency vibration

to a neighboring high frequency mode before dissipating to the low

frequency (v < 1200 cm

-1

) lattice modes. As an alternative to SPARS

probing of vibrational population (e.g. OH v=1 ) , we may try CARS probing.

The CARS method has the advantage of generating more detected signal

photons per pulse, but the disadvantage of requiring large excited state

densities and three coincident laser pulses.

To date our surface T.| measurements have been for molecules on collidal Si02 , a surface which has little catalytic activity. One way we

hope to explore the relation between VET and surface chemistry is to study

T-, for systems with important catalytic activity. For instance, the

hydroxyl groups inside zeolite catalysts are associated with active

BrCnsted acid sites, and there have been many transmission IR studies of

these OH sites in the 3700 cm

-1 region. In collaboration with Dr. R.

Wormsbecher at W. R. Grace (Columbia, MD) we hope to measure T^ times for

OH stretching vibrations in a number of zeolites of different catalytic

activity.

2. Dynamics of van der Waals Molecules

The same synch-pumped picosecond laser apparatus which will generate

IR pulses suitable for studying VET rates for bonds on metal surfaces,

will also generate pulses suitable for doing VET studies involving the

photodissociation of van der Waals molecules. An example of a van der

Waals experiment which we should do in 1986 is the photodissociation of

N0-C2H 4 by vj R

= 1900 cm

-1

1900cm-1

t C

2H 4 -N0 —> C2

Hi|( vy+vg) *N0 ——> C 2Hh + NO (J,G,A)

1870cm 1 t

> C

2H4*N0(v= 1) ——-> C

2

Hj4 + NO (J,0,A)

There is a significant controversy about the value of t, the predissociation lifetime. The photodissociation spectrum in the C

2 H^ vy region

is a Lorentzian (FWHM = 6cm

-1

), which has been interpreted as implying x * 0.9 ps. Our time-resolved measurements of product appearance using

nanosecond lasers proved that x < 10 ns. However our measurement of

isotropic flux distribution suggested that x £ x r , where the rotational

time of N0*C2H^ in our beam is xr

=* 35 ps.

We will generate tunable IR at 1900 or 1870 cm

-1

by generating the

difference frequency in LilO^ of the two amplified synch-pumped lasers.

One of them (at X = 575 nrn) is frequency doubled; the X = 288 nn is summed

with Nd-Yag (1.06 urn) in a KD*P crystal to generate light at 226 nm (P

1 head = 44176 cm

- "

1 ) suitable for LEF detection of the NO photoproduct on

69
the A<-X transition. Measurement of the NO LEF signal vs. pump/ probe delay

time (t D ), will determine t for excitation of C 2H4 (vy+vQ)*NO at 1900 cm

- ' compared to C

2 H4 *N0 (v= 1 ) at 1870 cm

-1

. It will be interesting to see if

the product yield has an exponential build-up as N0(t) = [ 1 -exp(-t/-r) ] , as predicted by statistical theories, and to see if there is any relationship

between t and photodissociation line widths. Excitation of the out-ofplane Vj motion which corresponds to the reaction coordinate, may give a short t , while there may be a significantly longer time for NO stretching

energy to flow into the van der Waals bond leading to dissociation.

Similar experiments are planned in which C-H stretching modes of van der

Waals clusters will be excited with picosecond IR pulses. The necessary 3 um light will be produced by the same dye lasers by difference frequency

generation.

3 . Desorption of Molecules from Surfaces

Several years ago we undertook a joint research project involving

staff and expertise of the Molecular Spectroscopy and Surface Science

Divisions of the Center for Chemical Physics to examine in greater detail

molecular dynamics at the gas-surface interface. The thrust of this

program was to use laser based, state and velocity selective diagnostics

to study molecules being thermally desorbed from well-characterized single

crystal metal surfaces in an ultra high vacuum environemnt. The results

of our work on N0/Ru(001) and NO/Pt (111) have been published and are reviewed herein. What is important about these experiments is that they

are extremely, perhaps uniquely, sensitive to the interactions between gas

molecules and different metallic surfaces.

The future directions of this research effort can be broken up into

two branches. First, in view of the marked differences for NO/Pt, where

there is only simple molecular desorption, versus Ru, where decomposition

competes with desorption, during FY ’86 we plan to explore the desorption

of NO from 1) co-adsorbed layers of NO and ammonia on Pt (111) where there

appears to be a co-operative interaction during the desorption process; and

2) Ir (111) where a major source of the desorbing NO is from atom-atom

recombination. Although all our present work has emphasized interactions

with the NO adsorbate, we hope to acquire a second dye laser to enable us to extend our laser probe capabilities into the vacuum ultra-violet and

study the molecule-surface dynamics of other, chemically interesting

species such as hydrogen and CO.

Thermal desorption studies are sensitive to the molecular dynamics

and energy flow in the region of the top of the chemisorption potential

energy well - a region important in the making and breaking of chemical

bonds. In the thermal desorption experiment the heating rates

traditionally used are much slower (e.g.'IO K/sec.) than other molecular

relaxation processes, which is good if one’s goal is to study the dynamics

of a highly energetic system under "equilibri um” conditions. A deeper

level of understanding can be obtained regarding molecule-surface energy

transfer by attempting to study non-equiblibri um conditions, such as might

be achieved following laser irradiation of the adsorbate covered surface.

70
We hope during FY'86 to successfully characterize the internal state

distributions, kinetic energy and angular flux distributions of the NO

driven off a P t ( 1 1 1 ) surface by non-resonant laser excitation at 532 nm. Estimates of the anticipated signal levels for desorption of 1 0

-i| of a monolayer per laser shot indicate that we should be able to determine the

vibrational as well as rotational partition functions. These experimental

results will provide a stringent test of the theoretical treatments

currently being applied to model laser-surf ace heating and energy

transfer processes.

4. Matrix Isolation Studies

The high sensitivity and extended spectral range afforded by the

Boraem interferometer will be crucial to the study of a number of important

systems during the coming year. Among the first systems to be studied

will be the photodecomposition- of acetylene to produce HC 2 and C]?.

Electronic transitions for both have recently been reported near 4000

cm

-1

. Observation of the absorptions resulting from these electronic

transitions in a rare-gas matrix will aid in the definitive identification

of the Z ~2 transition and in the assignment of the bands arising from

vibrationally unexcited HC 2 , which has an unusually low ground-state

bending vibrational frequency.

Priority will also be given to the studies of the H + HCN reaction,

in which the greater sensitivity of the Bomem interferometer will be

valuable for the definitive assignment of the infrared spectra of H

2CN and

of cis- and trans-HCNH. A search will also be made for near infrared

transitions of these species, each of which, like the isoelectroni

c

species HCO, may have a low-lying electronic transition.

As reported one year ago, the feasibility of detecting the infrared

and electronic spectra of molcular ions produced by codepositing a beam of

excited neon atoms with a suitable precursor molecule isolated in solid

neon has been demonstrated. The energy range accessible using excited

neon atoms (16.6-16.8 eV) will substantially broaden the range of

molecules whose ions are accessible over that covered in earlier

experiments in this laboratory using excited argon atoms, permitting a search for such important ions as HC0 +

, H

2 0

+

, NH^, and CHjJ. These

long-planned studies have been deferred until they could be conducted on th

Bomem interferometer, since the enhanced sensitivity of this instrument is

especially important when liquid helium must be used as the coolant. It

is hoped that studies of these experiental systems can proceed during

FY r 86. Such studies may provide much previously inaccessible information

on the vibrational and electronic spectra of molecular ions, important in

a variety of high energy systems ranging from combustion to high-speed

flight

.

The higher sensitivity of the Bomem interferometer will permit

studies of such other important classes of molecule as carbon-1 3 substituted species present in natural abundance and products of C-atcm

reactions in discharge sampling experiments. Observations of the

71
carbon-13 species will aid in product identification and will provide

valuable data for infrared spectral analysis. Since water and oxygen

impurities generally lead to the formation of 0 atoms in discharge

sampling experiments, the greater sensitivity of the Bomen inteferometer

will both facilitate and mandate a series of studies of the reactions of 0

atoms with small molecules. Such studies of the reactions of 0 atoms with

benzene and with nitrobenzene form an important part of the current

program of research being supported by the Army Research Office.

Preliminary studies have indicated that species of formula HCN

2 are stabilized by the reaction of photocheraically generted CH with a nitrogen

matrix. Further studies will be conducted in order to obtain a definitive

identification of this product, which may play important roles in

processes as diverse as the decomposition of explosive or propellant

molecules and the generation of "prompt" NO on atmospheric combustion of

hydrocarbons.

In order to obtain as complete as possible a characterization of the

spectra of the products of elementary reactions of silane, studies will be

continued not only on the reaction of excited argon atoms and of F atoms

with silane, but also on the corresponding H-, Cl- and 0-atom reactions.

Because F-atom reaction studies have provided an extremely valuable

tool for obtaining free radical spectra, they will continue. Emphasis will

be placed on studies of the ultraviolet absorption spectra of the products

of F-atom reaction with such species as CH

3 NO 2 , CH

3 CHO, CH^CN, and HCOOH,

for which infrared studies have provided definitive identifications of

free radicals.

As it becomes possible to stabilize a wide variety of molecular ions

and O-atom reaction products in rare-gas matrices, it will be increasingly

important to determine their laser-excited fluorescence spectra, in order

to provide a more complete characterization of their energy levels and to

establish a basis for their detection in atmospheric and combustion

systems. This need will mandate construction of a lasei—excited

fluorescence system dedicated to matrix isolation studies. This system

will also permit unique experiments to determine previously inaccessible

low-lying molecular electronic states; cage recombination of molecular

fragments formed on laser photodissociation of the matrix-isolated

molecule may populate such low-lying states, which may then be deactivated

by fluorescence or phosphorescence.

12
4.

QUANTUM CHEMISTRY GROUP

H. 3asch, P. Jasien, P. S. Julienne, M. Krauss,

F. H. Mies and W. J. Stevens

Introduction

Electronic Structure

1 . Theoretical Proton Affinities

2. Hydrogen Bonding of Water and Methanol

with Formamide

3. Localized Electron Pair Polarizabilities

4. Binding of Pt Complexes to DNA

5. Reaction Field Methods

6. Relative Energetics of Dipeptide Conformers

Scattering in the Presence of Radiation

1 . Radiative Redistribution

2. Application of Generalized Multichannel Quantum

Defect Theory (MCQDT)

3. Product Distributions in Molecular Photodissociation

4. Atomic Lineshapes and Resonance Fluorescence Spectra

a. Continuum-continuum Matrix Elements in Atomic Lineshape

Theory

b. Impact Limit in Weak Laser Fields

c. Impact Limit and Pressure Broadening of Mollow

Sidebands in Intense Fields

Multiphoton Effects in Resonance Fluorescence Spectra

Future Plans

73
A. Introduction

Atomic and molecular electronic structure and scattering methods are developed and applied to chemical and spectroscopic problems where

experiments are difficult to perform or so complicated that the theory is

needed to model the results.

The electronic structure method and code development and applications

now include chemical problems with large molecular systems including models

of biomolecules. The applications to the biotechnology area utilize the

recently acquired IBM-4381 in the Molecular Structure and Modeling Facility

(MSMF ) and are described in part in the competence summary.

B. Electronic Structure

1 . Theoretical Proton Affinities

(W. J. Stevens, P. Jasien)

A combination of several state-of-the-art ab initio quantum chemistry

methods has been applied to the quantitative determination of the proton

affinities of small molecules containing the Group VI elements, 0, S, Se, and

Te. Several series of molecules have been studied including CX, CX 2 , OCX,

and H

2 CX (where X is the Group VI atom). Comparisons with available

experimental data are very good, and the estimated accuracy of the

theoretical results is ± 2.5 kcal/mole. These are the first such accurate

calculations ever carried out for compounds containing selenium and

tellurium. In addition to the proton affinities, trends down the Periodic

Table are now available for favored protonation sites, vibrational frequency

shifts upon protonation, and structural distortions due to protonation.

Compact effective potentials (CEP), which were developed at NBS, allow

the chemically unimportant core electrons to be eliminated from the molecular

wavef unctions with little loss of accuracy. Thus, all of the Group VI

elements were treated as six electron atoms. Relativistic effects are incorporatd through the use of Dirac-Fock atomic wavef unctions in the

determination of the core potentials.

Full analytic energy gradient geometry optimizations of all neutral and

protonated species were carried out at the single configuration

self-consistent-field (SCF) level using accurate n double-zeta plus

polarization" basis sets. At the optimized geometries, harmonic force

constants were determined from numerical first derivatives of the analytic

energy gradients, and zero-point vibrational energy contributions to the

proton affinities were calculated. Electron correlation energy contributions

were evaluated by configuration interaction calculations including all single

and double excitations from the single conf i gur ation SCF wavef unct ions

.
This work is described in: P. G. Jasien and W. J. Stevens, J. Chem.

Phys. 83, 2984 (1985).

2. Hydrogen Bonding of Water and Methanol with Formamide

(W. J. Stevens and P. Jasien)

The characterization of the hydrogen bonding between water and formamide

has been of considerable interest to experimentalists and theoreticians

alike. This interest stems from the fact that formamide may be viewed as the

simplest molecule containing a peptide linkage. As such, the formamide-water

complex provides the simplest model for the hydration of proteins.

We have carried out detailed calculations on several conformers of the

formamide-water complex at a level of accuracy significantly beyond all

previous theoretical studies. Full gradient energy optimizations using

double-zeta plus polarization basis sets were carried out for each conformer.

The effects of electron correlation were determined by configuration

interaction including all single and double excitations from the SCF

configuration.

These accurate calculations show hydrogen bonding of water to the

carbonyl group to be favored by about 0.5 kcal/mol over bonding to the amide

group. However, a cyclic structure in which the water is hydrogen bonded to

the carbonyl and amide groups simultaneously is found to be most stable (9.5

kcal/mol) and is a likely candidate for molecular beam microwave spectrocopic

studies.

Fig. 4.1. Most stable hydrogen-bonded complexes of (a) water-formamide and

(b) methanol-formamide illustrating near coincidence of the two

structures.

Calculations were also carried out on methanol-formamide complexes to

determine if any fundamental differences exist in the hydrogen bonding of the

alcohol versus the water. The results show virtually identical structural

75
and energetic behavior of the methanol-formamide complexes when compared to

the water-formamide complexes. Again, the cyclic, doubly hydrogen-bonded

conformer is the most stable. Figure 4.1 shows the striking similarity of the

two cyclic structures.

The systematic study of basis sets, geometry optimizations, and

correlation energy for these hydrogen bonded complexes have allowed us to set

up general guidelines for future calculations of this type.

3. Localized Electron Pair Polarizabilities

(W. J. Stevens, P. Jasien)

The coupled, perturbed Hartree-Fock method (CPHF) has been used to

calculate the dipole polarizabilities of several prototypical non-aromatic

molecules containing H, C, 0, and N. The list of molecules studied and the

principal components of their calculated polarizability tensors are shown in

Table 4.1. All calculations were carried out with triple-zeta basis sets

augmented by two polarization functions on each atomic center. The resulting

polarizabilities are very close to the Hartree-Fock limit of accuracy, which

is generally within 15$ of experimental values for the isotropic part.

It is possible to transform the Hartree-Fock molecular orbitals, via a unitary transformation, into localized orbitals that can be identified as bonds and lone pairs. The same unitary transformation may be applied to the

orbital components of the CPHF molecular polarizability. In this way,

electron-pair polarizability tensors may be derived that are transferable

from molecule to molecule. Preliminary transferability studies have been

made using average bond and lone pair polarizabilities extracted from the

saturated molecules listed in Table 4.1. Table 4.2 compares spherically

averaged CPHF polarizabilities to polarizabilities obtained as a sum of

transferable bond and lone pair components. The agreement is within a few

percent. Anisotropic behavior and unsaturated bonds are still under

investigation but show similar promise. A functional group analysis (e.g.

methyl group, hydroxyl group, amide group) may be required in order to take

full advantage of anisotropic tensor contributions.

Transferable polarizabilities are a key ingredient in the ab initio

reaction field method being developed by the Quantum Chemistry Group. A

reaction field computer code will allow the study of very large molecular

systems by treating a small portion fully quantum mechanically while

representing the surrounding environment by a distribution of polarizable

point multipoles. The distribution of the point multipoles and their

polarizabilities will be determined from ab initio calculations on prototypical molecules and fragments whose properties are assumed to be

transferable.

76
Table 4.2. A comparison of CPHF molecular polarizabilities and

polarizabilities constructed from transferrable average

bond and lone-pair contributions.

Molecule aCPHF< A3) aTRANS( a3)

ch4 2.28 2.27

ch

3 oh 2.87 2.98

c

2h

5 oh 4.58 4.63

C 2H6 3-99 3.95

c

3 h8 5.68 5.62

c

2h6 o 4.58 4.73

4. Binding of Pt Complexes to DNA

(H. Basch, M. Krauss, W. J. Stevens)

The binding of the Pt diamine moiety to DNA was found to occur

predominantly in a cis intrastrand chelate between neighboring guanine bases.

The stereospecificity of the binding was recently called into question by the

observation of anti-tumor properties for both cis and trans Pt diammine

ascorbate molecules. Calculations of the relative trans effects and binding

energies for the carbon and oxygen ligand binding sites have shown that the

carbon site is less labile than the ammonia ligand. As a result cis chelates

can be formed with the DNA in reaction with either the cis or trans Pt

ascorbate molecules.

Substituted amines have also been considered in the binding of

/ 2 + Pt(R-NH2

) 2

to one or two imidazole ligands. The imidazole is used to model

the N7 binding site in guanine. The binding energies for R=CK

3 or cyclopropyl are found to each reduce in turn by 6J from the unsubstituted

values for R=H. These relatively slight variations in 'binding energy are not

expected to play a role in the binding behavior of the substituted Pt amines

to DNA. St eric effects are now being modeled to analyze the effect of the

aliphatic substitution. Preliminary results indicate that steric effects are not significant for R = CH^ or cyclopropyl. This work is supported by the

National Foundation for Cancer Research.

77
5. Reaction Field Methods

(W. J. Stevens, H. Basch, M. Krauss)

All-valence-electron quantum mechanical self-consistent-field (SCF)

calculations are limited to relatively small prototypes of bioraolecular

systems. Using the compact effective potentials (CEP), calculations are possible at the double zeta level of basis set for molecules with 30 to 40

atoms. Systematic generation of the CEP for the elements between Na and Rn

has been continued so that calculations including metal atoms can now be done

with a consistent, accurate set of CEP. Nontheless, calculations on systems

of the order of 1000 atoms are desirable and the reaction field method will

approximate the SCF for such a system. The system is divided into two

regions. In the quantum mechanical (QM) region all valence electrons are treated explicitly, while in the classical (CM) region, the charge

distribution contributes to the SCF hamiltonian through the interaction of

localized molecular orbital charges, moments, and polarizabilities centered

at the orbital centroids of the QM charge distribution. At the boundary

between the QM and CM regions, effective fragment potentials (EFP) emulate

the exchange and orbital exclusion of the orbital paris that are treated

classically. Approximate EFP have been generated for the interaction of

non-bonded systems and are now being tested for multimers of formamide and

imidazole.

6. Relative Energetics of Dipeptide Conformers

(M. Krauss, W. J. Stevens)

Double zeta level SCF calculations have been completed for a number of

conformers of the dipeptides,

0 H H 0

R H

with R = H (glycine-GLY) , CH

3

( alanine-ALA) , CH 2 0H (serine-SER) , CH 2 SH

( cysteine-CYS) , CH

2 C0*0H (aspartic acid-ASP) . The conformer geometries are

at equilibrium positions obtained with the molecular mechanics ECEPP code

(ECEPP data supplied by Dr. S. Chin, IBM Kingston) or at published quantum

optimized structures. In addition to providing data for the reaction field

code development, these calculations offer a comparison of energetics with

the available quantum and the classical mechanics values as seen in Table

4.3.

The agreement with the quantum results of Schfifer et . al. is

reasonably good. Since the presently used basis is more flexible, this

implies that the larger molecules allow for considerable basis superposition

and improved conformational energetics. For GLY 3d polarization functions

78
were added to all heavy atoms except the two methyl carbons. The second

GLY conformer was then found to be lowest in energy. This result can be

rationalized in terms of the importance of H-bonding in the conformers. The

GLY-1 conformer contains a seven member ring with the amide hydrogen of one

amide group bonded to the carbonyl oxygen in the other amide group. Double

zeta calculations have been found to overestimate H-binding, and adding

polarization functions provides lower and more accurate SCF values.

The comparison with the ECEPP energy orderings is not as good.

However, comparison with an empirical method requires the evaluation of the

dispersion interaction. Simple London type dispersion calculations for all

non-bonded interactions shows very little variation with conformation. For

example, the dispersion energy difference between the Cl, C2, and C6

conformers of GLY is less than 0.2 kcal/mole. More accurate analyses of the

correlation energy are sufficiently difficult that this estimate suggests

much effort would not be rewarded. The discrepancy with classical energetics

at specific conformers is likely to remain. Whether the topography of a i|>, $

map will be qualitatively different, needs to' be explored for specific

dipeptides

.

Table 4.3. Comparison of Dipeptide Conformer Energetics Energy (kcal/mole)

Peptide Conformer DZ DZP Ref. 1 Ref. 2 Ref. 3

GLY QM1 0.0 0.5 0.0 0.0

• QM2 0.02 0.0 0.8 0.8

QM3 4.5 3.8 4:7 1 .0

QM4 4.0 3.2 4.3 1 .2

Cl 0.3 0.0

C2 0.0 0.7

C6 8.7 1 .7

C8 7.7 5.1

ALA QM1 0.0 0.0 0.0

QM2 0.5 1.4 0.4

QM3 2.6 2.6 8.8

SER Cl 0.0 0.0

C2 3.8 1 .2

CYS Cl 0.0 0.0

C3 4.2 0.2

ASP Cl 2.8 o 0 o C3 0.0 0.3

1. Calculations with ECEPP; communicated by S. Chin, Kingston, I3M.

2. J. N. Scarsdale, C. V. Alsenoy, V. J. Klimkowski, L. Schafer, and F. A. Momany, J. Chem. Phys. 1 05 , 3438 (1983).

3. S. S. Zimmerman, M. S. Pottle, G. Nemethy, and H. A. Scheraga,

Macromolecules 10, 1 (1977).

79
C. Scattering in the Presence of Radiation

1. Radiative Redistribution

(P. Julienne)

We have continued close coupled scattering calculations to calculate

the fine structure branching ratios to 2 P j = 1/2 and j=3/2 when a Na atom is

excited to a dissociative 2

E or 2 II state during a collision with a rare gas

perturber atom, He, Ne, or Ar. We have also predicted the alignment

(orientation) of the 2P j=3/2 atom if a linearly (circularly) polarized

photon is absorbed. These calculations have been carried out in conjunction

with Drs. Linda Vahala and Mark Havey of Old Dominion University, Norfolk,

Va., and have successfully explained Dr. Havey's radiative redistribution

experiments. These calculations and experiments study the molecular dynamics

which occur between the time a photon is absorbed into a particular molecular

state and the time it is re-emitted later as fluorescence. The nonadiabatic

dynamics describe the way in which the coupling of orbital, spin , and

nuclear rotation angular momenta changes as the excited atoms separate.

This work is supported by AFOSR.

Fig. 4.2. Branching fraction f into Na ^P(j=1/2) when Na ^P j — 3/2 or 1/2

is produced following excitation of Na( 2P) by photon absorption

in the wings of the Na ^P «- 2 S transition broadened by

collisions with Ar (solid line and filled circles) or He

(dashed line and open circles). The detuning A is measured

relative to the mean energy of the fine structure doublet. The

lines give the results of close coupled scattering calculations

using ab initio potentials. The points represent the measured

values

.

SO
2.

Application of Generalized Multichannel Quantum Defect Theory

(MCQDT)

(P. S. Julienne, F. H. Mies)

The generalized MCQDT applied to molecular potentials gives a way of

relating the transition amplitudes for the closely related processes of

photodissociation and radiative redistribution. The theory also permits, for

suitable systems, the factorization of the transition amplitude matrix for

both phenomena into separate factors representing short range Franck-Condon

excitation and nonadiabatic final state interactions which occur at longer

range and control the distribution of product states. We have tested this

factorization for the Na( 2 P) + rare gas system studied above by applying the

recoil limit for fine structure branching and polarization. The recoil limit

for fine structure branching applies at sufficiently high fragment relative

separation velocity, and was found to apply at much lower relative fragment

kinetic energies for the light collision partner Kb than for Ar. Significant

departures from the recoil limit apply at thermal energies, reflecting the

influence of the molecular potentials on the nonadiabatic dynamics. An

invited paper on the MCQDT analysis of molecular dissociation was presented

at the International Conference on the Physics of Electronic and Atomic

Collisions.

3.

Product Distributions in Molecular Photodissociation

(P. S. Julienne and F. H. Mies)

For some time we have been using close coupled scattering calculations

to calculate the nonadiabatic free-free redistribution phenomena. We are now

generalizing our close coupling scattering codes to incorporate weak coupling

to a bound state so we can calculate bound-free matrix elements for a final

close coupled scattering state. This will enable us to calculate

product distributions in molecular photodissociation. We have obtained the

photodissociation code of Singer, Freed, and Band, and it is running on the

University of Maryland VAX + F?S array processor computer system. We are currently implementing the renormalized Numerov algorithm of this code in our close coupling scattering code on the N3S CYBER 205 super computer

.

This work is supported by AFCSR.

4.

Atomic Lineshapes and Resonance Fluorescence Spectra

(F. H. Mies and P. S. Julienne)

We are progr essi vely expanding the capabilities of cur close-coupled

scattering codes to include many aspects of radiative coupling and

laser-induced phenomena in molecular continuum spectroscopy . The programs

incorporate the effects of ncn-adiabatic couplings and the resultant

breakdown of the 3orn-0ppenheimer approximation. We directly extract the

photon scattering cross-sections and redistribution spectra from the

close-coupling codes, without any commitment to perturbation theory. Also we can systematically increase the intensity of the pumping laser field and

achieve saturation. Our calculations yield the resultant non-linear -espor.se

31
of the absorption cross-sections in the presence of inelastic collisions.

Further we have begun to incorporate two separate laser fields into our calculations. At present the second field is treated as weak and allows us to extract the resonance fluorscence spectrum in the presence of saturation

by the pumping laser field. We are now calculating the pressure-broadened

half-widths of the three peaked Mollow lineshape for the resonant transition

in Sr. The following are four separate accomplishments associated with this

program for FY85. This work is supported by AFOSR.

a. Continuum-continuum Matrix Elements in Atomic Lineshape Theory

(F. H. Mies and P. S. Julienne)

The calculation of the pressure-broadened lineshape for the atomic

transition A

i +yfiui^ + Af requires the evaluation of the following

continuum-continuum matrix element

= — iirfi <^-(E")| fl L |^(E+ )>. (1)

where the field-free multichannel wavef unctions 4>± incorporate non-adiabatic

effects and the breakdown of the usual Born-Oppenheimer Approximation. This

expression applies in the weak-field limit, when the Rabi frequency £2^ associated with thg photon is very small and when the radiative scattering

matrix element can be evaluated in perturbation theory. The precise

details of the lineshape analysis depends on the extent to which the incident

photon is detuned from the exact atomic resonance energy = (E^ -E^),

Afi

o “L

" u

fi ( 2 )

In the wings of the line, where A^ is very large compared to the

homogeneous width of the atomic transition, the absorption coefficient is

directly proportional to the radiative scattering cross section determined

by the transition matrix elements in Eq. (1). The impact approximation to

the pressure-broadened half-width in^he vicinity of the line center can can also be expressed in terras of S

fi .

For dipole allowed atomic transitions the radiative matrix element

which defines the pressur e-broadened atomic lineshape is only

conditionally convergent. Using a commutator technique to redefine the

integral we can isolate, and ultimately reject, the contribution of an indeterminant asymptotic surfce integral which is associated with the

energy normalization of the continuum wavef unctions which describe the

binary collision of the atom and its perturber. The remaining

contributions to the atomic lineshape are absolutely convergent. Since

the transformed integrals apply as well to the multichannel wavefunction

82
they encompass the effects of non-adiabatic and inelastic scattering.

Further we show the relationship of the commutator integral to the exact

’'equirements of close-coupled scattering theory for radiatively- induced

collisions. This scattering analysis suggests that we should interpret

the convergent lineshape as an expression of multichannel inelastic

collisions between field-dressed atomic states. The same interpretation

applies both in the impact and the static limit. We would advocate that,

since stable close-coupling codes are required to generate the

multichannel wavef unctions in Eq. (1), it is a simple matter to merely

expand the set of coupled equations to directly include the radiative

coupling between channel states as required by the dressed atom boundary

conditions and completely avoid explicit numerical quadrature of the

multichannel matrix elements in Eq. (1).

b. Impact Limit in Weak Laser Fields

(F. H. Mies and P. S. Julienne)

Care must be taken when one attempts to apply the perturbation

expression near the impact limit for allowed transitions. As the detunings

»

.• t approach zero, the major contributions to the matrix element (1) occur

in the asymptotic regions R=® and S^

Gubecomes proportional to 1/Af

t,t,

. i

A A

This divergent limiting form is predicted by the impact theories of Shafer

and Gordon and 3en-Reuven. In fact we can numerically extract this matrix

element from the full close-coupling codes for small Af fl- T and then employ

these same impact theories to obtain accurate estimates of the coilisional

half-widths of pressure broadened lines. Note that the near resonance photon

simultaneously causes a collective coupling among all the asymptotic channel

states and ^ » and we can never factor Eq. (3) into distinct products of

initial and final state terms.

We have performed multichannel close-coupled calculations of the

SrC^S) + Sr('P) transition broadened by Ar. A comparison of the exact

close-coupled raditive scattering matrix elements to eq. (3) confirms the

validity and utility of our numerical procedures for obtaining pressure

broadened half-widths in the weak field limit. This procedure should enjoy

substantial application throughout lineshape theory.

Sfi (A, 0 ) - 1 Qt 1 U>-» > > 2Af i £

* ^ S f ,f

6 f ,f*• t S-i * i ] (3)

33
c. Impact Limit and Pressure Broadening of Mollow Sidebands in Intense

Fields

(F. H. Mies)

r

1 Our close-coupled procedures for evaluating S avoid any commitment to

ra(j weak fields or perturbative estimates of S

fi . Of course, in the weak field

limit, we should choose a representative Rabi frequency sufficiently small to

insure that Eq. (1) remains linearly proportional to and yet sufficiently

large to insure numerical accuracy of the close-cgupled radiative scattering

matrix elements. As long as the magnitude of remains below about 10“3

there seems to be a one-to-one correspondence to Eqs. (1) and ( 3 )*

Of course true deviations from linearity must ultimately prevail as the

actual field strength increases. Such effects become particularly important

in the impact limit, and ultimately become manifest as pronounced sidebands

in the fluorescence spectrum of the pressure-broadened atomic lineshape

symmetrically located at frequencies ±UL from the scattered Raleigh line.

Experimental observations of the three-peaked Mollow lineshape have

been made for Sr+Ar. We have made seme preliminary calculations for Sr+Ar in

the strong-field limit and confirm that our close-coupling procedures

reproduce the dressing of the asymptotic Sr states and yields a three-level

version of Eq. ( 3 )* We are now in the process of calculating the complete

impact limit half-width for each of the separated peaks as well as the

expected polarization properties of the intense field fluorescence.

In addition to the spectrum there have been experimental estimates (and

semi classical calculations) of the inelastic collisional energy transfer

between the dressed Sr atomic states which causes collisional redistribution

of the resonance fluorescence. These cross-sections are also being extracted

from our calculations.

D. Multiphoton Effects in Resonance Fluorescence Spectra

(F. H. Mies)

Until now we have only allowed for a single photon transition in our close-coupled equations. In order to calculate the redistribution spectrum

in intense fields we have introduced a second laser field with Rabi frequency

Up and frequency aip

. So far we have only permitted a weak probe intensity U

p and have obtained a beautiful mapping of the Mollow lineshape for Sr+ Ar as a function of the probe detuning.

In view of our encouraging results with a weak probe field our ultimate

goal is to also saturate this field and examine the close-coupled

calculations for effects associated with four wave mixing. Again, such

effects have been observed in Sr + Ar and we are confident that our techniques can derive reliable estimates of the appropriate cross-sections.

84
E. Future Plans

In the area of electronic structure calculations the Quantum Chemistry

Group intends to move ahead in 1) the development of the reaction field

methods and code, 2) the calculation of accurate prototype energy and

properties for amides and nucleic acid components, and 3) model problems on H-bonding in enzymes, metal binding to both proteins and nucleic acids, and

the analog behavior of phosphates and vandates in nucleus-substrate binding.

The reaction field development is the major thrust. As we improve the

methods and the code we intend to apply the model calculations in category

three to enzyme binding and spectroscopy problems. In the interim

calculations in category two will provide insight into polarizabilities and

dispersion interactions in large aromatic systems, the electronic structure

of phosphorus and vanadium five-coordinate molecules, accurate

hydrogen-bonding energies comparing H, 0, and S sites, and metal binding to

small amides and isolated nucleic acid bases.

Future plans in the scattering analysis call for further development

and implementation on the CYBER 205 of efficient numerical algorithms for

calculating accurate photodissociation transition amplitudes to a manifold of

strongly coupled final states. Such codes enable us to calculate product

state distributions following photodissociation. Initial calculations will

be on diatomic systems with later generalization to small polyatomic systems.

Specific calculations are planned for the NaK molecule in conjunction with

experimental studies to be done at the University of Maryland. In addition

to our numeri cal studies, we also will continue to develop an analytic

half-collision theory of photodissociation and photo-assisted collisions

based on generalized multichannel quantum defect theory. This approach leads

to much insight into photodissociation dynamics and suggest various limiting

approximations which need further testing, such as recoil or frame

transformation approximations for product alignment or fine structure

branching. We will also continue to develop the nonadiabatic theory of atomic

line broadening in two areas. First, we will study the Lorentzian asymmetry

of the core profile. Second, we will incorporate the effect of strong laser

fields, including the effect of collisions in broadening the Mollow line

shapes and the effect of AC Stark shifts in modifying inelastic collision

cross sections.

85
.

*

-

.
5. PUBLICATIONS

(a) Publications of Past Year

Casassa, M. P. , Heilweil, E. J. , Stephenson, J. C., and Cavanagh, R. R.

,

"Time-Resolved Measurements of Vibrational Relaxation of Molecules on Surfaces: Hydroxyl Groups on Silica Surfaces," J. Vac. Soc. Tech. A3_,

1655 (1985).

Heilweil, E. J., Casassa, M. P. , Cavanagh, R. R. , and Stephenson, J. C.,

"Temperature Dependence of the Vibrational Population Lifetime of

OH ( v=l ) in Fused Silica," Chem. Phys. Letters, 1 17, 185 (1985).

Heilweil, E. J., Casassa, M. P., Cavanagh, R. R. , and Stephenson, J. C.,

"Vibrational Deactivation of Surface OH Chemisorbed on Si0

2

: Solvent

Effects," J. Chem. Phys. 82_, 521 6 (1985).

Heilweil, E. J. , Casassa, M. P., Cavanagh, R. R. , and Stephenson, J. C.,

"Vibrational Energy Relaxation of Adsorbates on Surfaces," SPIE

Proceedings 533 » 15 (1985).

Heilweil, E. J. , Casassa, M. P. , Cavanagh, R. R., and Stephenson, J. C.,

"Picosecond Vibrational Energy Relaxation of Surface Hydroxyl Groups on Colloidal Silica," J. Chem. Phys. 8l_, 2856 (1984).

Hougen, J. T. and Ohashi, N., "Group Theoretical Treatment of the Planar

Internal Rotation Problem in (HF) 2

," J. Mol. Spectrosc. 109, 134-165

(1985).

'

Hougen, J. T. , "Summary of Group Theoretical Results for Microwave and

Infrared Studies of H

2

0 2

," Can. J. Phys. 62_, 1392-1402 ( 1984).

Jacox, M. E. , "Ground-State Vibrational Energy Levels of Polyatomic Transient

Molecules," J. Phys. Chem. Ref. Data 1 3 > 945-1068 ( 1984).

Jacox, M. E. , "Spectroscopy and Photochemistry of Free Radicals Formed by the

Reaction of F Atoms with Small Molecules," Rev. Chem. Interned. 6_, 77-120 (1985).

Julienne, P. S. , "Collision Induced Radiative Transitions at Optical

Frequencies," in Phenomena Induced by Intermolecular Interactions , ed.,

by G. 3irnbaum (Plenum Press, 1985).

King, D. S. and Stephenson, J. C., "Kinetic Energy Disposal in the

Unimolecular IRMPD of Methyl Nitrite," Chem. Phys. Letts. 114, 461

(1985).

37
King, D. S. and Stephenson, J. C., "Infrared Multiphoton Dissociation of

Methyl Nitrite in a Molecular Beam; Internal States of the Nitric Oxide

Fragment,” J. Chem. Phys. 82, 2236 ( 1985).

King, D. S. , Mantell, D. A., and Cavanagh, R. R. , ”N0 Thermally Desorbed from

a Saturation Coverage on Pt(111): Internal State Distributions,” J.

Chem. Phys. Q2_, 1046 (1985).

King, D. S. , "Product State and Kinetic Energy Distributions in the

Ultraviolet Photodissociation of the N0-Ar van der Waals Molecule,” J.

Chem. Phys. 82_, 3629 (1985).

King, D. S. and Stephenson, J. C., "Energy Distribution in the Nitric Oxide

Fragments from the Vibrational Predissociation of NO-C 2H 1}," J. Chem.

Phys. 82, 5286 (1985).

Krauss, M. and Stevens, W. J., "Electronic Structure of FeO and RuO," J.

Chem. Phys. 82_, 5584 (1985).

Lovas, F. J., "Microwave Spectra of Molecules of Astrophysical Interest XXII.

Sulfur Dioxide (S0 2 )," J. Phys. Chem. Ref. Data j_4, 395-488 ( 1985).

Lovas, F. J. , "Microwave Spectra of Atmospheric Species," Proc. of CMA/NBS

Workshop on Atmospheric Spectra, (ed., by A. Weber, June 1985)

p. IIIA14.

Lovas, F. J., "Critically Evaluated Microwave Spectral Data," Proc. of

CMA/NBS Workshop on Atmospheric Spectra, (ed., by A. Weber, June 1985)

p. IV21

.

Maki, A. G., "Infrared Spectra and Atmospheric Chemistry," Proc. of CMA/NBS

Workshop on Atmospheric Spectra, (ed., by A. Weber, June 1985) p. IIIB1.

Maki, A. G., Pine, A. S. , and Dang-Nhu, M., "A Doppler-Limited Study of the

Infrared Spectrum of Allene from 2965 to 3114 cm

--'," J. Mol. Spectrosc.

112, 459-481 (1985).

Maki, A. G. and Wells, J. S. , "High Resolution Spectrum of the Band of

Nitric Acid (HNCU) Near 880 cm

-1

," J. Mol. Spectrosc. 108, 1 7~30

(1984).

Mies, F. H., "Adiabatic Analysis of Distant Perturbations: Application to

Herzberg-Teller Vibronic Coupling Theory," Molec. Phys. ^4_, 1 423-1 436

(1985).

Mies, F. H. and Julienne, P. S., "A Molecular Half Collision Analysis of

Atomic Lineshapes in Spectral Lineshapes ," Vol. 3. (ed., F. Rostas) W.

de Gruyter and Co., Berlin, pp. 393“420 (1985).

Miller, K. J., Taylor, E. R. , Basch, H. , Krauss, M., and Stevens, W. J. , "A

Theoretical Model for the Binding of cis- Pt(NHg)^ + to DNA," J.

Biomol. Struct. Dynam. 2, 1157 (1985).

88
Ohashi, N. and Hougen, J. T. , "The Torsional-Wagging Tunneling Problem and

the Torsional-Wagging-Rotational Problem in Hydrazine," J. Mol.

Spectrosc. 112 , 384-MOO (1985).

Petersen, F. R. , Wells, J. S. , Jennings, D. A., and Maki , A. G. , "Improved

Deuterium Bromide 1-0 Band Molecular Constants from Heterodyne Frequency

Measurements," J. Mol. Spectrosc. 1 07 , 48-61 (1984).

Pine, A. S. , Fried, A., and Elkins, J. W. , "Spectral Intensities in the

Fundamental Bands of HF and HC1," J. Mol. Spectrosc. 1 09 , 30-45 (1985).

Pine, A. S. , Lafferty, W. J. , and Howard, B. J., "Vibrational Predissociation,

Tunneling and Rotational Saturation in the HF and DF Dimers," J. Chem.

Phys. 81_, 2939-2950 (1984).

Pine, A. S. , "Tunable Laser Spectroscopy of Upper Atmospheric Molecules,"

Proc. of CMA/NBS Workshop on Atmospheric Spectra, (ed„, by A. Weber,

June 1985) p. Ill B8.

Pollock, C. P., Petersen, F. R., Jennings, D. E., Wells, J. S. , and Maki, A. G., "Absolute Frequency Measurements of the 00°2-00°0, 20°1-00°0 Bands

of NpO by Heterodyne Spectroscopy," J. Mol. Spectrosc. 107, 62-71

(1984).

Raab, M. and Weber, A., "Amplitude Modulated Heterodyne Polarization

Spectroscopy," J. Opt. Soc. Am. B2_, 1476 (1985).

Ratcliff, L. , Konowalow, D. D., and Stevens, W. J. , "Transition Moments for

Low-Lying Excited States in NaK," J. Mol. Spectrosc. 110 , 242 (1985).

Rumble, J. R. , Jr., Stevens, W. J. , and Truhlar, D. G. , "Effective

Electron Correlation in the Target Wavefunction on Electron-Molecule

Scattering," J. Phys. B17_, 31 5 (1984).

Snyder, L. E. , Henkel, C., Hollis, J. M. , and Lovas, F. J. , "Observations of

the Si02 Radical Toward IRC + 10216 at 1.27 Centimeters," Astrophys. J. (Letters) 290, L29-L33 (1985).

Stevens, W. J., Basch, H. , and Krauss, M., "Compact Effective Core Potentials

and Efficient Shared-Exponent 3asis Sets for the First- and Second-Row

Atoms, Li-Ar," J. Chem. Phys. 8l_, 6026 ( 1984).

Suenram, R. D., Lovas, F. J. , and Stevens, W. J. , "Reaction Products from a Microwave Discharge of N

2 and H 2

S, The Microwave Spectrum Two Confcrmers

of Sulfur Diimide (H-N=S=N-H) , " J. Mol. Spectrosc. (1985).

Weber, A., "Proceedings of the CMA/NBS Workshop on Atmospheric Spectra,"

November 3 _

4, 1 9 S 3 * (June 1985).

Wells, J. S. , Jennings, D. A., Hinz, A., Murray, J. S. , and Maki, A. G.,

"Heterodyne Frequency Measurements on N

2

0 at 5.3 and 9.0ym," J. Opt.

Soc. Am. B2 , 857-861 (1985).

89
(b) Publications in Progress

Basch, H., Krauss, M., Stevens, W. J. , and Cohen, D., "Electronic and

Geometric Structures of Pt(NH

3 )|

+

, Pt(NH2 ) 3 Cl 2 , Pt(NH3

) 3X and

Pt(NH

3

) 2XY (X , Y = H2 0,0H)," J. Inorg. Chem. (in press).

Basch, H., Krauss, M. , and Stevens, W. J. , "Cation Binding Effect on H-Bonding," J. Am. Chem. Soc. (submitted).

Basch, H., Krauss, M. , Stevens, W. J. , and Cohen, D., "Binding of

Pt(NH

3

) 3 + to Nucleic Acid Bases,” J. Inorg. Chem. (submitted).

3asch, H., Krauss, M. , Stevens, W. J., and Cohen, D., "Electronic and

Eleometric Structures of Pt(NH

3 )|

+

, Pt(NH

3

) 2 Cl2 , Pt(NH3

) 3X,

Pt(NH

3

) 2XY (X,Y = OH

-

, H

2 0)," Inorg. Chem. (submitted).

Basch, H. Krauss, M. , and Stevens, W. J. , "Cation Binding Effect on H-Bonding," J. Am. Soc. (submitted).

Bodaness, R. S., Heller, D. F. , Krasinski, J., and King, D. S. , "Direct

Two-Photon Excitation of the Tumor Localizing Photo-Sensitizer

Hematoporphyrin Derivative at 750 nm via the Soret Band System," J.

Biological and Biochemical Sciences (WERB review).

Casassa, M. P., Heilweil, E. J., Stephenson, J. C. , and Cavanagh, R. R.,

"Time-Resolved Measurements of 0H(v=1) Vibrational Relaxation on Si0 2

Surfaces: Isotope and Temperature Dependence," Phys. Rev. Lett,

(submitted)

.

Casassa, M. P. , Heilweil, E. J. , Stephenson, J. C., and Cavanagh, R. R.

,

"Time-Resolved Measurements of Vibrational Relaxation at Surfaces," J.

Electronic Spectry. (accepted).

Dang-Nhu, M. and Pine, A. S. , "Intensities des Raies d'Absorption dans la

Bande Chaude vg + v-| -j - v-j -j de l'Allene," Can. J. Phys. (in press).

Heilweil, E. J. , Casassa, M. P., Cavanagh, R. R. , and Stephenson, J. C.,

"Vibrational Energy Decay of Surface Adsorbates," Proceedings of the

1985 Conference on Time-Resolved Vibrational Spectroscopy

,

Springer-Verlag (in press).

Heilweil, E. J. , Casassa, M. P. , Cavanagh, R. R. , and Stephenson, J. C.,

"Time-Resolved Vibrational Energy Relaxation of Surface Adsorbates," J.

Vac. Sci. Tech. B_, (in press).

Hougen, J. T. , "A Generalized Internal Axis Method for High Barrier Tunneling

Problems, as Applied to the Water Dimer," J. Mol. Spectrosc. (accepted).

Howard, B. J. and Pine, A. S, "Rotational Predissociation and Libration in

the Infrared Spectrum of Ar-HCl," Chem. Phys. Letters (submitted).
Jacox, M. E. , "Comparison of the Ground-State Vibrational Fundamentals of

Diatomic Molecules in the Gas Phase and in Inert Solid Matrices," J.

Mol. Spectrosc. 113 . (in press).

Jacox, M. E. , "Reaction of F Atoms with Methyl Halides. Vibrational Spectra

of CH^XF and of H 2CX«»*HF Trapped in Solid Argon," J. Chem. Phys . 83_,

(in press)

.

Jasien, P. G. and Stevens, W. J. , "Calculated Proton Affinities for Some

Molecules Containing Group VI Atoms," J. Chem. Phys. (in press).

Jennings, D. E. , Weber, A., and Brault, J. W. , "Raman Spectroscopy of Gases

with a Fourier Transform Spectrometer : The Spectrum of D

2

," Appl. Opt.

(in press).

Karpas, Z. , Stevens, W. J. , Buckley, T. J., and Metz, R. , "The Proton

Affinity and Gas Phase Ion Chemistry of Methyl Isocyanate, Methyl

Isothiocyanate, and Methyl Thiocyanate," J. Phys. Chem. (in press).

King, D. S. and Cavanagh, R. R. , "Laser Studies of Molecule-Surf ace Interactions," Review prepared for Springer Series in Chemical Physics

(submitted)

.

Krauss, M., Stevens, W. J. , and Basch, H., "Relativistic Effective Potential

SCF Calculations of AgH and AuH," J. Comp. Chem. (in press).

Lovas, F. J., "Recommended Rest Frequencies for Observed Interstellar

Molecular Microwave Transitions," J. Phys. Chem. Ref. Data (submitted).

Mantell, D. A., Cavanagh, R. R., and King, D. S. , "Internal States

Distributions of NO Thermally Desorbed from Pt (111): Dependence on Coverage and Co-adsorbed CO," J. Chem. Phys. (submitted).

Mialocq, J.-C. and Stephenson, J. C., "Picosecond Laser Study of the

Collisionless Photodissociation of Dimethylni tranine at 266 run," Chem.

Phys. Lett, (submitted).

Mialocq, J.-C. and Stephenson, J. C., "Picosecond Laser Induced Fluorescence

Study of the Collisionless Photodissociation of Nitrocompounds at 266

nm" Chem. Phys. (in WERB review).

Mies, F. H. , Julienne, ?. S. , 3and, Y. 3., and Singer, S. , "A Convergent

Analysis of Radiative Matrix Elements in Atomic Lineshape Theory," J.

Phys. B (submitted).

Miller, K. J. , Taylor, E. R., Basch, H. , Krauss, M., and Stevens, W. J. , "A

Theoretical Model for the 3inding of Cis-PttNK^)!'1’ to DNA , " J.

3iomol. Str. Dyns. (in press).

Ohashi, N. , Lafferty, W. J. , and Olson, W. 3., "Fourier Transform Far

Infrared Spectrum of Hydrazine," J. Mol. Spectrosc. (submitted).

Pine, A. S. , Maki, A. G., and Chou, N.-Y., "Pressure Broadening, Lineshapes

,

and Intensity Measurements in the 2<-0 Band of NO," J. Mel. Spectrosc.

(in press).

91
Pine, A. S. and Fried, A., ’’Self Broadening in the Fundamental Bands of HF

and HC1," J. Mol. Spectrosc. (in press).

Pine, A. S. and Howard, B. J. , "Hydrogen-Bond Energies of the HF and HC1

Dimers from Absolute Infrared Intensities," J. Chem. Phys. (submitted).

Rosenkrantz, M. E. and Krauss, M., "Damped Dispersion Interaction Energies

for He-H 2 , Ne-H 2 , and Ar-H 2 ," Phys. Rev. (submitted).

Suenram, R. D., Lovas, F. J. , and Pickett, H. M. , "Millimeter Wave Spectrum

of Peroxynitric Acid," J. Mol. Spectrosc. (submitted).

Thompson, G. A., Maki, A. G. , and Weber, A., "The Infrared Spectrum of GeO,"

J. Mol. Spectrosc. (submitted).

Vahala, L. , Julienne, P. S., and Havey, M. D., "Nonadiabatic Theory of

Fine-Structure Branching Cross Sections for NaHe, NaNe, and NaAr," Phys.

Rev. A (submitted)

.

Wells, J., Hinz, A., and Maki, A. G., "Heterodyne Frequency Measurements on

N

2

0 Between 1257 cm

-1 and 13^0 cm-1

," J. Mol. Spectrosc. (in press).

92
6. TALKS

Casassa, M. P., ’’Time-Resolved Measurements of Vibrational Relaxation on Molecules on Surfaces,” 31st National Meeting of the American Vacuum

Society, Reno, NV, December 1984.

Casassa, M. P., ”Time-Resolved Measurements of Vibrational Relaxation on Surfaces,” Gordon Conference on Dynamics of Gas-Surface Interactions,

New London, NH, July 1985.

Casassa, M. P., "Time-Resolved Measurements of Vibrational Relaxation on Surfaces," Exxon Research and Engineering Co., Annandale, NJ, May 1985.

Casassa, M. P.,

Surfaces ,

"Time-Resolved Measurements of Vibrational Relaxation on IBM Watson Research Center, Yorktown Heights, NY, June 1985.

Casassa, M. P.,

Surfaces ,

"Time-Resolved Measurements of Vibrational Relaxation on General Motors Research Labs, Warren, MI, May 1985.

Casassa, M. P.,

Surfaces ,

"Time-Resolved Measurements of Vibrational Relaxation on Sandia National Laboratory, Livermore, CA, May 1985.

Casassa, M. P.

,

Surfaces,

"Time-Resolved Measurements of Vibrational Relaxation on Aerospace Corporation, Los Angeles, CA, May 1985.

Casassa, M. P. , "Dynamics of Vibrational Energy in Molecules: Vibrational

Relaxation of Molecules Chemisorbed on Surfaces," Sandia National

Laboratory, Albuquerque, NM, March 1985.

Casassa, M. P., "Picosecond Measurements of Vibrational Relaxation on Surfaces," Surface Science Division, National Bureau of Standards,

Gaithersburg, MD, February 1985.

Casassa, M. P. , Heilweil, E. J., Cavanagh, R. R., and Stephenson, J. C.,

"Vibrational Energy Relaxation of Adsorbates on Surfaces," Symposium on Surface 3onds, ACS National Meeting, Chicago, IL, September 1985.

Heilweil, E. J. , "Time-Resolved Vibrational Energy Decay of Surface

Adsorbates," Microphysics of Adsorbates Conference, Santa Fe, NM,

February 1985.

Heilweil, E. J., "Time-Resolved Vibrational Energy Decay of Surface

Adsorbates," Sandia National Labs, February 1985.

Heilweil, E. J., "Time-Resolved Vibrational Energy Decay of Surface

Adsorbates," Howard University, April 1985.

Heilweil, E. J., "Vibrational Energy Decay of Surface Adsorbates," Virginia

Commonwealth University, April 1985.

93
Heilweil, E. J., "Vibrational Energy Decay of Surface Adsorbates,"

Time-Resolved Vibrational Spectroscopy Conference, Bayreuth, West

Germany, June 1985.

Heilweil, E. J., "Picosecond Vibrational Relaxation of Adsorbates at

Colloidal Interfaces," Gordon Research Conference, Chemistry at

Interfaces, July 1985.

Hougen, J. T. , "Point-Groups, Permutation-Inversion Groups and Extended

Groups," IMS Colloquium, Okazaki, Japan, November 1984.

Hougen, J. T., "The Philosophy and Mathematics of Constructing Effective

Hamiltonians for Molecules with Large Amplitude Vibrational Motions,"

Seminar for the Theoretical Group at IMS, Okazaki, Japan, November 1984.

Hougen, J. T. , "Selected Topics from Atomic and Molecular Spectroscopy," IMS,

Okazaki, Japan, November 1984.

Hougen, J. T. , "The Avoided Crossing Technique of Meerts and Ozier: Its

Application to A values and Barrier Heights, and One Selection Rule

Puzzle," Seminar for the High Resolution Group at IMS, Okazaki, Japan,

December 1984.

Hougen, J. T. , "A Generalized Internal Axis Method for High Barrier Tunneling

Problems," Seminar for High Resolution Group at IMS, Okazaki, Japan,

December 1984.

Hougen, J. T. , "Application of Vibration-Rotation Tunneling Theory to the Gas

Phase Water Dimer Spectrum," Molecular Spectroscopy Division Seminar,

NBS , January 1985.

Hougen, J. T. , "Tunneling Motion in the Water Dimer," Lawrence Berkeley

Laboratory and Department of Chemistry, University of California,

Berkeley, CA, April 1985.

Hougen, J. T. , Four lectures on "Group Theory in Molecular Spectroscopy," at

the Chemistry Department, University of California, Berkeley, CA, April

1985.

Hougen, J. T. , "A Generalized Internal Axis Method for High Barrier Tunneling

Problems, as Applied to the Water Dimer," Washington Meeting of the

American Physical Society, Washington, D.C., April 1985.

Hougen, J. T., "A Generalized Internal Axis Method for High Barrier Tunneling

Problems, as Applied to the Water Dimer," 40th Symposium on Molecular

Spectroscopy, Coiambus, OH, June 1985.

Hougen, J. T. and Ohashi, N. , "The Torsional-Wagging Tunneling Problem and

Torsional-Wagging-Rotatinal Problem in Hydrazine," 40th Symposium on Molecular Spectroscopy, Columbus, OH, June 1985.

Hougen, J. T. , "The Use of Extended Permutation-Inversion Groups for

Constructing Hyperfine Hamiltonians for Symmetric Top Internal Rotor

Molecules Like H^C-SiH?," 40th Symposium on Molecular Spectroscopy

,

Columbus, OH, June 1985.

94
Hougen, J. T. , "Symmetry Beyond Point Groups in Molecular Spectroscopy,"

Department of Chemistry, University of Chicago, Chicago, IL, July 1985.

Hougen, J. T. , "Symmetry Beyond Point Groups in Molecular Spectroscopy,"

Seventeenth International Symposium on Free Radicals, Granby, CO, August

1985.

Hougen, J. T. , "A Generalized Internal Axis Method for High Barrier Tunneling

Problems, As Applied to the Water Dimer," American Chemical Society,

Chicago, IL, September 1985.

Hougen, J. T., "The Use of Extended Permutation-Inversion Groups for

Constructing Hyperfine Hamiltonians for Symmetric-Top High-3arrier

Internal Rotor Molecules," Ninth Colloquium on High Resolution Molecular

Spectroscopy, Riccione, Italy, September 1985.

Howard, B. J. and Pine, A. S. , "Near-Infrared Spectra of Rare-Gas-HCl

Complexes," 40th Symposium on Molecular Spectroscopy, Columbus, OH, June

1985.

Jacox, M. E., "Spectroscopic Studies of the Early Stages in the Decomposition

of Energetic Materials," Workshop on the Initiation of the Chemical

Decomposition of Energetic Materials, Office of Naval Research,

Annapolis, MD, October 1984.

Jacox, M. E. , "Spectroscopy of Intermediates in the Decomposition of

Energetic Materials. Experimental Studies at the National Bureau of

Standards," Workshop on Diagnostics for Ignition Mechanisms, Army

Research Office, Aberdeen, MD, November 1984.

Jacox, M. E. , "Spectra of Free Radicals Formed in the Primary Reaction of F Atoms with the Substituted Methanes," 7th Winter Fluorine Conference,

Division of Fluorine Chemistry, American Chemical Society, Orlando, FL,

February 1985.

Jacox, M. E. , "Dolphus E. Milligan - A Scientific Retrospective , " Dedication

Symposium, Dolphus E. Milligan Science Research Institute, Atlanta

University Center, Atlanta, GA, March 1985.

Jacox, M. E. , "Matrix Isolation Studies of the Decomposition and Ionization

of Small Molecules at Energies Between 11.5 and 16.8 eV," Symposium on Ultraviolet Photochemistry, 68th Canadian Chemical Conference, Kingston,

Ontario, Canada, June 1985.

Jacox, M. E. , "Spectra of Free Radicals Formed in the Primary Reaction of F Atoms with the Methyl Halides," 40th Symposium on Molecular

Spectroscopy, Columbus, OH, June 1985.

Jacox, M. E. , "Spectra of Free Radicals Formed in the Primary Reaction of F Atoms with the Substituted Methanes," 5th International Colloquium on the Spectroscopy of Matrix Isolated Species, Fontevraud, France, July

1985.

95
Jacox, M. E. , "The Vibrational Spectrum of the COOH Free Radical Trapped in

Solid Argon," 17th International Symposium on Free Radicals, Granby, CO,

August 1985.

Jacox, M. E. , "Vibrational and Electronic Spectra of Transient Species

Trapped in Solid Argon and Neon," Symposium on Theoretical and

Experimental Studies of Transient Species, 190th National Meeting of the

American Chemical Society, Chicago, IL, September 1985.

Jasien, P. G. and Stevens, W. J., "Proton Affinities of Some Molecules

Containing Group VI Atoms," 40th Symposium on Molecular Spectroscopy

,

Columbus, OH, June 1985.

Jennings, D. E. , Weber, A., and Brault, J. W., "Fourier Transform Raman

Spectroscopy of H

2 and D 2 in Flames," 40th Symposium on Molecular

Spectroscopy, Columbus, OH, June 1985.

Julienne, P. S. , "Theory of Atomic Collisions in Laser Fields," Physics

Department Seminar, University of Missouri, Rolla, MO, October 1984.

Julienne, P. S., "Nonadiabatic Half-Collision Theory of Line Broadening,

Laser-induced Collisions, and Photodissociation," Atomic and Molecular

Theory Group Seminar, Lawrence Livermore Laboratory, Livermore, CA,

December 1984.

Julienne, P. S., "Analytic Multichannel Theory of Molecular Dissociation,"

International Conference on the Physics of Electronic and Atomic

Collisions," Stanford University, Palo Alto, CA, August 1985.

King, D. S. , "State Selective Studies of Molecular-Surface Dynamics," Optical

Society of America National Meeting, San Diego, CA, October 1984.

King, D. S. , "State Selective Studies of Molecule-Surface Interactions,"

University of California, Irvine, CA, November 1984.

King, D. S. , "Dynamics in Chemical Bond Breaking Processes," Molecular

Spectroscopy Division Seminar, NBS, February 1985.

King, D. S. , "Energy Distributions in Thermally Desorbed Molecules,"

Conference on the Dynamics of Molecular Collisions, Utah, July 1985.

Krauss, M. , "Electronic Structure of Metal Oxides," IBM, Kingston, NY,

February 1985-

Krauss, M., "Effective Potentials in Quantum Chemistry," Rensselaer

Polytechnical Institute, NY, April 1985.

Lafferty, W. J. and Olson, W. B. , "High Resolution Spectrum of the V

2 and

Bands of H0C1," 40th Symposium on Molecular Spectroscopy , Columbus, OH,

June 1985.

Lovas, F. J. and Suenram, R. D., "Pulsed Beam Microwave Study of the van der

Waals Complex Ar • • *CH 2 CHCN, " Spring Meeting of the American Physical

Society, Crystal City, MD, April 1985.

96
Lovas, F. J., "Rotational Studies of Weakly-Bound Molecules by Conventional

and Fourier Transform Microwave Spectroscopy," Institut fur

Physikalische Chemie der Universitfit Kiel, Kiel, Germany, September

1985.

Lovas, F. J. and Suenram, R. D., "Pulsed Beam Microwave Study of the van der

Waals Complexes Ar***CH 2 CHCN and Kr««*OCS," Ninth Colloquium on High

Resolution Molecular Spectroscopy, Riccione, Italy, September 1985.

Lovejoy, R. W. , Schaeffer, R. D., and Olson, W. B., "High Resolution Infrared

Spectrum on SiH^D," 40th Symposium on Molecular Spectroscopy , Columbus,

OH, June 1985.

Maki, A. G. , "Laboratory Studies of Atmospheric Molecules at NBS," Workshop

on the Spectroscopic Studies of Atmospheric Species, NASA-Langley

Research Center, Hampton, VA, October 1984.

Maki, A. G. , Pine, A. S. , Wells, J. S. , Jennings, D. A., Fayt, A., and

Goldman, A., "Laboratory Studies of the Infrared Spectra of Atmospheric

Species," Topical Meeting on Optical Remote Sensing of the Atmospheric,

Incline Village, Nevada, January 1985.

Maki, A. G. and Pine, A. S. , "Pressure Broadening and Intensity Measurements

lor the 2-0 Band of NO," 40th Symposium on Molecular Spectroscopy

,

Columbus, OH, June 1985.

Mies, F. H., "Collisional Redistribution of Resonant Radiation in Intense

Laser Fields," Conference on Dynamics of Molecular Collisions, Snowbird,

Utah, July 1985.

Ohashi, N., Lafferty, W. J., and Olson, W. B., "Far-Infrared Spectrum of the

Torsional Band of Hydrazine," 40th Symposium on Molecular Spectroscopy

,

Columbus, OH, June 1985.

Pine, A. S., "Difference-Frequency Laser Spectroscopy of Hydrogen-Bonded and van der Waals Complexes," Joint MIT-Harvard Physical Chemistry

Colloquium, Harvard University, Cambridge, MA, December 1984.

Pine, A. S. , "High-Resolution Studies of Predissociation in Hydrogen-Bonded

and van der Waals Complexes," Chemistry Department, University of

California, Berkeley, CA, January 1985.

Pine, A. S., "Infrared Spectroscopy of Hydrogen-Bonded and van der Waals

Complexes Using a Difference-Frequency Laser," Western Spectroscopy

Association Meeting, Asilomar, CA, February 1985.

Pine, A. S., "Difference-Frequency Laser Spectroscopy of Hydrogen-Bonded and van der Waals Complexes," American Physical Society Spring Meeting,

Washington, D.C., April 1985.

Pine, A. S. and Fried, A., "Intensities and Self-3roadenings in HF and HC1,"

40th Symposium on Molecular Spectroscopy, Columbus, OH, June 1985.

Stevens, W. J., "Compact Effective Potentials and Basis Sets," IBM

Laboratories, Kingston, NY, October 1984.

97
Stevens, W. J., "Cation Binding Effect on Inidizole Tautomerism, " 5th

International Congress on Quantum Chemistry, Montreal, Canada, August

1985.

Suenram, R. D. and Lovas, F. J., "Microwave Spectrum of the

Ar* • ‘Acrylonitrile van der Waals Complex" 40th Symposium on Molecular

Spectroscopy, Columbus, OH, June 1985.

Stephenson, J. C., "Vibrational Energy Transfer," Chemistry Department,

Howard University, April 1985.

Stephenson, J. C., "Vibrational Energy Transfer in Adsorbates on Surfaces,"

Chemistry Department, MIT, May 1985.

Stephenson, J. C., "Vibrational Energy Transfer in Adsorbates on Surfaces,"

Gordon Conference on Molecular Energy Transfer, July 1985.

Stephenson, J. C., "Vibrational Relaxation of Chemical Bonds in Liquids and

on Surfaces," International Conference on Photochemistry, June 1985.

Thompson, G. A., Maki, A. G. , and Weber, A., "More Diode Laser Spectra of

Diatomic Molecules," 40th Symposium on Molecular Spectroscopy, Columbus,

OH, June 1985.

Thompson, G. A., Maki, A. G. , and Weber, A., "Infrared Measurements of High

Temperature Diatomic Species," XVII European Congress on Molecular

Spectroscopy, Madrid, Spain, September 1985.

Weber, A., Lafferty, W. J. , and Olson, W. B. , "High Resolution Infrared

Spectrum of Cyanogen," 40th Symposium on Molecular Spectroscopy,

Columbus, OH, June 1985.

Weber, A., Jennings, D. E. , and Brault, J. W. , "Fourier Transform Raman

Spectroscopy of Gases," XVII European Congress on Molecular

Spectroscopy, Madrid, Spain, September 1985.

Weber, A., Jennings, D. E. , and Brault, J. W. , "Fourier Transform Raman

Spectroscopy of Gases," Ninth Colloquium on High Resolution Molecular

Spectroscopy, Riccione, Italy, September 1985.

Weber, A., "Recent Work in High Resolution Molecular Spectroscopy at NBS,

Physics Department, University of Bayreuth, Bayreuth, Germany, September

1985.

98
7. MOLECULAR SPECTROSCOPY DIVISION SEMINARS

Allen, L. C. , Department of Chemistry, Princenton University, Princenton, NJ,

"Electronic Mechanism of Carbonic Anhydrase," March 1985.

Andrews, L., Department of Chemistry, University of Virginia,

Charlottesville, VA, "High and Low Energy Species: Spectroscopy of

Molecular Ions and HF Complexes in Solid Argon," November 1984.

3and, Y. B., Ben-Gurion University, Israel, "Optical Properties of

Molecules with Large Excited State Absorption," July 1985.

Band, Y. B., Ben-Gurion University, Israel, "Photodissociation of Diatomic

Molecules to Atoms with Non-vanishing Angular Momentum," July 1985.

Bowen, K. H., Department of Chemistry, Johns Hopkins University, Baltimore,

MD, "Negative Ion Photoelectron Spectroscopy of Negative Cluster Ions,"

May 1985.

Evenson, K. , Time and Frequency Division, NBS, Boulder, CO, "Tunable Far

Infrared Spectroscopy," November 1984.

Fraser, G. T., Chemistry Department, Harvard University, MA, "Nonrigidity and

Vibrational Predissociation in Ammonia van der Waals Complexes," March

1985.

Herman, R., Physics Department, Penn State University, "Collisional

Interference in the Vibration-Rotation Spectra of H

2 and HD," October

1984.

Hirota, E. , Institute for Molecular Science, Myodaiji, Okazaki, Japan,

"Infrared Laser Kinetic Spectroscopy," April 1985.

Hochstrasser , R. M. , Department of Chemistry, University of Pennsylvania, PA,

"Ultrafast Relaxation Processes in Condensed-Phase Molecular Systems,"

May 1985.

Howard, B. J., Physical Chemistry Laboratory, Oxford University, England, "IR

Studies on Intermolecular Interactions in Weakly Bound Species," April

1985.

Jackson, W. M. , Chemistry Department, Howard University, "Laser Studies of

Reaction Dynamics in Gases and on Surfaces," September 1985.

Kunz, T., Department of Chemistry, Rice University, Houston, TX,

"Photochemistry of Formyl Fluoride," February 1935.

99
Ladik, J., University Erlangen-NUrnberg, Germany, "Quantum Theory of

Polymers: Solid State Aspects," September 1985.

Mills, I. M. , University of Reading, England, "Molecular Vibrations Seen as Local Modes," April 1985.

Moore, R., Department of Chemistry Virginia Commonwealth University, VA,

"Picosecond Intermolecular Vibrational Relaxation in the Gas Phase,"

January 1985.

Pickett, H. M. , Jet Propulsion Laboratory, Pasadena, CA, "Spectroscopy in the

World Above One Terahertz," March 1985.

Schweitzer, E., Chemistry Department, MIT, Cambridge, MA, "Spectroscopic

Probes of Infrared Multiphoton Dissociation," February 1985.

Shostak, S., University of Rochester, Rochester, NY, "Studies of Infrared

Laser Excited Molecules by MBER and Maser Spectroscopy," February 1985.

van Duijnen, P., Theoretical Chemistry Group, Laboratory of Chemical Physics,

Croraingen, The Netherlands, "Reaction Field Hamiltonians and

Applications to Biomolecules," June 1985.

TOO
8. VISITING SCIENTISTS

The Molecular Spectroscopy Division has been host in the past year to

the following scientists who have worked with NBS scientists on problems

of mutual interest.

Adams, George, A., Ballistics Research Laboratory, works with M. Krauss on electronic structure codes for the chemistry and spectroscopy of large

molecules

.

Basch, Harold, Bar Ilan University, Israel, is engaged in research on metal interactions with biomolecules. He works with M. Krauss.

Bevan, John W. , Chemistry Department, Texas A & M University, worked with

W. J. Lafferty and W. B. Olson on the Fourier transform infrared

spectra of hydrogen bonded molecules.

Bodaness, Robert S. , National Institutes of Health, worked with D. S. King

on laser photosensitization processes in biological samples.

Chou, N.-Y., Chemistry Department, Howard University, worked with A. S.

Pine on collisional line broadening problems.

Ho, P.-T., Electrical Engineering Department, University of Maryland,

worked with J. C. Stephenson on research with picosecond lasers.

Hollis, Jan Michael, NASA/Goddard Space Flight Center, works with

F. Lovas and R. Suenram on software development for automated microwave

spectrometer and collaborates on astronomical studies.

Howard, 3rian, Oxford University, Oxford, England, worked with A.

S. Pine on high resolution infrared spectroscopy of the rare gas-HCl

van der Waais molecules.

Jennings, Donald E. , NASA/Goddard Space Flight Center, works with

A. Weber on a project to do Raman spectroscopy of gases with the Fourier

transform spectrometer of the National Solar Observatory (Kitt Peak).

Konowalow, Daniel D., Professor at SUNY-3inghamton, New York, works

with M. Krauss on studies of electronic structure of alkali and alkaline

earth molecules.

Lengsfield, Byron H., Ballistic Research Laboratory, works with M.

Krauss on electronic structure codes for the chemistry and spectroscopy of

large molecules.

Leubner, Christoph, J., Institut fur Theoretische Physik, UniversitSt

Innsbruck, Austria, worked with J. T. Hougen on improved least squares

determination of potential energy surfaces from spectroscopic data.

101
Lovejoy, Roland W. , Chemistry Department, Lehigh University, works

with B. Olson on the infrared spectrum of deuterated silanes.

Mialocq, John-Claude, Department de Physico-Chemie , Centre d’ Etudes

Nucleaires de Saclay, France, worked with J. C. Stephenson on problems

of vibrational relaxation, photo-dissociation, and fluorescence and

absorption spectroscopy on a picosecond time scale.

Mills, Ian M. , Department of Chemistry, Reading University, Reading,

England, worked with W. J. Lafferty on the spectroscopy of hydrogen

bonded molecules and the theoretical relationships between normal and

local mode models of molecular vibrations.

Ohashi, Nobukimi, Kanazawa University, Physics Department, Japan,

works with A. S. Pine, J. T. Hougen, W. J. Lafferty and B. Olson on high

resolution infrared spectroscopy of hydrazine and of hydrogen bonded

molecules.

Sattler, Joseph P., Harry Diamond Laboratories, U. S. Army,

collaborates with W. Lafferty and A. Maki on precise line frequency

measurements in the infrared.

Vahala, Linda L. , Physics Department, Old Dominion University, Norfolk,

VA, worked with P. S. Julienne on the theory of molecular line

broadening and final state branching ratios for the resonance

transitions of alkali atoms perturbed by rare gas collision partnes.

Wofford, Billy A., Chemistry Department, Texas A & M University, worked

with W. J. Lafferty and W. B. Olson on the Fourier transform infrared

spectra of hydrogen bonded molecules.

102

I

.
U OIPT. O V cow 1. PUBLICATION OR 2. Performing Organ. Repbrt No. 3. Publication Date

BIBLIOGRAPHIC DATA

REPORT NO.

SHEET iSee instructions) NBSIR-86/3313 JANUARY 1986

v TITLE and SUBTITLE

Technical Activities 1985 - Molecular Spectroscopy Division

j. AUTHOR(S)

A. Weber

[ PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contraci/Gram No.

S. Type of Report & Period Covered

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

SUPPLEMENTARY NOTES

J Document describes computer program; SF-185, FIPS Software Summary, is attached. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here

)

This report summarizes technical activities of the NBS Molecular Spectroscopy

Division during Fiscal Year 1985. These activities span experimental and

theoretical research in high resolution molecular spectroscopy, quantum chemistry,

and laser photochemistry, and include the development of frequency standards,

critically evaluated spectral data, applications of spectroscopy to important

scientific and technological problems, and the advancement of spectroscopic

measurement methods and techniques. A listing is given of publications and

talks by the Division staff.

2. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words py semicolons)

Environmental spectroscopy; hydrogen-bonded molecules; energy transfer; surface

desorption studies; picosecond spectroscopy; quantum chemistry; multiphoton effects;

and scattering theory.

3. AVAILABILITY

IX j Unlimited

i

[ For Official Distribution. Do Not Release to NTIS

I

i Order From Suoerintendent of Documents, U.S. Government Printing Office, Washington. D.C.

20402.

,X Order From National Technical Information Service (NTIS), Springfield. VA. 22161

14. NO. OF

PRINTED PAGES

110

15. Pr, C l
i \(