NBSIR 84-2982

Annual Report 1984

Center for Chemical Physics

P.J. Ausloos, R. A. Haines

U S. DEPARTMENT OF COMMERCE

National Sureau of Standards

National Measurement Laboratory

Gaithersburg, MD 2C8S9

October 1 384

issued December 1984

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Prepared for

11 S. DEPARTMENT OF COMMERCE

ionai Bureau of Standards

tnersburg, MD 20899

NBSIR 84-2982

ANNUAL REPORT 1984

CENTER FOR CHEMICAL PHYSICS

P.J. Ausloos, R.A. Haines

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Natioal Measurement Laboratory

Gaithersburg, MD 20899

October 1 984

Issued December 1984

Prepard for U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

INTRODUCTION

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

Center for Chemical Physics (CCP) for the period October 1, 1983 to

September 30, 1984. The Center is one of four units in the National

Measurement Laboratory. The Center for Chemical Physics (CCP), (1)

develops and maintains the scientific competences and experimental

facilities necessary to provide the nation with measurement methodologies

and measurement services in the areas of thermochemistry, chemical

kinetics, surface science and molecular spectroscopy; (2) measures,

compiles, critically evaluates, and disseminates reference data to the

industrial, government, and academic communities nationally and internationally; (3) develops theoretical methods and concepts, predictive

equations and models; (4) develops standards and techniques of measurement; (5) provides federal, state and local governments, industry and the

academic community with essential standards and certified standard

reference materials needed to maintain the integrity of the measurement

system; (6) provides consultative services and participates in collaboration efforts with other NBS Centers and the outside community.

The summary of activities is organized by division: Surface

Science, Chemical Kinetics, Chemical Thermodynamics and Molecular

Spectroscopy. Each division tells its own story and tells it in its own

way. In general, there is an introduction that outlines the main concerns

of the division, its goals and interactions. This is followed by a series

of short reports on current research and future plans, often with

explanations of why the work was undertaken. The publications during

the year are listed.

A detailed table of contents has been provided. It will permit the

reader to find those activities of greatest interest. More information

about particular work may be desired. To obtain this, the reader should

address the individual scientists or their division, c/o Center for

Chemical Physics, Chemistry Building, 3-162, National Bureau of Standards,

Gaithersburg, MD 20899.

i
Certain commercial equipment, instruments, or materials are identified in

this report to specify adequately the experimental procedure. Such

identification does not imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

11
Table of Contents

INTRODUCTION ii

SURFACE SCIENCE - - DIVISION 541

1 . Introduction . 1

2. Surface Standards Program 1

A. Reference Data 4

B. Reference Materials 13

C. Review Articles and Books 17

D. Standards-Committee Activities .... 25

3. Surface Measurements Program 30

A. Catalytic Surface Chemistry: Reaction Kinetics at High

Pressures over Single Crystal Catalysts 30

B. Electron-Stimulated Desorption: Relation to Surface Molecular

Structure and Reactivity 32

C. Nucleation, Growth and Surface Atomic Structure of Ultrathin

Films 34

D. Electron Spectroscopy of Surfaces 35

E. Ion-Surface Collisions and Atomic Excitations 35

F. Surface Theory 36

G. Fabrication of Fluorescent Screens 41

4. Surface Science Competence Program 42

A. Introduction 42

3. Reports of Surface Science Competence Program Activities ... 42

C. Surface Science Experiments at the Brookhaven National

Synchrotron Light Source 53

5. Publications 50

CHEMICAL KINETICS - - DIVISION 542

1. Introduction

Chemical Kinetics and Ion Kinetics Energetics Data Centers ...

iii

2 .
A. Introduction 74

B. Chemical Kinetics Data Center 74

C. Ion Kinetics and Energetics Data Center 77

D. Future Plans 78

3. Kinetics of Neutral Species 82

A. Introduction 82

B. Gas Phase Kinetics 83

C. Condensed Phase Kinetics 97

4. Ion Kinetics 103

A. Introduction 103

B. Gas Phase Ion Kinetics 104

C. Liquid Phase Ion Kinetics 112

5. Publications 119

CHEMICAL THERMODYNAMICS - - DIVISION 543

1. Introduction 125

2. Chemical Thermodynamics Measurements 135

A. Thermodynamics of Enzyme - Catalyzed Reactions 135

B. Weak Interactions in Model Enzyme - Substrate Systems . . . .138

C. Bomb Calorimetry of Key Biochemical Compounds 140

D. DSC Measurments and Standards 141

E. Calorimetric Standards/CSRM 1 41

F. Large Sample Combustion Flow Calorimetry 144

G. Drop Calorimetry of Non-Reacting Systems Below 2800 K .... 1 47

H. Raman Characterization of the Main Phase Transition in

Lipid Bilayers 149

I. Future Plans 150

IV
3. Thermodynamic Data Compilations 153

A. Chemical Thermodynamics Data Center 153

B. Electrolyte Data Center 157

C. Future Plans 159

4. Publications 161

MOLECULAR SPECTROSCOPY - - DIVISION 545

1. Introduction 165

2. High Resolution Spectroscopy Group 168

A. Introduction 169

B. Calibration Standards for the Infrared Spectrum 174

C. Spectroscopy in Environmental and Space Science 179

D. Spectroscopy of Hydrogen-Bonded Systems and Other Clusters . . 1 82

E. Fourier Transform Spectroscopy . . 189

F. Development and Applications of Theories of Molecular Spectra 192

G. Microwave Spectra 197

H. Molecular Spectra Data Center 198

I. Future Plans 201

3. Laser Photochemistry Group 201

A. Introduction . 201

B. Picosecond Studies of Molecular Energy Transfer 201

C. Desorption of Molecules from Surfaces 205

D. Photochemistry in Molecular 3eams 205

E. Matrix Isolation Spectroscopy 208

F. Future Plans 212

4. Quantum Chemistry Group 2

1 6

A. Introduction 216

v
3. Electronic Structure 216

C. Scattering 222

D. Future Plans 232

5. Publications 234

NBS Organization Chart

CCP Organization Chart

CCP Telephone Listing

Vi
SURFACE SCIENCE DIVISION

C. J. Powell, Chief

SUMMARY OF ACTIVITIES

Fiscal Year 1984

1 . INTRODUCTION

A. Background

Over the last ten years, there has been a dramatic growth of surface

science and its applications. At the national and local meetings of many

professional societies, there are typically multiple sessions of papers

concerned with the physical and chemical properties of surfaces.

Similarly, many current professional journals and books are devoted in

whole or in part to similar reports of recent advances in this field.

These advances result from investigations of basic surface properties

(such as composition, atomic structure, electronic structure) of surfaces

and the dependence of many complex surface processes (such as molecular

sticking, dissociation, diffusion, reaction, and desorption) on the basic

properties and other variables relevant to catalysis, corrosion, thin-film

systems, microelectronics, computer elements, and communications

applications. Investigations of this type range from fundamental studies,

at an atomic and molecular level, on clean single-crystal surfaces to

similar studies on appropriately modified surfaces or structures and lead

not only to the development of surface science but also to new materials,

devices, and processes. Similar investigations, principally measurements

of surface composition and of other needed properties, are frequently made

of a large variety of materials (e.g., metals, polymers, semiconductors,

oxides, glasses, etc.) after fabrication and at various time during their

service life for quality control, process optimization, and failure

analysis.

A large number of techniques are utilized for surface

characterization. A choice of one or more techniques is made typically on

the basis of the property to be measured, the specimen material, and the

specific needs (e.g., spatial resolution, chemical information,

sensitivity, degree of difficulty, and cost). The most common type of

surface measurement is the determination of surface composition. Most

surface analyses now are qualitative but there is a growing demand for

quantitative analyses with improved accuracy. At the present time, there

are very few standard procedures and limited reference data and reference

materials. A variety of techniques is employed for the determination of

surface atomic structures, surface electronic structure, and for

investigations of surface processes such as reactions, diffusion,

segregation, etc. Theories of the measurement process need to be

developed for the newer techniques. For all techniques, there is a need

1
for relevant procedures, tests to establish measurement reliability, and

applicable reference data. In most cases, the measurement science needs to

be developed since knowledge of the key concepts and parameters is often

extremely limited.

The techniques utilized in surface science have generally been

applied to characterize surfaces in an ultrahigh vacuum environment. The

solution of many scientific and technical problems often requires removal

of liquid- or gas-phase material prior to surface examination and/or the

removal of solid-phase material to expose a region of interest. While

this approach has often been successful, artifacts of many different types

can arise during the removal of one or more phases. There is therefore a growing need for improved measurement methods that can be used for the in

situ characterization of solid-solid, solid-liquid and solid-gas

interfaces with the sensitivity, elemental specificity, and spatial

resolution comparable to those attained with the current

surface-characterization methods.

B. Goal, Objectives and Division Program

The goal of the Surface Science Division is to improve the quality of

existing surface-characterization measurements and to extend the present

capability. Specific objectives of the Division are:

(1) Perform experimental and theoretical research in surface

science to provide a scientific base for surface-measurement

methodology

;

(2) Develop measurement methods and provide properties data for the

physical and chemical characterization of surfaces and surface

processes to meet identified needs of industry, government, and

other groups;

(3) Develop standard procedures, calibration data, and reference

materials in cooperation with national and international

standards organizations; and

(4) Develop expertise in selected new areas of science in order that

NBS can provide measurement services where needs are anticipated

in the near future.

Objectives (1) through (3) are met organizationally through an NBS

task titled '’Development of Measurement Methods for Surface Properties”

(Task 15502). The Division’s program for this task is divided into two

principal activities, a Surface Standards Program and a Surface

Measurements Program. Work performed during the year in these two areas

is reported in Sections 2 and 3. respectively. This work is supported by

NBS resources (including some funds transferred through the Office of

Standard Reference Data) and by contracts with three Federal agencies

(Office of Naval Research, Department of Energy, and National Aeronautics

and Space Administration)

.

2
Objective (4) is met as a result of the NBS Competence Program

through a task titled "Surface Science" (Task 26103). Some objectives of

this task are also supported by contracts with other agencies (Office of

Naval Research, Department of Energy). A separate report on these

activities is given in Section 4.

C. Technical Output and Professional Interactions

Sections 5 and 6 of this report contain listings of the publications

and talks, respectively, for the past year by Surface Science Division

staff and for projects supported by the Division. These listings were

prepared before the end of the fiscal year and include publications and

talks for the latter part of FY 83 not given in the previous Division

report.

Many members of the Division staff hold leadership positions in

professional societies and groups, as listed in Section 8. These

interactions not only reflect the professional esteem in which these staff

members are held but provide useful mechanisms for both disseminating the

results of NBS work and. for learning of significant measurement problems

being faced by those using surface-characterization equipment.

Interest in surface science and surface characterization at NBS

continues to grow. The Division has had an active Seminar program, as indicated in Section 7, and these seminars have been attended by other NBS

staff. The Division staff also participates In an informal weekly lunch

meeting, attended by scientists from a number of NBS organizational units,

at which developments in surface science are discussed. Members of the

Division staff are frequently consulted on a wide range of NBS problems.

Much of the Division's work is carried out in collaboration with

other scientists within NBS, within the U.S.A., and internationally . The

nature of these collaborations is shown in Section 9. These

collaborations are considered important because they enable a relatively

small Division staff to work synergistically with other experts, they

enable us to have access to unique facilities or expertise, and they

enable our expertise to be extended to a larger range of important

scientific and technical problems.

The Division has been fortunate in having eleven visiting scientists

work at NBS for varying periods during the past year, as indicated in

Section 10. These scientists enrich our staff and our projects. A

further enrichment of our programs comes from Postdoctoral Research

Associates who hold 1- to 2-year appointments; 3ix Postdoctoral Research

Associates worked in the Division during FY 1984.
2 . SURFACE STANDARDS PROGRAM

The term "surface standards" is intended to cover a broad range of

activities that are directed to improving the quality of

surface-characterization measurements through: (1) development of needed

reference data and standard reference materials; (2) leadership in

standards -committee activities, the development of standard practices, and

other actions to improve the accuracy and reliability of surface-analysis

measurements; and (3) publication of review articles and books that enable

scientists to make surface-characterization measurements of the needed

quality.

A. Reference Data

We describe here results of projects in which needed reference

data are being generated. Section 2.C includes recent review articles

that summarize and evaluate data required in a number of surface

spectroscopies

.

1 . Measurement of Absolute Ion Sputtering Yields

(J. Fine)

The principal aim of this work is to obtain absolute sputtering

yields of 1-5 keV ions on well-characterized elemental samples in order to

improve the accuracy of depth-profiling measurements by current surfacemethods. The accuracy of depth-profiling measurements is at present

limited by the unavailability of accurate, reliable, sputtering-yield

data and by the lack of knowledge of the topographical and morphological

effects produced by ion bombardment.

The use of quartz-oscillator techniques to measure mass loss due to

ion bombardment can result in very accurate determinations of

ion-sputtering yields. In addition to the third-harmonic oscillator

method developed at MBS, another system (operating in the fundamental

mode) has been developed at the J. Stefan Institute in Ljubljana,

Yugoslavia as part of a joint US/Yugoslavia program and is presently in

operation.

Measurements have been made using the quartz oscillator facilities in

Ljubljana to determine sputtering yields of Ni/Cr multilayered thin-film

materials of structure and morphology similar to those of the NBS Ni/Cr

SRMs (Section 2.B). Similar measurements have been initiated at NBS using

the third-harmonic method. Quartz crystal of our own design, having

highly polished surfaces, were used as substrates during the actual

production-run deposition of our SRM structures. These same SRM materials

will be used for mass-loss sputtering-yield determinations; such data will

then permit cur Ni/Cr SRMs to be useful in the calibration of depth scales

for other materials whose sputtering yields are known.

Similar measurements are planned on a variety of in-situ prepared

elemental targets to obtain ion-dose dependent sputtering yields.

4
2. Sputtering Yield Data Compilation

(J. Fine and G. P. Chambers)

Sputtering techniques for depth-profile analysis are presently

being used in conjunction with a number of surface analytical methods to

analyze compositional changes of materials at surfaces and at interfaces.

Many of these techniques have now progressed to the point of being able to

provide quantitative sputter-depth-profile information yet the necessary

sputtering yield data is not readily available in easy-to-use, evaluated

form.

We therefore will prepare a compilation of evaluated sputtering-yield

data for those parameters that will have specific use in surface analysis

and depth profiling. This work is supported by the NBS Office of Standard

Reference Data. Our compilation will be in tabular form for those ion

species (Ne, Ar, Kr, Xe, O

2 , N 2 , Cs) and energies (0.5 to 20 keV) and

angles of incidence used in surface characterization. We will initially

prepare such compilations of sputtering yields for elemental targets only.

We anticipate making use of the HP1000 Data System in the Center for

Chemical Physics.

This project was initiated in June, 1984. Progress to date and plans

are:

a. A system designed to compile sputtering yield data is being

developed and is to be implemented on the HP1000 System.

b. A bibliography of sputtering yield data is being assembled

and reprints are being collected.

In the coming year we intend to abstract, evaluate, and compile

sputtering-yield data of interest to the surface-analysis and sputterdepth-profiling community on elemental targets.

3- Measurement of Core-Level Binding Energies

(C. J. Powell and N. E. Erickson)

Accurate reference values of the binding energies of core-level

electrons are needed for the commonly used technique of x-ray photoelectron spectroscopy (XPS) and for other surface-characterization

methods. These values are needed for the calibration of instrumental

energy scales. For this application, data are required for a limited

number of pure materials that adequately cover the range of common

measurements.

We have previously made high-accuracy measurements of the principal

core-level binding energies cf Ni, Cu, Ag, and Au by XPS. These values

compare favorably with similar high-accuracy values measured independently

in other laboratories by other methods. We have similarly made

high-accuracy measurements of the kinetic energies of Auger electrons from

Mi, Cu, Ag, and Au.

5
At the time that these measurements were being made, we noticed that

literature values of XPS binding energies appeared to be systematically

higher, by about 1 eV, than those measured by appearance-potential

spectroscopy (APS). As systematic errors can occur and remain undetected

in binding-energy measurements by any one technique, we have measured

binding energies by XPS, APS, and core-level electron energy -loss

spectroscopy (EELS) with the same samples and the same reference power

supplies to eliminate the more obvious sources of possible error in

comparisons of measurements made in different laboratories.

We have determined the L, binding energies for Ti , V, Cr, Fe, Co, and

Ni by XPS, APS, and EELS. XPS measurements were made from the energy

positions of the photoelectron peaks and the EELS measurements were made

of the minimum energy loss for a particular core excitation. The latter

measurements were made as a function of incident electron energy, from

about 50 eV above the threshold for L^-shell excitation to 1500 eV; in

general, the measured EELS binding energies varied slightly with incident

energy. The determination of an APS binding energy requires comparison of

the measured spectrum with a calculated spectrum. The latter is based on

a one-electron model in which the core-electron and the scattered electron

both reach final states above the Fermi level. If agreement of the

calculated and measured APS data is obtained, it is then possible to

identify the electron energy on the experimental spectrum that corresponds

to a final state with two electrons at the Fermi level.

Table 1 summarizes our numerical results. For Ti and V, there was significant disagreement in the calculated and measured APS spectra so the

indicated APS binding energies must be regarded only as estimates. For

Cr, Fe, Co, and Ni, the differences between XPS and APS binding energies

range between 0.6 and 1.5 eV, a result similar (for Fe, Co, and Ni) though

different in detail to that found in the literature. In general, the EELS

energies do not interpolate between the APS and XPS binding energies, the

limits expected for adiabatic and sudden excitation, respectively.

Our principal conclusions are as follows: (1) There is a breakdown

of the simple one-electron model to describe the APS data for Ti and V.

(2) There may be different final states for each type of excitation. We

do, in fact, observe changes in EELS lineshapes for Ti , V, and Cr, as the

incident energy is varied. (3) There appears to be a varying screening

response of the solid for each type of excitation. (4) Our results

indicate that conventionally measured binding energies obtained by

different methods may differ. A more detailed knowledge of the applicable

final states and selection rules is needed to transfer binding energies

from one spectroscopy to another.

We plan to compare our present method of calibrating electron

energies using a thermionic electron source with a field-emission electron

source. The latter method may be more accurate since no work-function

correction is required. We also plan to use an existing x-ray

monochromator as the detection device in a bremsstrahlung isochromat

spectrometer. Core-level binding-energy measurements by this technique

will complement the current measurements with XPS, APS, and EELS.

6
Table 1 . Summary of L^-shell binding-energy data for 3d elements obtained

by APS, EELS, and XPS. The EELS binding energies were found to

depend on incident electron energy; the lowest and highest

measured EELS binding energies are shown here and correspond to

the lowest and highest incident energies, respectively, used for

each element. The uncertainty of each measurement is estimated

to be ±0.2 eV. The APS data for Ti and V are questionable as there was inadequate agreement of measured and calculated

spectra.

Element APS

Binding energy (eV)

EELS XPS

Ti 455.1 (?) 453.5-454.0 454.1

V 5 1 0 . 9 ( ?

)

512.0-512.4 512.6

Cr 573.9 573.7-574.5 574.6

Fe 705.8 706.7-707.0 707.3

Co 777.7 777.7-777.9 778.7

Ni 852.2 851 .6-851 .8 852.8

4. Reference Data for X-ray Photoelectron Spectroscopy and

Auger-Electron Spectroscopy

(C. J. Powell and N. E. Erickson)

A project was initiated in FY 82 with funding by the NBS Office

of Standard Reference Data to prepare compilations and evaluations of

core-electron binding energies and of the kinetic energies of selected

Auger electrons for a wide range of elements and compounds. In particular, data are being compiled of the ’’chemical shifts" that can give useful

information on the chemical state of a particular element. Data of this

type are now scattered throughout the scientific literature but there is a need both for a convenient single source and for evaluation of the

sometimes conflicting data. The reference data are being compiled

Dr. C. D. Wagner, a contractor to NBS, and the Surface Science Division is

providing technical review. Dr. Wagner has currently generated over 8000

"lines" of data in his data files. It is expected that a report will be

prepared for the Journal of Physical and Chemical Reference Data.

5. Inelastic Mean Free Paths and Attenuation Lengths of Electrons

in Solids

(C. J. Powell)

The inelastic mean free path (IMFP) of low-energy electrons in

solids is a key parameter in quantitative surface analysis by Augerelectron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS).

For electrons of energy between 100 and 2000 eV, the range of practical
interest for AES and XPS, the IMFP is typically between 3 and about 30A.

Experimental measurements of the related quantity, the attenuation length

(AL), are difficult and most published AL values are believed to have

substantial uncertainties. (The IMFP is the average distance between

successive inelastic collisions while the AL results from measurements

analyzed with a model in which elastic electron scattering is assumed to

be insignificant.) Theoretical IMFP calculations may also be in error on account of approximations and assumptions made in the analysis. It has

therefore been difficult to reconcile available IMFP calculations and AL

measurements and, in particular, to establish how IMFP and AL values vary

with material and electron energy.

IMFP calculations have been made for eight materials (C, Mg, Ail,

A&2O 3

, Cu, Ag, Au, and Bi) from tabulations of dielectric data. The

dielectric data, from the infrared to the x-ray region, satisfies various

sum rules. As assumption needs to be made concerning the

momentum-transfer dependence of the complex dielectric constant but this

choice does not affect the conclusions appreciably.

Previous authors have fitted values of IMFP, X, to a relation of the

form X a En

, where E is the electron energy. A widely-used empirical

formula based on AL data suggests that n = 0.5 while simple theory

indicates n = 0.75. The present calculations show that n = 0.75 for the

"free-electron-like" solids (e.g., Mg, Al, Bi) but that n = 0.6 for the

noble metals. These differences, which are in accord with trends of the

available AL data, can be explained by different spectral distributions in

the dielectric data. Differences in the absolute values of IMFP from one material to another (for a constant electron energy) appear to be greater

than expected from the empirical formula. These results indicate that

additional IMFP calculations and more AL data are needed in order to

perform quantitative surface analyses by AES and XPS with the desired

accuracy.

6 . Electron Attenuation Lengths in Condensed Molecular Films

(R. L. Kurtz, R. Stcckbauer, N. Usuki, T. E. Madey)

Measurements of the electron attenuation length, X, in con- densed molecular layers have been made for electron energies from 18-70 eV

at the NBS SURF-II synchrotron radiation facility. The molecules (H 2 0,

CH^OH, and CcH-jp) were condensed with unit sticking probability at 90 K on clean Cu(100) by dosing with the known flux of a calibrated micro-capillary array doser. The drop in photoelectrcn emission intensity from the

Cu 3d band was measured as a function of increasing layer thickness. By

varying the incident photon energy, the resultant photoelectron kinetic

energy was varied. Semi-logari thmic plots of the emission intensity

versus the molecular dose yield straight lines whose inverse slopes give

the attenuation length directly. Plots of X vs. electron energy for each

molecule exhibit weak energy dependences, with broad minima in the range

30-40 eV. The mean minimum values of X for each film are: C

5 H5 , (6.1 ± 1.5) A; CH^OH, (7-3 ± 1-8) A; H2

O, (9.0 ± 2.2) A. These are the first

8
such measurements for these molecules in this energy range, and have

important implications for the theory of electron transport in solids, as well as radiation damage processes in biomaterials.

7. Angular-Anisotropy Effect in Measurements of Electron

Attenuation Lengths in Crystalline Films

(W. F. Egelhoff)

Attenuation lengths of lew-energy electrons in solids, important

for quantitative surface analysis by Auger-electron spectroscopy (AES) and

x-ray phctoelectron spectroscopy (XPS), have been frequently measured from

the characteristic decay or growth of an AES and XPS signal as a film of

increasing thickness is deposited on a substrate (see section 2.1(f)). We

have recently found evidence for a serious source of error that may arise

in such measurements when the overlayer film is ordered.

Figure 1 shows a striking example of the observed effect. The lower

curve shows XPS data for the nickel 2p^/2 photoelectron peak measured

after 0.3 monolayers of nickel had been deposited on a Cu(100) substrate.

The top curve shows similar data, recorded under the same measurement

conditions, after the nickel film had been covered by 1.5 monolayers of

copper. Instead of tne expected decrease in the Ni signal due to electron

attenuation in the Cu overlayer, there is a sharp increase if, for this

case, the photoelectrcns are collected at a polar angle of 45° in the

<100> surface azimuth. For these conditions, the overlayer film acts tc

increase rather than attenuate the Ni signal.

Fig. 1. X-ray photoelectron spectrum from 0.3 monolayers of nickel on a Cu(100) substrate (bottom) and following the addition of 1.5

monolayers of copper (top).
The basis of this new phenomenon is the forward-scattering

enhancement of photoelectron intensities along internuclear axes that is

described in Section 4.B.5. The overlying Cu atoms redirect the Ni

core-level intensity from being nearly isotropic to being peaked at the

45° polar angle. At other polar angles, the Ni XPS signal decreases as expected.

These measurements indicate that conventional attenuation-length

measurements by the overlayer-film method may have substantial errors. These errors will depend on the particular measurement geometries and the

degree of crystalline order in the overlayer film. Work is in progress to

determine the magnitude of the forward-scattering effect in other systems

and to examine its impact in more detail on attenuation-length

measurements.

8. Background Estimation in X-Ray Photoelectron Spectroscopy and

Auger-Electron Spectroscopy

(W. F. Egelhoff

)

A difficult problem that arises when x-ray photoelectron

spectroscopy (XPS) and Auger-electron spectroscopy (AES) are used for

quantitative surface analysis is the estimation and subtraction of the

"background” intensity under the peaks of interest. No rigorous experimental or theoretical method has as yet been devised for making separations of the XPS and AES signals from the often large and sloping

background of secondary and scattered electrons. Common practice now consists of making arbitrary (but reasonable) estimates of this background

guided by inelastic electron scattering data.

A new approach to the background-estimation problem has been

discovered based on the forward-scattering phenomena described in section

4.B.5. Figure 2 shows XPS data for Ni(100) that has been decomposed

into two parts, XPS intensity which exhibits enhancement at a polar angle

of 45° and XPS background intensity (shaded) which does not. The basis of

the decomposition is the observation that the primary pnotoelectron

intensity is enhanced along internuclear axes whereas background electrons

are emitted isotropically. Although this method of background estimation

can be accomplished only with single-crystal materials, the background

shape can be compared with results from electron transport calculations.

9. Theory of the Quantum Hall Effect: Application to Measurement

of the Fine-Structure Constant and to Realization of a New

Resistance Standard

(S. M. Girvin)

Theoretical investigations of the quantum Hall effect have been

undertaken in support of the NBS Electricity Division's quantized Hall

resistance project. The quantization of the Hall resistance is being used

to measure the fine structure constant to very high accuracy and to

realize a universal standard of electrical resistance based solely on

C
Fig. 2. X-ray photoelectron spectrum of Ni(100). The shaded (background)

region indicates the intensity that does not show enhancement at

a polar angle of 45°.

fundamental constants of nature. We have addressed both fundamental

physics questions (see section 4.2 (g) as well as practical problems

concerning possible sources of experimental error in the extremely high

accuracy measurements being made. Recent topics of investigation in the

latter category include thermoelectric effects and the role of finite

temperatures and disorder. Another important area has been the successful

interpretation of the sudden, extremely non-linear onset of dissipative

breakdown at high current densities. This phenomenon occurs when the

carrier drift speed exceeds the speed of sound resulting in a phonon

lasing effect.

10. Meeds for Reference Data

The growth of surface science and its applications has led to

needs for expanded reference data. These needs can be summarised in the

following three categories.

a. Surface Analysis

Four techniques are now in common use for surface analysis:

Auger-electron spectroscopy (AES), x-ray phctoelectron spectroscopy (XPS)

or electron spectroscopy for chemical and analysis (ESCA) , seccndary-ion

mass spectroscopy (SIMS), and ion-scattering spectroscopy (ISS). With

each technique, there is need for the following reference data:

7 7
evaluation of spectra from pure elements for common conditions

of measurement

evaluation of chemical or matrix effects on spectra

benchmark data for the calibration of the energy and intensity

scales of instruments

reference data on line positions in compounds

reference data for key parameters in quantitative analysis

(e.g., ionization cross sections (AES, XPS) , ion scattering

cross sections (ISS), electron inelastic mean free paths and

attenuation lengths (AES, XPS), electron backscattering (AES),

ion neutralization cross sections (SIMS, ISS), and multiple

ionization (SIMS)

algorithms for data analysis (e.g., determination of background

due to multiple scattering (AES, XPS, ISS) and fragments (SIMS,

ISS), deconvolution to obtain source lineshape (AES, XPS, ISS),

inelastic scattering (AES, XPS), and double scattering (ISS))

data for ion sputtering yields in elements, alloys, and

compounds as a function of ion type, ion energy, and bombardment

geometry

data for radiation damage rates during analysis, including cross sections for desorption from surfaces and decomposition of

surface layers.

b. Surface Science

Many techniques are in common use for the determination of

surface properties and processes. Data are needed in the following areas:

reference data on vibrational energies and intensities for

molecular species on surfaces (as measured by electron

energy-loss spectroscopy, infrared spectroscopy, Raman

scattering, neutron inelastic scattering)

reference data for determination of surface structures by

low-energy electron diffraction and other techniques (e.g.,

library of basic data, algorithms to compute surface structure)

reference data for absolute determination of surface coverages

(atoms/cm 2

) on surfaces of different structure and atomic

composition

reference data for surface processes (kinetic data for catalytic

and surface reactions, desorption, diffusion, segregation)

12
Ion-surface interactions are of importance in sputtering and ion

scattering spectroscopy. Reference data are needed to

characterize the electron and photon spectroscopy of scattered

and sputtered ions, as well as of excited neutral species

leaving the surface.

c. Interface Science

There is rapid development now of techniques that can be

used for the in situ characterization of solid-solid, solid-gas, and

solid-liquid interfaces. A variety of techniques (e.g., Rutherford

backscattering spectroscopy, infrared spectroscopy, Raman spectroscopy,

extended x-ray absorption fine structure, neutron inelastic scattering,

particle-induced x-ray emission) are being utilized and reference data and

algorithms will be required for their effective application.

The Division plans to establish priorities for these data needs in

collaboration with the interested groups including the NBS Office of

Standard Reference Data, the recently formed Standard Reference Data

Subcommittee of ASTM Committee E-42 on Surface Analysis, and the Surface

Chemical Analysis Project of the Versailles Agreement on Materials and

Standards (section 2.D.2). A plan for generation of high-pr iori ty data

will then be prepared.

B. Reference Materials

1 . Development and Production of Standard Reference Materials

(J. Fine)

The objective of this project is to develop standard reference

materials (SRMs) that will allow convenient determinations of ion-beam

sputter-removal rates in commonly used surface-analysis instrumentation.

Thin film reference materials of known thickness will be used to calibrate

ion-gun current densities for particular operating conditions. This

information together with sputtaring-yield data for a range of materials

will establish depth scales in the measurement of depth profiles.

Prototype thin-film structures were initially fabricated in order to

evaluate their usefulness as depth profiling SRM’s and to develop

appropriate characterization techniques. These structures were made by

sputter deposition as part of a joint US/Yugoslavia program between NBS

and Dr. B. Navinsek at the J. Stefan Institute in Ljubljana. Analyses of

these prototype materials at NBS and by members of ASTM Committee E-42 on Surface Analysis were done using Auger-electron spectroscopy, x-ray

fluorescence, secondary-ion mass spectroscopy, Rutherford backscattering

spectroscopy, and proton-induced x-ray emission. The results were

satisfactory and a stock of similar thin-film SRM’s were fabricated in

1982 using the sputter deposition facility at the J. Stefan Institute in

Ljubljana.
The analyses obtained on our nickel-chromium multilayered th in-film

structures indicate that this material should be of considerable benefit

as a depth-profiling SRM. Work on its characterization and calibration is

now completed. We have determined the uniformity and periodicity of the

thin-film layers, absolute film thicknesses, sputtered interface depth

resolution, and structural stability. Measurement methods used to

characterize this Ni/Cr SRM include N(E) Auger sputter profiling,

Rutherford backscattering spectroscopy (RBS), and neutron activation

analysis (NAA)

.

Previous results of absolute film thicknesses and uniformity

obtained by RBS indicate that samples taken from the same production

batch, as well as from 3 separate batches are all uniform to ±3 percent

for a given layer material. These values now have been further verified

with NAA techniques by Dr. R. Fleming of the NBS Inorganic Analytical

Research Division. This method analyzes the total Ni and Cu content of

the multilayered films by using a gravimetrically calibrated reference;

the resulting accuracy is better than 1 percent. NAA results obtained on

the Ni layers indicate that the average Ni layer thickness (areal density)

determined from three batches is 58.3ug/cm^ (±2 percent). The

uncertainty in the film thickness variation in samples of a given batch

is ±1 percent. The average Ni layer thickness as determined by NAA is 4 percent greater than the RBS value. This difference is probably due to

the uncertainty in the stepping power data used in the RBS analysis; this

uncertainty is generally recognized to be between 5 and 10 percent.

Statistical analysis of our characterization results and calibration data

is being completed. This Ni/Cr structure will be issued as NBS Standard

Reference Material 2135 once the certificate has been written.

Future work includes characterization and calibration of a "marker”

layer-type sputter depth profile SRM which we have recently fabricated in

production quantities. This material also was fabricated in Dr.

Navinsek’s laboratory and consists of eight Cr layers (- 40 nm each)

separated by about 2 monolayers of C^O^.

2. Quantitative Sputter Depth Profiling of Interfaces

(J. Fine)

Determination of composition-versus-depth at virgin solid/solid

interfaces is at present difficult to achieve with sputter-depth-profiling

techniques. This is due, to a large extent, to a lack of understanding of

the dependence of the sputtered-interface depth resolution on ion

bombardment conditions (i.e., ion beam energy, species, angle of incidence, current density, and on sputtered depth). A careful evaluation of

such factors which affect interface resolution is not available for any

given interface system. A consistent set of such measurements obtained

for various ion beam parameters, all done on the same material, would

significantly enhance our ability to assess the abruptness of interfaces

before sputter-profile analysis took place and would be of value in

determining optimum sputter-profile conditions.

lb
Our development of Ni/Cr multilayered SRMs with known layer thickness

and periodicity makes it both possible and practical to systematically

examine the dependence of interface resolution on ion beam parameters.

Use of the computer-controlled, sequential-mode N(E) Auger data system

which has been previously developed allows us to obtain reliable and

accurate measurements of the interface depth resolution for the Ni/Cr

material

.

We have made the following measurements during the past year:

a. Sputtered interface depth data has been obtained for 1 and

2 keV argon ion bombardment at various ion current densities at 50° angle of incidence as a function of sputtered

depth. We have demonstrated that measured interface depths

(using the 10 to 90 percent intensity-change criterion) as narrow as 5.0 nm can be obtained with these SRMs.

b. In collaboration with Dr. B. Navinsek and his group at the

J. Stefan Institute in Ljubljana we have made use of quartz

crystal microbalance techniques to monitor the rate of

mass-loss during sputter profiling. This method allows us

to observe interface depths as evidenced by the change in

sputtering rate due to the differences in the sputtering

yields of Ni and Cr at an interface. Our results indicate

that interface depths as narrow as 3.5 nm can be observed

on our Ni/Cr SRMs during sputtering with 8 keV argon ions.

c. We also are collaborating with Dr. P. Lindfors of the

Perkin-Elmer Corporation - Physical Electronics Division

and Dr. D. Mitchell of the National Research Council of

Canada, Ottawa to document interface depth resolution

dependence on ion beam angle of incidence and on current

density. Due to the outstanding structural composition of our SRMs, we have been able to collect a large 3et of data,

and we will be able to define interface "shapes” as well as interface depths. This data should be adequate to warrant

calculations of the collision cascade broadening effect at

sputtered interfaces.

3. Needs for Reference Materials

The following needs for additional reference materials have been

identified.

a. Depth-Profiling SRMs

Surface analysis is frequently combined with ion sputtering

to measure composition versus depth and to investigate solid-solid

interfaces. It is believed that sputter-depth-profiling applications will

continue to grow and, with the development of new techniques for deposition of materials (e.g., molecular-beam epitaxy), the development of

better depth-profiling techniques, and demands for better accuracy, use of

1 C — ^
new materials, and desire for sharper interfaces, there will be an increased demand for suitable SRMs to ensure the reliability of the

relevant measurement system.

NBS has recently developed a multi-layer periodically-modulated SRM

consisting of alternate layers of Ni and Cr (section 2.2 (a)). Similar

structures consisting of Ni/Ag and Cr/CrpCb have been fabricated but

remain to be characterized. We have also fabricated films of Ta205 on Ta

and these also have to be characterized.

There are needs for similar SRMs involving other types of materials

in high-technology industries, for example, Si0 2 on Si, epitaxial

modulated-thickness films of GaAs/GaAMS, thin layers of metals on semiconductors (Schottky barriers), and polymer films on metal substrates.

b. Calibration of Surface Analysis Instruments

There is a growing need for the characterization and

calibration of surface-analysis instrumentation, particularly since the newer computer-controlled instruments require greater system stability and

reproducibility than before. Until recently, most surface analyses have

been qualitative but there is increasing demand for quantitative analyses

of defined accuracy. These developments require SRMs for instrument

calibration (e.g., of the instrument transmission functions for Augerelectron spectrometers (AES) and x-ray photoelectron spectrometers (XPS)

and for the determination of instrument performance (e.g., the detection

of trace constituents).

Pure elemental and multicomponent materials can be utilized for

calibration of the energy and intensity scales (and instrument

transmission functions) of AES and XPS instruments. A suite of pure

elements (e.g., Cu, Ag , and Au) could be used for both purposes. Liquid

alloys (e.g., Ga-Sn or Ga-In) can be cleaned readily to generate

two-component surfaces of known composition for checks of instruments and

algorithms for analyses. Multicomponent materials with known quantities

of trace elements are needed for determining the ability of an instrument

to detect trace species. Suitable materials could include ion-implanted

impurities in silicon (with a specified depth distribution and maximum

concentration) and multi -component liquid alloys (with homogeneous

compositions or with segregation of trace species at the surface).

c. Surface Science

There is a need for SRMs that would lead, with the user

following a prescription, to surfaces of known composition (e.g., to known

coverage of a chemisorbed layer of molecules on a metal substrate or to a monatomic layer of a metal on a substrate). A single crystal SRM (e.g.,

Ni) could be cleaned and exposed to different gases (e.g., 0

2

, N 2 , H 2 , CO,

^S) to establish known surface coverages of such gases. Such an SRM

would enable calibration of AES and XPS equipment now used for measurements of surface coverages. Another use of the SRM could be through the

deposition of several monolayers of a metal (e.g., Ag) , again after

16
cleaning, with appropriate thermal treatment to remove all but a monolayer

of the deposit. This process could also be used for the calibration of

AES and XPS equipment used for investigations of novel properties of

ultrathin metal films.

The Division intends to establish priorities for new SRMs in

collaboration with the NBS Office of Standard Reference Materials and the

Standard Reference Materials Subcommittee of ASTM Committee E-42 on Surface Analysis. Plans for producing additional SRMs will also depend

on sales of the recently developed Ni/Cr SRM 2135 (section 2.2 (a)) that

will be announced shortly.

C. Review Articles and Books

1 . Review Articles

The Division staff authors many review articles and book

chapters that provide information on measurement methods, relevant

reference data, reference materials, and theories of the measurement

process for many surface spectroscopies.

Division staff members have authored 24 review article recently. The

titles and abstracts of review articles published since 1983 are listed

below:

a. "Adsorption on Metal Surfaces: Some Key Issues," J. W.

Gadzuk, in Atomistic3 of Fracture, eds : R. M. Latanision

and J. R. Pickens (Plenum Press, NY), 391-418 (1983).

The physical picture and basic philosophy of the principal

techniques for performing theoretical calculations of the electronic

structure of adsorbates on metal surfaces are presented. A discussion of

spectroscopic probes and non-adia’oatic effects is given. These ideas are illustrated via specific case studies.

b. "Fundamental Excitations in Solids Pertinent to Desorption

Induced by Electronic Transitions," J. W. Gadzuk, in

Desorption Induced by Electronic Transitions DIET-I , eds

:

N. H. Tolk, M. M. Traum, J. C. Tully and T. E. Madey

(Springer-Verlag, NY), 4-25 (1983).

Various aspects of the importance of treating the initial

electronic excitation and subsequent atomic, ionic, or molecular nuclear

motion as a coherent single process is emphasized. Theoretical modeling

is carried out in terms of rranck-Condon factors for both translational

degrees of freedom as well as intrinsic substrate excitations. Several

desorption scenarios are discussed and model desorbate energy distributions which follow from the theory outlined here are presented.

7 7
c. "The Determination of Molecular Structure at Surfaces using

Angle Resolved Electron and Photon Stimulated Desorption,"

T, E. Madey, F. P. Netzer, J. E. Houston, D. M. Hanson, and

R. L. Stockbauer, in Desorption Induced by Electronic

Transitions DIST-I , eds: N. H. Tolk, M. M. Traum, J. C.

Tully, and T. E. Madey (Springer-Verlag, NY), 120-138

(1983).

We review recent data and theoretical models related to the use of angle-resolved electron and photon stimulated desorption in

determining the structures of molecules at surfaces. Examples include a variety of structural assignments based on ESDIAD (electron stimulated

desorption ion angular distributions), the observation of short-range

local ordering effects induced in adsorbed molecules by surface impurities, the influence of electron-beam damage on surface structure, and a direct comparison of ESD and PSD ion yields for the same system.

d. "The Quantum Hall Effect I," M. E. Cage and S. M. Girvin,

Comments on Solid State Physics, _1J_, 1 ( 1983).

The recent surprising observation of a quantization of the

Hall resistance in units of h/e 2 in quasi-two-dimensional conductors

presents the possibility of obtaining an improved value of the fine

structure constant and development of a quantum standard of resistance

using a solid state device.

e. "The Quantum Hall Effect II," S. M. Girvin and M. E. Cage,

Comments on Solid State Physics 1

1

, 47 (1983).

The recent surprising observation of a quantization of the

Hall resistance in units of h/e2 in quasi-two-dimensional conductors has

necessitated a major rethinking of our theoretical picture of transport in

these systems. The central problem is understanding why ideal behaviour

persists even in the presence of strong disorder.

f. "Thin Film Reference Materials for Sputter Depth Profile

Calibration," J. Fine and B. Navinsek, in Proc. of Invited

Lectures and Progress Reports, Symposium on the Physics of

Ionized Gases, ed: G. Pichler, Institute of Physics of the

University (Zagreb, Yugoslavia), 223 (1983).

Standard reference materials for calibrating depth-oferosicn scales in ion beam sputtering are presently being developed

jointly by the National Bureau of Standards, the Josef Stefan Institute,

and ASTM Committee E-42 on Surface Analysis. Four separate types of thin

film structures have been selected and produced in order to satisfy

various needs in depth profile analysis using surface spectroscopies.

Advantages of these specific structures as well as details of their

fabrication will be discussed. Preliminary results on the characterization and compositional analysis of these prototype materials have been

obtained regarding uniformity and periodicity of multilayered films,

interface widths, absolute film thickness, film stoichiometry, and film

18
stability and diffusion. The measurement methods used to characterize

these thin-filra structures include compositional sputter depth profiling

using Auger spectroscopy as well as x-ray fluorescence, Rutherford

backscattering spectroscopy, chemical analysis, weight -loss measurements

due to sputter erosion using quartz oscillator techniques, and proton

induced x-ray emission. The results of these analyses indicate that such

materials may be well suited for use in sputter depth profile calibration.

g. "Electron and Photon Stimulated Desorption: Benefits and

Pitfalls," T. E. Madey, D. L. Doering, E. Bertel, and R.

Stockbauer, Ultramicroscopy _n_> 187 (1983).

Electron beam irradiation of solids often results in damage-producing events along with information-producing events. In the

present paper we explore mechanisms of beam damage in solids, as well as examples of the benefits of electron (and photon) stimulated processes to

study molecules at surfaces. Information about the geometrical structure

of adsorbed species can be obtained from measurements of the angular distribution of ions released by electron or photon stimulated desorption.

The directions of ion emission are directly related to the orientations of

the surface bonds which are ruptured by the excitation. The method of

Electron Stimulated Desorption Ion Angular Distributions (ESDIAD) has

proven particularly useful in characterizing local molecular structure at

surfaces in the absence of long range order; recent measurements of

bonding configurations stabilized by impurities or by lateral interactions

are discussed. Photon stimulated desorption (PSD) studies using

synchrotron radiation are providing new insights into fundamental electronic excitation processes at surfaces. Mechanisms for the excitation

and desorption of ions are examined (valence, shallow core level and

deep-core-level excitations), and examples include ion desorption from

adsorbed monolayers, as well as ion formation and desorption processes in

condensed molecular films.

h. "An Overview of Research at NBS using Synchrotron Radiation

at SURF-II," D. L. Ederer, R. P. Madden, A. C. Parr, S. B.

Saloman, G. Rakowsky, J. Copper, R. Stockbauer, T. E.

Madey, and J. L. Dehmer, Proc. of the Conference on the

Applications of Accelerators in Research and Industry, IEEE

Trans, on Nuclear Science NS30 > 1020 (1983).

The National Bureau of Standards (NBS) Synchrotron Ultraviolet Radiation Facility (SURF-II) is used in conjunction with several

high throughput monochromators to study the interaction of vacuum ultraviolet photons with solids and gases. Recent work has been concerned with

the photon stimulated desorption of atomic and molecular ions from

surfaces, with the effect of electric fields on molecular photcabsorption

and with the study of molecular photoionization by angle-resolved photoelectron spectroscopy. These research programs yield new informacion

about molecular bonding at surfaces, molecular dynamics near ionization

thresholds, and the coupling of the electronic and nuclear motion near

resonances in molecules. In addition to these programs in basic research,

SURF-II is used for the calibration of transfer standard detectors over a

19
photon energy range 20-250 eV. Calibration of monochromator systems is

achieved over the photon energy range 5-250 eV by using the now calculable

spectral intensity radiated by the electrons, which are confined in a nearly circular orbit.

i. "The Use of Angle Resolved Electron and Photon Stimulated

Desorption for the Determination of Molecular Structure at

Surfaces,” T. E. Madey and R. Stockbauer, Rev. Brasil. Apl.

Vac. (J. of Brazilian Vacuum Society) 3_, 47 ( 1983).

This paper is a brief review of recent data related to the use of angle-resolved electron stimulated desorption and photon stimulated

desorption in determining the structures of molecules at surfaces.

Examples include a variety of structural assignments based on ESDIAD

(electron stimulated desorption ion angular distributions), the observation of short-range local ordering effects induced in adsorbed molecules

by surface impurities, and the application of photon stimulated desorption

to both ionic and covalent adsorbate systems.

j. "Desorption of Ions from Surfaces: Mechanisms of Photon

Stimulated Desorption,” R. Stockbauer and T. E. Madey,

Annals of the Israel Phys. Soc. 6_, 483 ( 1983).

A review is given of the mechanisms of Photon Stimulated

Desorption (PSD) from ionic, covalent and van der Waals bonded surface

molecules. An interatomic Auger decay process describes desorption from

ionically bonded, maximal valency compounds. The mechanism for ion

desorption from covalently bended systems is thought to involve relatively

long-lived two-hole states. Ion desorption from thick molecular films

presents a challenge to theory since heavy fragments (Cx

Hy) desorb from

some (cyclohexane) and not from others (water, methanol).

k. "Instrumentation for Photon Stimulated Desorption,” R.

Stockbauer, Nucl. Instr. Meth. 222, 284 (1984).

Photon stimulated desorption (PSD) refers to the ejection

of ions, atoms or molecular fragments from a surface initiated by the

absorption of single photons; to date, most studies have concentrated on detection of ions. The properties of PSD which make it attractive as a surface characterization tool are its extreme surface sensitivity, the

ions being ejected only from the topmost layer and the rapidity with which

the ions are ejected. Since the desorption is fast (-10”' 4 s), with

respect to molecular vibrations (-10

_1 “ s), the ion trajectory reflects

the initial bonding geometry of the particle to the surface.

To study the ion desorption process, one would like to know the mass,

kinetic energy distribution, angular distribution and the yield

(desorption rate vs. photon energy) of the desorbing ions. Time-of-f light

(TCP) analyzers are used where the time between the light pulses from the

synchrotron light source is on the same order as the flight time of the

ion from the surface to the detector. One can measure easily ion mass and

yield but not kinetic energy or angular distributions with this class of

20
analyzer. Electrostatic deflection analyzers are used to measure ion

yields and kinetic energy distributions and, in some instruments, ion

angular distributions. In general, they are useful where the time

structure of the synchrotron light is too short to allow TOF analysis.

Optical methods have recently been applied to the detection of excited

states of neutral Li seen in PSD of LiF.

l. "Inelastic Scattering of Electrons in Solids," C. J.

Powell , in Electron Beam Interactions with Solids for

Microscopy , Microanalysis , and Microlithography , D . F

.

Kyser, H. Niedrig, D. E. Newbury and R. Shimizu, eds. (SEM

Inc., Chicago), 19-31 0984).

The principal mechanisms and available data for the

inelastic scattering of electrons in solids are reviewed. The processes

relevant for electron-probe microanalysis, electron energy-loss spectroscopy, Auger-electron spectroscopy, and x-ray photoelectron spectroscopy

are described and examples of relevant electron energy-loss data are

shown. The discussion is based on the dielectric description of inelastic

scattering and treats processes important in the excitation of both core

electrons and valence electrons. Information is given on the cross

sections for ionization of core levels, inelastic mean free paths of Auger

electrons and photoelectrons in solids, and radiation damage.

m. "Non-adiabatic Effects in Elementary Surface Reactions:

State-to-State Molecular Beam Experiments as a Probe," J.

W. Gadzuk, in Many-Body Phenomena at Surfaces, D. Langreth

and H. Suhl , eds. (Academic Press, NY), 517 (1984).

A theory of elementary chemical reactions at solid surfaces

is described in terms of surface induced diabatic transitions between

"reactant" and "product" potential curves. It is then shown how the

internal vibrational state distribution of a diatomic molecule scattered

from a surface could provide unique dynamical information required as input to the diabatic transition theory.

n. Early Applications of Vacuum, from Aristotle to Langmuir,"

T. E. Madey, J. Vac. Sci. Tech. k2, 110 (1984).

Highlights of the development of vacuum science and

technology from ancient times to the early twentieth century are reviewed.

The view of the Greek philosophers that vacuum was an impossibility

hampered understanding of the basic principles of vacuum until the mid

17th century. Verifiable vacua were first produced in Italy by 3erti and

Toricelli ; von Guericke's dramatic experiments vividly demonstrated

atmospheric pressure. Piston-like "air pumps" were widely used in England

and the European continent through the 18th and early 19th centuries to

produce and characterize the properties of vacuum (lack of sound transmission, inability to support life, gas discharges, etc.). The Industrial

Revolution was made possible through the genius of Newcomen, who designed

huge atmospheric engines (based on condensation of steam to form a vacuum

beneath a pistcn, which was then driven by the pressure of the atmos-
phere) . A system of "atmospheric railways" propelled by vacuum pistons

was built in England in the mid 19th century. Serious scientific developments of the 19th century which necessitated vacuum included Crooke’s and

Faraday’s gaseous discharge measurements, the first sputtering experiments

by Grove, the isolation of the rare gases by Ramsey, the standards work of

Miller, the discovery of the electron by Thomson and of X-rays by Rontgen.

The development of the incandescent light by Edison provided a background

for the remarkable achievements of Langmuir in vacuum and surface science

at the dawn of the 20th century. An appendix is included which lists

museums containing vacuum-related exhibits.

2. Review Articles in publication

We list below the titles and abstracts of review articles in the

course of publication.

a. "The Chemisorption of Carbon Monoxide by the Transition

Metals," J. T. Yates, Jr., T. E. Madey , and J. C.

Campuszno, The Chemical Physics of Solid Surfaces and

Heterogeneous Catalysis, eds: D. A. King and D. P. Woodruff.

A comprehensive review of the adsorption of carbon monoxide

by transition and noble metals is presented. The systematics of CO

adsorption and dissociation are discussed, along with detailed examinations of the surface structures, electronic and vibrational spectra, and

kinetic behavior of adsorbed CO.

b. "The Thermal Desorption of Adsorbed Species," J. T. Yates,

Methods of Experimental Physics , ed: R. L. Park.

The experimental and theoretical basis of thermal

desorption spectroscopy as a tool for surface characterization is described. Methods of analysis of thermal desorption spectra to obtain

kinetic information (desorption energies, desorption order, coverages of

adsorbed species) are included, together with numerous examples of

desorption of small molecules from single-crystal metal surfaces.

c. "Experimental Methods in Electron and Photon Stimulated

Desorption," T. E. Madey and R. Stockbauer, Methods of

Experimental Physics, ed : R. L. Park.

We review experimental techniques and applications of

electron stimulated desorption (ESD) and photon stimulated desorption

(PSD) of atomic, molecular, and ionic species from surfaces. The physical

principles of these methods, as well as details of experimental measurements and data analysis are discussed. We close with a discussion of beam

damage in surface analysis.

22
d. "Laser Studies of Surface Chemical Reactions," R. R.

Cavanagh and D. S. King, Chemistry and Physics of Solid

Surfaces V, Springer-Verlag.

A review of laser studies of surface chemical dynamics with

emphasis on thermal desorption processes is presented. The correlation of

gas-phase and liquid-molecular dynamics with analogous surface processes

is demonstrated with primary emphasis on experimental techniques for

monitoring relevant quantum state populations. Recent results for the

desorption of NO from Ru(001) and NO from oxidized Ru crystals are compared in terms of rotational populations, and velocity and angular flux

distributions.

e. "Final-State Resolved Studies of Molecule-Surface Interactions," D. S. King and R. R. Cavanagh in New Laser and

Optical Investigations of Chemistry and Structures at

Interfaces , Verlag Chemie.

Quantum state-specific diagnostics as applied to surface

dynamics are discussed. The NO molecule is used as an illustration of the

experimental determination of gas-surface interactions . Topics reviewed

included beam-surface scattering, thermal desorption, vibrational accommodation, Doppler profiles, and Fourier-transform infrared emission spectroscopy .

f. "Characterization of Surfaces Using Electron and Photon

Stimulated Desorption," T. E. Madey, D. E. Ramaker , and R.

Stockbauer, Ann. Rev. Phys. Chem.

We review various mechanisms of electron- and photonstimulated desorption of ions and neutrals from surfaces. Examples

include desorption from ionic surfaces, from covalent adsorbates on metal

surfaces, and from layers of physically adsorbed atoms. The use of the

electron stimulated desorption ion angular distribution (ESDIAD) method

for determining local structures of surface molecules is described.

g. "Inelastic Mean Free Paths and Attenuation Lengths of

Low-Energy Electrons in Solids," C. J. Powell, Scanning

Electron Micrcscopy/1984

.

A review is given of calculations of inelastic mean free

paths and measurements of attenuation lengths of low-energy electrons in

solids. The emphasis of the review is on the systematics of the

dependences of these quantities on material and electron energy. Calculations of inelastic mean free paths from experimental dielectric data

indicate that different dependences on electron energy occur in different

materials and that deviations from simple theoretical or empirical

expressions are to be expected.
h. "Innershell Ionization Cross Sections,” C. J. Powell in

Electron Impact Ionization , T. D. MSrk and G. H. Dunn,

eds., Springer-Verlag.

A review is presented of cross-sections for the ionization

of inner-shell electrons by electron impact. Calculations, measurements,

and empirical formulas are described and compared. Recommendations are given for the selection of cross-section data. Finally, applications of

inner-shell ionization cross-section data to materials and surface

analysis problems are described.

i. "Core Level Binding-Energy Shifts in Solids at Surfaces,”

W. F. Egelhoff, Jr., Surface Sci. Reports.

This review presents an overview of the theory and of

various successful approaches to the interpretation of core-level binding-energy shifts observed in photoelectron spectroscopy. The theoretical

background is presented at a level readily accessible to the general

reader. Particular attention is paid to relative merits of the two

basically different conceptual frameworks for interpreting core-level

binding-energy shifts, the initial-state-final-state approach and the

equivalent -core-Born-Haber-cycle approach.

j. "Methods for Producing Clean Liquid Surfaces,” J. Fine and

' S. Hardy, in Treatise on Clean Surface Technology , K. L.

Mittal, ed.

A review of techniques which have been used to produce

atomically clean surfaces on liquids is presented. Methods are discussed

for the handling and pre-cleaning of liquid specimens. Ultrahigh vacuum

techniques for obtaining clean liquid surfaces are divided into two types:

those which do so by creating a new surface and those which act by

removing impurities that are present. The techniques include a number of

variations on drawing pure liquid from the interior of a reservoir,

deposition of liquid films, skimming, evaporation, dissolution, chemical

reactions, and sputtering by ion bombardment. A number of specific liquid

systems are discussed.

3 . Book s (C. J. Powell, T. E. Madey)

While there are many books and review articles on different

aspects of surface science and its applications, there is a dearth of

information on how to make reliable surface-characterization measurements.

There is, in essence, a large information gap between review articles in

which the latest scientific advances are discussed and manufacturers * handbooks in which the operation of a commonly used

surface-characterization equipment is described. Arrangements have been

made with a commercial publisher to produce a series of books, "Methods of

Surface Characterization,” which will address thi3 need. Two Division

2b
staff members (Powell and Madey) and three other scientists (A. W.

Czanderna at the Solar Energy Research Institute; D. M. Hercules and J. T.

Yates at the University of Pittsburgh) constitute an editorial board that

will oversee the production. A series of four volumes is currently

planned which will describe the principles, techniques, and methods

considered important for surface characterization. It is intended to

describe how important surface-characterization measurements are made and

how to ensure that the measurements and interpretations are satisfactory

(to the greatest extent possible). The approach of the series will be

pedagogical or tutorial.

Volumes 1 and 2 of the book series will describe the techniques and

methods for electron spectroscopy and ion spectroscopy, respectively,

which are in widespread use for surface analysis. These volumes will be

concerned largely with techniques for which commercial instrumentation is

available. It is hoped that writing for Volume 1 will be completed in

1984 and that for Volume 2 by early 1985.

Volume 3 will describe techniques for specimen handling and depth

profiling. For specimen handling and treatment, this volume will provide

a compilation of methods that have proven useful. This volume will also

address the common artifacts and problems associated with the bombardment

of solid surfaces by electrons and ions. Finally, a description will be

given of methods for depth profiling. Authors have been selected to write

chapters for this volume.

Volume 4 will bring together a description of methods for vibrational

spectroscopy of molecules on surfaces. Most of the techniques are currently under active development; commercial instrumentation is

available for some techniques but this situation could change in the next

few years. The current state-of-the-art of each technique will be

described as will the relative capabilities. An important component of

the volume will be the summary of the relevant theory. Authors have been

selected to write chapters for this volume.

D. Standards-Committee Activities

Three staff members of the Division hold offices on two ASTM Committees, the E-42 Committee on Surface Analysis and the D-32 Committee on Catalysts, and on the recently approved Surface Chemical Analysis Wording

Party established under the auspices of the Versailles Agreement on Materials and Standards. Participation in the activities of these groups

provides an opportunity for leadership, for dissemination cf MBS expertise, and to learn of significant measurement problems in relevant areas.

1. ASTM E-42 Committee on Surface Analysis

(C. J. Powell, J. Fine)

The ASTM Committee E-42 on Surface Analysis founded in 9 T 5, now

has seme 140 members; in addition, a large number of non-members, both

within the U.S. and throughout the world, participate in Committee

activities. The purpose of the Committee is to advance the field of

25
surface analysis and the quality of surface analyses through the

development of appropriate standards, standard practices, reference data,

reference materials, round robins, symposia, workshops, and publications.

There is substantial activity in all of these areas.

Committee activities of the past year include approval of the

following standards:

• E673-84: Standard Definitions of Terms Relating to Surface

Analysis (includes additions of terms for AES, ESCA, and SIMS)

• E983~84: Standard Guide for Electron Beam Effects in Auger

Electron Spectroscopy

• E984-84: Standard Guide for Identifying Chemical Effects and

Matrix Effects in Auger Electron Spectroscopy

• E995-84: Standard Guide for Background Subtraction Techniques

in Auger Electron Spectroscopy

• E996-84: Standard Practice for Reporting Data in Auger Electron

Spectroscopy

• El 01 5-84: Standard Practice for Reporting Spectra in ESCA

(Electron Spectroscopy for Chemical Analysis)

• El 01 6-84: Standard Practice for Determining and Specifying the

Properties of Electrostatic Electron Spectrometers

The following proposed standards are at various stages of Committee

or Subcommittee ballot:

• Additions to Standard Definitions of Terms Relating to Surface

Analysis (for ESCA and Energetic Ion Analysis methods)

• Standard Guide for Specimen Handling

• Standard Practice for Reporting Depth Profile Data in Secondary

Ion Mass Spectrometry

Other standards are being drafted.

The Committee sponsors two symposia each year, one at the annual

Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy in

the spring and the other at the National Symposium of the American Vacuum

Society in the fall. Papers presented at these symposia are published. A workshop is held at each meeting to discuss particular measurement

problems. At the 1983 American Vacuum Society Symposium the topic was

"Case Studies in Ion Sputtering: Applications and Limitations," and the

topic at the 1984 Pittsburgh Conference was "Solving Industrial Problems

Using Surface Analysis."

26
Technical Subcommittees of the E-42 Committee have organized a number

of round robins to compare the results of nominally identical measurements

in different laboratories. The following round robins are currently in

progress

:

• Comparison at the relative intensities of Auger-electron peaks

in a series of Cu-Au alloys

• Comparisons of depth distributions measured by secondary-ion

mass spectroscopy (SIMS) for 1

1

B implanted in Si

• Comparisons of the relative intensities of peaks in SIMS data

using metallic glasses

• Comparisons of peak intensities determined by ion-scattering

spectroscopy (ISS) for Cu-Ag-Au alloys

• Comparisons of elemental sensitivity factors for ISS signals of

Cu and Au

• Comparisons of core-level binding energies determined by x-ray

photoelectron spectroscopy with instruments calibrated using a field-emission electron source.

The E-42 Committee created a Standard Reference Data Subcommittee

during the past year. This new Subcommittee will: (i) establish needs for

reference data in the area of surface analysis; (ii) develop criteria for

the evaluation of data; (iii) set priorities for needed data evaluations;

and (iv) identify groups and experts to perform the evaluations. It is

anticipated that the Subcommittee will give advice on these matters to the

Division and the NBS Office of Standard Reference Data.

Work at NBS on the production and characterization of standard

reference materials for surface analysis (see Section 2.2) and on other

surface-standards activities is reported to and disseminated by the E-42

Committee.

2. Surface Chemical Analysis Working Party of the Versailles

Agreement on Materials and Standards

(C. J. Powell)

The "Versailles Agreement" arose from a meeting of the Heads of

Government (Economic Summit; of industrialized nations at Versailles in

1982. The participants represented Canada, Federal Republic of Germany,

France, Italy, Japan, United Kingdom, United States of America, and the

Commission of the European Communities. A Summit Working Group on Technology, Growth, and Employment was establishing consisting of

government science advisers. Thi3 group has established Working Groups in

areas such as photovoltaic solar energy, food technology, advanced

robotics, biotechnology, high speed trains, remote sensing from space, ind

27
advanced materials and standards. A summary of developments resulting

from the Versailles Agreement was published as a News and Comment article

recently in Science 224 , 1317 (1984).

The Versailles Project on Advance Materials and Standards (VAMAS) has

considered the establishment of Technical Working parties in specific

materials areas. A proposal for a Surface Chemical Analysis Working Party

was prepared by Dr. M. P. Seah of the UK National Physical Laboratory and

Dr. C. J. Powell of MBS and approved by the VAMAS Steering Committee in

June, 1984. The proposal described the growth and diversity of surface

analysis in the development of advanced materials in modern technologies

and, additionally, the use of surface analysis for improved films and

coatings. The principal techniques of surface analysis in common use were

identified and the technical limitations to accurate surface analyses

summarized. Accurate surface analyses require: (i) knowledge of the

principles of the measurement method, (ii) knowledge of the behaviour of

the measuring instrument, and (iii) correct interpretation and expression

of the final measured result. Standards for the measurement of surface

composition with known accuracy and for the reliable determination of

chemical state will thus require: (a) a base of principles, definitions

of terms, and suitable equations, (b) reference procedures for the

measurement of intensities and spectral features, (c) procedures and data

for instrument calibrations, (d) reference data for material parameters

such as elemental sensitivity factors, matrix terms, chemical state

spectra, ion sputtering rates, etc., and (e) standards for specifying an analysis. Specific needs were identified for the common methods of

surface analysis. Auger-electron spectroscopy, x-ray photoelectron

spectroscopy, and secondary- ion mass spectrometry together with the needs

for ion sputtering which is used to obtain composition versus depth

information in films and coatings. Existing standards activities in the

member countries were reviewed and suggestions were made for additional

standards for surface chemical analysis by scientists in the member states

of VAMAS.

As yet, the VAMAS Steering Committee has not developed appropriate

policies concerning the mechanisms of operation of the different Working

Parties. Although the precise nature of the relationship between national

groups (such as the ASTM E-42 Committee) and VAMAS is not clear, it

appears that VAMAS will provide the means to develop surface analysis

standards on an international basis and with support from a high level in

each government. There should therefore be greater participation in the

development of surface analysis standards than would happen through the

ASTM E-42 Committee alone. At present, technical representatives in the

VAMAS nations are being contacted to identify areas for technical

activities and possible experts who might contribute. Current activities

of the ASTM E-42 Committee will be coordinated with corresponding VAMAS

activities

.

28
3. ASTM D-32 Committee on Catalysts

(R. D. Kelley)

The activities of the ASTM Committee D-32 on Catalysts are

concerned with the physical and chemical properties of commercial

catalysts and with developing suitable procedures for catalytic activity

measurements. The membership of the Committee numbers approximately 120.

Semi-annual meetings are attended by between 60-80 percent of the

membership including West Europeans and South Americans. The Subcommittee

on Chemical Analysis is chaired by R. D. Kelley.

In 1984, the committee celebrated the tenth anniversary of its

founding and continues to maintain an active pace. Since its inception

the Committee has focused its activities on tests which are used routinely

in the buying and selling of catalysts. The types of tests which have

been examined include surface area, pore-size distribution, density, crush

strength, particle size, attrition and abrasion, chemical analysis,

amongst others. These tests involve methods which were thought to be n well

established", and thus particularly suitable for standardization.

However, experience has shown that many of these tests have not yielded

satisfactory results in round robin testing. Two examples of these

problems can be seen in the standardization of the multipoint BET method

for surface area and the chemical analysis for platinum in reforming

catalysts. In both of these cases, standardization activity in progress

for about six years and involving many problems in both procedures and

materials has been substantially completed in 1984.

Highlights of the past year include:

• the certification of surface area standards by the NBS Office of

Standard Reference Materials (to be available shortly) and the

initiation of a program to certify a platinum-containing

catalyst reference materials (to be completed in 1-2 years).

• completion of satisfactory round-robin testing for the chemical

analysis of nickel (a wet chemical method and an atomic

absorption method) and of palladium (wet chemical method) in

catalysts.

• initiation of standardization of a procedure for multi-element

chemical analysis of catalysts by atomic absorption, an ambitions project which will be of great value to both producers

and users of catalysts.

• Twenty test methods have been submitted for Committee ballot.

29
3. SURFACE MEASUREMENTS PROGRAM

The Division undertakes a number of projects directed to the improved

physical and chemical characterization of surfaces and surface processes.

These projects meet Division objectives through the development of new

measurement techniques, improvement of existing techniques, development of

theories of the measurement process, applications to important scientific

and national problems, development of measurement procedures, tests of the

measurement system, development of new concepts, and provision of needed

data.

A. Catalytic Surface Chemistry: Reaction Kinetics at High Pressures

over Single Crystal Catalysts

(T. E. Madey, R. D. Kelley, T. J. Udovic, M. J. Wax)

The overall objective of this research is to provide a molecular

understanding of heterogeneous catalytic chemistry. Among the areas studied are the catalytic synthesis of hydrocarbons from carbon monoxide

and hydrogen on metal surfaces, the mechanism of catalytic poisoning and

promotion, and the molecular and electronic nature of chemisorbed species.

Results of the past year are described below:

We have completed a study designed to address a controversy concerning the catalytic activity of tungsten as a methanation catalyst. We had

previously found that polycrystalline W wire exhibits a high activity for

CHi| synthesis from H2 and CO, whereas van Langeveld and Ponec have

recently reported that evaporated W films are very poor methanation

catalysts. In neither of these studies were surface analysis techniques

used to ascertain the surface composition of the catalyst. In an attempt

to resolve this controversy, a closer look was taken at the methanation

activity of W(110) (the most densely-packed crystal face of W) over a range of reactant partial pressures and temperatures in conjunction with

pre- and post-reaction surface analysis utilizing Auger electron spectroscopy (AES). The results of this study indicate that the W(110) surface

does, indeed, have the ability to hydrogenate CO to form methane as well

as higher hydrocarbons, but the appearance of this activity requires

higher hydrogen partial pressures than were utilized by van Langeveld and

Ponec. Figure 3(a) illustrates the methanation behavior of W(110) as compared with that of Ni(100) determined earlier in this laboratory. The

plot of turnover number for methane production versus inverse temperature

indicates a lower apparent activation energy for W( 11 0 ) (56 kJ mol

-1

) compared to that for Ni(100) (103 kJ mol

-1

) as well as a higher

activity for W( 1 10) at lower temperatures. The plot of methanation

turnover number versus partial pressure of hydrogen, figure 3(b), indicates the 3trong dependence on hydrogen partial pressure for W( 1 1 0

)

compared to Ni(100). For low partial pressures of hydrogen (ca. 1 Torr),

the regime studied by van Langeveld and Ponec, W(110) behaves like a poor

methanation catalyst. For higher hydrogen partial pressures (ca. 100

Torr), the regime previously studied in this laboratory, W(110) behaves

like a good methanation catalyst. These results demonstrate the importance of analyzing a wide range of reaction parameters when characterizing

the catalytic activity of a surface.

50
Figure 3. (a) Methanation activity as a function of temperature for

W ( 110) and Ni(100) (partial pressure of hydrogen = 100 Torr

and partial pressure of carbon monoxide = 0.1 Torr). (b)

Dependence of methanation turnover number on the partial

pressure of hydrogen for W(110) and M i ( 100) (reaction

temperature 500 X, partial pressure of carbon monoxide = 0.1

Torr)

.

31
Research has been initiated on the reactivity of model supported

catalysts. Equipment has been constructed which allows controlled

deposition of aluminum and platinum on tungsten single crystals. After

each experiment, a clean tungsten surface may be obtained by heating above

the desorption temperature of any deposited materials. Various amounts of

platinum, used commercially for hydrocarbon reforming, may be evaporated

directly onto tungsten, or onto thin films of alumina prepared in situ.

Use of these well-defined materials will allow examination of catalytic

activity as a function of support structure and of catalyst electronic

structure, which, at least in the submonolayer region, varies with

platinum coverage.

We plan to examine the hydrogenolysis activity of platinum deposited

on tungsten and on thin layers of alumina on tungsten. We will vary the

Pt particle size by appropriate heat treatment and attempt to correlate

activity with particle size. We will also complete a study of the

hydrogenolysis activity of two single-crystal tungsten surfaces, (110) and

(100). In addition, we will study the hydrogenation of CO over refractory

metal surfaces (Mo single crystals) and over alloy catalyst surfaces

(e.g., Cu/Ni single crystals).

B. Electron-Stimulated Desorption: Relation to Surface Molecular

Structure and Reactivity

(T. E. Madey, F. P. Netzer, K. Bange)

A continuing challenge in surface science is the determination of the

structures of molecules on surfaces, particularly in the absence of long

range order. The goal of this project is to provide a direct determination of the structures of adsorbed molecules using the Electron Stimulated

Desorption Ion Angular Distribution (ESDIAD) method, and to characterize

the fundamental electronic excitations resulting in the stimulated

desorption of ions. ESDIAD has the advantage of being sensitive to the

local bonding geometry of adsorbed molecules, even in the absence of long

range order in the adsorbed layer. The present primary thrust of this

project is to use ESDIAD to characterize the influence of surface additives (e.g., catalyst promoters and poisons) on the local structure of

small molecules on metal surfaces. Another objective is to determine

molecular structures in controversial cases.

32
During the last year, we found clear evidence for a local interaction

between CO and Na adsorbed on Ru(001). For low coverages of Na(9Na £ 0.15

ML) and saturation coverages of CO at 80 K, a fraction of the CO molecules

undergo a substantial change in bonding configuration: molecular CO bound

perpendicular to the Ru(001) surface changes to an "inclined" configuration in the presence of low coverages of Na. The bonding interaction which

causes the tilting is believed to be primarily electrostatic in nature.

We also examined the influence of two electronegative additives, 0

and Br, on the structure and reactivity of H

2 o adsorbed on an Ag (110)

surface. The interactions of Ag + 0 and Ag + Br with H20 are of interest

in electrochemistry, and this study has provided a new view of the

structure and kinetics.

We found that H

2o adsorbed on clean Ag (110) forms hydrogen-bonded

arrays having short-range local order; H2 0 desorbs in molecular form at

170 K, with no evidence for dissociation. When H

2 0 is coadsorbed with

atomic 0 on Ag (110), there is evidence for new oxygen-stabilized molecular

H

2 o having different local structure, as well as a reaction between H2 O

and 0 to form OH, i.e., H

2 o + 0(ad) -*• 20H(ad). In addition, the H 2O

interacts so strongly with molecular 0

2 (ad) that dissociation of O 2 and

formation of OH appear to occur even at - 100 K. The 0H(ad) is "tilted"

on Ag (110), with the molecular axes inclined along [001] azimuths.

Desorption of H

2 o due to recombination of 0H(ad) occurs at - 300 K.

In contrast, there is no hydrogen-abstraction reaction between H

2 0

and Br on Ag (110). The H

2 0 forms locally-ordered structures, and the

binding energy increases, but no reaction to form OH (or HBr) occurs.

We have used ESDIAD to address a controversy concerning the structures of atomic and molecular oxygen on Ag (110) . Many previous authors

have suggested that both 0 and 0

2 are adsorbed in the troughs, in high

symmetry positions; 0

2 was believed to be bonded with the 0-0 axis

parallel to the surface. Our data indicate that both atomic oxygen (T ads

> 300 K) and molecular oxygen (T ads = 80 K) are bonded in directions along

[001] azimuths. The structures based on these measurements (e.g.,

molecular 0

2 bonded via one 0 atom, and "inclined" slightly in the [001]

direction) are at variance with most existing models, and provide new

insights into the Ag-oxygen interaction.

There appear to be major differences in the "activity" of oxygen in

promoting hydrogen abstraction on different surfaces. On Ru(Q01), for

example, the interaction of H2 0 or NH^ with adsorbed oxygen causes an increase in molecular binding energy, "but no dissociation. On Ni ( 1 10) and

Ag(1l0), 0(ad) abstracts hydrogen from H

2 0 to form 0H(ad). During the

coming year, we will examine the ability of both atomic and molecular C 9

to promote hydrogen abstraction reactions on different surfaces. Specifically, we will study NH^ + 0 on Ag(IIG), where the oxygen is relatively

23
weakly bound, and NH^ + 0 on Fe(100), where the oxygen is more strongly

bound. Other polar molecules (H20, CH3OH) and other electronegative

additives (Br) will also be studied.

Iron is an excellent catalyst in NH^ synthesis, and adsorbed K is

known to "promote" the reaction by increasing the rate. In an attempt to

understand at the molecular level how K influences the bonding geometry of

NH^ to Fe, we will use ESDIAD to examine the structure and reactivity of

NH^ on clean Fe(100), and on K-dosed Fe(100). The influence of K on the

local structure of CO will also be studied; such interactions may be

relevant to Fischer-Tropsch synthesis.

Recent angle-resolved ultraviolet photoemission spectroscopy measurements have indicated that molecular 0

2 is bound to a Cu(110) surface

through one 0 atom, with the 0-0 axis perpendicular to the surface. These

results seem to be at variance with previous measurements, in which the 0

2 was presumed to be lying down. We will attempt to verify this structure

using ESDIAD; we will also study the influence of additives (0, Br) on the

structure and reactivity of H

2 0 .

We will examine the structure of H

2 o on Cu(110) and the influence of

additives (Na, 3r, 0) on the H

20 structure; these measurements will be

compared with ongoing theoretical calculations of the same system by Dr.

U. Landman of the Georgia Institute of Technology (and recently appointed

part-time Expert to the Division staff).

C. .Nucleation, Growth and Surface Atomic Structure of Ultrathin

Films

(A. J. Melmed, A. Ciszewski)

There is considerable interest in the nucleation and growth of

ultrathin films of metals and semiconductors. The interest is concerned

with relationships between particular physical and chemical properties and

the morphology and atomic structure of the film, particular for thick- nesses in the monolayer range. For example, surface atoms of samarium

metal have been found to be divalent, while the bulk atoms are trivalent,

and this has led to the suggestion that the surface lattice parameters

should be larger than the corresponding bulk values or that surface

reconstruction should occur. In order to investigate the surface atomic

structure of Sm, we determined the conditions for producing clean Sm

surfaces by epitaxial crystal growth from the vapor. The initial work was

done in a field electron microscope, and then the methods developed were

transferred to a low-energy electron diffraction apparatus, where ( 0001 ) Sm crystal layers were grown on a (011) W substrate. Initial results

indicate that the (0001) Sm surface is not reconstructed, but that is is

expanded relative to the bulk by 4-151, depending on layer thickness. We

plan to complete the determination of surface lattice parameter for ( 0001 ) Sm during the coming year and to clarify the homogeneity of Sm coverage

during the early stages of deposition on (011) W.
During the past year, methods developed earlier to prepare clean

films by epitaxy from the vapor phase in the ultrahigh vacuum of a

f ield-electron microscope were extended to relatively high-vapor-pressure

materials. In collaboration with Drs. V. Maurice, 0. Frank, and J. H.

Block of the Fritz-Haber Institute in Berlin, the rare-earth metal

europium was grown on rhenium and tungsten substrates. Growth modes,

nucleation sites, and epitaxial relationships were determined. An

unexpected type of film growth was observed to occur, namely the growth of

fee europium on one and only one rhenium plane [Re(203)3; this growth mode

could be correlated with the surface atomic structure of Re(203).

D. Electron Spectroscopy of Surfaces

(T. J. Jach)

Preliminary experiments have been conducted in an attempt to observe

the surface shifts of core-level binding energies by means of glancing-inc idence x-ray photoemission. In collaboration with Drs. P. Cowan of

the NBS Quantum Metrology Group and L. B. Holdeman (COMSAT), measurements

have been made of photoelectron spectra of nickel and gold for x-ray

angles of incidence less than and greater than the macroscopic critical

angle for total external reflection in order to observe shifts in binding

energies for predominantly surface or bulk atoms. Recent work has

centered on the requirement to have smooth surfaces at an atomic level in

order to get a significant contribution from the surface atoms. The

correlation of the x-ray reflectance of the surface with other means of

roughness measurement (stylus, scanning electron microscopy) may lead to a new method of measuring average surface roughness where the scale of the

roughness is less than 20A.

E. Ion-Surface Collisions and Atomic Excitations

(J. Fine, R. Klein)

Energetic ion-surface collisions generate extensive near-surface

collision cascades in a solid. The energy transfer and atomic excitations

that take place as a result of such collisions are not well understood in

part because, until rather recently, the primary method for monitoring

these collision phenomena was to measure macroscopic effects such as the

total sputtering yield. The collision cascade causes the ejection of

atoms, ions, electrons, and photons from the surface but it has only been

in the past few years that measurements of the ejected particles or radiation have been utilized to obtain detailed information of specific

atomic collisions that take place in condensed materials.

In recent experiments in which aluminum or silicon targets were

bombarded by argon ions, we have shown that the Auger decay of

collisionally excited A1 or Si atoms can take place outside the solid

depending on the distance from the surface (within the material) at which

the excitation occurred, the atom's velocity, and the lifetime of the

atomic excitation. As a result of our Monte-Carlo calculations to analyze

these experiments, we expect that such sputtered atoms will have rather

high kinetic energies - extending up to hundreds of electron volts. An

experimental determination of the kinetic energy distribution and state

35
identification of those specific atoms which have undergone Auger decay

would give more information concerning the atomic excitation and

collisional energy transfer mechanisms of ion collisions in solids.

In collaboration with Drs. L. J. Moore (NBS Inorganic Analytical

Research Division) and T. B. Lucatorto (NBS Radiation Physics Division), a new series of experiments has been planned and designed to determine the

kinetic energy distributions of specific sputtered atoms (ions) and their

states of excitation. Resonance multiphoton laser-ionization schemes have

been developed to detect sputtered atoms in specific excited states and

time-of -flight techniques will be used to measure their kinetic energy

distributions. Pulse timing and gating sequences have been designed and

estimates made of signal intensity indicate that such measurements are feasible. An apparatus has been designed for the preliminary

investigation of this problem. An ultrahigh vacuum chamber for

ion-beam-angle dependent measurements is under construction and will be

coupled to two ultraviolet pulsed laser systems. We plan to investigate

atoms and ions sputtered from surfaces of Mg, kl, and Si and to study the

distance beneath the surface at which collisional excitation occurs.

F. Surface Theory

(J. W. Gadzuk)

The theoretical physics projects of the Division are focused on understanding both static and dynamic behavior of atoms, molecules,

solids, surfaces, radiation and their mutual interactions . This understanding comes from consideration of the microscope atomic-scale properties of individual entities as well as from larger-scale statistical

properties of ensembles. Based upon our studies, phenomenological model

theories are constructed which relate to the chemical physics and statistical mechanical behavior and to the intrinsic properties of relevant

physico-chemical systems. In addition, a significant effort is invested

in the study of the role of an actual measurement process in determining

observed quantities in different surface spectroscopies. Ways in which

measurement-process-specific quantities such as line shapes, satellite

structures, etc. provide additional information on system dynamics are of

considerable interest.

A large-scale theoretical effort has been underway aimed at the

modeling of reaction dynamics at metallic surfaces and at interfaces of

electro-chemical significance. Major focus in the past has been on the

role of internal degrees of freedom (vibrational, rotational, and electronic) of molecules in determining energy flow and redistribution

occuring in elementary molecular processes at surfaces. Several major

advances deserve mention here.

An analytic and exact model which enables calculation of the classical trajectories for a diatomic molecular beam incident upon a surface has

been constructed in collaboration with Dr. S. Holloway of the University

of Liverpool, England. Account is taken of the possibility for the

molecule to: i) scatter in a vibrationally excited state; ii) form an adsorbed negative-molecular-ion resonance state; iii) aissociatively

36
adsorb; and iv) undergo electronic charge transfer than thus potential

surface hopping. In the spirit of past gas-phase work of Polanyi, we have

explored the ramifications of potential-energy-surface (PES) topological

features in determining the outcomes of dynamic processes at surfaces.

For instance, the PES shown in Fig. 4 has been useful in studying the

branching ratios for competing processes:

N

Fig. 4 Diabatic potential-energy surface for diatomic molecule A,

approaching a solid surface with z the center-of-mass distance from

the surface and p the intra-molecular separation relative to the

neutral molecule equilibrium distance. The entrance (z+*>) and exit

(p->®) channels correspond to an intact molecule (A?) and 2 adsorbed

particles (2A ads ), respectively, and are separated by a negative

molecular ion resonance (A 0 ~)

.

Transitions between the various

states can occur along the "seam" where the potential energies of

adjacent states are equal.

A2(gas) •* A2

" (ncndissoc. ads.) -* A? (gas),

A

2 (gas) •+ A

2 ~ (nondissoc. ads.),

A

2 (gas) -* A2

~ (non dissoc. ads.) * 2A (dissoc. chemisorbed)

We have studied the activated dissociative chemisorption of a diatomic molecule for the case where an activation barrier exists in the

exit channel along the seam (curve crossing) separating the A?~ resonance

from the dissociated state.

•57
Due to the fact that the incident A

2 molecule first goes through the

A

2

~ resonance, memory of the initial partitioning of energy (normal versus

parallel translational, vibrational, etc.) is erased. Consequently,

activated adsorption could depend upon the total energy available, not just

the normal translational component, as recently observed experimentally.

This conclusion runs counter to all previous surface-science intuitive

models based on one-dimensional potential-energy curves, thus

demonstrating the utility of new conceptual models. We have calculated

mean translational-to-vibrational energy redistributions, dissociative

sticking probabilities, and have included the role of substrate viscous

forces and mode mixing. We are in the process of establishing the role of

chaotic versus quasi-periodic intra-molecular dynamics on surface

processes.

Charge-transf er processes in molecule-surface interactions constitute

a large class of events which are significant in determining chemical

outcomes. From the point of view of quantum intra-molecular dynamics,

charge transfer (harpooning) between a surface and an incident molecule is

equivalent to potential surface hopping in which the wavepacket

representing the initial state of a particular molecular degree of freedom

is instantly placed at a non-equilibrium location on a new potential

surface. By following the time evolution of the wavepacket, observable

dynamic properties relevent both to spectroscopy and chemistry can be

calculated, often in a significantly more economic way than by using

traditional stationary-state techniques and almost always in a physical

more transparent manner.

Adapting the wavepacket-dynamics approach developed previously by

Heller to surface dynamics, we have constructed a wavepacket model for

describing intra-molecular dynamics in surface scattering processes

involving charge transfer. The essence of the model is shown in Fig. 5 where intra-molecular potential curves for a neutral diatomic molecule and

a negative molecule ion are shown. The center-of-mass translational

motion is described by some imposed surface scattering trajectory which

sets the time interval t R

. Initially the molecule A 2 is in its

vibrational ground state. At time t=0, an electron transfers from the

substrate to the molecule, the A

2

_ curve is switched on, and the wavepacket propagates on the new curve until time t=x^, when the electron

returns to the solid. This event has the effect of switching back on the

A

2 potential and allowing the now-moving wavepacket to propagate on the

neutral curve. We have developed a quantitative theory for the

vibrational excitation distribution in the scattered molecule in terms of

che molecular parameters and the lifetime of the A

2

~ intermediate state.

A typical set of vibrational distributions is shown in Fig. 6 for bromine

a3 a function of n, the vibrational quantum number, and with the intermediate state lifetime treated parametrically . Not only is substantial

excitation experienced, which varies with t^, but also significant

dissociation can occur due to curve hopping. The wavepacket dynamics

serves as a complement to the classical studies and holds promise for

being fairly easily generalized to polyatomic molecules with several

degrees of freedom. Work is in progress to extend these studies to more

complicated systems, and into the area of stimulated desorption.

38
Fig. 5 Intramolecular potential energy curves illustrating the

wave-packet propagation of the relative nuclear coordinates

associated with the existence of a negative-molecular ion within

the time interval 0<t<x a

. The return of the wave packet onto the

A

2 curve at t=x R , while possessing both kinetic and potential

energy, is responsible for vibrational excitation of the final

state of the A

2 molecule.

Fig. 6 Predicted vibrational excitation distributions of scattered

molecules as a function of vibrational quantum number for bromine

molecules which have experienced the sequence of curve hoppings

shown in Fig. 5. The resonance lifetime (via n = w with

u) the 3r 2 ~ frequency) is treated parametrically . The curve

labeled n=2 corresponds to an internal vibrational population

distribution of the bromine molecule containing - leV total

vibrational energy.

29
Vibrational spectroscopy of molecules adsorbed on solid surface is one of the dominant tools for obtaining microscopic chemical information

of chemisorption systems. While the principal use has been through

interpretation of the energy position of a loss feature, increasing

attention has been focused on the information content of the lineshape and

width. Aside from inhomogeneous broadening effects, the surface science

community has, almost without exception, assumed that the linewidth is a direct measure of inelastic damping of an excited oscillator outside a surface. Many debates on the relative roles of phonon versus electronhole pair contributions to dissipation have relied on the characteristics

of the observed linewidth as the final determinant. Although elastic

dephasing collisions are well known in other vibrational spectroscopies to

usually be the dominant source of observed linewidths in systems involving

condensed phases, their importance in surface vibrational spectroscopy has

not been recognized. In collaboration with Dr. A. Luntz of the IBM

Research Laboratory, San Jose, we have developed a theory based on the

exchange-coupling dephasing model due to Harris, in which there is

anharmonic coupling between intra-molecular (-2000cm-1

) modes and largeamplitude, low-frequency modes associated with bonding to the surface. The

latter modes, coupled to the stochastically fluctuating phonon both of the

substrate, are responsible for dephasing collisions that can yield large

(~10-50cm-1

) vibrational linewidths. The predicted temperature dependence

of the linewidth is in accord with very recent experimentally measured

values, adding strong support to our theory. From this work we conclude

that the vibrational linewidths of intra-molecular adsorbates modes cannot

generally be related to dissipative effects, as has been assumed previously, without independent measurements which distinguish between

dissipative and dephasing broadening mechanisms.

We plan to continue work in the area of surface reaction dynamics with

near-term emphasis on the determination of self-consistent reaction

trajectories, realistic electron-hole pair coupling constants, determination of energy-redistribution patterns amongst translational, vibrational,

rotational, and electronic degrees of freedom of ''reactant'’ molecules and

electron and phonon modes of surfaces under controllable dynamic and

reactive conditions, and synthesis of the various components of the

elementary reaction theories into theories of experimentally realizable

processes. Special emphasis will be placed on the phenomenon of

dissociative surface processes and on the role of non-linear dynamics.

In addition, analysis will be made, whenever possible, of novel

experimental results. Current interest includes laser-assisted surface

processes and state-to-state analysis, high-resolution surface vibrational

spectroscopy, neutron spectroscopy of metal hydrides, and other experimental probes of non-adiabatic effects.

ho
G. Fabrication of Fluorescent Screens

(A. J. Melmed)

Fluorescent phosphor screens are widely used in the Surface Science

Division and in other parts of NBS, and very often the phosphor screen

must be custom-made. The preparation process usually requires two or more

days, especially if a high-quality, uniform phosphor deposition is

required

.

In collaboration with Dr. 0. Frank of the Fritz-Haber Institute,

Berlin, a simple method was developed for preparing fluorescent screens on

glass or metal substrates. Whereas the former method required a careful

balance of large volumes of complex chemical solutions for maintaining

proper ion and binder concentrations in the phosphor settling solution,

the new process replaces the chemistry with rapid mechanical steps and

smaller volumes of liquid. The entire process can be completed in about

three hours.

Ul
4. SURFACE SCIENCE COMPETENCE PROGRAM

A. Introduction

The Surface Science Competence Program is a part of the NBS "Competence Program." The NBS program was introduced to provide long-term

support for the establishment and maintenance of areas of excellence in

science and technology contributing to the NBS mission and the projected

needs of the NBS clientele.

The Surface Science Competence Program N3S Task 26103 was established

with new staff 5 1/2 years ago. Initial plans envisaged new research

activities in three areas: (a) chemical dynamics on surfaces, (b) ion

interactions with surfaces, and (c) surface properties of novel materials;

there are, however, appreciable overlaps among these areas with other

Division projects. Specific projects have been selected based on a judgement of their4 scientific merit and on assessments of the future

scientific and technical challenges requiring NBS expertise and action.

Our strategy has been to involve existing Division staff in the new activities to the maximum extent possible and to implement both new experimental and new theoretical activities.

Section 4.B contains reports of the technical activities initiated as part of the Surface Science Competence Program. Section 4.C contains

reports of activities by Surface Science Division staff concerned with the

development of experiments at the Brookhaven National Synchroton Light

Source. These experiments are being conducted as part of NBS Task 26105

which was initiated in 1980.

B. Reports of Surface Science Competence Program Activities

1. ESDIAD-EELS Studies of the Structure and Reactivity of Adsorbed

Species (N. D. Shinn, T. E. Madey)

The determination of the local bonding geometry of adsorbed

species on surfaces is of fundamental importance to the development of a conceptual basis of surface chemistry. Fundamental studies of adsorbate

structures on single crystals are just beginning to give baseline information to technologies such as heterogeneous catalysis, corrosion prevention, electrochemistry, and semiconductor design.

We have constructed an unique apparatus combining ESDIAD (Electron

Stimulated Desorption-Ion Angular Distributions) and EELS (Electron

Energy-Loss Spectroscopy). These techniques in combination make a powerful facility for surface structural characterization.

This year high resolution electron energy loss spectroscopy (EELS),

electron stimulated desorption ion angular distributions (ESDIAD) , low

energy electron diffraction (LEED) and Auger electron spectroscopy (AES)

have been used to identify two distinct, sequentially populated, molecular

CO binding states at 1 20 K on Cr(11G). The first is a new molecular state

42
(a-, ) with a substantially weakened CO bond, believed to be a precursor

state to dissociation, whereas the second state (a2 ) is comparable to

terminally bonded CO on other transition metals. The CO stretching

frequencies of the a 1 state (- 1150-1330 cm

-1

) are substantially shifted

from the gas phase (2143 cm

-1

), and are believed to be the lowest values

ever reported for adsorbed molecular CO on any clean or promoted

transition-metal surface.

Initially, CO adsorbs with a sticking probability of - 0.9 which

decreases upon formation of a c(4x2) CO overlayer. EELS spectra reveal a weak 450 cm

-1 loss and, surprisingly, two strong features at 1150 and 1330 cm

-1

. All losses increase with additional CO exposure, shifting to - 475,

1200, and 1330 cm

-1

at full c(4x2) CO overlayer formation. With further

CO exposure, the c(4x2) LEED pattern reverts to (1x1) symmetry with high

background intensity, and a new triplet of EELS loss features appears at

495, 1865, 1975 cm . ESDIAD measurements as a function of CO exposure

show atypically weak 0

+ ion emission along [110], increasing in intensity

only after the appearance of the latter triplet of EELS features.

These data are suggestive of a-| CO molecules oriented nearly parallel

to the Cr(110) surface, while a2 CO molecules are oriented normal to the

surface at either atop or bridge-bonded sites. Figure 7 illustrates

representative EELS spectra for the sequential a-j , a 2 CO population

experiment. Further investigation of the unique a-j , CO state and the

decomposition of both a-j and a-2 CO are planned.

Also this year, an EELS/ESDIAD/LEED (low-energy electron diffraction)

study of 0

2 adsorption on Cr(110) has found an unexpected molecular

adsorption state below 200K. As seen in figure 8, the vibrational

spectrum at 120 K includes losses at 615 cm

-1 (due to atomic oxygen) and

1020 cm

-1

, the latter being attributed to the 0-0 stretch of 0

2 (ads). At

300 K, only atomic oxygen remains with a characteristic loss at 605 cm

-1

. Low oxygen exposures at 300 K result in the formation of a p(4x2) 0

overlayer and the observation of a 290 cm

-1 loss feature, possibly due to

a surface phonon of the ordered overlayer.

ESDIAD measurements of the low temperature adsorption have identified

six azimuthaily ordered cones of 0

+ ion emission away from the surface

normal. These correlate with the appearance of the 0-0 vibrational loss,

indicating that there are six discrete tilted orientations for 0

2 (ads),

reflecting the pseudo -hexagonal symmetry of the Cr(110) surface. In

contrast, atomic oxygen results in a single [110] directed cone of 0*

emission observable at all temperatures.

In addition, an example of co-adsorbate interactions inducing a significant molecular reorientation has been found in C0/02 titration

experiments on Cr(110). Cr(11) pre-dosed with atomic oxygen to form a p(4x2) structure completely blocks the a 1 CO adsorption state. Only the

495, 1865, and 1975 cm 1 CO vibrational features are seen, in addition to

the Cr-0 stretch at 605 cm

-1

. A more dramatic result is found when a c(4x2) CO adlayer - i.e., the saturation coverage of the a-j binding state

- is subsequently exposed to oxygen. The stepwise increase in surface
Cr(1 10)/CO

Fig. 7. Summary of EELS/ESDIAD/LEED data for CO adsorption on Cr(110) at

120 K. Panel at left shows representative spectra for two stages

of chemisorption, a-j CO is characterized by spectrum (a) with CO

stretching frequencies at 1150 and 1330 cm

-1

, as well as a Cr-CO

mode at - 475 cm

-1 and a residual Cr-0 stretch at 600 cm-1

. Further CO exposure populates the a 2 CO state with corresponding,

site-specific CO stretching frequencies at 1865 and 1975 cm

-1

. The Cr-CO stretching mode is observed at 495 cm

-1

.

44
EELS

Cr(1 10)/Q,

3 1000 2300 Energy Loss (cm-1 )

g. 8 Vibrational spectra for oxygen chemisorption on Cr(llO) with

corresponding HSDIAD patterns. At 300 K, atomic oxygen resul

a single [110] directed G

+ ion beam and a Cr-0 stretching fre

of 605 cm

-1

. The 2909 cm

-1 shoulder is observable only at low

oxygen coverages. Low-temperature adsorption populates a new molecular state, with 0-0 stretching frequency of 1020 cm

- and 3

new off-normal 0

+ ion beams.

.O

rr
oxygen causes the 1200-1300 cm" 1 CO intensity to gradually decrease while

a commensurate increase is seen in loss features at - 470, 1865 and 1975

cm" 1. This oxygen-induced conversion of CO from the first binding state

to the second (terminally bound) state further demonstrates the existence

of two molecular CO states at 120 K. Detailed coverage-dependent studies

are underway to elucidate these apparent site-blocking and site-competition effects.

We plan to continue of the Cr(110) chemisorption studies with

particular attention to the effects of steps, sub-surface oxide, and

contaminants. Further investigation of the complex interaction of CO and

0

2 is necessary to confirm evolving models for this system. Additional

experiments with No, C0 2 , and H 2S are anticipated to assess the chemical

activity of Cr(110j for dissociative adsorption and the role of common

bulk impurities (N,S) in adsorbate ordering. The investigation of Cr(100)

surface chemistry and adsorbate-induced surface reconstruction continues.

In view of the surprising results of the Cr ( 1 1 0) /C0/02 investigation,

comparable experiments are planned for the less dense Cr(100) surface in

order to address the question of site-specific CO chemisorption modes on chromium. To date, the only work on molecular adsorption on singlecrystal chromium surfaces is that done in this laboratory, although there

is a wider interest in assessing chemisorption trends in the family of Cr,

Mo, and W transition metals.

We plan to interface the high-resolution EELS spectrometer to an LSI

11 microcomputer. This will enable semi-automated data acquisition in a pulse-counting mode as well as facilitated and expanded data-manipulation

capabilities. These improvements to the EELS instrument are necessary for

the satisfactory performance of relevant, low-signal experiments. Initial

experiments planned will probe the low coverage interactions of H

20 and

0(ads) on Ag (110) and also NH^ on oxygen pre-dosed Ni(110). In both

cases, vibrational data is needed to confirm the models developed in

previous ESDIAD/LEED/TDS (thermal desorption spectroscopy) studies for the

respective hdyrogen abstraction reactions H

2 0 + O(ads) -* 2 OH(ads) (on

Ag( 1 1 0) ) and NH^ + 0(ads) •* NH2 (ads) + OH(ads) (on Ni(110).

We will complete the fabrication, testing, and incorporation of the "high

pressure" reaction chamber and transfer mechanism to the EELS/ESDIAD

system. This powerful combination of ultrahigh vacuum spectroscopic

techniques and actual kinetic measurements

on the 3ame model catalyst will allow the correlation of reaction rates to

models derived from detailed chemisorption studies. Of particular

interest is the hydrogenation of CO over the atomically corrugated Ni(110)

and Ni(311) surfaces. Although recent studies of other structured model

catalysts have revealed interesting chemisorption results, the significance of these results has not been established. Our studies will

directly test the catalytic importance of an atomically rough surface as well as expand our knowledge of chemisorption on "defective" surfaces.

46
2. Laser Diagnostics of Surface Dynamics

(R. R. Cavanagh)

The pathways and rates of energy transfer at surfaces are not

readily accessible with conventional surface-characterization techniques.

A variety of laser-based techniques, however, have been applied to similar

questions of molecular dynamics in both the gaseous and liquid states. In

this program, the focus is on extending established laser methods and

developing new laser-based techniques for addressing the issue of energy

transfer at surfaces. The program includes exploitation of both the

spectral and temporal resolutions available in state-of-the-art lasers.

Two types of experiments are underway in which laser diagnostics are employed to probe dynamical processes at surfaces. This work is

performed in collaboration with Drs. King, Stephenson, Mantell, Heilweil,

and Casassa of the NBS Molecular Spectroscopy Division.

a. Laser-Excited Fluorescence as a Probe of Surface

Dynamics

Molecular desorption from surfaces is representive of

chemical conversion processes which transform reactants (adsorbed molecules on surfaces) to products (gas-phase molecules and bare surfaces).

In order to characterize and utilize such reaction processes, it is

essential to have state-specific diagnostic probes suitable for such

measurements. The focus of this work is to explore the application of

laser-based probes (such as laser-excited fluorescence and multiphoton

ionization) to adsorbate and desorption processes. Processes such as thermally activated desorption, chemically induced desorption, laserinduced desorption, and electron-stimulated desorption exemplify the scope

of the project. This work is being performed collaborativeiy with Drs.

King and Mantell.

A high-resolution excimer-pumped tuneable dye laser is being used to

study thermal desorption in a new ultrahigh vacuum chamber. The laser

operates under microprocessor control, giving single-mode tunability over

0.5 cm

-1

, at a resolution of 0.005 cm

-1

, or over 20 cm

-1

at a resolution

of 0.03 cm

-1

. The output energy is typically 100 pJ at 226 nm. Combining the present tuning range of the laser (800-215 nm) with the improved

surface diagnostics in the vacuum chamber, a much broader range of

desorption diagnostics i3 possible.

Experiments to probe the spatial orientation of the NO molecular axis

following thermal desorption from Ru(Q01) have been performed. Since

inhomogeneous magnetic fields on the order of 10 mGauss are sufficient to

perturb the measurements, the magnetic shielding in the new chamber was

critical, as was the ability to take data only at the zero crossing of the

current used to heat the Ru crystal. We have not found any preferred

alignment (i.e., the molecular axi3 is neither predominantly perpendicular

nor parallel to the crystal plane).
The influence of adsorbate-adsorbate interactions on the dynamics of

thermal desorption is essentially unexplored. Our current sensitivity

level of - 5 percent of a monolayer will permit us to examine the magnitude of such effects. In addition during the coming year thermal

desorption from substrates which are inactive for NO decompostion (such as

Pt ( 111) or Pd( 111)) will be characterized. In such systems (where

dissociative channels are not in competition with thermal desorption),

strong polarization effects are anticipated. Laser surface heating will

be explored. Heating pulses of 10 ns duration will be used to initiate

molecular desorption. The resulting internal-state distribution will

provide an unique characterization of the heat bath sampled during the

desorption process. Also, the extension of state-specific diagnostics to

molecules such as carbon monoxide or hydrogen will be initiated with the

construction of the appropriate four-wave mixing facilities for working in

the 150 nm wavelength region.

b. Real-Time Measurements of Energy Transfer at Surfaces

The time scale for energy transfer at surfaces remains a major uncharacterized parameter of chemistry at interfaces. The application of time-resolved laser techniques to a variety of solid surface

problems holds promise for providing real-time diagnostic of relaxation

mechanisms. With a variety of techniques, the surface response to laser

excitation (using infrared, visible, or ultraviolet pulses) is probed in

real time.

In collaboration with Drs. Stephenson, Heilweil, and Casassa, the

first real-time measurements of the vibrational relaxation of a surface

species have been made. The vibrational relaxation of OH (3690 cm

-1

) groups on colloidal silica (-120A diameter particles, 200 m

2 /gm) dispersed

in carbon tetrachloride has been found to be 150 ps. This relaxation time

is forty times shorter than for comparable diatomic oscillators at this

wavelength, yet the time is three orders of magnitude longer than that

obtained from linewidth arguments. The implications of the observed

lifetime will only be fully realized when a variety of systems with

different chemical properties have been explored. It is already apparent,

however, that in this system vibrational relaxation is slow on the time

scale of rotational reorientation.

Infrared pump-pro’oe experiments are a general method for establishing

rates and mechanisms of energy transfer. Three issues will be addressed

next. First, the measurement technique will be extended to pressed discs

in vacuum to determine why the vibrational relaxation at the solid-liquid

interface is substantially different from that at the solid-vacuum

interface. Second, the relaxation rates for adsorbed OH species on other

insulator surfaces such as AI 2O

3

, ZnO, and zeolites will be examined to

determine the correlation between vibrational relaxation and substrate.

Third, two-color pump-probe techniques will be applied to establish the

pathways of vibrational relaxation on surfaces.

48
3. Neutron Inelastic Scattering by Adsorbed Species on Surfaces

(R. R. Cavanagh, R. D. Kelley, T. J. Udovic)

Neutron scattering is regarded generally as a tool of solid

state physicists. Recently, however, there have been increasing neutron

scattering efforts in the areas of material and chemical sciences. At

NBS, the Surface Science Division has been working with the Reactor

Radiation Division to apply neutron scattering to molecular processes at

surfaces. Our efforts in the past year can be divided into several

categories

.

The Neutron Inelastic Scattering, (NIS) spectra have been measured

for the first time during a steady-state catalytic chemical reaction. In

this case the hydrogenation of CO to hydrocarbons. The "in situ"

measurements of adsorbate vibrations have been made using a Raney nickel

catalyst (a high surface area material). Reaction products were measured

with gas chromatography while the vibrational spectra were being recorded.

Measurements were made as a function of temperature and H

2 /C0 ratio.

Analysis of the spectral changes which accompany changes in reaction

parameters is not complete. However, it is quite clear that the introduction of CO into the hydrogen gas stream results in a substantial reduction

in the concentration of atomic hydrogen on the nickel surface.

Vibrational modes due to hydrocarbon species are also detected. The

intensity of these modes varies in a complex way with changes in reaction

conditions (partial pressure of carbon monoxide, temperature, and flow

rate) ..

NIS from adsorbed species on platinum black is being investigated.

The hydrogen spectra are being analyzed and the chemical activity of

"defect" hydrogen is being probed. Present data permits the identification of H

2 0 formation at room temperature due to reaction of oxygen with

preadsorbed hydrogen. In addition, NIS of NH? adsorbed on such a "defect"

surface exhibits vibrational spectra in distinct contrast to that anticipated for close-packed platinum planes.

The NIS spectra of the chemisorption of C

2 H 2 and C?tin on Raney nickel

were analyzed. Spectra were taken as a function of surface coverage, of

adsorption temperature, and of subsequent heat treatment. These data

indicate that at low temperatures C

2 H2 adsorbs molecularly on the nickel

surface. In the case of ethylene adsorption, some bond dissociation takes

place even at 120 K leading to a mixed adsorbed phase composed predominantly of a molecular C

2

Hi| species. The vibrational spectra of the

molecularly adsorbed species are very similar to those observed on flat

Ni(111) single crystal surfaces with electron energy-loss spectroscopy.

Thus, as was found on high surface area platinum in an earlier NIS study,

steps and other crystal defects (which are likely to be present in a heterogeneous material like Raney nickel), have little influence on the

structure of the molecularly adsorbed hdyrocarbon. However, the presence

of crystal defects can account, by analogy with single-crystal work, for

the low-temperature (C2 H2 , T > 200 K; C 2 H 4 , T > 120 K) decomposition of

the molecularly adsorbed species on the Raney nickel surface.

1+9
NIS experiments have been initiated on a variety of new materials

including ruthenium black and zeolite H-rho. Novel behavior of hydrogen

has been observed in each case. In the zeolite, for example, a pronounced

reversible conversion of bound hydrogen is apparent when the sample is

heated from 275 K to 550 K. The removal of a peak at 1060 cm

-1 is

accompanied by an increase in the intensity of a peak near 200 cm

-1

.

We plan to extend NIS adsorption studies to supported catalysts,

e.g., Pt/Al 20j or Pt in zeolite Y. We will examine the changes in the

vibrations of chemisorbed hydrogen with particle size. The supported

catalysts will be prepared and characterized in collaboration with various

external laboratories. Further analysis of the material will be provided

by neutron diffraction and possibly small-angle neutron scattering. We

will continue the "in-situ" reaction studies of H

2 /C0 reactions on the

nickel catalyst. Both increased and decreased reaction temperatures and

increased partial pressure of CO are obvious reaction-parameter changes

which could clarify the nature of the surface species which we have

detected on the catalyst surface during reaction.

Next year, we will examine the adsorption of hydrogen on Pd black.

Neutron scattering can distinguish between chemisorbed hydrogen and

hydrogen absorbed into the bulk. We will study the vibrational spectroscopy of hydrogen on the surface and the conversion to the bulk hydride.

NIS will be utilized to probe the surface chemical behavior of RuS?, a potentially promising hydrodesulferization catalyst. The adsorptive

behavior of H

2 and H2S on both Ru black and sulfided Ru black will be

further investigated. Future plans involve investigations the adsorption

of thiophene on RuS2 in an attempt to elucidate the adsorbate-surface

geometry. Knowledge of the exact adsorption scheme (i.e., o-bonded

"edgewise adsorption" through the sulfur heteroatom vs. x-bonded "flatwise

adsorption" through the aromatic ir-cloud) provides insights concerning the

type of reaction favored (i.e., hdyrogenolysis vs. hydrogenation) during

hydrodesulfurization.

Further studies of adsorbate diffusion will be facilitated with the

installation of a cold neutron source on the NBS reactor during the fall

of 1984. Coupled with a redesigned time-of -flight spectrometer , the

capability to probe a wider range of momentum transfers will be significantly enhanced. The increased beam flux, number of detector channels,

and the simultaneous reduction of constraints on sample size will all

increase the flexibility of the technique.

4. Photon-Stimulated Desorption and Ultraviolet Photoeraission

Spectroscopy Using Synchrotron Radiation

(R. Stockbauer, R. L. Kurtz, T. E. Madey)

A major goal of this effort is to identify the electronic

excitations which result in the photon-stimulated desorption (PSD) of ions

from molecules adsorbed on surfaces as well as from condensed films. When

possible, PSD ion yields are correlated with ultraviolet photoemis3ion

spectroscopy (UPS) of both valence states and core-electronic states of

the adsorbed species. These studies provide tests of recent theoretical

50
models of electron- and photon-stimulated desorption of ions. In addition, the synchrotron radiation techniques (PSD, UPS) are combined with

other surface spectroscopies to study adsorption and reactions on solid

surfaces.

Several '’firsts” were achieved in FY 1984: the first studies of a single-crystal stoichiometric oxide surface (Ti02 (110)) using synchrotron

radiation, the identification of a new ion-desorption mechanism in

oxidized Cr(110), and the design of a new two-dimensional display analyzer

for angle-resolved PSD.

During the past year, we have used synchrotron radiation to make

detailed studies of the electronic excitation spectra of the oxidized

Cr(110) surface above the 3p ionization threshold ( ,> 42 eV) . We have

correlated these measurements with PSD of 0

+ ions, and have found a new

ion-desorption mechanism: only certain core-hole states initiate ion

desorption. These findings have important implications for the theory of

PSD. We were the first to apply synchrotron radiation to characterize the

electronic structure of a single-crystal metal oxide surface, Ti02 (110).

Preliminary photoemission results have clarified the d-’oand contributions

to the oxide valence band, and have indicated the presence of a shake-up

resonance - 19 eV below the Fermi level. We also made the first PSD study

of well-characterized TiO? single-crystal surfaces having the (110)

orientation. This major test of the Knotek-Feibelman model of ion

desorption reveals shortcomings in the basic theory. The studies involve

"perfect” Ti02 surfaces, and "defect" surfaces produced by Ar

+ sputtering

and indicate the importance of local-site geometry in PSD and electronstimulated desorption.

This year we installed and tested a new high-flux grazing incidence

monochromator at SURF-II. The new monochromator proceeds a photon flux 20

to 50 times more intense than our present instrumentation in the photon

energy range 25 to 80 eV. The range can be extended to 175 eV with a new

grating. We also completed the design and initiated construction of a new

display-type ion and electron energy analyzer. It is a bandpass analyzer

which employs an ellipsoidal mirror; the design is based on an analyzer

pioneered by Eastman. When complete, it will be the second such analyzer

in operation in North America.

This coming year we plan to use variable wavelength UPS to determine,

for the first time, the influence of oxide surface structure and surface

defect concentration on the electronic properties of transition metal

oxides. Nearly-perfect , annealed TiO? and TijpC^ will be compared with

defective, Ar

+ -bombarded surfaces. Single crystals having different

orientations will be studied; both thermally-stable and thermally-unstable

surfaces (e.g. Ti02 (110) and (100)) will be examined.

We will complete the construction of the new angle-resolving,

display-type ion and electron energy analyzer. We will employ this

special analyzer to measure the surface core-level shifts in several

related series of oxidized substrates, e.g., ranging from clean Ti to

oxidized Ti metal, to stoichiometric Ti 203 and Ti02- This will provide
new insights into the local electronic environment of both cations and

anions. We will employ the same analyzer to initiate the first angleresolved PSD studies on both nearly-perfect and defective oxides, as well

as on molecules adsorbed on these surfaces. Surface structure inferred

from these studies will be correlated with other measurements.

Next year we also plan to extend extend chemisorption measurements on single-crystal oxides to the temperature range below 300 K. Surprisingly,

there are virtually no chemisorption measurements reported at low temperatures. Our goal is the accurate determination of adsorption energetics

and molecular structure for CO, H

20, NH

3 on oxides.

5. Surface Chemical Physics of Single Crystals, Alloys and

Ultrathin Films

(W. F. Egelhof f

)

Interest in ultrathin (down to one atomic layer) metal films on

the part of the surface-science community has greatly increased in the

past three years as it has become clear that an understanding of such

structures will contribute to solving a wide range of problems in surface

science. Among these are an understanding of the differences between

surface and bulk electronic structure, the changes of surface electronic

structure associated with chemisorption, the basis of surface magnetism,

the modification of 3urface-layer properties by the substrate, the

relationship between electronic structure and epitaxial growth, and the

surface properties of alloys. The experimental program presently underway

is using an integrated approach to study single-crystal surfaces, alloys

and ultrathin films to contribute to an understanding of all of the above

problems

.

This past year a new aspect of x-ray photoelectron spectroscopy (XPS)

and Auger-electron spectroscopy (AES) of thin films was discovered that can be used to characterize the morphology of films and to provide a new

approach to analyzing XPS and AES data. The phenomenon takes the form of

enhanced intensities of photoelectrons or Auger electrons from an atom in

the film along the internucleus axes towards neighboring atoms. This is

illustrated in Fig. 9. For 2 ML of copper on Ni(100), the inner Cu layer

will have its core-level peak intensities enhanced at a polar angle of

45°, as shewn, due to forward scattering by the top Cu layer. Figure 10

presents experimental data of the angular distributions of the Cu 2pj/?

signal as a function of the film thickness that show a strong peak at

8=45° when the Cu film thickness exceeds 2 ML. The effect is more

pronounced for electron energies greater than 500 eV; for example, in the

angular profiles of the Cu AES peak several peaks are observed that can be

interpreted as arising from specific layers of Cu atoms. Numerical

calculations demonstrate that this type of enhancement is not due to

electron channeling (the Kikuchi effect) as has been suggested previously.

52
Fig. 9 Illustration of enhanced forward scattering at a polar angle of

45° from the bottom Cu layer by the top Cu layer of a 2ML copper

film on a Ni(100) substrate.

EPITAXIAL Cu ON Ni(IOQ)

Fig. 10 Intensity of the Cu 2

p

3/2 X?S signal as a function of polar angle

for different thicknesses of copper (in monolayers) epitaxially

deposited on Ni(100).
This forward scattering phenomenon has been found to be an easily

used, new approach to gaining structural information about surfaces. The

enhanced intensities along internuclear axes provide a very useful

diagnostic of whether epitaxy, agglomeration, surface alloying, or interdiffusion are occur ing at a multicomponent surface.

The forward-scattering phenomenon can be utilized to provide additional, new data important for surface and interface science. The XPS

signal originating from a particular layer can be selected by appropriate

choice of angle. This provides a convenient means of measuring the

core-level binding energies of surface atoms and comparing them with the

binding energies of atoms in other layers and in the bulk. The phenomenon

also provides a new way of determining the background in XPS and AES data;

this method is described in Section 2. A. 8. Finally, this work has shown

that conventional methods of measuring electron attenuation lengths for

quantitative surface analysis by XPS and AES can be subject to appreciable

errors, as described in Section 2. A. 7; the ”eff ective” attenuation length

for the analysis of thin ordered films will depend on the film morphology

and crystalline perfection as well as the instrumental geometry.

A major, invited review article, ’’Core-Level Binding-Energy Shifts in

Solids and at Surface,” is currently being written for Surface Science

Reports. In addition work will continue on the development of the

forward-scattering effect as a tool for structural and electronic studies

of surfaces and interfaces; the refinement of the use of the equivalent-core 3orn-Haber-cycle method for determining thermochemical quantities at surfaces and interfaces; and the initiation of a program to make

nonequilibrium, exotic alloy structures such as alternating single,

epitaxial atomic layers of Cu/Ni/Cu/Ni/Cu. . . . This plan represents an extension of the field of ordered alloy systems to a potentially unlimited

number of nonequilibrium ordered systems. Novel materials such as these

have not been prepared previously but are expected to have unusual

physical and chemical properties.

6. Atcm-Probe Field-Ion Microscopy of Materials

(A. J. Melmed, R. Klein, M. E. Twigg)

We are completing the construction of an Atom-Probe Field-Ion

Microscope (APFIM) facility which will enable microscopic determinations

of composition and atomic structure to be made on a wide range of materials in both the surface and the bulk regions of a sample. The atom probe

facility will consist of three instruments: (1) A rapid turnover lowtemperature field-ion microscope (built earlier) for initial specimen

surveys and microstructure characterization; (2) A general-purpose atom

prcbe (GPAP) for initial microccmposition surveys; and (3) A high-resolution energy-corrected atom probe (ECAP) for accurate microcomposi tional

analyses, especially for very small (< 10 nm) precipitates, alloy phases,

etc.

The general purpose atom probe (GPAP), completed last year, has been

applied to the determination of the composition and the micro-structure of

superalloys. With the imaging and variable-aperture capability of the

5^
GPAP , selected ares of the alloy specimens, that is single-phase regions,

have been isolated for analysis. This is important to alloy designers

since the composition of microprecipitates strongly influences the

properties of alloys. The GPAP is especially suited for this type of

research into structures and composition for precipitates, etc. whose

major dimensions are less than about 50 nm. The GPAP is thus extending

our measurement capabilities to an important area of materials characterization.

An evaluation of the instrumental factors influencing the

compositional analysis of alloys has been initiated and i3 more than half

completed. This work will lead to an optimization of procedures to

improve the confidence limits in determinations of composition with our GPAP.

Considerable progress has been made in the construction of the

energy-corrected atom probe (ECAP). The main chamber, the straight-path

detector, the energy-compensation lens, and the end-flight ion detector

have been assembled. The vacuum manifold, gas-supply systems and

electronic control units are being constructed.

During the past year, a nickel-base superalloy, RSR 143

(Ni , A1 ,Mo,Ta) , has been analzyed with the GPAP and an analytical transmission electron microscope (TEM) . The compositions of the three phases of

this alloy, namely the o matrix, the o’ cuboids and the DO 22 platelets

were determined, and it was found that the platelets (fine-scale

precipitates) were rich in aluminum and tantalum. The conclusion is that

both elements provide for the stabilization of the precipitates in this

alloy.

The alloy RSR 224a (Ni,Al,Mo,W) has been studied in cooperation with

the Pratt and Whitney Company. It was found that W is present in excess

in the small precipitates and indeed is absent from the surrounding

matrix. The inference is that further addition of W should provide

additional precipitate growth, so that particular mechanical properties

may be enhanced.

In conjunction with the comparisons of analyses made using the GPAP

and TEM, calculations have been made of the effect of bremsstrahlunginduced fluorescence on x-ray micro-analysis in the TEM. This fluorescence

should not be overlooked in the analysis of thin-foil specimens composed

primarily of heavy elements.

Measurements have been made on Ni^Al to assess factors influencing

the analysis of composition with the GPAP. As found by others, increasing

the ratio of pulse voltage to dc bias voltage (from 10 percent to 40

percent) results in increases of the measured Ni/Al ratio towards the

nominal value. Further analysis of these experiments is in progress.

During the next year we will initiate micro-diffusion measurements

for metal/metal and metal/semiconductor systems. We plan the completion

of construction and testing of the ECAP and its application to grain-
boundary segregation studies in nickel-base superalloys and to

interdiffusion measurements of metal layers on semiconductors. We will

also continue studies establishing and extending the confidence limits of

atom-probe analysis of complex systems.

7. Surface Theory

(S. M. Girvin, P. K. Lam, T. J. Jach, D. E. Ramaker)

Theoretical research is conducted in areas judged useful for the

experimental program and to develop a conceptual base for surface and

interface properties, processes, interactions, and measurements.

A principal area of investigation has been the quantum Hall effect.

This phenomenon occurs in a two-dimensional electron gas (inversion layer)

found at an atomically smooth interface between two semiconducting

materials. At very high magnetic fields (10T) and low temperatures (IK),

the Hall resistance of such devices is found to take on universal

quantized values h/e^i where h is Planck's constant and e is the elementary charge. The quantum number i can be an integer or a simple rational

fraction.

The main problem of current interest is to develop an understanding

of the origin of the fractional quantum numbers. These are believed to

arise from a collective many-body ground state due to Coulomb interactions. A new formalism has been developed for doing quantum mechanics in

high magnetic fields in two dimensions. The work centers on the use of

coherent states which allow formulation by semi-classical wave packet

dynamics. This method reduces the complexity of computer calculations

which so far have limited the number of interacting particles for which

the Schroedinger equation can be solved. We are presently in the process

of applying this formalism to the fractional quantum number problem using

both analytical and numerical Monte Carlo methods.

This year we have predicted the critical value of magnetic field for

the liquid-solid transition (Wigner crystallization) of the inversion

layer. For this work, a new variational wave function was developed to

describe the solid phase. This wave function includes correlations and i3

thus an improvement over previous Hartree-Fock results. Numerical

evaluation of the ground state energy shows that above a critical magnetic

field corresponding to 6.5 flux quanta per electron, the inversion layer

solidifies (see Figure IT) and the fractional quantum Hall effect will no longer be observed. This prediction is in good agreement with the limited

experimental data presently available and has stimulated considerable new

interest in attempts to directly observe Wigner crystallization in

inversion layers.

Work also continued on the elucidation of the mechanisms that can give rise to electron- and photon-stimulated desorption of ions from

adsorbates on surfaces as well as from single crystals. Particular

attention was given to the competitive core-hole decay processes (i.e.

resonant photoemi3sion and Auger decay) either before or after escape of

the excited electron. The adsorption systems studied included OH and H-0

5o
V

1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1/10

8.28 16.56 24.84 33.12 41.40

B (Tesla)

ig. 11 Ground^ state energy vs. magnetic field B at an electron density

cf IQ 1 '/cm2

. The corresponding Landau level filling factor v is

shown on the upper horizontal axis. The dashed line is the

Hartree-rock energy for the solid. The solid line labelled e7 is

the improved solid energy incorporating correlations. E^ i3 :ne

liquid state energy. Note that for > 6.5, the 30lid i3 more

stable than the liquid.

57
on Ti(001), Cr(110), and Cu(100) and 0 on Cr(110). A study has also begun

to explain the relative 0

+ desorption yields from single-crystal and

ion-sputtered Ti02 surfaces. All of these systems have been studied

experimentally at NBS (Section 4.B.4). The role of excitonic or defect-like states in the localization of either the initial excited

electrons or the two final-state holes, and hence in the desorption

process, will be further studied.

Next year we plan to complete the analytic and numerical

Monte Carlo descriptions of the fractional quantum number ground state and

see the application to inversion layer systems of the semi-classical

wave-packet techniques pioneered by Keller for chemical physics problems.

Work will continue (in collaboration with Prof. M. W. Cole of the Pennsylvania State University) to develop a scattering theory formalism to

describe the repulsive part of the rare-gas metal-substrate interaction.

C. Surface Science Experiments at the Brookhaven

National Synchrotron Light Source

A major undertaking since FY 1981 has been the initiation of an NBS-Naval Research Laboratory collaboration to establish four beam lines

at the Brookhaven National Synchrotron Light Source (NSLS). The Surface

Science Division is directly involved in experiments on two of the four

beam lines on the x-ray ring. These experiments have been delayed due to

machine difficulties at Brookhaven.

1 . Photon-Stimulated Desorption and Photoemission Spectroscopy

(T. E. Madey, R. Stockbauer)

This work is an extension of the experiments performed at the

NBS SURF-II facility (see section 4.B.4) to the higher photon energies

and more intense light available at the NSLS. Light from a unique,

extended-range monochromator (5-5000 eV) will be used to study surface

structure and surface bonding of adsorbed molecules. Experimental probes

will include photoemission spectroscopy, surface extended x-ray absorption

fine structure, and, particularly, photon-stimulated desorption.

The main experimental ultrahigh vacuum chamber has been designed and

constructed, and the sample preparation chamber has been designed and

acquired. Ancillary electronics and uhv apparatus (manipulator,

LEED/ESDIAD optics, argon ion gun, etc.) have been purchased. The first

efforts will be to characterize the roles of both core-level and valence

excitations in the photon-stimulated desorption (PSD) of ions from

covalently-bonded surface molecules.

2. Dynamical Effects Associated with Core-Level Ionization

(T. J. Jach)

The NBS Quantum Metrology Group under the direction of Dr. R. D.

Deslattes is developing an experiment to study electronic effects

associated with core-level ionization in atoms, molecules, and simple

solids. Specific effects to be studied include dynamical effects in
inner-shell vacancy production, relaxation of the extra-atomic

environment, and production of multiple vacancies with or without

ionization. One member of the Surface Science Division staff has been

assigned to work half-time on this project.

The experiment will consist of a high-resolution crystal

monochromator designed to operate between 0.8 and 5 eV in an ultrahigh

vacuum. It is planned to study photoemission. Auger-electron emission,

and x-ray fluorescence from gas and solid targets, particularly in the

vicinity of thresholds for core-level ionization. The facility will also

be used for extended x-ray absorption fine structure and x-ray

standing-wave studies of clean surfaces and interfaces.

We have become the first user group to obtain radiation from a port

on the NSLS x-ray ring. Present work involves the extensive modification

and positioning of our beam output facilities in collaboration with machine

physicists at NSLS. We have developed a new generation of photodiodes

particularly designed for x-ray synchrotron experiments. These diodes are compatible with ultrahigh vacuum (UHV) systems and are bakeable. We are

designing the second-generation UHV experimental chamber for

surface-sensitive x-ray experiments. This chamber will be particularly

suited for x-ray standing wave, surface diffraction, and reflectance

studies. It will include a manipulator capable of being set to very small

angles with high accuracy.
5. PUBLICATIONS

(a) Publications of Past Year

Andreadis, T., Fine, J., and Matthew, J. A. D. , " Auger Electron

Emission from the Decay of Collisionally-Excited Atoms Sputtered from

A1 and Si,” Nucl. Instr. and Meth. 209/21

0

, 495 (1983).

Andreadis, T. D., Roush, M. L. , Davarya, F., Chambers, G. P., Goktepe,

0. F., and Fine, J., "Shape of the Target Region Affected by the

Cascade Recoils,” Nucl. Instrum., and Meth. 21 8, 74? (1983).

Bange, K. , Grider, D. E., Sass, J. K., and Madey, T. E. , "The Surface

Chemistry of I^O on Clean and Oxygen-Covered Cu(110),” Surface Sci.

136, (1984).

Benndorf , C. and Madey, T. E. , "Adsorption and Orientation of NPU on

Ru( 001 )

, " Surface Sci. 135, 164 (1983).

Benndorf, C. and Madey, T. E., "Interaction of NH^ with Adsorbed Oxygen

and Sodium on Ru(001): Evidence for both Local and Long-Range Interactions," Chem. Phys. Lett. 101, 59 (1983).

Benndorf, C., Ndbl, C., and Madey, T. E. , "H 2O Adsorption on Oxygen-Dosed

Ni ( 1 1 0 ) : Formation and Orientation of 0H(ad)," Surface Sci. 138, 292

(1983).

Bertel, E. , Stockbauer, R., and Madey, T. E. , "Electron Emission and Ion

Desorption Spectroscopy of Clean and Oxidized Ti(0001)," Surface Sci.

141 , 355 (1984).

Blaszczyszyn, R., Blaszczyszyn, M. , Meclewski, R. , Melmed, A. J. , and

Madey, T. E., "Interactions of Sulfur with Nickel Surfaces:

Adsorption, Diffusion, and Desorption," Surface Sci. 131, 433

(1983).

Cage, M. E. , Dziuba, R. F., Field, B. F., Williams, E. R. , Girvin, S. M.

,

Gossard, A. C. , T3ui , D. C., and Wagner, R. J., "Dissipation and

Dynamic Nonlinear Behavior in the Quantum Hall Regime," Phys. Rev.

Lett. 51_, 1 374 (1 983).

Cage, M. E. and Girvin, S. M. , "The Quantum Hall Effect I," Comments on Solid State Physics 1

1

, 1 (1983).

Cage, M. E., Field, B. F., Dziuba, R. F., Girvin, S. M. , Gossard, A. C.

,

and Tsui, D. C., "Temperature Dependence of the Quantum Hall

Reistance," Phys. Rev. 3. Rapid Commun. 3£, 2286 (1984).

Cavanagh, R. R. and King, D. S., "Molecule-Surface Interactions and

Dynamics," J. Vac. Sci. Tech. A2, 1036 (1984).

oO
Cavanagh, R. R. f Rush, J. J., Kelley, R. D., and Udovic, T. J.,

"Adsorption and Decomposition of Hydrocarbons on Platinum Black:

Vibrational Modes from NIS," J. Chem. Phys. _80_, 3478 (1984).

Cavanagh, R. R., Rush, J. J., and Kelley, R. D., "Comment on Quantum

Motion of Chemisorbed Hydrogen on Ni Surfaces," Phys. Rev. Lett. 52 , 2100 (1984).

Cowan, P. L., Hastings, J. 3., Jach, T. J., and Kirkland, J. P., "A

UHV-Compatible Two-Crystal Monochromator for Synchrotron Radiation,"

Nucl . Instr. and Method. 208 , 349 (1983).

Doering, D. L. , Semancik, S., and Madey, T. E., "Coadscrption of Water and

Sodium on the Ru(001) Surface," Surface Sci. 1 33, 49 (1983).

Ederer, D. L. , Madden, R. P., Parr, A. C., Saloman, E. B., Rakowsky, G.

,

Cooper, J., Stockbauer, R., Madey, T. E. , and Dehmer, J. L. , "An

Overview of Research at NBS Using Synchrotron Radiation at

SURF-II," IEEE Trans. Nucl. Sci. NS30, 1020 (1983).

Egelhoff, Jr., W. F., "Surface Electronic Structure and Screening of

3d-Band Holes in Cu(100)," Phys. Rev. 3. 329 , 4769 (1984).

Egelhoff, Jr., W. F. , "A Core-Level Binding-Energy-Shift Analysis of

Adsorption and Desorption," Phys. Rev. 3. 529 , 3681 (1984).

Egelhoff, Jr., W. F. , "A Core-Level Binding-Energy Shift Analysis of N 0 on

Ni ( 1 00) , " J. Vac. Sci. Technoi. A2, 1013 (1984).

Egelhoff, Jr., W. F., "Surface Electronic Structure Changes Induced by

Chemisorption," J. Vac. Sci. Technoi. A2, 932 (1984).

Egelhoff, Jr., W. F., "Growth Morphology Determination in the Initial

Stages of Epitaxy by XPS," J. Vac. Sci. Technoi. A2 , 350 (1984).

Egelhoff, Jr., W. F., "N 2 on Ni(100): Angular Dependence of the N-j 3 XPS

Peaks," Surface Sci. 141, L324 (1984).

Egelhoff, Jr., W. F. , "X-Ray Photoelectron and Auger Electron Forward

Scattering: A New Tool for Studying Epitaxial Growth and

Core-Level Binding Energy Shifts," Phys. Rev. 3 RaDid Comm. 30, 1052

(1984).

Erickson, N. E. and Powell, C.

Shell-Excitation Energies

AEAPS , and EELS," J. Vac.

J., "Summary Abstract: Comparison of

of 38, Transition Metals Obtained by

Sci. Technoi. A2, 840 (1984).

6l
Fine, J., Andreadis, T. D., and Davarya, F., "Measurement of

Time-Dependent Sputter-Induced Silver Segregation at the Surface

of a Ni-Ag Ion Beam Mixed Solid," Nucl. Instr. and Meth. 209/210,

521 (1983).

Fine, J., Andreadis, T. D., and Matthew, J. A. D., "Anomalous N

2 ^ Auger

Spectra of In and Sn," J. Phys. C. 17 , L257 (1984).

Fine, J. and Navinsek, 3., "Thin Film Reference Materials for Sputter

Depth Profile Calibration," Proceedings of Invited Lectures and

Progress Reports, Eleventh Symposium on the Physics of Ionized

Gases , edited by G. Pichler, Institute of Physics of the

University (Zagreb, Yugoslavia) 1983, p. 223.

Gadzuk, J. W. , "Shape Resonances, Overtones, and Electron Energy Loss

Spectroscopy of Gas Phase and Physisorbed Diatomic Molecules," J.

Chem. Phys. 79* 3982 (1983).

Gadzuk, J. W. , "Vibrational Excitation in Molecule-Surface Collisions due

to Temporary Negative Molecular Ion Formation," J. Chem. Phys. 79,

6341 (1983).

Gadzuk, J. W. and Holloway, S., "Trajectory Approach to the Hydrogen

Evolution Reaction," Phys. Scripta T4_, 36 (1983).

Gadzuk, J. W. , "Non-adiabatic Effects in Elementary Surface Reactions:

State-to-State Molecular Beam Experiments as a Probe," in

Many-body Phenomena at Surfaces , D. Langreth and H. Suhl, eds.,

Academic Press 517 (1984).

Girvin, S. M. , "Hall Effect," Mc-Graw-Hill Encyclopedia of Science and

Technology (1984).

Girvin, S. M. , "Particle-hole Symmetry in the Anomalous Quantum Hall

Effect," Phys. Rev. B. Rapid Commun. _29, 6012 (1984).

Girvin, S. M. , "Anomalous Quantum Hall Effect and Two-Dimensional

Classical Plasmas: Analytic Approximation for Correlation Functions

and Ground State Energies," Phys. Rev. 3 .30, 558 (1984).

Girvin, S. M. and Cage, M., "The Quantum Hall Effect II," Comments on Solid State Physics 1

1

, 47 (1983).

Girvin, S. M. and Jach, T. , "Interacting Electrons in Two-Dimensional

Landau Levels: Results for Small Clusters," Phys. Rev. 3. 28 , 4506 (1983).

Girvin, S. M. and Jach, T. , "Formalism for the Quantum Hall Effect:

Hilbert Space of Analytic Functions," Phys. Rev. B. 29, 5617

1984).

-62
Hardy, S. and Fine, J., "Surface Segregation in Liquid Ga-Sn Alloys by

AES," Surface Sci . 134, 184 (1983).

Jach, T. and Cowan, P. L., "PIN Diodes as Detectors in the Energy Region

500 eV - 10 eV," Nucl. Instr. and Methods 208 , 423 (1983).

Jach, T. and Powell, C. J., "X-Ray Photoemission Spectroscopy of

Environmental Particles," Environmental Sci. & Technol. 1

8

, 58

(1984).

Jonson, M. and Girvin, S. M., "The Thermoelectric Effect in a Weakly

Disordered Inversion Layer Subject to a High Magnetic Field,"

Phys. Rev. B. 29, 1939 (1984).

Kelley, R. D. and Semancik, S., "Gn the Mechanism of Fischer Tropsch

Synthesis on a Single Crystal Nickel Catalyst," J. Catal. _84_, 248

(1983).

Kelley, R. D., Cavanagh, R. R., and Rush, J. J., "Coadsorption and

Reaction of H

2 and CO on Raney Nickel: ' Neutron Vibrational

Spectroscopy," J. Catal. 83 > 464 (1983).

Klaua, M. and Madey, T. E. , "The Adsorption of H

20 on Clean and

Oxygen-Dosed Silver Single Crystal Surfaces," Surface Sci. 136, 42

(1984).

Lam, P„ K. and Girvin, S. M., "Liquid-Solid Transition and the Fractional

Quantum Hall Effect," Phys. Rev. 3. Rapid Commun. 30 » 473 (1984).

Landman, U. , Kleinman, G. G. , Cleveland, C. L. , Kuster, E. , Barnett, R. N.

and Gadzuk, J. W. , "Hindered and Modulated Rotations of Adsorbed

Diatomic Molecules: States and Spectra," Phys. Rev. B. 29 , 4313

(1984).

Madey, T. E. , "Early Applications of Vacuum, From Aristotle to Langmuir,"

J. Vac. Sci. Tech. A2, 110 (1984).

Madey, T. E., Doering, D. L. , Bertel, E., and Stockbauer, R., "Electron

and Photon Stimulated Desorption: Benefits and Pitfall3,"

Ultramicroscopy JJ_, 187 (1983).

Madey, T. E. , and Stockbauer, R., "The Use of Angle Resolved Electron and

Photon Stimulated Desorption for the Determination of Molecular

Structure at Surfaces," Rev. Brasil. Apl. Vac. (J. of Brazilian

Vacuum Society) 3, 47 (1983).
Madey, T. E. , Benndorf, C., Doering, D. L. , and Semancik, S. , ’’Influence

of Surface Additives, Sodium and Oxygen, on the Structure and

Bonding of H

2o and NH 3 on Ru(001),” Proc. of 1984 Internat. Catal.

Cong., Berlin (DECHEMA, Frankfurt, 1984), Vol. IV, p. 51 .

Melmed, A. J. and Carroll, J. J., ”An Approach to Realism in Field Ion

Microscopy via Zone Electropolishing,'’ J. Vac. Sci. Tech. A 2, 1388

(1984).

Miskovic, Z., Vukanic, J., and Madey, T. E. , ’’Influence of the Image

Interaction of Ion Desorption Processes,” Surface Sci. 141, 285

Navinsek, B., Panjan, P., Zabkar, A., and Fine, J., "Determination of

Sputtering Yields from a New Procedure for Depth Profiling of

Multilayered Structures,” Nucl. Instr. and Meth. B2_, 670 (1984).

Netzer, F., Doering, D. L., and Madey, T. E. , "Evidence for Molecular

Reorientation on a Surface: Coadsorption of CO and Na on Ru(OOI),” Surface Sci. 143, 363 (1984).

Powell, C. J., "Inelastic Scattering of Electrons in Solids,” in Electron

Beam Interactions with Solids for Microscopy, Microanalysis, and

Microlithography, D. F. Kyser, H. Niedrig, D. E. Newbury and R.

Shimizu, eds. (SEM Inc., Chicago, 1984), pp. 1 9 — 31-

Ramaker, D. E. , "Comparison of Photon Stimulated Dissociation of Gas

Phase, Solid, and Chemisorbed Water,” J. Chem. Phys. 80_, 133 (1983).

Roush, M. L., Davarya, F. , Chambers, G. P., Anareadis, T. D., He, J.,

Goktepe, 0. F., and Fine, J., "Distribution of Origins of

Sputtered Particles and the Shape of the Target Region Affected by

the Cascade Recoils,” Nucl. Instr. and Meth. B2_, 693 (1 984).

Rush, J. J., Cavanagh, R. R., and Kelley, R. D., "Neutron Scattering from

Adsorbates on Platinum Black,” J. Vac. Sci. Technol. AJ_, 1245 (1983)

Simons, D. G., 3rcwn, M. D., Fine, J. , Andreadis, T. D., and Navinsek, 5.

"Rutherford Backscattering Analysis of Multilayered Cr-Ni

Structures to be used for Sputtering Standards,” Nucl. Instr.

and Meth. 213, 585 (1983).

Stockbauer, R. and Madey, T. E. , "Desorption of Ions from Surfaces:

Mechanisms of Photon Stimulated Desorption,” Annals of the Israel

Phys. Soc. 6_, 483 (1 983).

Stockbauer, R. , Hanson, D. M., Flodstrom, S. A., Bertel, E. , and Madey,

T. E., "Photon Stimulated Desorption of Ions from Water and Methanol

Adsorbed on a Ti(001) Surface,” Physica Scripta T4_, 126 (1983).

Stockbauer, R. , "Instrumentation for Photon Stimulated Desorption,” Nucl.

Instrum. Meth. 222, 284 (1934).

64
Stockbauer, R. f Ramaker, D. E. , Bertel, E. , Kurtz, R. L. , and Madey, T.

E. , "Mechanisms for Photon Stimulated Desorption of 0

+ from

Cr ( 1 1 0 )

, " J. Vac. Sci. Tech. A2, 1053 (1984).

Twigg, M. E. , "The Effect of Continuum Fluorescence on X-Ray Microanalysis

in the Analytical Transmission Electron Microscope," J. Microscopy

134, RP1-RP2 (1 984).

65
(b) Publications in Progress

Bange, K. , Madey, T. E. , and Sass, J. K., "Characterization of OH(ad)

Formation by Reaction Between H

20 and 0(ad) on Ag(110)," Proceedings

of ECOSS-6, Surface Sci. (in press).

Bertel, E. , Kurtz, R. L. , Stockbauer, R., Ramaker, D. E. , and Madey, T. E.

"Photon Stimulated Desorption of H

+ Ions From OH on Ti , Cr, Cu, and

From Bulk Solid H

20," Phys. Rev. Rapid Commun. , (submitted).

Bertel, E. , Stockbauer, R., Kurtz, R. L. , Madey, T. E., and Ramaker,

D. E. , "The Decay Channels of the 3p Resonance in the 3d

Transition Metals and Their Relevance to the Mechanisms of

Electron- and Photon Stimulated Desorption," Proceedings of ECOSS-6,

Surface Sci. (in press).

Bertel, E. , Stockbauer, R., Kurtz, R. L. , Ramaker, D. E. , and Madey, T.

E. , "Resonant Photoemission and the Mechanism of Photon Stimulated

Ion Desorption in Transition Metal Oxides," Phys. Rev. Lett,

(submitted)

.

Butler, J. J., Holland, D. M. P., Parr, A. C., Stockbauer, R., and Buff,

R., "Automation of the MBS Threshold 'Photoelectron-Photoion

Coincidence Mass Spectrometer," J. Phys. E. (submitted).

Butler, J. J., Holland, D. M. P., Parr, A. C., and Stockbauer, R. , "A

Threshold Photoelectron-Photoion Coincidence Spectrometric Study of

Dimethylether (CH^OCH^)," Int'l. J. Mass Spectrom. Ion Phys. (in

press )

.

Cavanagh, R. R. and King, D. S. , "Laser Studies of Surface Chemical

Reactions," Chemistry and Physics of Solid Surfaces V , Springer

Verlag, New York, Inc. (in press).

Ceyer, S. T. and Yates, J. T. Jr., "Ammonia Adsorption on the Ag (311)

Surface," Surface Sci. (submitted).

Ceyer, S. T. , Melmed, A. J., Carroll, J. J., and Graham, W. R., "On the

Atomic Structure and Optical Constants of (001) Tantalum," Surface

Sci. (in press).

Ciszewski, A. and Melmed, A. J., "Epitaxial Growth and Some Properties of

Samarium Crystals on Tungsten," J. de Physique (submitted).

Ciszewski, A. and Melmed, A. J., "Epitaxial Crystal Growth of HCP Metals

on BCC Metals: Dysprosium on Tungsten," J. Cryst. Growth (in press).

Ciszewski, A. and Melmed, A. J., "Epitaxial Crystal Growth of Gadolinium

on Tungsten," Surface Sci. Lett, (in press).

Ciszewski, A. and Melmed, A. J., "Surface Self-Diffusion of Dysprosium and

Gadolinium," Surface Sci. Lett, (in press).

66
Egelhoff, Jr., W. F., "Core Level 3inding-Energy Shifts in Solids at

Surfaces," Surface Sci. Reports (submitted).

Egelhoff, Jr., W. F. , "Angle Resolved XPS of the Epitaxial Growth of Cu on

Ni ( 1 00) , " Proceedings of the 1st Int. Conf. on the Structure of

Surfaces, (submitted).

Egelhoff, Jr., W. F. and Armstrong, R. A., "The Role of Forward Scattering

in Angle Resolved XPS," Surface Sci. (submitted).

Fine, J. and Hardy, S., "Methods for Producing Clean Liquid Surfaces,"

Treatise on Clean Surface Technology , ed. K. L. Mittal,

(submitted)

Gadzuk, J. W. and A. C. Luntz, "On Vibrational Lineshapes of Adsorbed

Molecules," Surface Sci. (in press).

Gadzuk, J. W. and Ndrskov, J. K., "Vibrational Excitation, Harpooning,

and Sticking in Molecule-Surface Collisions," J. Chem. Phys.

(in press).

Heilweil, E. J. , Casassa, M. P., Cavanagh, R. R. , and Stephenson, J.C.,

Picosecond Laser Studies of Energy Transfer in Molecules on Surfaces," J. Chem. Phys. (in press).

Heinonen, 0., Taylor, P. L. , and Girvin, S. M. , "Electron-Phonon

Interactions and Breakdown of the Dissipationless Quantum Hall

Effect," Phys. Rev. B. (in press).

Holloway, S. and Gadzuk, J. W. , "Energy Redistribution and Dissociative

Adsorption in Molecule-Surface Collisions Involving Charge

Transfer/Surface Hopping," Surface Sci. (in press).

Kelley, R. D., Cavanagh, R. R., Rush, J. J., and Madey, T. E. , "Neutron

Spectroscopic Studies of the Adsorption and Decomposition of C

2 H2

and C

2H 4 on Raney Nickel," J. Catal. (submitted).

King, D. S. and Cavanagh, R. R. , " Final-State Resolved Studies of

Molecule Surface Interactions," in New Laser and Optical Investigations of Chemistry and Structures at Interfaces. Veriag Chemie,

( in press )

Madey, T. E. and Benndorf, C., "Influence of Surface Additives (Na and 0) on the Adsorption and Structure of NH^ on Ni(110)," Proc. of

EC0SS-6 (Surface Science, in press).

Madey, T. E. , Ramaker , D. E. , and Stockbauer, R., "Characterization of

Surfaces Using Electron and Photon Stimulated Desorption," Ann.

Rev. Phys. Chem. (in press).
Melmed, A. J., Maurice, V. , Frank, 0., and Block, J. H. , ’’Rare-Earth

Crystal Growth from the Vapor: Eu/Re and Eu/W," J. de Physique

(in press).

Miskovic, Z., Vukanic, J., and Madey, T. E. , ’’Influence of the Image

Interaction on Electron Stimulated Desorption Ion Angular

Distribution (ESDI AD) Proc. of Symposium on Physics of Ionized

Gases, SPIG (Dubrovnik, Yugoslavia, August 1984), (in press).

Powell, C. J., ’’Inelastic Mean Free Paths and Attenuation Lengths of

Low-Energy Electrons in Solids,” Scanning Electron

Microscopy/ 1 984 (in press).

Powell, C. J., ’’Innershell Ionization Cross Sections,” in Electron Impact

Ionization , T. D. Mark and G. H. Dunn, eds. Springer-Verlag (in

press)

.

Shinn, N. D. and Madey, T. E., "Unusual C-0 Bond Weakening on a Clean

Metal Surface: CO on Cr(110),” Phys. Rev. Lett, (submitted).

Twigg, M. E. , Melmed, A. J., Klein, R., Kaufman, M. J. , and Fraser, H. L.

,

"A Microanalytical Study of Secondary Precipitation in a Nickel-Base

Superalloy Using Atom Probe Field Ion Microscopy and Analytical

Transmission Electron Microscopy,” Journal de Physique (in press).

Twigg, M. E. , Melmed, A. J., Klein, R., Kaufman, M. J., and Frase, H. L.

,

”A Microanalytical Study of Secondary Precipitation in a Nickel-Base

Superalloy With Use of Atom Probe Field Ion Microscopy and Analytical

Transmission Electron Microscopy,” Proceedings of Workshop on Electron Microscopy (submitted).

Twigg, M. E., Melmed, A. J. , Klein, R., Kaufman, M. J., and Fraser, H. L.

,

"A Microanalytical Study of Secondary Precipitation in RSR 143 using

Atom Probe Field Ion Microscopy and Analytical Transmission Electron

Microscopy,” Proc. of Fifth Internat. Symp. on Superalloys (in

press)

.

Udovic, T. J., Kelley, R. D., and Madey, T. E. , "The Methanation

Activity of W(110)," Surface Sci. Lett, (submitted).

68
(c) Recent Publications of New Staff Members (Kurtz, Lam, Shinn, Twigg,

Udovic, and Wax) Describing Work in Previous Positions

Chou, M. Y., Lam, P. K. , and Cohen, M. L. , "Calculation of Compton Profile

of Beryllium," Phys. Rev. B. 28 1626 (1983).

Chou, M. Y., Lam, P. K. , and Cohen, M. L. , " Ab initio Study of Structural

and Electronic Properties of Beryllium," Phys. Rev. B. _28_, 4179

(1983).

Henrich, V. E. and Kurtz, R. L. , "The Electronic Structure of Oxide

Surfaces and Surface Defects," Proc. IXth Int. Vacuum Congress and

Vth Int. Cong, on Solid Surfaces, Madrid, Spain, 100 (1983).

Henrich, V. E. , Kurtz, R. L., and Sadeghi, H. R., "Summary Abstract:

Chemisorption on Transition-Metal Oxides and Oxide Supported Noble

Metals," J. Vac. Sci. Technol. A1_, 1074 (1983).

Janowicz, A. H., Periana, R. A., Buchanan, J. M., Kovac, C. A., Stryker,

J. M., Wax, M. J., and Bergman, R. G. , "Oxidative Addition of Soluble

Iridium and Rhodium Complexes to Carbon Hydrogen Bonds in Methane and

Higher Alkanes," Pure Appl. Chem. 56., 13 (1984).

Kurtz, R. L. and Henrich, V. E. , "Geometric Structure of the Fe^WOOl)

Surface: A LEED and XPS Study," Surface Sci. 1 29 , 345 (1983 k

Kurtz, R. L. and Henrich, V. E. , "Summary Abstract: The Influence of 0

2

and H

20 on the Surface Electronic Structure of Corundum

Transition-Metal Oxides," J. Vac. Sci. Technol. A2, 842 (1984).

Lam, P. K. and Cohen, M. L., "Calculation of High-Pressure Phases of A1,"

Phys. Rev. B 26, 5986 (1983).

Lam, P. K. and Cohen, M. L. , "Correlation of Superconductivity and

Material Properties," Phys. Lett. 97A, 114 (1983).

Lam, P. K., Chou, M. Y., and Cohen, M. L. , "Temperature and Pressure

Induced Phase Transition in Be," J. Phys. C. VJ_, 1984 (in press).

Lam, P. K. and Chen, M. L. , "Dependence of Lattice Constants and Bulk

Moduli on Pseudopotential Properties," J. Phys. C. (in press).

McDonald, G. W. , Udovic, T. J. , Dumesic, J. A., and Langer, S. H.,

"Equilibria Associated with Cupric Chloride Leaching of Chalcopyrite

Concentrate," Hydrometal, (in press).

McFeely, F. R., Morar, J. F., Shinn, N. D., Landgren, G. , and Himpsei,

F. -J., "Synchrotron Photoemission Investigation of the Initial Stages

of Fluorine Attack on Si Surfaces: Relative Abundance of Fluorosilyl

Species," Phys. Rev. B. 30, 764 (1984).

69
Morar, J. F., McFeely, F. R., Shinn, N. D., Landgren, G. , and Himpsel,

F. J. , "Synchrotron Photoemission Investigation: Fluorine on Silicon

Surfaces,” Appl. Phys. Lett. 45 , 174 (1984).

Shinn, N. D., Morar, J. F., and McFeely, F. R., "Spectroscopic

Characterization of Fluorinated Silicon Single Crystal Surfaces," J.

Vac. Sci. & Technol. (in press).

Wax, M. J., Stryker, J. M. , Buchanan, J. M. , Kovac, C. A., and Bergman,

R. G. , "Reversible C-H Insertion/Reductive Elimination in

( n^-pentamethylcyclopentadienyl) ( trimethylphosphine) iridium

Complexes. Use in Determining Relative Metal-Carbon Bond Energies

and Thermally Activating Methane," J. Am. Chem. Soc. 106, 1121

(1984).

70 ;

.
CHEMICAL KINETICS DIVISION

S. E. Stein, Acting Chief

SUMMARY OF ACTIVITIES

Fiscal Year 1984

1 . INTRODUCTION

A. Objectives

The objective of chemical kinetics is to understand the critical

factors controlling chemical change and reactivity. It seeks to both

interpret chemical change in terms of individual molecular events as well

as develop quantitative theoretical models for these events. The importance and vitality of this area are documented in the "Pimentel Committee

Report" to the National Academy of Sciences. In this report, "Chemical

Reactivity" was chosen as one of the three "Priority Areas" in chemistry

where the greatest opportunities for major advances lie (see, Chemical and

Engineering News, Jan. 4, 1984, p. 8).

Exciting experimental breakthroughs and theoretical developments have

been made over the last few years that have dramatically expanded the

breadth and depth of the discipline. Experimentally, largely as a result

of advances in laser and computer technology, a new array of instrumentation has been developed with improved precision and sensitivity which are now yielding increasingly varied and detailed kinetic information. Spurred

on by practical concerns such as combustion, these new instruments,

alongside more established techniques, have brought certain areas of

kinetics to a high degree of development where practitioners are on the

verge of devising reliable theories with broad predictive power. The

development of these theoretical tools is of critical importance to the

field, since they provide frameworks for interpreting as well as generalizing results of individual experiments.

While the number and diversity of reactions investigated by

kineticists are increasing at a rapid pace, the area has actually become

more cohesive a3 interrelations between formerly divergent areas become

increasingly clear. For instance, unimolecular reaction rate theories

developed for neutral species are being widely used to explain critical

features of gas-phase ion-molecule chemistry. In a similar manner,

results of gas-phase ion chemistry are beginning to provide a 3olid

conceptual foundation for solution ion chemistry, a poorly understood area

of immense practical importance.

71
In addition to its role as a major fundamental area of science,

chemical kinetics has long been an area directly associated with the

solution of practical problems. The chemistry of combustion, radiation

and the atmosphere are examples of areas whose development depends

directly on advances in chemical kinetics.

Over the past several years the scope of the Chemical Kinetics

Division has expanded significantly. Just three years ago its primary

emphasis was on gas-phase reactions of neutral species. Through recent

additions of staff and changes in research direction, it is now roughly

equally divided into groups studying ion and neutral reactions and on studies in the gas and condensed phases. In addition, the data centers

(ion and neutral) are broadening their efforts in kinetic data evaluation.

Since predictive theories are required by both experimentalists and

’’data evaluators”, the advancement of these theories provides a key

unifying factor for the diverse activities in the division. In fact, it

is expected that the Division may ultimately be able to offer not only NBS

evaluated data but also NBS Calibrated” predictive models for data not

currently available.

Central to the broadening of the Division's scope have been two

competency development programs. One, entitled "Transformations of

Complex Organic Compounds”, has enabled us to add personnel and equipment

in order to apply the exact principles of small molecule kinetics to

reactions of more complex substances. The second program, "Physical and

Chemical Effects of Ionizing Radiation on Matter” has enabled the growth

of existing programs on gas-phase ion-molecule chemistry to include more

complex reactions in both the gas phase and solution.

3. Goals and Activities

The scientific objective of the Chemical Kinetics Division is to

understand the molecular factors governing chemical change and reactivity.

This involves both the measurement of fundamental kinetic properties in

well-defined chemical systems and the development of theoretical concepts

and predictive methods. The Division is particularly interested in areas which either lead to broad improvements in theory or assist in the

solution of important national problems.

In pursuit of the above objective, our work involves the following

types of research: (1) developing new experimental systems for expanding

the scope of kinetic measurement; (2) measuring rates of reactions needed

for interpreting more complex processes; (3) developing theory and

determining rate constants needed for theoretical advancement, (4)

compiling, evaluating and disseminating chemical kinetics data, (5)

interpreting complex chemical reactions in terms of single-step molecular

events.
c. Services to Other Bureau Programs and Other Agencies

The Chemical Kinetics Division devotes a part of its efforts to

serving as a technical resource for other bureau or other agency programs.

We consider this activity to be an integral part of our mission, and a key

means of applying our scientific expertise to industrial, defense, and

environmental problems. Moreover, these projects are directly related to,

and strongly supportive of, the basic long-range scientific objectives of

the division. In FY 84 we participated in the following NBS and OA

programs

Office of Standard Reference Data

• The Chemical Kinetic Data Centers (ion and neutral) for data

compilation and evaluation.

Department of Energy

• Compilation and evaluation of combustion chemistry data

• Chemistry of hazardous waste incineration

• Pulse radiolysis studies of electron transfer in solution

• High pressure mass spectrometric studies of ion kinetics and

thermodynamics.

Department of Defense

• Laser-induced chemistry

• Chemical theory of carbon

• Kinetics of the decomposition of nitroaromatics

• Multiphoton ionization processes

• Tandem mass spectrometric studies

• Ion-cyclotron resonance studies of carbon cluster ions

National Aeronautics and Space Administration

• Elementary kinetic studies of stratospheric reactions

• Chemical kinetics of planetary atmospheres

• Chemical fundamentals of soot formation

• Evaluation of stratospheric chemical kinetic data

7"5
2. CHEMICAL KINETICS AND ION KINETICS ENERGETICS DATA CENTERS

J. T. Herron, R. F. Hampson, Jr., W. Tsang, F. Westley, G. W. Zumwalt

R. J. Cvetanovic, S'. G. Lias and R. D. Levin

A. Introduction

The Chemical Kinetics Data Center and the Ion Kinetics and Energetics

Data Center are responsible for providing compilations and evaluations of

numerical data on the kinetics of chemical reactions, and the thermochemical properties of ionic and free radical species. They provide this

information to scientists in other disciplines, such as the analytical and

biological sciences, and to modelers analyzing very complex chemical

systems such as encountered in combustion and atmospheric chemistry.

Although the major source of information contained in the existing data

bases is derived from experimental studies, our goal is the development

and application of theoretical methods for both the analysis and extrapolation of existing experimental data, and the prediction of the behavior

of systems for which there is no data. Over the past year the Data

Centers have been physically relocated to make them contiguous. This will

allow the two centers to more easily share facilities and equipment as well as to encourage joint planning and programs.

3. Chemical Kinetics Data Center

(J. T. Herron, R. F. Hampson, Jr., W. Tsang, F. Westley

G, W. Zumwalt and R. J. Cvetanovic)

The goals of the Chemical Kinetics Data Center are to provide

compilations, bibliographies and evaluated chemical kinetics data bases

for neutral species in support of science and technology, to develop new

or improved theoretical models for the prediction of chemical kinetic

data, and to develop cost effective methods for the acquisition, evaluation, and dissemination of chemical kinetic data to the scientific and

technical community. This center has served for many years as a national

resource in supplying evaluated chemical kinetic data, data compilations,

and annotated bibliographies covering a broad range of scientifically and

technologically important systems. We have been involved in the chemistry

of stratospheric ozone depletion since it was first recognized as a serious problem, and continue to assist in providing the authoritative

chemical kinetic data base used in modeling stratospheric chemistry. We

provide data compilations and evaluations which address critical problems

in combustion chemistry and allow a more thorough testing of kinetic

theory.

One of our major goals in the area of data evaluation is to provide a definitive set of evaluated rate data for use in combustion modeling. This

activity is supported by the Office of Basic Energy Sciences of DoE. The

first major output of that program, "Chemical Kinetic Data Base for

Methane Combustion", is complete and has been submitted to WERB for

publication in JPCRD. This compilation contains evaluated data on over

250 chemical reactions. In this evaluation, a common theoretical framework, transition state theory, was used to fit data measured over a wide

* "

7 ^
range of temperature and pressure. An example of the results of this

application of theory to combustion reactions can be seen in Fig. 1, where

data on the reaction C

2 Hg < 2CHo are shown. Remarkably, the theoretical

framework shows that the majority of the data in the older literature are

valid, contrary to earlier conclusions, and that one can derive rate

expressions for this reaction over the entire range of temperature and

pressure encountered in a practical combustion systems. This powerful

approach to data evaluation has been widely used in this new data evaluation, and will be used in extensions of this data base.

The extension of this work to include the reactions of propane,

propyl radicals, methanol, and the CH2 0H radical with all species

considered in the methane system is in progress. Over ninety reactions

are involved.

In order to increase the pace of our data evaluation efforts and

focus efforts in the area we have organized an informal, voluntary

committee of chemical kineticists and modelers to provide external inputs

for this work. The present members of our committee are:

David Gutman

Norman Cohen

David Golden

Joe Michael

James Miller

Charles Westbrook

Bruce Klemm

Thomas Dunning

Robert Hampson

Wing Tsang

Illinois Institute of Technology

Aerospace Corporation

SRI International

Brookhaven National Laboratory

Sandia Corporation

Lawrence Livermore Laboratory

Brookhaven National Laboratory

Argonne National Laboratory

NBS, Chemical Kinetics Division

NBS, Chemical Kinetics Division

A related project on developing a compilation of experimentally

determined rate constants for combustion reactions continues. Papers for

the period 1977-1981 have been abstracted and all data entered into the

HP-1000 computer system. This compilation is being extended to include

all data published through 1982. This bibliographic activity is the

prelude for the critical evaluation of combustion data for systems more complex than methane.

An evaluation of stratospheric reactions of halogen containing

species was completed as part of the Data Center contribution to the NASA

Panel for Data Evaluation and has been published by the Jet Propulsion

Laboratory. These data were also reviewed and evaluated for the CODATA

Task Group on Chemical Kinetics. The complete report of that task group

has now been prepared and edited by the Data Center and submitted to JPCRD

for publication.

The development of a data base management system continues under a contract to R. J. Cvetanovic. Most work on the identification and

nomenclature of inorganic and organic compounds is complete. Work on the

l J
LOG

K{/sec)

LOG P(torr)

2 3 LOG POorr)

Figure 1 Comparison between experiments and calculations for the reaction

C

2 H

6 t 2CH

3

A. 2CH

3 + C2 Hg

3. 2CH

3 * C

2 H6

C. C2 H6 -*• 2CH3

370 K (strong collider)

1400 K (argon, 300 cm" step size)

840 K and 913 K (1000 cm 1 and strong collider)

- 76
actual searching routines for chemical reactions is underway and should be

largely complete this year. Each component of the system is being tested

on a suitably designed data base.

C. Ion Kinetics and Energetics Data Center

(S. G. Lias and R. D. Levin)

The goals of the Data Center are to collect and maintain an upto-date file of experimental data pertaining to the thermochemistry of

ions (ionization potentials, appearance potentials, and equilibrium

constants) and the kinetics of ion-molecule reactions, to make compilations of these data and to publish evaluations of the collected data for

use by the scientific and technological communities.

The Data Center is closely associated with the Ion Kinetics Group

which, as described in Section 4., can supply benchmark measurements of

ion thermochemical quantities to aid in data evaluation activities.

(1) Major publications this year were:

"Evaluated Gas Phase Basicities and Proton Affinities of Molecules; Heats of Formation of Protonated Molecules" by Sharon G.

Lias, Joel F. Liebman, and Rhoda D. Levin has been accepted for

publication in the Journal of Chemical and Physical Reference

Data, where it will appear in the third issue of 1984.

This publication includes:

(a) A comprehensive review of the field of gas phase proton

affinity measurements.

(b) A table containing evaluated gas basicities and proton

affinities, as well as the original literature data from

which these were derived, for 784 atomic and molecular

species.

(c) A table containing evaluated heats of formation of all the

molecular species and of the protonated molecules.

(d) An annotated bibliography of references on gas phase proton

affinity determinations.

(e) A bibliography of references to heats of formation of the

relevant neutral molecules.

"Ion Thermochemistry" by S. G. Lias, J. E. Bartmess, J. F.

Liebman, J. L. Holmes, and R. D. Levin is a comprehensive and

complete compilation of evaluated heats of formation of more than

3000 positive and negative ions, now being prepared for puoli ca- tion in the Journal of Physical and Chemical Reference Data:

77
Preliminary work on the evaluation of ion heats of formation is

completed, and a computer printout of a first draft of the tables

for the manuscript is in hand. Figure 2 shows an example page

from the publication. An exhaustive and comprehensive review,

proof-reading and editing effort, coupled with a cross-checking

of recent literature, is now under way in the Data Center.

Outside collaborators J. F. Liebman (University of Maryland,

Baltimore County), J. E. Bartmess (University of Tennessee), and

J. L. Holmes (University of Ottawa) are also reviewing the

tables.

(2) Archive maintenance included literature searching, identification of articles containing data, copying, abstracting, and

encoding data into the computer. Approximately 250 articles

containing approximately 2000 measurements were processed this

year,

(3) Data center automation included the implementation of a researchable data base for the archival data on ionization

potential and appearance potential data, in collaboration with

Dr. David Neumann of the Chemical Thermodynamics Data Center and

the completion of programs permitting the formatting of encoded

evaluated data into forms suitable for publication.

D. Future Plans

As part of the implementation of our long range plan, there will be

major changes in the management and operation of the Chemical Kinetics

Data Center in FY 1985. John Herron has become the Director, and will

spend full time on the design and implementation of Data Center programs.

The broadening of the scope of the Chemical Kinetics Data Center continues

to be a priority objective, as is the increased involvement of other data

evaluators, from inside and outside NBS. To help foster this development,

the Data Center will be heavily involved in the planning and organization

of the International Conference on Chemical Kinetics, to be held at NBS in

June, 1985. Other changes in the direction and operation of the data

cencer will be put in place as plans are developed over the coming year.

A major objective in the coming year will be the implementation of

the database management system and the conversion of existing data bases

into forms suitable for inclusion in the new system. Progress in developing software for the new system has reached the point at which search

routines for chemical reactions will be tested and modifications made

based on an analysis of the test runs. We will continue to work closely

in this endeavor with the Ion Energetics and the Chemical Thermodynamics

Data Centers. We will also be considering various options concerning the

identification, abstraction, and entry of data into the new system.
> CD J in 1 PJ

1 m

i O*

} rs. o <r O* in PJ

Cv

i ! cr <x 1 1 pj > pj T— PJ PJ ! m 1 l 03

1 i 5 O'* : — UJ t\ o pj 1 CD n PJ i m <r IN CD i co ca i Pv i ! i T— 1 i nO <r — m >0 CD 1 — I 21 ! 1 <r 03 n ”"2 *n 1 Ps f J ! i — 1 O 3 CM PJ O'* *0 CD T-> 03 CM in pj o <r —

3 'T J LU2: 1 o o N >0 ?0 :-0 pj >0 o 0 O'* 9*. s >D 1 Qd

1

1

in n >0 PJ "0 N pj Cs =n PJ

1

cn m

<

< -s

3 o

Q— >

— CM PJ PJ PJ

rrs rv

CD 03 CD

•! -?! *H

<r <r PJ CD e

-¥1 CM

>0 O

PJ M

CP C5

o c o <s

1-' pj — PJ PJ o o =

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= I>

PJ **- ID

C c*

Si

C. c

in -

€

t a* pj >0 ^ CD

flj 3

S- 5

\

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cr

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i U

T9

Figure

2.

Sample

page

from

forthcoming

publication:

"Ion

Thermochemistry"

by

S.

G.

Lias,

J.

E.

Martmess,

J.

F.

Liebman

J.

L.

Holmes,

F.

P.

Lossing,

and

R.

D.

Levin,

to

be

submitted

to

J.

Phys.

Chem.

Ref.

Data.
Evaluation activities in the coming year will center around the

continuing development of the combustion chemistry data base. The data

base will be expanded to include additional reactions of three and four

carbon compounds including propene, the butanes and butenes, and

iso-propyl and butyl radicals. The number of reactions of these new

species with species already in the data base is about 192. The total

data base will then consist of over 500 evaluated chemical reactions.

Since relatively little information is available for larger species,

extension to larger molecules will increasingly require the development

and application of theoretical methods.

Work will also continue at a much reduced scale on the maintenance of

the stratospheric chemistry data base, particularly with that part

concerned with halogen containing molecules. In connection with this

activity, Bob Hampson will continue to serve a member of the NASA Panel

for Data Evaluation and the CODATA Task Group on Chemical Kinetics. This

work is supported by NASA.

A new project on the evaluation of chemical kinetic data related to

the industrially important problem of SFg dielectric breakdown was initiated in FY 1984 by John Herron and will be continued as a Data Center

activity this year. This is part of a much broader area of the applied

chemistry of plasma deposition and plasma etching as related to electronics manufacturing and the coating industry.

On-going work on the publication of evaluated heats of formation

of ions will be completed in the Ion Kinetics Data Center. The

routine literature-searching, abstracting, and data encoding

activities of the Data Center will continue in order to maintain

up-to-date archives of ionization potential, appearance potential, and proton affinity data.

Formulation of plans for an evaluated compilation of data on ion-molecule rate constants and reaction cross sections, and the

initiation of a literature search for that project will begin in

the Ion Kinetics Data Center. This work will complement

previous compilations in the field, which are now seriously out

of date. Data from these earlier publications will be combined

with that from the literature of the past five years, and will

be organized for inclusion in a computerized data base.

In addition, a data base to aid in the standardization of

literature reference formats, and the design of a small auxiliary data base to aid in retrieval of papers from the library

will be implemented for the joint use of this Data Center and

the Chemical Kinetics, Chemical Thermodynamics and Electrolyte

Data Centers.

-80
Dr. J. E. Bartmess of the University of Tennessee has compiled

and evaluated the data on thermochemistry of negative ions for

the forthcoming publication on heats of formation of ions. He

has expressed an interest in permanently maintaining computersearchable up-to-date archives of anion thermochemistry as an activity of our data center. Such an arrangement will be

formalized.

8l
3. KINETICS OF NEUTRAL SPECIES

R. Billmers, W. Braun, R. L. Brown, A. Fahr, L. L. Griffith,

J. W. Hudgens, M. J. Kurylo, A. Laufer, M. Manka, R. I. Martinez,

D. Robaugh, M. D. Scheer, S. E. Stein, L. E. Szegvary, W. Tsang

and J. A. Walker

A. Introduction

There are two main parts to this program. The first deals with the

kinetics of the reactions of atoms and small polyatomic molecules. It is a traditional area of research at NBS. The second is our competence

program entitled "Transformation of Complex Organic Compounds". In this area we are attempting to develop an understanding of the behavior of

large organic molecules under severe conditions at the single-step

elementary reaction level.

Reactions of small molecules and atoms are of interest for their key

role in numerous gas phase chemical processes as well as for their use in

the development of chemical kinetic theory. The ability to unambiguously

detect trace concentrations of small reactive intermediates by optical

techniques is largely responsible for the high degree of development of

this area. Included in this component of our program are the development

and application of lasers for studies of high temperature kinetics (B4),

studies of small oxygen containing free radicals (B5) rate measurements of

reactions involving unsaturated free radicals (36) and oxidation chemistry

( 37 ).

The competence development part of our program is devoted to studies

of larger, more complex species under severe reaction conditions. These

studies represent an expansion of the scope of research in our Division,

in which the exact methods of small molecule kinetics are applied to

reactions of more complex molecules. Specifically, studies are intended

to lead ultimately to the capability of prediction and thus permit

detailed understanding of the whole range of problems where the stability

and reactivity of large organic molecules are of key importance. This

program has now reached its full planned scope. Shock tube, very low

pressure pyrolysis and liquid phase pyrolysis systems are all fully

operative and producing data. We are currently concentrating on aromatic

systems. These range from systems where phenyl, phenoxy and benzyl

radicals are of key importance, to the pyrolysis of anthracene and the

reactions of large resonance stabilized radicals in solution and, at the

large molecule limit, theoretical studies on graphite. In all cases our fundamental approach has yielded significant new insights. These will be

discussed in detail in B1-B2 and C. The MS/MS system has finally been

delivered and appropriate reactors have been constructed (Section B3a)

.

Dr. J. W. Hudgens has joined us from the Naval Research Laboratory. He is

bringing into the division expertise in multiphoton ionization of transient reaction intermediates (Section B3b). This powerful new technique

will lead to a determination of structures and reactivities of intermediates not detectable by other means and is expected to play an important

role in our program.

32
B. Gas Phase Kinetics

1 . Elementary Kinetic Studies of Aromatic Species

(A. Fahr, D. Robaugh, S. E. Stein and W. Tsang)

This is an important aspect of our competence program. Under

severe conditions aromatic type compounds are of key importance. Of

concern here are the stability and reactivity of benzyl type radicals and

the phenyl radical. There is at present considerable uncertainty regarding their properties. We have used two techniques, the heated single

pulse shock tube and the very low pressure pyrolysis reactor to study such

systems. Our results during the current year have demonstrated the

validity of our experimental procedures. We have discovered a new mechanism for nitrobenzene decomposition, obtained a new value for the

heat of formation of phenyl radical, demonstrated that literature reports

of a new mechanism for the decompostion of alkybenzene are in error and

provided the first data on radical combination reactions involving phenyl

radicals

The heated single pulse shock tube has been used to study the

mechanism and rates of decomposition of a number of aromatic compounds.

These include phenyl iodide, nitrobenzene, ethylbenzene and isopropylbenzene. Complementing these studies, the decomposition of ethylbenzene

and isopropylbenzene has been studied using the very low pressure

pyrolysis (VLPP) technique.

The work on phenyl iodide was aimed at establishing the validity of our procedures for studying reactions involving phenyl radical production.

We have found cyclopentane to be a satisfactory radical trap. A limit has

been found (.1 percent) above which all phenyl radicals will be converted

to benzene through abstraction from cyclopentane. The rate expression for

phenyl iodide decomposition has been found to be k (<f> I -*<!> *+I • ) = exp

(-33200/T)/sec. The rate constants are within 50 percent of earlier

measurements in a flow system. However, the rate expression differs

considerably. Assumption of a temperature independent recombination rate

leads to AHf(4>») = 3^1 kJ/mol (298 K) . Thi3 is about 13 kJ higher than

the number recommended in a recent review. On the basis of detailed

balance the new thermochemistry leads to a recombination rate of 10

|fj

2,/mol-sec. This is an order of magnitude slower than comparable reactions

of resonance stabilized radicals. If the lower heat of formation is

used a recombination rate of 2x1 0^2,/mol ‘sec is predicted.

For nitrobenzene decomposition, the main process involves the

splitting of the weak C-N bond. However from mass balance considerations

and the detection of phenol and its relative concentration under a wide

variety of concentrations we find that there is an important minor channel

(30 percent) involving isomerization to nitrate followed by cleavage of

the 0-NQ bond. The rate expression for C-N bond fission is found to be

k(<j>-N0 2-*fr*+N02 ) = 10 15 * 4 exp( -33COO/T)/sec.
Combination of this rate and the thermochemistry leads to a reverse rate

of 2.5x10^Z/mol-sec (1100 K) and a l/T^2 dependence. Here, as above, we are obtaining combination rates that are close to an order of magnitude

slower than that from alkyl and resonance stabilized radicals. The

important question is thus whether this is characteristic of reactions

involving unsaturated radicals.

These studies of nitrobenzene permitted us to assess the capabilities

of the heated shock tube. Nitrobenzene has a boiling point of 210°C. It

also is known to be extremely "sticky". After some initial problems we found no obstacles in preparing and holding mixtures in the sub-hundred

ppm levels. We have also been able to make quantitative determinations of

compounds such as diphenyl, diphenylether and phenol in the sub ppm level.

Thus it would appear that heating the shock tube opens up a vast number of

low volatility organic compounds to study. The problems that we have en- countered deal with the storage and transport of these substances at

150-250°C range. Under these conditions surface enhanced pyrolytic

reactions can occur. We feel that these represent soluble problems and

are exercising much greater care in these matters.

The ethylbenzene and isopropylbenzene studies originate from the

remarkable claims by Troe's group in Gottingen that ethylbenzene pyrolysis

proceeds via C-H bond cleavage. This unexpected result, if true, would

considerably alter current ideas of hydrocarbon pyrolysis mechanisms. We

employed both heated shock tube and VLPP techniques to test these claims.

In our shock tube studies of ethylbenzene pyrolysis, in the presence

of propyne as a radical trap, we found the main reaction product to be

toluene. Clearly, benzyl radical must be the precursor. In the absence

of the radical trap we find large quantities of styrene. In the inhibited

decomposition of isopropylbenzene styrene is also the main reaction

product. This demonstrates that the 1 -phenylethyl radical is a precursor

to styrene. The implication of this study is that the 1 -phenylethyl

radical and styrene detected in real time by Troe and coworkers are products of a radical chain decomposition and are not products of an initial decomposition processes. In our VLPP studies of alkylbenzene

decomposition we pyrolyzed ethylbenzene and isopropylbenzene with the

objective of making definitive mass balance measurements. In the case of

ethylbenzene, methyl and benzyl radicals were the sole detectable products

and, using di-tert-butyl peroxide as a CH^ radical source, > 85 percent of

the decomposed ethylbenzene generated methyl radicals. In the case of

isopropylbenzene decomposition, methyl radicals and styrene were the only

products observed. These studies showed that > 85 percent of the

1 -phenylethyl radicals formed by C-C bond breaking rapidly lost H-atoms

and were detected as styrene. This demonstrates that if, as proposed by

Troe, the 1 -phenylethyl radical were formed in ethylbenzene pyrolysis by

benzyl C-H bond breaking, it would have been efficiently detected in our experiments as styrene. Since no styrene was observed in our ethylbenzene pyrolysis experiments, benzylic C-H bond breaking is not

competitive with C-C bond breaking.
Together, VLPP and shock tube studies convincingly demonstrate that

no revision to conventional pyrolysis mechanisms are needed for

alkylbenzenes

We now plan to use our heated 3hock tube to study the reactions of

phenyl and OH radicals with a number of unsaturated systems. These are reactions that involve two possible reaction channels, displacement and

abstraction. We believe that our comparative technique can yield high

accuracy absolute rates and branching ratios. A systematic study of such

systems should yield information of great scientific and practical

interest. For abstraction processes, it will lead to detailed comparisons

with saturated systems. Displacement can be considered a combination of

addition and radical decomposition. Can the data be quantitatively

rationalized in such a context? Finally, we intend to continue with

thermal decomposition studies. In particular, we are interested in

thermal sources of OH and phenyl radical (in addition to that discussed

above). We will also carry out additional studies on ni troaromatic

systems.

VLPP studies of phenyl radicals with unsaturated molecules,

specifically, acetylene and ethylene, are planned for the next year.

Phenyl radical recombination will serve as a competitive channel against

which to compare these measurements. Carbon deposition by phenyl

radicals will also be investigated. Thi3 is an entirely unexplored area

which is expected to interface with our theoretical studies of graphite

molecules (see Section C3)

2. Thermal Stabililty of Alkyl Radicals

(W. Tsang)

In earlier work we reported on our conclusions regarding the

necessity for higher heats of formation for all the simple alkyl radicals

(except for CHo*). A paper has been completed and submitted for publication. Figure i illustrates the magnitude of the differences in terms of

the equilibrium heats of formation of the radicals as deduced from our analysis of existing data for alkane and alkyl radical decomposition and

the reverse processes and the currently used values. These results are of

key importance as we enlarge our data base to include an increasingly

larger range of compounds, since a minimal requirement must be that the

thermodynamics be satisfied. Our analyses lead to the following best rate

expressions for alkyl radical decomposition.

k (nC^Hy • * C2Hjj + CH^*) =

k ( iC^Hy • -*• C^Hg + C 4

#

) =

k(sC4

Hci * -* C^Hg + CHg») =

10 13 * 1 exp( -1 5259 /T) /sec

10 13 * 34 exp( -181 04/T) /sec

1q12.97 eXp( -i 4886/T) /sec

k(sC 4 H9

- ** 1 ~C 4 Hq + H •

)

10U.11 exp( -1 8498/T ) /sec

K ( SCjjflg « -* C-CI4H3-2 + H*) -j 0

1 2 . 62 8xp(-17652/T)/sec

'35
k ( sC^Hg • -» t-CiiHg-2 + H*) = 10 12 * 66 exp( -1 7225/T) /sec

kCtC^Hg* -» iCi^g + H*) = lO 1 ^- 92 exp( -1 891 0/T) /sec

k(tC5H-|i * 1 C 4 H8 + CH3

) = 1

1 ^ • °3 exp(-1 501 1 /T) /sec

The systematic trends in the these expressions, within the context of

thermochemical kinetics, provides the basis for the prediction of the

rates of decomposition of all alkyl radicals.

3. New Kinetic Measurement Techniques

(J. W. Hudgens, R. I. Martinez)

Progress in the area of kinetics depends to a large degree on advances in our ability to detect reactive intermediates and products.

Part of our Competence Initiative is devoted to developing new detection

methods for polyatomic species of concern in our program. In particular,

we have begun to apply the methods of multiphoton ionization (MPI) and

tandom mass spectrometry (MS/MS) for the analysis of large organic

reaction intermediates.

a. Multiphoton Ionization of Free Radicals

(J. W. Hudgens)

The development of multiphoton ionization over the past few

years has given kineticists a new method with which to probe the spectroscopy and dynamics of molecules which could not be detected by currently

available, laser-induced fluorescent methods. Since most molecules and

free radicals do not fluoresce, MPI greatly expands the number of different chemical species that can be detected with lasers.

Multiphoton ionization works by subjecting molecules to intense

visible or ultraviolet laser light that causes them to absorb photons

until they ionize. Usually the first step involves a simultaneous

absorption of two or more photons which promote the molecule into an excited electronic state. The excited state molecule absorbs an additional photon and ionizes. Since ions are easy to detect, the technique is

very sensitive.

The most important reaction systems involve free radicals. At

present, the data base for radical detection by MPI is small, but we are developing a base useful for future kinetic studies. Research efforts

this year have produced methods for the MPI detection of allyl, 2-methyallyl CH2F, CF, SiF, CIO, BrO, and CH radicals (and their deuterated

counterparts). The detection of allyl and 2-raethylallyl radicals is very

significant because it extends MPI detection to the important class of

conjugated pi-bonded radicals.

Allyl radicals appear in many thermal, photochemical, and surface

catalytic reactions. Yet no laser methods were available for their

optical detection. Allyl and 2-methylallyl radicals were detected in the

86
-

PROPYL

75

y •o 1 Q. C. y 03

r-l 3 03 03

y

>»

O 03 75 4J t. n 13 05 3 Mil) X5

CO 3 75 1 cH i—H 4-> 73

4-> >, 03 c 4-3 .c > 03 r-H • 4-3 4—3 3

rH <U • 73 75 >. S 35 c y

.u

y o s_ 3 — o

CD

O SO •a 1 G E y

03 c y 3 4-3 o c *H 03 « —1 U5 i. i—

rH 4-3

a 3

>> 03

C -*H

O

0.4-3 0'

H r-H

O

O

— •H 03 . S- S- 4J 3 o C

Q. 03 CT

y

O

r~* *^4

yy 03

75

y o c. i— -4 c o U3 03 O

S_ 75 c u - y 75 y r-H <—t J—3 -—4 75 o

>) c a

y —3 O. — L. 3 3 O T3 a CD

s_ o c y 1 —-H a. 4J o L 43 75 1 CD 75 C 5- ^ c y 75 —

0=

C— 3 O t- o c ^-1 C — T 03 i. 1 03 C O 03 o =» 1 c

-4 x: 1 3

4-3 4J S, 75 y — o £ S 3 in y

s_

w

O -

Cz- i—* I >, ^ 05 ^ C

<•-1 4-3 iei I—i 03 O 3^ nj c

CO o y

—< ^—V i v* T-» «— E 4-3 S_ ^H CO

4-3 3 C c c

03 *3 4J

—

o CTJ

C 03

4—. 03 o y tt o y s_ c £—

4-4

a 3 CTJ 75 75 E

4-3 G.

y 75 c Q

— 3 E—

y 1) c O 73 — J* 3 !_ 4_ y r—< a) o x O c 4-3 2: 5 r H OIC c

W* .coy

X

O

— t_

75 4-3 flj _3 50 O O.

q

C c c

03— 3 C C)

Q

4-> 43 El. -4

g

75 03

Q

C 4-3 03 o O 7) •4 o £- 7)a y -7 E >* -D c 3 ~» ^ E

--H o a 3 7' jQ

L.

y -3 2

-O C. 4- • •-H O -5 1) C *—

43 cr —4 C

C

u 3 0 —

h

'A 901

my

U)

37
effluent of a flow reactor. The allyl radicals were formed by the

reactions of fluorine atoms with cyclopropane and propene. The 2-methylallyl radicals were formed by the reactions of fluorine and chlorine

atoms with isobutene. Focused laser light between 480-535 nm induced two

photon absorptions preparing the 3s 2 A-, Rydberg states of the radicals.

Absorption of two additional photons ionized the excited Rydberg radicals.

The ionization process produced only molecular ions —an important result

because it enhances detection sensitivity. Before this work the ns Rydberg states of allyl radicals were unknown.

CIO and BrO radicals are important in reactions involving stratospheric ozone. MPI detection of CIO and BrO was developed using a tunable

dye laser between 417-465 nm. Halogen atoms were reacted with ozone in

the flow reactor to generate the radicals. The MPI spectra were generated

by simultaneous three photon absorption which prepared excited Rydberg

states of CIO and BrO. The excited Rydberg radicals then absorbed one or two more photons and ionized. All of the states of BrO detected in this

study had not been previously reported.

The work cited above are typical experiments used for the development

of the MPI radical detection data base. By generating a catalog of MPI

detectable species we can use selective and sensitive laser techniques to

probe reactions systems which involve these radicals. Particularly

important are the class of aromatic free radicals and their reaction

products (which includes allyl radicals). Attempts to detect members of

this class of radicals will be made this year. Laser strategies used to

selectively ionize them may include two frequency MPI, production of

doubly charged ions, and laser induced charge exchange schemes.

b. MS/MS Tandem Mass Spectrometry

(R. I. Martinez)

A new long-range study of the dynamics of neutral-neutral and

ion-neutral reaction systems has been initiated. The focus of thi3 sutdy

is the development of a real-time analytical methodology which involves

the combination of species-specific ionization via 2-color multiphoton

ionization and structural analysis via MS/MS tandem mass spectrometry

(i.e., 2-color MPI-MS/MS) . The intended use of this technique is as a diagnostic tool for monitoring directly in real-time unknown polyatomic

reactive transients in complex reacting mixtures.

The technical basis for developing 2-color MPI-MS/MS is that the

direct, real-time analysis of multicomponent mixtures by mass spectrometry

can be divided into two parts:

(1) the ionization of the mixture components and

(2) the quantitation of each component on the basis of its respective

ion signal intensity.

For quantitation, one must be able to do either (a) or (b):

(a) find a parent or fragment ion which is unique for a particular

mixture component, or

(b) ionize only one component at a time in the presence of all the

the other mixture components.

88
None of the ionization techniques used routinely (electron impact,

photoionization, chemical ionization, etc.) can satisfy requirements (a) or (b) for all components of complex mixtures which can give rise to

isomeric and/or isobaric parent and/or fragment ions. The only ionization

technique which presently can provide species-specific ionization (i.e.,

satisfy requirement (b) above) is 2-color MPI (muliphoton ionization

wherein one X-j is used for up-pumping and a second X 2 is used for

ionization)

While 2-color MPI can provide specific and ultra-sensitive

photoionization, it cannot provide unique structural identification of an unknown polyatomic component in a complex multicomponent mixture

containing isomers and/or isobars unless a synthetic sample exhibits all

the same optical 2-color MPI transitions as the unknown. Clearly,

synthetic reference samples are not possible for complex polyatomic

radicals. To date, only small, simple radicals have been studied.

In dealing with unknown species, it is necessary to characterize the

molecular structure of the ion precursor. That may be accomplished

through the use of the complementary technique of MS /MS tandem mass spectrometry . Tandem mass spectromeetry exploits the ion fragmentation

patterns which are characteristic of the structure of a compound. Thus

the detailed ion-fragmentation mapping obtained in the MS/MS by

collision-induced dissociation (CID) of ions formed by '’ soft " 2-color MPI

should allow one to uniquely identify any component in a complex mixture.

The specially designed NBS MS/MS tandem mass spectrometer is now

in-house and we have been proceeding with the requisite testing, and

modifications needed to accommodate the MPI.

The first phase of the planned research program involves (1)

development of 2-color MPI schemes; (2) measurement and analysis of

2-color MPI "breakdown curves"; and (3) measurement and analysis of CID

"breakdown curves".

During FY 1985, we will initiate studies on the CID dynamics of

selected molecules. Also, we will attempt to develop MPI detection

schemes for the propargyl radical for use in neutral kinetics studies

pertinent to the atmosphere of Titan. This work is supported in part by

NASA.

4. Laser Studies of High Temperature Chemical Kinetics

(W. Braun and M. D. Scheer)

The following three projects describe our use of CO? lasers for

studying high temperature reactions. The first two projects use a laser

as a unique heating source, while the last use the laser to per turn

reactions already occurring in high temperature environments. Thi 3 work

was supported in part by the Air Force Office of Scientific Research.
a. A New High Temperature Reactor.

During the past year a very rapid, high temperature heating

method has been developed. Small volumes of fused silica can be heated to

incandescence and beyond through the absorption 10 micron infrared

radiation at power density levels in the neighborhood of 1 kW/cm^. By

using the focused beam of a cw C0 2 laser (

(

P20 ) line of the 001-100 band)

to heat only the orifice end of a fused silica reactor tube, one can achieve very short heating times for gases that flow through the tube and

expand adiabatically out of the orifice into an evacuated region.

When chemically reactive gases are allowed to flow at near sonic

velocity through a small volume heated by quartz walls irradiated in the

above fashion, high temperatures and short reaction times are readily

achieved. When the gas is subsequently cooled by adiabatic expansion into

a low pressure region, all collision dependent secondary processes are arrested particularly those with significant activation energies. Such a reactor can be used to obtain rate parameters under high temperature,

short time conditions not readily attained with other heating methods. By

thus extending the range of experimental temperatures at reduced reaction

times, additional bond breaking processes can be made to occur and the

kinetics of higher activation energy decomposition channels may be

isolated and characterized. The multichannel decomposition of cyclobutanone was used to demonstrate the applicability of such a reactor.

It was shown that when a dilute mixture of cyclobutanone in helium

flows through the laser heated orifice, it is exposed to the high temperature for only about 20 microseconds. During such short reaction times

secondary reactions tend to be reduced and it was demonstrated that the

formation of propylene from the secondary isomerization of cyclopropane

could be minimized. At orifice temperatures near 1200 K, propylene + CO was shown to be formed directly from the cyclobutanone. The activation

energy for this high temperature channel was found to be about 330

kJ/mole compared to 242 and 218 for the two lower channels that form

cyclopropane + CO and ethylene + ketene respectively.

b. A New Comparative Method for Determining High Temperature

Activation Energies

The new rapid flow, high temperature reactor can be used to precisely

determine activation energies by comparing the rate of product formation

with the rate of emission of black body photons from the heated orifice at

a given wavelength and temperature. Towards this end, a thin film of

graphite was deposited on the inside surface of the orifice. The laser

was 3lowly modulated at a frequency of 1/2 Hz with a 50 percent duty

cycle. It was found that when a 60:1 mixture of helium and t-butyl iodide

flowed through the cyclically heated orifice, the I atoms produced were in

phase with the temperature modulation. This temperature modulation was detected by measuring the spectral radiancy of the quartz orifice at 660

nm and the I atcm3 by resonance fluorescence at 1 62 nm using an iodine

resonance lamp and a solar-blind photomultiplier. For temperatures

between 1000 and 1500 K, the black body emission of photons at wavelengths

90
below 600 nm may be described by the Wien approximation. Under these

circumstances, the ratio (

2 /Ri

)

of spectral radiancies at two different

temperatures, T

2 and obeys the following relation:

T 2-?l

T 2Tl

kA , Rp > £n ( he Hi

Since the production of I atoms is governed by the Arrhenius law, the

ratio (

2 /I -] ) for their rates of formation at the same two temperatures is

given by the similar relation:

T 2~T 1 T

2 t 1

The activation energy in terms of the measured quantities (R2 , R-] , 1 2 , 1

)

and relative to the energy of the black body photons emitted at 660 nm is

therefore

he 2,n( ) h

A in(^)

R

1 nc where at 660 nm, ( ) is 43.3 kcal/mole. As an example of the application

of this measurement

A methodclcgy the t-butyl iodide decomposition yielded

an activation energy of about 170 kJ/mole for scission of the C-I bond in

the neighborhood of 1000 K.

c. Enhancement of Unimolecular Decompositions Near the

Second Order Limit.

The departure of systems undergoing chemical change from an equilibrium distribution of energies among their molecular degrees of freedom

occurs only when rates of reaction are very large. Ignition and diffusion

flame phenomena, occurring in heterogeneous and unmixed fuel-oxidant

systems, as well as flame propagation through thoroughly mixed gaseous

combustibles, are examples of such rapid high temperature chemical

processes. Cne important property of these rapidly reacting systems is

the existence of a competition between the rate of collisional activation

and the rate of surmounting the activation barrier leading to successful

reaction. At low temperatures and high pressures, collisional activation

maintains an equilibrium Boltzmann distribution of energy required to

overcome the activation barrier so that the process is activation limited.

1 / p Since the collision rate varies as T while the rate of overcoming the

barrier varies as exp(-E/RT), it is readily seen that for any given

pressure there exists a temperature beyond which the latter rate will

surpass the former. Under these circumstances the high-energy "tail" of

91
the Boltzmann distribution becomes depleted with a consequent lowering of

the overall reaction rate. Unimolecuiar decompositions are the simplest

examples exhibiting such fall-off behavior and are characterized by a depletion of those molecules with vibrational energies near and beyond

that required to surmount the activation barrier. The pumping of infrared

laser energy into the high end of the vibrational energy distribution of

the decomposing molecules provides a means of enhancing their rates of

decomposition. A quantitative comparison of these rates' with and without

measured doses of laser radiation will yield relative efficiencies for

the transfer of vibrational energy at levels near the activation energy

barrier by IR photons compared to heat-bath collisions. Recombination

processes on the other hand demonstrate the inverse effect where the

reaction exothermicity must be removed by collision with a heat-bath

molecule in order to stabilize the adduct. Thus, only collisions which are efficient in removing energy rather than supplying it will enhance the

recombination reaction rate and the absorption of IR photons will decrease

the recombination rate.

Intense infrared radiation is therefore capable, in principle, of

pumping the vibration-rotational manifolds of a reactive molecules and

thereby provide a means for decreasing the depletion factor in the

Boltzmann distribution at high energies. It has been experimentally

demonstrated that pumping the lowest vibrational energy levels of a reactive molecule with an appropriate resonant wavelength can, under

favorable circumstances, enhance the rate of reaction. The very high

vibrational-rotational energy levels near the top of the activation

barrier on the other hand, are very closely spaced in a quasi -continuum so

that pumping the high energy part of the Boltzmann distribution should be

virtually wavelength independent.

Preliminary evidence for such an effect has been obtained when ethyl

iodide diluted twenty fold in 5 torr of helium was passed through a one ram diameter fused silica orifice coated with a thin film of graphite. The

orifice was heated by a 1 5 watt C0 2 laser beam (P20 line of the COT -100

band at 10.6 micrometers) that was chopped at a frequency of 1/2 Hz with a 50 percent duty cycle. The temperature excursion of the orifice walls

during this modulated heating cycle was found radiometrically to be

between 1025 and 1275 K, where heating to the temperature maximum occurred

during the one second that the laser was on and cooling to the temperature

minimum occurred during the one second that the laser was off. It was found that the I atoms produced at a given temperature during the modulation cycle were always greater when the laser was on compared to when it was off. A much smaller effect was observed for t-butyl iodide under

similar modulation conditions. This observation is consistent with the

fact that the ethyl iodide is much further in the falloff than is the

t-butyl iodide with twice as many active oscillators. Since alkyl iodides

do not have strong ground state absorptions at 10.6 micrometers, it is

unlikely that radiative heating of the substrate alkyl iodides can account

for these results. Further measurements at different pressures, orifice

temperatures, and laser wavelengths are planned in order to determine

92
whether the observed enhancement of the rates of these high temperature

decompositions can be ascribed to a laser induced repopulation of the high

energy end of the depleted vibrational energy distribution.

5. Kinetics of Free Radical Reactions

(M. J. Kurylo)

The scope of this program is the high sensitivity detection and

chemical characterization of small gas-phase free radicals (atoms,

diatomics, and triatomics). These reactions are important because of

their ubiquitous involvement in gas-phase chemistry ranging from acid

rain to the depletion of stratospheric ozone, the incineration of hazardous waste to the design of improved combustors, the effects of increased

internal energy on kinetics and mechanisms to the elucidation of the

chemical composition of planetary atmospheres.

Studies completed the past year, supported by NASA, include measurements (by flash photolysis resonance fluorescence) of the kinetics of

acetonitrile (CH^CN) with Cl atoms and OH radicals. These studies were

aimed at providing input for calculations of CH^CN lifetimes in the

stratosphere and troposphere. The results, however, when compared with

available literature data on Cl and OH reactivity with other substituted

methanes, provides evidence for decreased reactivity between strong

electrophiles. This lowering of the rate constant with increasing electron

affinity of the substituents is at odds with some of the commonly used

semi-empirical algorithms for rate constant estimation. As such, our findings represent an important contribution to the needed data base for

improved estimation schemes.

Additional studies have focused on the measurement of H0 2 reaction

rates utilizing flash photolysis kinetic absorption spectroscopy. In

particular, cur determinations of the H0 2 self reaction rate as a function

of N

2 and O 2 pressure yield a low pressure extrapolation in good agreement

with a number of other low pressure studies. In addition, the pressure

dependences we observe quantitatively support recent rate constant

assessments. Interest in this reaction stems not only from it3 importance

in atmospheric and combustion chemistry but also from its consideration a 3 a subject of extensive theoretical treatment regarding both the stability

of the H

2 0

dimer reaction intermediate, and the accelerated reactivity

attributed to water vapor complexation.

In the coming year we plan to continue these experiments with

emphasis on the H0 2 /NG2 combination reaction in order to explain the

discrepancies in the reported fall off behavior. In addition, we will

study the cross-disproportionation reaction of the H0 9 and CH^O? radicals. During the past year, we have gained considerable expertise in

monitoring H0 2 via absorption spectroscopy. Information regarding our detection sensitivity as a function of wavelength and the sensitivity of our calculated second order rate constants to fluctuations in analysis

lamp intensity and signal to noise ratios have permitted us to define the

optimum conditions for investigating the cross-disproportionation react.cn

between H0 2 and CH^ 02 * This reaction is important not only in the
atmospheric oxidation of methane and other hydrocarbons but also in

low-temperature combustion systems. Furthermore, given the rather good

data bases for the H0 2

+ H02 and CH3O2 + CH3O2 reactions, the cross-system

lends itself for interesting theoretical considerations regarding the

factors influencing radical radical disproportionation. Experimental

measurements will commence in the near future under conditions specified

by our recent extensive modeling calculation. Ideally, we would like to

have supporting results from monitoring the decay of both radical species.

However, due to the large differences in self reaction rates of the two

species, precise kinetic measurements will not always be possible for both

radicals at each set of initial concentrations. Nevertheless, our calculations indicate that suitable complementary measurements can be made

to fully characterize the kinetics of this reaction.

During the forthcoming year we will collaborate with members of the

Center for Analytical Chemistry on the application of diode laser spectroscopy (second harmonic detection) to kinetic systems. While our goal

is for an apparatus of general utility in monitoring free radical kinetics

or delineating reaction mechanisms through trace product analysis, our initial thrust will be at high sensitivity H0 2 detection to complement our kinetic spectroscopic work. Our planned experiments will differ from some very recent work in other laboratories by focusing on a different H0 2 bond

system and on the sensitivity of second harmonic detection. Inherent to

such measurements will be the determination of pressure broadening

parameters and the effect of water vapor on the infrared line strengths.

The effective coupling of the expertise of the collaborating individuals

will hopefully pave the way for successful future investigations.

6. Reactions of Biradicals and Unsaturated Free Radicals

(A. Fahr and A. Laufer)

The objective of this program is to produce and characterize

biradicals and unsaturated free radicals. The efforts are specifically

concerned with reactions of these unique species leading to a quantitative

understanding of their chemistry. The experimental approach is to use flash photolysis to generate free radicals and/or excited species, kinetic

spectroscopy to follow species concentrations and gas chromatography for

end-product analysis. Isotopic substitutions are applied to clarify

mechanistic processes. In these cases, mass spectroscopy of the final

products is the analytical method of choice.

Our recent efforts, supported by the NASA Planetary Atmospheres

Program, have been directed towards understanding the mechanism of

formation and the chemistry of the vinylidene unsaturated biradical

(H 2 CC) . This species, the smallest of a large class of unsatur3ted

biradicals, was initially identified by us on the basis of its characteristic absorption in the vacuum ultraviolet. We have been able to determine that the particular state of vinylidene under observation is that of

the lowest lying excited triplet, about 1.5 eV (55 kcal/mol) above the

ground singlet state. Our initial examination of the chemistry of this

unique species involved a study of its rate constant for removal, via

9U
collisional quenching by He, and a determination that the commonly invoked

intermediate in C2H2 systems (often referred to as C2H2 ) is chemically

equivalent to vinylidene. In a recent paper, which has appeared this year

in Icarus, we have applied both BEBO and BSBL techniques to calculation of

hydrogenation abstraction rate constants from hydrocarbon molecules by

vinylidine which are important in the atmospheres of the outer planets.

In addition, we have been able to identify alternate sources of

triplet vinylidene. These results suggest that the species may, in fact,

be significantly more important in the photochemistry of hydrocarbons than

previously expected. In particular, the spectrum attributable to H2C=C

has been observed following the vacuum ultraviolet photolysis of C2H4. In

the ethylene system, the H

2 C=C is not a primary photofragment but increases rapidly to a maximum and then decays with a rate constant characteristic of its quenching by helium. The buildup of H?c=C with time, in

this system, suggests that the previously observed absorption is due, not

only to vinylidene, but also it is a vibrationally cold molecule.

A rate constant of the order of 10 -11 cm3 molecule -

s - "

1 which represents the sum of both quenching and reaction (probably addition) of both

protonated and deuterated triplet vinylidene with C2H4 and C2D4, respectively, has been derived. The work will soon appear in the Journal of

Photochemistry

While the direct observation of vinylidene is of great scientific

interest, the importance of the radical in hydrocarbon chemistry remains

unknown. This year we have determined the extinction coefficient of

vinylidene from which we will be able to ascertain the quantum yields for

vinylidene formation in the photodissociation of hydrocarbons of planetary

importance. The extinction coefficient measurements are based on a study

of the photodissociation of vinyl chloride. Though triplet H

2 CC now has

been observed from C

2 H2 and C2H4 photolysis and as a product of the

reaction of two triplet CH2 radicals, vinyl chloride has certain advantages. Conceptually, if the photodissociation proceeds through a 1,1

elimination of HC1 , the remaining fragment is vinylidene whose concentration will be equal, to that of the HCJi. Since HC1 can be determined

absolutely, the absolute concentration of H9CC(3 b

2

) becomes a tractable

quantity. The possible products of the dissociation of C2H3CI:

C1(P^? i/2^’ c

2 k2’ HC1 and H2CC(^B2) all have well characterized

absorptions in the vacuum ultraviolet. We have obtained time-resolved

spectra of these several species from which we deduce the primary process

proceeds by direct absorption to an excited singlet

hv , * C

2 H

3 C1 > C2 H

3

C 1 ( n , t ) (

1 )

followed by an intersystem crossing to a high vibrational level of tne

vinyl chloride triplet (tt,tt ) at 4.0 eV above the ground singlet.

(^HqCI ' (h,ir ) * C2H

3 C1 ) ( 2 )

95
The unimolecular dissociation of the triplet, k uni = 4 x lO^s -1

, leads, in

a spin-allowed process, to triplet vinylidene (H 2 CC) and HC1(XI+

)

C2H3CI 3 ( tt

,

-

rr* ) * H2 CC( 3 B

2 ) + HC1(XZ+

) (3)

The equivalence of HC1 and H2CC having

e(H2 GC), the extinction coefficient, = studies have been completed and we are reduction.

been determined allows us to obtain

420 atm

-1 cm-1

. The experimental

in the process of final data

In the coming year, we will use our measured vinylidene extinction

coefficient to study the kinetics of the reactions of vinylidene with H

2 , CHi| , and N2. In the case of H2 and CH4 the reactions are of great

interest in planetary chemistry and will provide a check on our earlier

theoretical calculations of these rate constants. For N

2 , the interest is

in any reaction which can lead directly to the formation of HCN.

7. Oxidation Chemistry

(R. I. Martinez)

The general objective of this program is to understand in detail

the chemical kinetics of oxidation reactions, particularly those reactions

which involve 0, 0

2 , ana O3.

Most of our efforts in the past several years have been directed

towards a detailed understanding of the reactions of ozone with alkenes in

the gas phase. Photochemical air pollution is driven by the oxidation of

organic compounds, and the oxidation of alkenes by 0o is among the most

important of all classes of atmospheric reactions. The kinetics and

energetics of dioxymethylenes and their isomers, which we derived from

studies on Oo-alkene systems, led to our 'unifying postulate that the three

reaction systems R*R ,f C:+02 , 0+R ’ R"C=0 , and O3 + olefins proceed via a common intermediate dioxymethyiene (R , R"CC0*), i.e.,

0o + Olefins

• R’R"C: + 02

> R * R

T *C00 • *• R’R"C=0 + 0

(carbene +0 2 ) * ( 0+oxoalkane)

products

Additional support for this unifying postulate has been derived from

studies on the photooxidation of formaldehyde and the reaction of 0^ with

tetramethylethylene (TME)

.

96
In the case of the reaction of ozone with tetramethyiethylene

(TME), temporal dependences of the products at 4 torr suggest that the

process Me 2 c00 •-Me 2 CQ + 0 accounts for 20-30 percent of the dissociative

branching of the Me 2 C00* produced via O3 +TME. It also suggests dissociative branching to OH to the extent of ^ .5—1 0 percent following the known

intramolecular rearrangement of the Me 2C00* to the hydroperoxide

H

2C=C(CH 3 )-0

- 0H. At atmospheric pressure and in solution it has been

found that the hydroperoxide further isomerizes to hydroxyacetone by

intramolecular OH migration. It thus appears that at low pressures the

intermediate hydroperoxide may lose the OH to some extent in competition

with its isomerization to hydroxyacetone.

The photooxidation of formaldehyde plays an important role in diurnal

photochemical smog cycles, and the elementary processes involved have been

the focus of intense research in recent years. The unifying postulate

above formed the basis for an extended version of our previously proposed

mechanism for the photooxidation of formaldehyde. This mechanism provides

a reinterpretation consistent with all observations to date. It involves

photoisomerization of the H

2C0 to the hydroxymethylene HCOH which

subsequently reacts with 0

2 to form the hydroxy-substituted dioxymethylene

HC(0H)0O which subsequently decomposes to the products observed.

Dissociation of the HC(0H)00* to 0 + HC(0)0H can be compared with the same process for the isoelectronic analogue HC(C&)00*-K) + H(C£)=0 which

accounted for ca. 20 percent of the dissociative branching of the

HC(C£)00* studied in collaboration with Dr. Hiromi Niki of Ford Motor Co.

C. Condensed Phase Kinetics

1 . Thermal Reactions of Aromatic Compounds

(S.E. Stein, L. Griffiths, M. Manka, R. Billmers and A. Fahr)

This program represents one of the main thrusts of our competence program. As in the gas phase, under severe conditions aromatic

compounds are of key importance. Our work in this areas has led to a new

and unexpected mechanism for anthracene pyrolysis. Rate constants for

elementary reactions involving aromatic molecular and free radical species

in liquids have been determined. Our ESR spectrometer has proved to be

extremely valuable in providing a direct means of detecting radicals at

high temperature. A database of kinetics information is being constructed

and some significant general kinds have been observed. Our work on graphite using self consistent field calculations have now covered systems

up to 1000 carbon atoms. Thi 3 is close to two orders magnitude larger

than systems that have previously been studied by this method. Some

extraordinarly suggestive insights on the effect of edges and resonance

energies have been obtained.

(a) Mechanistic Studies of Aromatic Pyrolysis

We are examining reactions in organic fluids at high

temperatures with the aim of both finding reaction pathways leading to

polymerization and carbonization and using this information to develop

predictive theory. Our continuing experimental and tneoretical studies

97
the anthracene pyrolysis system has clearly demonstrated that dimerization

occurs by a simple and entirely unexpected coupling of two ground state

molecules described by the bimolecular rate constant k/l*mol

-

^ *s -

^ = 10^*^ exp(-45,200 (cal)/RT). We propose a novel mechanism in which the

following biradical intermediate,

transfers an H-atom from one anthracene unit to the other thereby forming

a stable dihydro-diaryl compound. This mechanism suggests that larger

polyaromatic molecules can form analogous biradicals of lower energy and

therefore react even faster. This process provides a ubiquitous means of

initiating decomposition reactions of species that are otherwise unreactive. This latter point has been demonstrated in preliminary experimental studies of two other polyaromatic molecules, phenanthrene and

pyrene.

During the next year we intend to determine detailed polymerization

mechanisms for polyaromatic molecules other than anthracene in order to

determine the generality of the above findings as well as to develop more general predictive models.

(b) Elementary Kinetic Studies of Polyaromatic Species in

Solution.

These experiments are designed to yield rate constants and

thermodynamics of elementary chemical reactions involving polyaromatic

molecules and free radicals. Reactions are studied when they have been

identified as key processes in the thermal chemistry of aromatic substances and their rate constants are not available or reliably estimatable.

The transfer of hydrogen from one species to another in liquid

pyrolysis environments is a critical feature of many pyrolysis mechanisms.

We have examined a number of such processes in detail. In one study, we made what we believe is the first direct measurement of the rate constant

for H-atom transfer from one closed-shell molecule to another. This often

proposed radical formation step was found to occur without complication

in the following reaction,

98
H H

Because of the exceptionally low exothermic ity of the reaction ( -35 kcal

mol -1 ), side reactions and chain processes which normally obscure such

processes did not occur. These measurements now enable us to place rate

constants for a wide range of such reactions on an absolute basis.

In another series of studies we devised and used a new kinetic system

for measuring rate constants for H-atom abstraction by resonance stabilized radicals from aromatic molecules. Despite the significance of these

reactions in pyrolysis chemistry, only one absolute measurement of a race constant for such a reaction had been reoorted in the literature (namely

4>CH 2 + 4) 1 CH^ -* 4>CH

3 + 4

) 1 CH 2 , k/2, mol

-1 s -

' = 10 10, 5 exp( -1 1 000/T) ) . Our

measurements were made in a reaction system involving an equilibration

• • between R 2 molecules and R* radicals (R* = <p <p and ). Addition of

H-donor molecules to these system opened up the desired reaction channel,

namely H-atom abstraction by R*, the rate of which could be measured by

end product analysis. A series of H-donors were studied and a strong

correlation between reaction exothermicity and rate was found. For

thermoneutral reactions, absolute rate constants were comparable to the

above mentioned value for the benzyl radical. The most surprising

observation was that these reactions have unusually high pre-exponential

factors <-10 10 -5 2,/mol «s). These values are at least 1.5 orders of

magnitude higher than expected on the basis of either transition state

theory or gas-phase measurements for related reactions. The origin of

this discrepancy is now under study.

Over the next year rates and mechanisms of hydrogen transfer reactions which transform one hydroaromatic structure into another (e.g.,

©2® * @2^ ) wlll be determined. We are particularly interested in extracting rate constants for H-atora transfer

from a radical to a molecule. These processes are often assumed to occur,

but a rate constant for such a reaction has never been unambiguously

measured

2. ESR Detection of Aromatic Radicals at High Temperatures

(R. L. Brown)

As part of a continuing attempt to measure high temperature

equilibria involving aromatic radicals, the pyrolysis of xanthene

( (QlalS) ) in benzene solution was examined. The ESR spectrum of the

rather staole xanthyl radical was easily observed over a temperature range

of 250° to 330°C. A kinetic computer model of this reacting mixture was

99
developed using a xanthyl dimer dissociation rate constant measured in

this laboratory by other means. The xanthyl radical concentration at

various temperatures predicted by this method was found to be in agreement

with the concentrations observed in preliminary experiments.

The use of ESR in measuring aromatic radicals at high temperatures

and pressures has been hampered by the low inherent sensitivity of this

method. Measurements are limited to systems containing relatively stable

radicals. In an attempt to circumvent this problem, we have begun to

explore a different approach. An easily generated radical such as

t-'outoxy can be used to generate unstable radicals by hydrogen abstraction

directly in the ESR cavity. From the observed radical concentration and

the concentration of dimer product it should be possible in favorable

situations to measure hydrogen abstraction rates and radical recombination

rates. To test the feasibility of this technique a mixture of t-butyl

peroxide initiator in isopropyl alcohol was pyrolysed over a temperature

range of 150 to 275 °C. A spectrum of hydroxy isopropyl radical was seen at

180°C; it increased in intensity up to 215°C, and then decreased,

disappearing at 250°. The decrease at higher temperature can be

attributed to the destruction of the peroxide initiator before the sample

reached the ESR cavity. Since hydroxy isopropyl is not a particularly

stable radical, this experiment provides encouraging support for this

approach. In addition, by using the initiator with a mixture of two

reactants, it may be possible to observe two different unstable radicals

at the same time. This could make possible the measurement of

radical-radical equilibria.

3. A Chemical Theory of Graphite

(R. L. Brown and S. E. Stein)

Graphitic materials can be assumed to consist of loosely bound

layers of very large planar polyaromatic molecules of varying size and

having a variety of edge structures. Theories of aromatic molecules

separate the bond joining the carbon atoms into two types; the o-bcnds,

which are localized in the region between the adjacent nuclei, and the

Tr-bonds arising from the u-electron3 which lie above and below the plane

of the molecule. These ^-electrons are metallic in nature and are the ones which give graphite most of its characteristic properties. There are

a number of theories which have been successfully used to model such

Tr-electron networks. However, they have been applied almost exclusively

to the relatively small molecules of interest to organic chemists. Their

application to very large graphitic molecules is the goal of this program.

Assistance for this research was provided by the Air Force Office of

Scientific Research.

Part of our work during the past year involved the systematic

application of the simplest theories to the largest molecules possible.

In addition to finding general structure/reactivi ty relations, this work

provides a logical foundation on which to build more sophisticated

theoretical analysis. Selected results of these calculations for 1/12

"slices" of hexagonal molecules are shown in Figure 4.

00
Senes /

®*®*y*9

*9*9

* 9*

»

9

*9

*9*9*9

<*„d.9

*9*9

9

*9

*9

*9

*9*9

*9*9*9

*9

*9*9

*9*9

-aY*9*9*f*9*9*9*9*9*9*9*9*9*

Fig. 4. Properties of Graphite-Like Molecules. Results are shown as 1 '_

"slices" of hexagonal molecules. Such a slice i3 shown for a typical molecule in the top structure. Four different edg*- tyoes

were studied (series 1 to 4) . The upper left set of four

structures show ring aromaticities - the area of each ring

proportional to its aromaticity. The upper right set 3how sc n: orders - the width of each bond is proportional to its benc

order. The lower set shows localization energies - the ar-.-j : each circle is proportional to the energy required to i -

. • electron at the position of the circle.
Much of our work over the past year was devoted to the development of

efficient computer codes to adapt a sophisticated theory, namely the Pople

SCF method, to very large molecules. All of the theories we have used

require constructing a so-called bond matrix which describes how the

carbon atoms are connected. For the simpler theories the key to their

extension to large molecules was the discovery of a unique numbering

system for the carbon atoms which results in a narrow banded bond matrix

which greatly simplified the computations. The corresponding matrix for

the SCF method, however, is not banded, and in practice contains few, if

any, zero elements. Calculations using this type of matrix require far

more computational time than the banded cases. Furthermore, the SCF

method is iterative and converges more slowly the larger the molecule.

Thus the computational problem rapidly becomes enormous for molecules of

even modest size. The only way to use this method on large molecules is

to require them to have the maximum symmetry. Then, group theoretical

methods can be applied to greatly simplify the problem. Another advantage

of using symmetrical structures is that they are easily formed into

homologous series each consisting of a progression of increasingly larger

molecules. This facilitated the extrapolation of various electronic

properties to infinite size molecules (graphite) and the development of

structure/reactivity relations at the "edges” of large polyaromatic

molecules. By this means we have developed computer codes which allows us

to use this SCF technique on molecules containing up to 1000 carbon atoms.

For complete convergence of this iterative method, such large molecules

require three to six days on a high speed computer. To our knowledge, the

largest molecule to which this method had previously been applied was coronene, which contains 24 carbon atoms.

Our results so far with this more complex model are usually in

qualitative agreement with simpler theories. One critical difference,

however, is that SCF methods, in accord with intuition, indicate that

resonance energies per C-atom approach a unique value with increasing

molecular size, independent of edge type. For the simplest theory used in

previous studies, different limiting values were approached which depended

on edge type. There are other general important qualitative differences

whose implications we are now examining in detail.

102
4. Ion Kinetics

D. Brault, J. 3utler, A. Harriman, R. S. Huie, Z. Karpas,

R. D. Levin, S. G. Lias, C. Lutz, ML Mautner, P. Neta,

H. Okabe, I. Price, M.-C. Richoux, F. P. Schwarz,

L. W. Sieck and C. Speller

A. Introduction

The scientific emphasis and focus of the work of the Ion Kinetics

Group is derived largely from the competence-building program, ’’Physical

and Chemical Effects of Ionizing Radiation on Matter", which is a joint

effort with the Center for Radiation Research. The Ion Kinetics Group was organized in 1979 under the direction of Dr. P. Auslocs to fulfill the CCP

component of the Competence. Originally called the Ionizing Radiation

Group and assigned to the Chemical Thermodynamics Division, the nucleus of

the group was transferred in 1983 to the Chemical Kinetics Division where

the work on the kinetics of ionic processes became a natural complement to

the existing programs on the kinetics of neutral species. The preexisting Chemical Kinetics Division program on Aqueous Chemical Kinetics

was incorporated as an integral part of the Liquid Phase Ion Kinetics

activities. The group also has been historically affiliated with the Ion

Kinetics and Energetics Data Center, with members of the scientific staff

collaborating on compilations and evaluations of data necessary to the

understanding of the chemistry of ions and the effects of ionizing

radiation on matter. The activities of the Data Center are described in

Section 2.

The long range goal of the Competence on the Physical and Chemical

effects of Ionizing Radiation on Matter is to obtain experimental and

theoretical results which contribute to the understanding of the response

of matter to ionizing radiation in sufficient detail for application to

radiation biology, radiation dosimetry, and radiation processing.

The approach to the study of radiation effects involves diverse

experimental tasks which examine individual facets of the problem area in

detail. These individual tasks are summarized below:

Energy Deposition

Experiments aimed at achieving a fundamental understanding of the

initial ionization event.

Radiation-Induced Fragmentation of Organic and Biochemical Ions

Studies of the energetics, rates and modes of fragmentation of

ions

Kinetics of the Interactions between Ions and Molecules

Fundamental investigations of the chemistry of ions, including

elucidations of reaction rates and mechanisms, thermochemistry.

103
Energetics of Solvation and the Chemistry of Solvated Ions

Studying the link between the kinetics and thermochemistry of

ions in the gas phase and in the liquid phase; development of

predictive capacity to apply results of gas phase mass spectrometric studies to condensed phase systems.

Charge Recombination of Ionized Transients

Providing insight into energy transfer and radiation damage

mechanisms.

Radiation Chemistry of Organic and Biochemical Solutions

Energy transfer, radiation damage in biologically important

substances

.

Kinetics and Radiation Chemistry of Aqueous Inorganic Solutions

Electron transfer reactions in aqueous solutions of inorganic

salts, particularly emphasizing the kinetics of reactions

involving SO^

-

, SO^”, and HSO^

-

.

Ion Kinetics and Energetics Data Center

Building and maintaining up-to-date evaluated data base relating

to ionization processes and ionic reactions.

The description of the scientific activities of the Group which

follows will be organized under headings corresponding to the various

activities listed above, although an arbitrary subdivision has been made

between Gas Phase Ion Kinetics and Liquid Phase Ion Kinetics. Data Center

activities are discussed in Section 2. It will be noted that there is

considerable overlap between the various areas of activity, with many

individuals contributing to more than one task.

B. Gas Phase Ion Kinetics

The program aims at achieving a fundamental understanding of the

unimolecular and bimolecular reactions of ionic species generated by high

energy radiation, and further, at examining the effects, of ion-molecule

clustering, i.e., of solvation. Aspects of the program currently supported by the Department of Energy and the Air Force Office of Scientific

Research give particular emphasis to examinations of processes involving

small organic molecules, while experimental work tied in to the Liquid

Phase Ion Kinetics effort (described below) focuses on model compounds

which give information about processes in biochemical systems.
1 . Radiation-Induced Fragmentation of Organic Ions

(J. Butler, in collaboration with R. Stockbauer, 541 , and A.

Paar, CRR; S. G. Lias, L. W. Sieck)

The overall objective of this research is to provide a detailed

understanding of the disposal of energy by the excited ions which

initially result from the absorption of ionizing radiation, and to

determine the kinetic and thermodynamic parameters which influence the

fragmentation processes.

Results obtained during the past year include:

(a) A determination of the energetics of the reaction:

+ + (tert-C H )CH -> C H C(CH ) + CH 4965 65 32 3

which led to an accurate value for the heat of formation of the isopropylbenzene (cumyl) ion. This is the same ion which is formed by the

protonation of C^f^CCCH^^C^, and therefore, the experimental result

provided a comparison standard for the evaluation of the scale of proton

affinities completed by the Ion Kinetics and Energetics Data Center last

year. This result was obtained from, photoelectron-photoion coincidence

experiments, in which ions of known excitation energy are sampled by

examining those positively-charged species formed in coincidence with zero

energy . electrons , where the ionization event was effected by photons of

known energy. The experiments also yield rate constants and entropy

changes associated with the dissociation processes, as well as accurate

heats of formation of the fragment ions.

(b) In order to evaluate information about the structures and

thermochemistry of ions isomeric to protonated benzene the fragmentation

processes leading to the formation of this ion were studied in 1,3.5-

nexatriene, 1,3- ana 1 , 4-cyclohexadiene using the photoelectron-photoion

coincidence technique. It was found that the slow fragmentation process:

+ + C H > C H + H

6 3 6 7

proceeds in all three systems through the 3ame cyclic transition 3tate,

which lies at an energy level higher than the (c-C^Hy + H) products.

+ Figure 5 shows the normalized abundances of the parent C^Hg ions and the

-L. fragment ions formed in the three subject compounds as a function of

energy, examined after 1.2 microseconds and 6.2 microseconds. The

bimolecular chemistry of ions was also investigated.
Relative

Abundance

(Normalized)

1.0

0.8

0.6

0.4

0.2

3.8 10.0 10-2 10.4 10.6 10.8 10.4 10.6 10.8 11.0 11.2 11.4 11.6 10.4 10.6 10.8 11.0 11.2 11.4 11.6 eV eV eV

Fig. 5. Photoelectron-photoion coincidence results elucidating the

fragmentation process: C^Hq* + hv -+ CgHy* + H in the CgHg isotners

1 , 3 ,5-hexatriene, 1 ,

3

-cyclohexadiene , and 1 ,4-cyclohexadiene.

Shown are the normalized relative abundances of the parent CgHg

+ ions (decreasing curves) and daughter CgHy

+ ions (increasing

curves) observed in coincidence with zero-energy electrons,

plotted as a fu notion of the energy „of the ionizing photons. The

ions are observed after 1.22 s x 10"° and 6.20 3 x 10

- °. In all

three cases, the degree of fragmentation observed is greater after

6.20 s x 10~° than after 1.22 s x 10~ , demonstrating that the

fragmentation processes are slow. RRKM modelling of the

fragmentation (solid lines) demonstrates that the dissociation

processes in the three systems all proceed through the same cyclic

transition state.

106
(c) Other work carried out to provide benchmark measurements

for evaluations of ion thermochemistry included a study of ionic

fragmentation processes in dimethylether . An accurate heat of formation

for the CH0OCH2 ion was obtained. It was al30 demonstrated in a photoion-pnotoelectron coincidence study that the HC 0

+ ion formed in

dimethylether results both from dissociation of H^CCT and of CH^OCi^*

(d) A detailed evaluation was made of thermal activation

efficiencies at the low pressure limit of the decomposition of protonated

methoxyethers for a variety of organic and inorganic bath gases. The

measurements, made on the high pressure pulsed electron mass spectrometer

,

provide the only data available on thermal activation for ionic systems in

the vapor phase. Although the ordering of efficiencies is qualitatively

similar to that generally found for vibrational activation of neutrals,

the spread in values is greater for the ionic system. The work was sponsored by the Department of Energy.

Future plans include the examination of ionic fragmentation mechanisms in styrene and cyclooctatetraene which will resolve conflicting

information in the literature about heats of formation of ions, and aid in

the evaluation of data for a forthcoming Data Center evaluation effort.

Plans also include the determination of ionic fragmentation energetics in

phenylacetylene, which will tie in with an effort described below in which

the ion-molecule kinetics in this system will be elucidated.

2 . Kinetics of the Interactions between Ions and Molecules

(S. G. Lias, P. Ausloos, M. Mautner, T. Buckley, L. W. Sieck, F.

Schwarz)

Irradiation at energies above the ionization threshold produces

positive ions which may undergo rapid ion-molecule reactions. This task

involves the elucidation of the bimolecular chemistry of ions, including

an examination of reaction mechanisms, and measurements of rate constants

and equilibrium constants as a function of temperature. The latter

experiments lead to quantitative information about the thermochemistry of

proton transfer, charge transfer, and other ionic reactions. Much of the

work ties in directly with the above-described task on ionic fragmentation

processes, as well as with the work of the Ion Kinetics and Energetics

Data Center. Accomplishments of the past year are discussed below.

(a) In conjunction with the Data Center evaluation of the thermochemistry of ions, questions arose about the structured) and +possible

isomerization mechanisms of ions of the empirical formula C^H7

. In most

systems where CgHh species occur, it has previously been assumed that the

ions will predominantly have the thermodynamically most stable structure,

that of protonated benzene. A study was made of the reaction kinetics of

this important species originating (a) from fragmentation of parent

radical cations in 1 , 3 , 5 -hexatriene, 1 , 4- and 1 . 3 -cyelohexadiene , and a variety of CgH-|Q isomers or (b) in ion-molecule reactions such as:

+ + O3H4 + C2H4 •* C5H7 + H
(where C

3 H4 is allene or propyne). It was demonstrated that in each of

these systems, the

+ ion exhibits at least two structures, and the second,

non-benzenium CgHy structure is different in different systems. Probable

structures for the other isomers are protonated fulvene (protonated

methylenecyclopentadiene) and protonated dimethylenecyclobutene.

(b) In molecules which undergo large geometry changes upon

ionization, ionization energies are difficult to determine by techniques

which detect the onset of ionization as a function of energy; in such

cases the onset of ionization is a slowly increasing function of energy and

the exact onset energy is difficult to pinpoint. For this reason, the

adiabatic ionization energies of hydrazines have never before been

determined successfully. In work carried out during the past year, the

ionization energies of a variety of substituted hydrazines were determined

in the pulsed high pressure mass spectrometer by measuring the equilibrium

constants of charge transfer reactions:

A

+

+ B -* 3

+ = A

(where A is a hydrazine and B is a reference compound with a well-established ionization energy) . The enthalpy change of the charge

transfer reaction is the difference in the adiabatic ionization potentials

of B and A. The effects of steric constraints on ionization energies in

these compounds was also examined.

Similar experiments carried out in the ion cyclotron resonance

spectrometer resulted in the first determination of the adiabatic

ionization energy of cyclopropane, which undergoes a Jahn-Teller

distortion upon ionization.

(c) As an adjunct to an extensive evaluation of proton affinity

data completed during the past year in the Ion Kinetics and Energetics

Data Center (Section 2), a large number of proton affinity determinations

for species of special interest or importance have been carried out

through measurements of equilibrium constants of proton transfer reactions

using both the pulsed high pressure mass spectrometer and the ion

cyclotron resonance spectrometer.

(d) In a project sponsored by the Air Force Office of Scientific Research, the rates and mechanisms of consecutive ion-molecule

condensation reactions in diacetylene, cyanoacetylene , and cyanogen have

been examined in both the ion cyclotron resonance spectrometer and the

pulsed high pressure mass spectrometer. These compounds, which contain

conjugated triple bonds undergo efficient condensation and condensation-dissociation sequences leading to the formation of high molecular

weight hydrogen-deficient ions such as C-|gHg in diacetylene. The ion

cyclotron resonance instrument was modified so that the trapped ions cculd

be irradiated by photons from a pulsed dye laser, and the photodissociation mechanisms of the product condensation ions were mapped.

108
(e) The reactions of radical cations in excited electronic

states have been investigated using pulse radiolysis to generate the

excited ions, and fluorescence measurements to follow the abundances of

the excited ions as a function of time. Fluorinated benzene ions emit in

the visible (470 nm) with appreciable yields. In the presence of excess

krypton, argon, neon, or helium, the charge transfer from the inert gas,

+ +*

X + C

6 (H,F) 6 + X + C6 (H,F) 6

(where X is the rare gas) produced the fluorescing cation; this reaction

can not be studied by any other technique. The results yield rate

constants for this ion-molecule reaction which are close to the

ion-molecule collision rates.

Future plans include additional work on the ionization energies of

hydrazines which will examine such effects as rigid geometry, constrained

angles and bulky substituents on ionization energies. In this group, the

effects of the large geometry changes on the kinetics of electron transfer

will be also be investigated. Preliminary results show that the reactions

are unusually slow for exothermic charge transfer processes. In addition,

a comprehensive study of ion-molecule collision kinetics including

highly accurate determinations of the rate constants for collision-rate

processes in a variety of organic compounds will be made. We will also

investigate the kinetics of the reactions of the phenyl ion and other

isomers. The chemistry of this important species, which is generated

in irradiated aromatic compounds, has never been systematically explored.

Studies will be made of ion-molecule mechanisms in aromatic molecules with

unsaturated side chains.

3. Solvation Thermodynamics and the Kinetics of Solvated Ions

(M. Mautner, L. W. Sieck, S. G. Lias, and C. Speller)

Ionic species, such as those generated in acidic or basic

environments or by radiation are ubiquitous intermediates in biochemical

and organic reactions. The stabilities and reactivities of ionic

intermediates are affected by their intrinsic molecular properties

(studied in the gas phase under tasks (a) and (b), above), and by

interactions with the condensed phase environment. This task utilizes the

high pressure mass spectrometer to bridge the gap between the

understanding of ionic chemistry attained in the gas phase studies and

that required for understanding and prediction of processes occurring in

irradiated liquids, acid-catalyzed reactions, and enzyme catalyzed

processes. In this instrument, it is possible to generate a given ion in

the presence of a solvation species (such as H

2 0, for example), to

determine the thermochemistry of solvation by an examination of equilibria

such as

A

+

+ M * (

+ *M)

(

+

*M) + M - (A + -M2 ), etc.

—

*1 OQ^ s
and to study the kinetics and mechanisms of reactions of the solvated

ions.

Work has been carried out during the past year on the following

problems

:

(a) A comprehensive investigation was conducted of the ionic

hydrogen bond, which plays a dominant role in ion-solvent interactions.

For structurally simple organic species, it was shown that the energy of

the ion-solvent association bond is directly proportional to a molecular

property, the proton affinity of the molecule. The proportionality

constants for a great variety of A***H+ ***B association ions (where A and

B represent heteroatoms in different classes of molecules) have been

generated in this laboratory. Such correlations can be used in

estimations of cluster ion stabilities from proton affinity data. The

strengths of ionic hydrogen bonds involving complex molecules were also

determined. The results revealed structural effects such as hydrogen

bonds between parts of the same molecule, as well as the occurrence of

multiple bonding between ions and large molecules.

Further studies of this type sponsored by a contract with the

Department of Energy were concerned with establishing the thermochemistry

of hydration of low molecular weight sulfides and mercaptans, as w^ll as the hydrogen bond strengths associated with the solvation of CH0 NH3 by

several mercaptans (N***H + ***S and S***H + ***S interactions). This data on hydration energies was used to correlate the relationship between the

difference in proton affinity between H

2 O and any mercaptan as a function

of the dissociation energy (stability) of the proton-bound hydrate, for

use in estimating properties of other clusters, as described above.

(b) An interpretation of the observed correlation between proton

affinity and the energy of ion-molecule association was provided by a study carried out on the ion cyclotron resonance instrument. The H/D

exchange processes:

AH

+

+ BD -*• [AH *BD] + - AD

+

+ BH

were examined for some of the association ions, [AH*3H], whose

thermochemistry was determined in the high pressure mass spectrometer (see

(a) above). It was shown that (a) the mechanism of the exchange reaction

involves the occurrence of an endothermic proton transfer reaction in the

[ AH *BD] complex; (b) just as there is a relationship between the proton

affinity of AH and the strength of bonding with BH (see (a)), the

probability of occurrence of the H/D exchange also correlates with the

strength of bonding; (c) both effects can be interpreted in terms of

a double-welled potential surface, the major features of which can be

predicted simply from the classical ion-molecule interaction potential.

(c) The work described under (a) and (b) was concerned with systems

in which the molecules contain atoms with free electron pairs which

interact with a proton, i.e. systems with interactions of the types

N* , *H

+ ***N, 0 ,,, H+ ***0, S***H + ***0 and N** # H

+ ***S. However,

no
In some systems, the results indicated that the proton interacts with a carbon atom or a carbon-based electron system. In this report period,

extensive studies were dedicated to these types of unconventional hydrogen

bonds, using as reactant quarternary ammonium ions for which only this

type of interaction is possible. Such interactions were found to be

unexpectedly strong, in the range of 40-80 kJ/mol. The results suggest

that in solution, hydrogen atoms from solvent molecules may bond not only

to a proton in the ionic species, but also to hydrocarbon chains or aromatic rings, which were thought not be significantly interactive. The

results are pertinent to understanding interactions in biological systems,

especially to the interactions of the quarternary neurotransmitter ion

acetylcholine with neuroreceptors. The model developed also implies that

interactions of amide groups with phenyl rings in proteins may play a role

in peptide conformation.

(d) Rocket-born experiments have indicated the occurrence of

tropospheric ion clusters which involve molecules of water, hydrogen

cyanide and acetonitrile. Clusters of this kind can be generated in the

high-pressure mass spectrometer. The stabilities of one of the major type

of clusters, those containing only water and acetonitrile, were measured.

The results are relevant not only to ionospheric chemistry, but also to

the solvation of the proton in mixed solvents, such as the

water-acetonitrile system, which is commonly used in liquid

chromatography

(e) The pulsed high pressure mass spectrometer was converted to

negative ion operation in early 1984. The positive ion capability was retained, and switching between the two modes of operation can be

completed in less than one minute using external circuitry only.

There is at present a striking lack of information about the

association and clustering reactions of negative ions. This is

particularly true for association ions incorporating relatively complex

organic molecules. Such information is of fundamental scientific

importance as well as impacting directly on negative chemical ionization

analytical mass spectrometry, which utilizes association (adduct) ion

formation for diagnostic purposes both in traditional mass spectrometric

and tandem MS/MS instrumentation. Determination of the stabilities of

association ions formed by the interactions of Cl~ and NO? with a variety of organic molecules will be carried out. The N0^ type anions

are easily generated in mixtures containing trace quantities of

nitroalkanes , and they may provide a viable alternative to other reagent

ions such as Cl", which is commonly used in negative chemical ionization

analysis.

In the future the bonding in ionic charge transfer complexes will be

examined. Of particular interest is the contribution of resonance charge

transfer between components of the complex, and the dependence of

resonance contributions on molecular parameters such as ionization

potentials and steric hindrance. Also clustering studies will be extended

to further clusters of ionospheric interest, and also to clusters where

unconventional hydrogen bonds may occur.

Ill
4. Charge Recombination of Ionized Transients

(F. Schwarz, H. Okabe, P. Ausloos)

In systems under high energy irradiation, the ions formed

ultimately recombine with electrons or negative ions. In recent years,

this Group has examined in some detail the kinetics and chemical consequences of ion-electron recombination in organic liquids (’’geminate

recombination”). During the past year, attention has been given to such

processes occurring in the gas phase. Systems were selected for study in

which the excited neutral species formed in the recombination:

M

+

+ e N + M*

dissociates to give excited fragments which fluoresce:

M* -> F* + R F + R + hv

In such systems the electron-ion recombination process can be followed by

monitoring the emission intensity as a function of time.

This past year fluorinated methane and ethane gases were exposed to 3

nanosecond pulses of high energy (0.6 MeV) electrons from a Febetron 706,

and the lifetime and spectrum of the resulting emission from excited

fragments were monitored. In CF2 H2 , CF2 HC&, CF 3H, CF

3 C 2,, and C2 F5 , the

dissociative process following recombination could be established by

examining the fluorescence of excited fragment species, i.e., in CF2h2 and

CF2HC! the recombination emission results from excited CH2 fragments,

while CF

3 H, CF3

CJL, and C2 Fg produce CF

3

emission. Quenching constants of

the fluorescence by various gases were also obtained. The only previous

exploitation of recombination fluorescence as a means of monitoring

ion-electron recombination in the gas phase was in inert gases at high

pressure

.

C. Liquid Phase Ion Kinetics

1 . Radiation Chemistry of Organic and Biochemical Solutions

(P. Neta, M.-C. Richoux, D. Brault, A. Harriman, and M. Mautner)

Pulse radiolytic techniques are used to characterize the

chemical properties of short-lived species in irradiated solutions.

Particular attention is given to reactions of reactive transients which

have been implicated as intermediates in radiation-induced and thermal

processes in a variety of chemical and biological systems. An important

aspect of this program is the study of inter- and intramolecular electron

transfer processes aimed at understanding similar phenomena in biological

systems as well as in photochemical systems of interest for possible use

in solar energy conversion; this aspect of the work is supported by the

Department of Energy. By studying the kinetics and mechanisms of the

various transient species and by understanding the parameters that control

them, we can hope to be able to predict the behavior of these species in more complex systems.

112
(a) The toxicity of halogenated hydrocarbons (such as carbon

tetrachloride and the anesthestic agent halothane, CF^CHCZBr) is believed

to originate from cytochrome-P^Q-mediated generation of peroxyl radicals

with subsequent attack on biological targets, especially unsaturated

lipids. The feasibility of this proposed mechanism was demonstrated in a pulse radiclytic study of the reactions of peroxyl radicals with iron

porphyrins and with fatty acids. Fe^-^-deuteroporphyrin was used as a model for cytochrome P45Q and its reactions with peroxyl radicals were

studied by pulse radiolysis. Peroxyl radicals derived from the

halogenated hydrocarbons were found to oxidize the ferric porphyrin by an electron transfer mechanism to produce a Fe-^-^-porphyrin pi-radical

cation. The rates of reactions of oleic acid, linoleic acid, linolenic

acid, and cholesterol with halogenated peroxyl radicals were much slower

than the rates for reaction with the porphyrin. Furthermore, the peroxyl

radicals derived from these fatty acids react with the porphyrin rapidly.

The latter reaction leads to degradation of the cytochrome P450 but is not

a termination reaction, since it produces hydroperoxides which may, in

turn, produce additional radicals to attack the lipids.

(b) Photogeneration of oxygen from water is an integral part of

green plant photosynthesis and many laboratory models aimed at the storage

of solar energy have tried to mimic this natural process. In this problem

area, an experimental examination of the catalytic oxidation of water to

oxygen by metalloporphyrin pi-radical cations has been carried out. In

the ideal case, a metalloporphyrin (synthetic analogue of chlorophyll)

absorbs the light and forms an excited state which then donates an electron to another compound in the system. The electron acceptor then

reacts with a catalyst to form hydrogen from water. The metalloporphyrin

radical-cation (formed after donating an electron) should react with

another catalyst that will produce oxygen from water. The overall effect

should be splitting of water to hydrogen (a fuel) and oxygen by sunlight

with no net change in the other components of the system. For this

purpose, it is necessary to find the proper catalyst, the best

metalloporphyrin, and the optimal experimental conditions for oxygen

production. This was done using pulse radiolysis and monitoring the rate

of reaction of various porphyrins and catalysts to determine the factors

that influence these rates. With this approach, a tedious search for

oxygen production under different conditions was unnecessary. After

optimization of the kinetic factors by pulse radiolysis, it was found that

the optimal kinetic conditions also gave the highest oxygen yield.

The best porphyrin found for this purpose is not the one predicted on

a txhermodynamic base to be the strongest oxidant (ZnTMPyP), but rather

that which forms a longer-lived radical-cation (ZnTPPS) . The best

catalyst was found to be colloidal Ru0 2

, and the rate of reaction

increased with pH, especially at pH> 9 .

In conjunction with these studies, it was found that the radical

cations produced by one-electron oxidation of porphyrins in aqueous

solutions tend to form complexes with any anions present in solution.
This complexation affects both the absorption spectra and the lifetime of

the radical cation (and thus its ability to participate in oxygen

production via the catalyst).

(c) During the past year, the program of study of intramolecular

electron transfer processes, supported by the Department of Energy, was expanded to include investigations of temperature effects on such processes. The studies were carried out on nitroaromatic halides in which

electron attachment on the nitro group is followed by electron transfer

through the molecule leading to halide loss. Earlier work had demonstrated that the rate of the intramolecular electron transfer depends on

the respective geometries and the relative electron affinities of the

nitro and halide functions in the molecule. Since the dependence of the

rate constants on molecular geometry is governed by energy barriers and

entropies of activation, a physical understanding of structural effects

can be achieved only if both of these factors are known, i.e., if the rate

constants are known as a function of temperature. A temperature-controlled pulse radiolysis cell was constructed and a series of reactions

were measured. The results show that both the energy barriers and the

entropies of activation vary significantly from reaction to reaction.

Some specific structural trends emerge, such as the decrease of reaction

probabilities when steric factors interfere with optimal orientation of

the substituents for electron transfer.

The studies of intramolecular electron transfer processes have

recently been extended to include additional types of compounds, namely

covalently-bound dimers of metalloporphyrins (MP-M'P), viologens (V-V T ),

and mixed dimers (Mp-V), as well as related compounds with two reducible

metal centers. The purpose of this work is to achieve an understanding of

similar natural processes, occurring for example during photosynthesis,

with the hope of synthesizing a model that will mimic the natural process

effectively.

In order to determine the enthalpy and entropy changes that

contribute to the electron affinities of organic and biological molecules

in aqueous solution, the temperature dependences of charge transfer

equilibrium constants in the pulse radiolysis of liquids was studied using

the temperature-controlled pulse radiolysis cell. Preliminary results show

that the major differences between molecular electron affinities are due

to enthalpy effects, but in large molecules entropy effects are also

significant. This is of interest to biological systems related to the

action of quinone containing co-enzymes, which are large molecules and

which participate in biological electron transfer processes.

Planned extensions of this work include the study of reactions of

peroxyl radicals with iron porphyrins and fatty acids, aimed at examining

the factors causing the toxicity of halogenated hydrocarbons . This study

will be continued with similar experiments on other possibly harmful

halogenated hydrocarbons, such as other anesthetic agents and the

pesticide ethylene dibromide. The reactions of the alkyl and perioxyl

radicals derived from such compounds with iron porphyrins in their various

oxidation states will be examined.

Ilk
In addition, the study of the catalytic oxidation of water to oxygen

by metalloporphyrin pi-radical cations (aimed at mimicking the

photogeneration of oxygen from water in natural photosynthesis) will be

continued with an examination of the behavior of the cation radicals of

several other porphyrins, in which either the ligand or the metal will be

varied. Thus, the structural parameters which govern the process will be

systematically examined. In conjunction with these experiments, the

behavior of those porphyrins in reduction processes will also be studied,

since recent results have demonstrated that the radical anion lifetimes

vary over a very wide range, depending on the ligand and particularly on

the central metal ion.

The temperature dependences of charge transfer equilibria in irradiated organic liquids will be determined for a large group of quinones and

ni troaromatic molecules. The enthalpy and entropy changes and the

resulting effects on molecular electron affinities, as well as the effects

of alkyl, aryl and halogen substitution on these factors will be investigated. This work may be also extended to include acid-base dissociation

equilibria. Exploratory investigations will be carried out on charge

transfer through macromolecules. Specifically, the feasibility of

measuring charge transfer through pulse irradiated polymer-coated

electrodes will be explored. The studies of intramolecular electron

transfer processes have in covalently-bound dimers of metalloporphyrins

(MP-M'P), viologens (V-V*), and mixed dimers (Mp-V), and related compounds

with two metal centers will be expanded to examine the effects of molecular structure on the kinetics of these processes.

2. Kinetics and Radiation Chemistry of Aqueous Inorganic Solutions

(R. E. Huie (in collaboration with Dr. N. C. Peterson of

Polytechnic Institute of N.Y.) and ?. Neta)

The current experimental work in aqueous phase chemical kinetics

is an outgrowth of an effort to understand the induced oxidation of iron

by SC>2 in aqueous solution. In this system, SO 2 and ferrous ion are

simultaneously oxidized under conditions such that neither would be

oxidized alone. As described in la3t year's annual report, the mechanism

previously proposed to explain this phenomenon has the radical anions SO?

and SO^ as key intermediates, along with HSO^, peroxymonosulfate

Therefore, the current emphasis of the experimental work is to develop a broad understanding of the chemistry of these 3pecie3 to assess their role

in the mechanism of SO o oxidation. The practical importance of this

problem area lies in the probable role of these reactions in causing acid

rain, and the possibility of achieving an improved understanding of

lime-limestone flue gas scrubbing.

In this work, pulse radiolysis is used as a technique to selectively

generate reactive intermediate species of interest. Stopped flow and

kinetic spectrophotometry experiments are performed to obtain complementary information on aqueous kinetics.
(a) For understanding the oxidation of S02 in solution, the chemical

reactions of SO^ must be understood, and much effort was devoted to

studying the chemistry of this species during the past year.

The reaction with 0

2 is of particular interest. Pulse radiolysis

experiments established that this reaction has a rate constant of 1 . 5x10 ^ M

-

‘s _1

, and gives SOg as a product:

SO^ + 0

2 * SO5

Rate constants for the oxidation of a variety of organic compounds

by SO

3

have been measured, and equilibrium constants for the reactions

of SOj with chlorpromazine and phenol were also obtained. SO

3 was found to be a mild oxidant. The oxidation of several compounds by SO5 was found to occur more rapidly than the corresponding oxidation by SO

3

.

The reaction of S0 2 with the ferrous ion has been investigated by

monitoring the product ferric sulfite complex at 350 nm. A product which

absorbs at this wavelength initially builds up rapidly (k~2x 1 O^M

- 1 s

-1

) and

at longer times continues to build up at a slower rate (k-2x 1 O^M

- 1 s

-1

)

.

These results may be explained by the rapid formation of a ferrous sulfite

radical complex followed by a slower electron transfer step.

(b) The oxidation rate of S(IY) by various radicals has been

measured. In addition to showing the expected increase in rate with

increasing reduction potential of the radical, the ratio of the rates of

oxidation of sulfite to that of bisulfite, k(S0- )/kHS0_) decreased. For

mild oxidants, bisulfite is quite unreactive. This information may help

explain some observations on the pH dependence of S0 2 antioxidation in

aqueous solutions.

The formation and reaction of sulfite radicals has been investigated

more thoroughly by studying the one-electron redox reactions involving

sulfite ions and the aromatic amines, aniline, dimethylaniline

,

diphenylaraine , phenylenediamine , and tetramethyl-phenylendiamine (TMPD)

.

The behavior of the aromatic amines varied from aniline, whose radical

cation reacts rapidly with sulfite and slowly with bisulfite to TMPD,

which is oxidized by the sulfite radical and whose radical cation reacts

with SO

3

to form a new species which absorbs at 455 nm.

(c) The kinetics of the reactions of peroxymonosulf ite with ferrous

and cobaltous ions have been studied using the techniques of stopped flow

and kinetic spectrophotometry. The reaction of ferrous ions with

peroxymonosulfate is monitored by following the build-up of ferric ion in

the stopped flow spectrophotometer. The rate of the reaction, which has

been determined at several acidities and with either ferrous ion or peroxymonosulfate in excess, appears to be a complicated function.

The cobalt-catalyzed decomposition of peroxymonosulfate has also been

studied. The results show that the Co(III) builds up as the

peroxymonosulfate is depleted, suggesting a mechanism such as:

116
2+ -

3 + - - Co + HSO * Co + OH + SO

5 4 — 2 + 2- 3+

SO + Co + SO + Co

4 4

3 + - 2+ - + Co + HSO + Co + SO + H

5 5

2 SO -0 2 ,SO +0

5 4 2

The study of this slow reaction is complicated by the reduction of Co(III)

by water.

(d) The reaction of bisulfite with hydrogen peroxide was examined to

determine if the reaction is a radical chain process. In experiments in

which ascorbic acid or pyridine were added to scavenge radicals, there was

no apparent effect on the measured rate, supporting a non-radical reaction

mechanism.

In the future the systematic exploration of the chemistry of SO^

will be continued with a pulse radiolytic examination of reactions with

the ferrous and ferric ions and with other metal ions such as manganese,

vanadium, and copper. The reaction of ferric ion with bisulfite will also

be studied using an anaerobic mixing system and kinetic spectrophotometry

(in collaboration with Dr. Carl Clark of South Carolina State College).

In addition, computer modelling will be applied to analyze the

experimental data on the kinetics of the reactions of peroxymonosulf ite (in

collaboration with Dr. N. C. Peterson of Polytechnic Institute of N.Y.).

3. Fluorescence Studies

(C. Lutz, F. P. Schwarz, and P. Ausloos)

Many organic liquids excited by absorption of radiation undergo

fluorescence or energy transfer to solute molecules in competition with

fragmentation. Measurements of fluorescence quantum yields, energy

transfer efficiencies, fluorescence lifetimes, and yields of phctofragments

in irradiated organic liquids provide information about the nature of the

excited states formed by direct excitation, and about the nature of the

excited molecules resulting from ion-electron recombination.

In the past year, the following projects were brought to completion:

(a) The study of p-dioxane aggregation with water and with methanol

was completed with the determination of the flourescence lifetimes of the

emitting aggregates. The aggregate formed in pure dioxane can incorporate

small amounts of H

20 or CH

3 OH into its structure, which significantly

alters its fluorescence properties.

117
(b) The charge transfer from trans-decalin radical cation to a variety of aromatic compounds was monitored through the fluorescence

resulting from recombination of the product cation with an electron:

D

+ + M -» M

+ + D

M

+ + e > M*

M* -» M + hv

It was found that the relative rate constants for charge transfer from the

trans-decalin ion to the various solutes increase with increasing

exothermicity of the reaction.

118
5. PUBLICATIONS

(a) Publications of Past Year

Ausloos, P. and Lias, S. G., "Isomerization of Carbonium Ions in Collision

Complexes," Int. J. Mass Spectrom. Ion. Proc., 58_, 165 ( 1984).

Baral, S., Hambright, P., and Neta, P., "One- and Two-Electron Reduction of

Aluminum and Tin Pyridyl porphyrins . A Kinetic Spectrophotcmetric Study,"

J. Phys. Chera., 88_, 1595 (1984).

Brault, D. and Neta, P. , "Reactions of Fe1 11 -Porphyrins with Peroxyl Radicals

Derived from Haloethane and Halomethanes , " J. Phys. Chem., 88_, 2857

(1984).

Braun, W., McNesby, J. R. , and Scheer, M. D., "A Comparative Rate Method for

the Study of Uni molecular Fall-Off Behavior," J. Phys. Chem. 88_, 1846

(1984)

Brown, R. L. , "Counting of Resonance Structures for Large Benzenoid Polynuclear Hydrocarbons," J. Computational Chem., 4_, 556 ( 1983).

Cvetanovic, R. J., and Singleton, D. L., "Reaction of Oxygen Atoms with

Olefins," Reviews of Chemical Intermediates _5, 183 ( 1984).

DeMore, W. B., Golden, D. M., Hampson, R. F., Howard, C. J., Kurylo, M. J.,

Molina, M. J., Ravishankara, A. R. , and Watson, R.T., "Chemical Kinetics

and Photochemical Data for Use in Stratospheric Modeling," Evaluation No

6 of the NASA Panel for Data Evaluation , JPL Publication 83-62, Jet

Propulsion Laboratory, Pasadena, CA.1983.

Grodkowski, J. and Neta, P. , "One-Electron Oxidation in Irradiated Carbon

Tetrachloride Solutions of ZnTPP, TMPD, and Phenols," J. Phys. Chem., 38^

1205 (1984)

Hambright, P. , LeMelle, S. , Alston, K,, Neta, P. , Newbali, H. H. , and

DiStefano, S. , "A Dissociative Mechanism for the Dithionite Reduction of

Cobalt (II I) -Myoglobin," Inorg. Chim. Acta, 92_, 167 ( 1984).

Kurylo, M. J. and Knable, C-. L., "A Kinetics Investigation of the Gas Phase

Reactions of Cl(^P) and OH(X^u) with CH^CN : Atmospheric Significance and

Evidence for Decreased Reactivity Between Strong Electrophiles," J. Phys

Chem. , 88_, 3305 ( 1984) .

Laufer, A. H. and Lechleider, R. , "The Reaction of Ethynyl Radicals with 0 o . Rate Constant for Formation of CO," J. Phys. Chem., 88_, 66 ( 1984).

Laufer, A. H., Gardner, E. P. Kwok, T. L. and Yung, Y., "Computations and

Estimates of Rate Coefficients for Hydrocarbon Reactions cf Interest to

the Atmospheres of the Outer Solar Systems," Icarus, 56_, 560 ( 1983).

Lias, S. G., "Ion Thermochemistry - A Summary of the Panel Discussion," Gas

Phase Ion Chemistry, Ed. M. A. A. Ferreria, Amsterdam: Reidei, 1984.

119
Lias, S. G. and Buckley, T. J., "Structures and Reactions of C^^* Ions

Generated in Cyclopropane," Int. J. Mass Spectrom. Ion Proc. 56_, 123

(1984).

Martinez, R. I. and Herron, J. T., "Acid Precipitation: The Role of

0 o-Alkene-S02 Systems in the Atmospheric Conversion of SO2 to H2 SO4

Aerosol," J. Environ. Sci. Health, A1

8

, 739 (1983).

Martinez, R. I. and Herron, J. T., "Methyl Thiirane: Kinetic Gas-Phase

Titration of Sulfur Atoms in S

x

oy Systems," Int. J. Chem. Kinetics, _15_, 1127 (1983).

Martinez, R. I. and Niki, H. , "O-Atcm Production via Dissociation of a Dioxymethylene Diradical," Bull. Soc. Chim. Beiges, 92_, 657 ( 1983).

Meot-Ner (Mautner) , M., "The Ionic Hydrogen Bond. 4. Intramolecular and

Multiple Bonds. Proton Affinities, Hydration and Complexes of Amides and

Amine Acid Derivatives," J. Am. Chem. Soc. 106, 278 (1984).

Meot-Ner (Mautner), M., "Structurally Complex Organic Ions: Thermochemistry

and Non-Covalent Interactions," Accounts of Chem. Res., VJ_, 186 ( 1984).

Meot-Ner (Mautner), M., "The Ionic Hydrogen Bond and Ion Solvation. 1.

-NH

+ ***0, -NH

= ***N- and -0H

+ ***0- Bonds. Correlations With Proton

Affinity. Deviations Due to Structural Effects," J. Am. Chem. Soc. 106,

1257 (1984).

Meot-Ner (Mautner), M., "The Ionic Hydrogen Bond and Ion Solvation 2.

Solvation of Onium Ions by 1-7 H

2 O Molecules. Relations Between Monomolecular, Specific and Bulk Hydration," J. Am. Chem. Soc. 106, 1264

(1984).

Morehouse, K. M. and Neta, P. , "Redox Reactions of Manganese Porphyrins in

Aqueous Solutions. Steady-State and Pulse Radiolysis Spectrometric

Studies," J. Phys. Chem., 88_, 1575 ( 1984).

Niki, H. , Maker, P. D., Savage, C. M., Breitenbach, L. P., and Martinez, R.

I., "Fourier Transform Infrared Study of the Gas-Phase Reaction of ^0^

with trans-CHCl=CHCl in ^^^-Rich Mixtures. Branching Ratio for 0-Atcm

Production via Dissociation of the Primary Criegee Intermediate," J.

Phys. Chem., 88 , 766 (1984).

Norris, R. K., Barber, S. D. , and Neta, P. , "Steric Effects on Rates of

Daholgenation of Anion Radicals Derived from Substituted Nitrobenzyl

Halides," J. Am. Chem. Soc., 106, 3140 (1984).

Scheer, M. D., McNesby, J. R. and Braun, W., "A Study of the Collisional

Activation of Cyclobutanone by the Transient Heating of SiF^," J. Phys.

Chem. 88_, 1850 ( 1984) .

Schwarz, F. P. and Meot-Ner (Mautner), M., "Fluorescence Quenching of Liquid

Alkylbenzenes Excited by Nonionizing and Ionizing Ultraviolet Radiation

and by 3 Radiation," J. Phys. Chem. 87, 5206 (1983).

120
Shaub, W. and Tsang , W. , "Dioxin Formation in Incinerators," Environmental

Science and Technology 1

, 72 (1983).

Sleek, L. W., "Screening of Aromatic Components in Petroproducts by Chemical

Ionization Mass Spectrometry with Chlorobenzene as Reagent Gas," J. Anal.

Chera., 55, 38 (1983) .

Singleton, D. L. , Irwin, R. S. , and Cvetanovic, R. J. , "Mechanism of the

Reaction of Oxygen Atoms, (O^p) with Dimethyl Disulfide," Can. J. Chem.,

6l_, 968 ( 1983)

Siskos, P. A., Peterson, C. C. and Huie, R. E. , "The Kinetics of the Manganese

(III) - Sulfur (IV) Reaction in Aqueous Perchloric Acid Solutions,"

Inorg. Chem., 23_, 1134 (1984).

Stein, S. S. , Gardiner, W. C., Frenklach, M., and Clary, D. W., "Detailed

Kinetic Modelling of Soot Formation in the Shock Tube Pyrolysis of

Acetylene," Twentieth Symposium on Combustion, University of Michigan,

Ann Arbor, Michigan, August (1984).

Stein, S. E. , Brown, R. L. , Griffith, L. and Manka, M., "Chemical Fundamentals

of Coal Char Formation," ACS Div. Fuel Chem. Preprints, St. Louis,

Missouri, 29(2), 42-8 (1984).

Tsang, W., "Single Pulse Shock Tube Studies on the Decomposition of 1,2—

Dibromoperfluoroethane and Allyl Bromide," J. Phys. Chem., 88 281

( 1984).

Walker, J. A., Tsang, W. and Szegvary, L. , "Comparative Response of Re-refined

and Virgin Lube Oils to Additives by DSC," Measurements and Standards for

Recycled Oil - IV NBS Special Publication 674, 1984, pp. 201.

Wasik, S. P., Schwarz, F. P., Tewari , Y. B., and Miller, M. M., "A Head-Space

Method for Measuring Activity Coefficients, Partition Coefficients, and

Solubilities of Hydrocarbons in Saline Solutions," J. Res. NBS 89 , 120

(1984).

121
(b) Publications in Progress.

Baral, S. , Neta, P. , and Hambright, P. , "Spectrophotometric and Kinetic

Studies of the Radiolytic Reduction of Several Pyridylporphyrins and

Their Metal Complexes," Radiat. Phys. Chera., (in press).

Baulch, D. L., Cox, R. A., Hampson, R. F., Kerr, J. A., Troe, J. and Watson,

R. T., ’’Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Supplement II," CODATA Task Group on Chemical Kinetics, J. Phys.

Ref. Data, (in press).

Brault, D., Neta, P. , and Patterson, L. K., "The Liquid Peroxidation Model for

Halogenated Hydrocarbon Toxicity. Kinetics of Peroxyl Radical Processes

Involving Fatty Acids and Fe(III) Porphyrius ,’’ Nature, (submitted).

Brown, R. L., "Rate Constant Uncertainties in Chemical Reaction Models,"

Computers and Chemistry, (submitted).

Harriman, A. and Neta, P., "Catalyzed Decay of Zinc Porphyrin ir-Radical

Cations with Colloidal Ru02 *2H2 0 in Aqueous Solutions," J. Phys. Chem.,

(submitted)

.

Huie, R. E. and Neta, P. , "Oxidation of Ascorbate and a Tocopherol Analogue by

the Sulfite Derived Radicals SO^

- and SO

5

-

," Environ. Res., (in press).

Huie, R. E. and Neta, P. "The Chemical Behavior of SO^

- and SO^- Radicals in

Aqueous Solutions," J. Phys. Chem., (in press).

Huie, R. E. and Neta, P., "Reaction of the Canadate Ion With Chloroprcmazine

and Chi or promazine Free Radical With the Vanadly Ion," Inorg. Chem.

Acta. , (in press )

.

Laufer, A. H. , "Vinylidene (^B2 ) - An Active Intermediate in the Photolysis of

Ethylene," J. Photochemistry, (in press).

Lias, S. G., Liebman, J. F. , and Levin, R. D., "Evaluated Gas Phase Basicities

and Proton Aff if ini ties of Molecules; Heat of Formation of Protonated

Molecules," J. Phys. Chem. Ref. Data, (in press).

Lias, S, G., "Thermoneutral Isotope Exchange Reactions in Proton-Bound

Complexes of Water with Organic Molecules: Correlations with Energetics

of Formation of the Corresponding Association Ions," J. Phys. Chem., (in

press )

.

Lias, S. G., Jackson, J. A., Argentar, H. , and Liebman, J. F. , "Substituted

N,N-Dialkyl Anilines: Relative Ionization Energies and Proton Affinities

through Determination of Ion-Molecule Reaction Equilibrium Constants," J.

Org. Chem., (submitted).

Lias, S. G. and Ausloos, P. , "Structures of C^H^-

+ Ions Formed in Unimolecular

and Bimolecular Reactions," J. Chem. Phys., (submitted).

Manka, M. J. and Stein, S. E. , "Disproportionation-Recombination Rate Ratios

for Hydroaromatic Radicals," J. Phys. Chan., (in press).

122
Martinez, R. I., "The Photooxidation of Formaldehyde: A Different Perspective," Int. J. Chen. Kinetics, (submitted).

Meot-Ner (Mautner), M., and Deakyne, C., "Unconventional Ionic Hydrogen Bonds

CH <S+ ***X and NH+

«*«ir Interactions. Complexes of Quarternary Ions with

Solvents and of Onium Ions with n-Donors", J. Am. Chem. Soc. (submitted)

Meot-Ner (Mautner), M., Nelsen, S. F., Willi, M. F., Frigo, T. B., "Special

Effects of an Unusually Large Neutral to Radical Cation Geometry Change:

Adiabatic Ionization Energies and Proton Affinities of Alkylhydrazines ,

J. Am. Chem. Soc., (submitted).

Meot-Ner (Mautner), M. and Sieck, L. W., "The Ionic Hydrogen Bond and Ion

Solvation. 3- SH+ ***0 and NH+ ***S Bonds. Correlations with Proton

Affinity. Mutual Effects of Weak and Strong Ligands in Mixed Clusters,"

J. Phys. Chem. (submitted).

Morehouse, K. M. and Neta, P. , "Kinetics of Demetallation of Manganese (II)

Porphyrins in Aqueous Solutions," J. Phys. Chem., (in press).

Neta, P. and Siraic, M. G., "Free Radical Chemistry of Natural Products,"

Proceedings of Institut of Food Technologists Basic Symposium, (in

press )

Neta, P. and Harriman, A., "Zinc Porphyrin u-Radical Cations in Aqueous

Solution. Formation, Spectra and Decay Kinetics," J. Phys. Chem.,

(submitted)

Scheer, M. R. , Braun, W., and McNesby, J. R. , "The Application of a New High

Temperature Reactor to Unimolecular Decompositions," Chem. Phys. Lett.,

(submitted)

Scheer, M. D., "A Kinetic Isotope Effect in the Thermal Dehydration of

Cellobiose," Proceedings of the International Conference on Fundamentals

of Thermal 3iomass Conversion, (in press).

Schwarz, F. P. and Smith, D., "The Yields of Excited States of Aromatic

Derivatives in the Radiolysis of Hydrocarbon Solutions," Radiat. Phys.

and Chem., (in press).

Schwarz, F. P. , "Positive Charge Transfer in the UY Photolysis and g Radiolysis of trans-Decalin Solutions," J. Phys. Chem., (submitted).

Schwarz, F. P. , Okabe , H. , and Ausloos, P. , "Fluorescence Studies of the Puls

Radiolysis of the Halogenated Derivatives of Methane and Ethane," J.

Phys. Chem., (submitted).

Shaub, W. M. and Tsang, W., "Overview of Dioxin Formation In Gas and Sella

Phases Under Municipal Incinerator Conditions," ACS Symposium Series

Special Publication, (in press).

Shaub, W. M. and Tsang, W., "Hazardous Waste Aspects of Municipal Solid Waste

Incineration," Proceedings of Conferences on Hazaraou3 Waste Management,

Pergamon Press (in press).

123
Sieck, L. W. and Meot-Ner (Mautner), M., "Thermal Decomposition of Ions;

1. Pyrolysis of Protonated Ethers-Activation Energies and A Factors," J.

Phys. Chem., (in press).

Sieck, L. W. and Meot-Ner (Mautner), M., "Thermal Decomposition of Ions;

2. Pyrolysis of Protonated t-Butylmethyl ether at the Low Pressure Limit.

Activation Efficiencies for Various Bath Gases," J. Phys. Chem., (in

press)

.

Stein, S. E. and Brown, R. L. , "A Chemical Theory of Graphite," Carbon, (in

press)

.

Stein, S. E. , Brown, R. L. , and Greenberg, A., "Relative Thermochemical

Stabilities and Reactivities of Benzo[ A] pyrene and Selected Isomers," The

Chemistry of the Total Environment, (in press).

Stein, S. E. , "Free Radicals in Coal Conversion," Coal Conversion Chemistry , R. Schlosberg, Ed., Pergamon Press, 1984, (in press).

Tsang, W., Macek, A., Domalski , E. , Walker, J. A., Charagundl, S. R. , Colbert,

J. C., Kirklin, D. R., Ledford, A. E. , Deiker, P. H. and Ryan, R. V.,

"Combustion Characteristics of Refuse Derived Fuels," NBSIR, (in press).

Tsang, W. and Shaub , W. M., "Tracers for Quality Assurance in Municipal Solid

Waste Incineration," NBSIR, (submitted).

Tsang, W. and Hampson, R. F., "Chemical Kinetic Data Base for Methane Combustion," J. Phys. Chem. Ref. Data, (submitted).

Tsang, W., "Single Pulse Shock Tube Study on The Thermal Stability of

Ketones," Int. J. Chem. Kinet., (in press).

Tsang, W., "Hazardous Waste Aspects of MSW Combustion," Proceedings of Energy

from Municipal Waste Workshop, NBSIR, (in press).

Tsang, W. and Walker, J. A., "Chemical Processes During the Starved Air

Incineration of Municipal Solid Waste," NBSIR, (submitted).

T3ang, W., "Stability of Alkyl Radicals," J. Am. Chon. Soc . , (submitted).

Walker, J. A. and Tsang, W., "RDF Characteri zation Using Thermal Methods of

Analysis," NBSIR, (submitted).

Westley, F. , "Tables of Experimental Rate Constants for Chemical Reactions

Occurring in Combustion (1978-1982). Part I. Reactions of Inorganic

Compounds with Other Species. Part II. Reactions of Organic Compounds

with Other Species," (submitted).

124
CHEMICAL THERMODYNAMICS DIVISION

Eugene S. Domalski, Acting Chief

SUMMARY OF ACTIVITIES

Fiscal Year 1984

1 . INTRODUCTION

A. Objectives

The Center Thermodynamics Division is a discipline-oriented branch of

the Center for Chemical Physics and is concerned primarily with the

quantitative relationships that govern the transformation of energy from

one chemical system or form to another. The Division seeks to establish

itself as a leader in this field through the certification of calorimetric

standards, performance of precise thermochemical measurements, and

evaluation and compilation of thermodynamic data. Particular emphasis is

directed toward the implementation of a research program in biothermodynamics which offers fundamental data for industrial applications to the

production of chemicals, food, and fuels by means of biocatalytic processes .

Specific objectives of the Chemical Thermodynamics Division are:

• to advance the science of calorimetry through the design of

novel calorimeters, development of improved procedures, and

attainment of enhanced measurement precisions and accuracies,

• to develop and certify standard reference materials for

calorimetric measurements.

• to carry out calorimetric and related measurements from which

thermodynamic properties can be derived on key chemical

substances or processes.

• to provide compilations on the chemical thermodynamic properties

of inorganic and organic compounds as well as miscellaneous

materials from evaluation of the data reported in the literature.

to develop estimation methods for the prediction of chemical

thermodynamic properties of organic and inorganic compounds in

the gaseous, liquid, solid, and aqueous phases.

to respond to the needs of industry, academia, and government

for thermodynamic, and related information.

125
(b) To determine the equilibrium constants for the isomerization reactions mentioned in (a) as a function of temperature using HPLC methods.

(c) To determine the enthalpies of the enzymatic elimination of

ammonia by aspartic acid, phenylalanine, and tyrosine as a function of temperature using microcalorimetry.

(d) To determine the equilibrium constants for the enzymatic

elimination of ammonia using HPLC or GC methods.

Deliverables

(a) Thermodynamic properties (AH, AS, AG, K, percent conversion)

as a function of temperature.

(b) Generalizations regarding: common molecular substrate

sites vulnerable to enzymatic conversion, separation of

enthalpy and entropy effects.

2. Energetics of Nucleic Acid Bases and Nucleosides

Rationale

Thermodynamic properties must be available on key nucleic acid

moieties so that an understanding of the intra- and intermolecular bonding

energetics can be derived. The availability of a sufficient set of

thermodynamic properties will allow for the development of estimation

schemes so that the energetics of other related heterocyclic nitrogen

compounds can be predicted without the need for experimental measurements.

Experimental Approach

(a) To determine the enthalpies of combustion and solution of

crystalline adenine, purine, guanine, adenosine, and

guanosine using combustion bomb calorimetry and solution

calorimetry

.

(b) To develop an estimation procedure which accommodates the

available enthalpy of formation (At-H) data on nucleic acid

bases and nucleosides and can be used to predict the A^H's

for other related heterocyclic nitrogen compounds. The

development of estimation procedures is oriented toward

ultimately providing the biochemist/bioengineer with a predictive capability for thermodynamic properties in the

aqueous phase.

Deliverables

(a) Expansion of the predictive capability for the enthalpy of

formation to nucleic acid bases and nucleosides.

126
B. Organization

The Chemical Thermodynamics Division is organized into two major

activities: thermodynamic measurements and thermodynamic data compilation. These activities are under the supervision of two group leaders,

Eugene S. Domalski , Acting Division Chief, and David Garvin, Director of

the Chemical Thermodynamics Data Center, respectively. The Division has

15 full time staff, 2 guest workers, 3 part time employees, 4 contractors,

1 consultant, 2 rehired annuitants, 1 research associate, and 1 summer

student. Three persons have been reassigned to other Divisions within

MBS, which leaves the Division with an overall staff of 32 persons.

C. Biothermodynamics Research Program

A research program in biothermodynamics has been developed in FY 1984

which constitutes a major part of the overall MBS effort in biotechnology.

The biothermodynamics research program should provide the measurements and

data which will be useful to the chemical industry as it introduces

bio-processes into the production of chemicals, foods, and fuels. Certain

research areas were easily implementable in the program for the Division

while others required longer periods to initiate because of the need for

the purchase of an instrument, reassignment of manpower, or establishment

of a research agreement with a private vendor.

The biothermodynamics research program consists of the following

tasks

1 . Thermodynamics of Enzyme-Catalyzed Reactions

Rationale :

Studies carried out on enzyme-catalyzed reactions have shown

that enzymes vary in the degree of their substrate specificity. Some

enzymes are very specific while others can tolerate significant changes in

the substrate with little change in the attainment of chemical equilibrium

or in the general nature of the reaction product formed. The formation

and breaking of bonds, as a result of an enzyme interacting with a substrate, centers around specific sites in the substrate molecule.

As a result of examining a general reaction type (NHq elimination)

catalyzed by several related enzymes and several reactions (isomerization)

caused by the same enzyme, we hope to provide fundamental information

regarding the specific products, the energetics, and equilibria of these

reactions

Experimental Approach

(a) To determine the enthalpies of reaction for the isomerization of glucose to fructose, xylose to xylulose, and

ribose to ribulose as a function of temperature using

glucose isomer ase by microcalorimetry.

1 07
M. Measurements and Standards on Biochemical Systems using DSC

Rationale

For many years differential scanning calorimetry (DSC) has been

used to study conformational energies and phase changes in proteins,

lipids, and biopolymers. Such transitions provide useful characterizational information which often show relationships between thermal

phenomena and molecular structure. Thermal interactions between enzymes

and various substrates can be measured and should yield information

regarding the nature of the chemical binding in such systems. The need

for temperature and enthalpy of transition standards in research on biochemical substances in aqueous solution demands additional focus on DSC

measurements for selected systems.

Approach

(a) To determine whether interactions between glucose iscmerase

and various monosaccharide substrates can be shown to have

a stabilizing effect on the enzyme-substrate complex.

(b) To determine whether the DSC thermogram can serve as standard char act erizational tool which may identify enzyme

stability, enzyme deactivation (denaturation) , or specific

enzyme transitions.

(c) To assess whether phase transitions in selected lipids

(such as DPPC ) or proteins can serve as standards for

temperature of transition or enthalpy of transition for

biochemical systems in aqueous solution.

Deliverables

(a) Characterization of enzyme-substrate interactions as well

as a clearer understanding of enzyme stabilization.

(b) Development of temperature and enthalpy of transition

standards for biochemical systems in aqueous solution.

5. Compilation of Tables of Thermodynamic Properties of Important

Biochemical Substances

Rationale

Tables of critically evaluated thermodynamics properties of

biomolecules in aqueous solution are needed by biologists, biochemists,

and biochemical engineers involved in making thermodynamic assessments as calculating the energetics of chemical reactions or establishing the

equilibria for a collection of reacting species. A variety of compilations have been published (although none recently) but have been omnibus

in nature and not specifically oriented toward the biochemical or biotechnological community.

128
(b) Extension of thermodynamic data on nucleic acid bases and

nucleosides by combining available A^H data, AH solution,

solubilities, entropy, AH fusion, AH vaporization.

3. Weak Interactions in Model Enzyme-Substrate Systems

Rationale

Research is in progress in which the solubilities of nonelectrolytes are being measured in aqueous solution as a function of

concentration and temperature using the generator column technique. These

measurements will allow for the derivation of values indicative of the

weak interactions responsible for such phenomena as enzyme stabilization,

enzyme-substrate binding, enzyme immobilization, base stacking and

intercalation in polynucleotides. Systematic measurement of a key set of

values for weak inter-functional group interactions should provide the

foundation for a predictive capability useful in explaining some aspects

of the phenomena mentioned above in aqueous solution.

Experimental Approach

(a) To determine the solubilities of non-electrolytes in

aqueous solution as a function of concentration and

temperature using the automated coupled-column chromatographic apparatus.

(b) To focus on interactions between aromatic hydrocarbons and

organic compounds containing -OH, -CH^, and -CONH? functional groups.

Deliverables

(a) Development of an estimation methodology based on the

additivity principle will be derived from the set of values

for interactions obtained experimentally and will allow for

the prediction of weak interactions between small molecules

containing these groups.

(b) As data accumulates from the measurements and larger

molecules are examined, computer modeling with functionalgroup interaction potentials can be used. From the

estimation methodology based on larger molecules, prediction of model interactions of phenomena such as enzymesubstrate binding, enzyme stabilization, and protein

folding will be possible.

129
7. Gibbs Energy Maps for Biochemical and Aqueous Solutions

Rationale

Gibbs energy maps are of importance in determining product

yields for biochemical reactions, for the meaningful comparison of

thermodynamic measurements made on biochemical solutions under a wide

variety of experimental conditions, and for the calculation of the

equilibrium properties of solutions.

Approach

R. A. Alberty (J. Biol. Chem. 243 , 1337 (1968); ibid. 244 , 3290

(1969)) in a series of classic papers described the construction of Gibbs

energy maps and their application to biochemical systems. A shortcoming in

his treatment was that he did not include activity coefficients in his

model, i.e., he assumed ideal behavior of the solute species. Since

Alberty* s paper was published in 1969, there have been very significant

advances in computation and in numerical analysis software which makes the

inclusion of activity coefficients and other improvements possible.

In addition to obtaining amounts of species in solution, it is also

possible to calculate activity and osmotic coefficients and the thermodynamic excess properties of aqueous solutions. A paper describing this

type of generalized procedure is now in preparation and it has also been

applied to a publication on the treatment of the thermodynamics of the S02

+ H

2

0 system.

Deliverables

(a) R. N. Goldberg, "On the Use of an Equilibrium Model for the

Calculation of the Activity and Osmotic Coefficients in

Aqueous Solutions," J. Res. Nat. Bur. Stand., 89,251

(1984).

(b) R. N. Goldberg and V. B. Parker, "Thermodynamics in

Solution of S02 (g) in Water and Aqueous Sulfur Dioxide

Solutions," in review.

The fundamental idea which is used is that all of the equilibrium

properties of a solution can be calculated from a knowledge of the

equilibria in solution and assumption^ ) concerning the non-ideal behavior

of the solute species in the solution.

At present we are able to handle equilibrium calculations involving

up to about ten different species in solution (including trace amounts) as well as being able to calculate the excess thermodynamic properties.

These calculations can be performed on multi-component as well as binary

solutions. Generic types of interactions which are accounted for include

ion-pairing, complexation , hydrolysis, and hydration. The temperature

range over which calculations have been performed is 0 to 150°C.

130
Approach

(a) To prepare a table of critically evaluated thermodynamic

data (AH, AG, S, C

p

, K at 298 K) on amino acids, peptides,

and heterocyclic compounds.

Deliverables

Availability of critically-evaluated thermodynamic data on amino

acids, peptides, and heterocyclic compounds from a common up-to-date

reference source.

6. Estimation of Thermodynamic Properties of Important Biochemical

Substances

Rationale

The usefulness of compilations on critically evaluated thermodynamic data of organic and bio-organic compounds can be maximized when such

compilations become the foundations for predictive or correlational

schemes. The availability of evaluated data on A

fH, S, and C

p

for organic

and bio-organic compounds in the condensed and aqueous phases offers a unique opportunity to develop estimation and correlational schemes for

such compounds. As mentioned before, the ultimate goal is to provide the

biochemist/bioengineer with a predictive capability for thermodynamic

properties in the aqueous phase.

Approach

(a) To develop Benson- type group contribution values for the

estimation of the enthalpy of formation, entropy, and heat

capacity at 298 K in the condensed phase and in aqueous

solution.

(b) To use the Benson approach to develop an estimation method

for heterocyclic nitrogen compounds in the gaseous, liquid,

solid, and aqueous phases.

Deliverables

(a) Establishment of a predictive capability for organic and

bio-organic compounds at 298 K in the condensed and aqueous

phases

(b) Expansion of the CHETAH Program (developed by ASTM Subcommittee E-27.07 on Estimation Methods) for the prediction of

thermodynamic properties (A^H, S, C D

) of organic compounds

in the condensed and aqueous phases' with a focus on bio-organic compounds.

131
8. Revision of Thermodynamic Data for Waste Incineration

Rationale

A report (NBSIR 79—1 479 ) lists thermal properties (primarily

specific heat capacity and gross heat of combustion) of over 300 waste

components including a large number of biomass species. An up-to-date

edition of this report is underway as part of a SERI-NBS Research Agreement to expand the listing of waste materials to between 500-600.

Particular attention has been will be placed upon expansion of the

available data on biomass materials. New data on other waste components

will also be included.

Approach

(a) Past research and evaluation efforts on thermodynamic data

compilation will be used. About 100 to 150 materials have

been evaluated previously.

(b) The recent literature will be searched for available

thermodynamic data on waste components. Special emphasis

will be placed on the collection and evaluation of biomass

materials

.

Deliverables

(a) An up-dated compilation containing thermodynamic data on waste components and biomass materials, entitled: "Thermodynamic Data for Biomass and Waste Incineration."

9. Thermodynamics of the Krebs Cycle and Related Reactions

Rationale

The Krebs or citric acid cycle is the major metabolic pathway in

aerobic organisms for the oxidation of sugars, fatty acids, and some amino

acids. Many of its reactions and substrates are involved in biosynthetic

pathways for carbohydrates, lipids, and proteins. It is the central

mechanism for metabolic processes in almost all cells. A fundamental

understanding of the thermodynamic equilibria of these processes has

applicability toward the optimization of industrially-related bioconversion reactions for current and future needs.

Approach

(a) Thermodynamic data of the species involved in the Krebs

cycle reactions are being evaluated.

(b) A least sums - least squares solution to the network of

reactions which comprise the Krebs cycle processes are being studied. Emphasis is on the C-i'obs energies of

reactions in aqueous solution.

1
Deliverables

(a) Evaluated thermodynamic properties for the Krebs cycle

reactants and products in aqueous solution at 25°C (Af

H,

A

f G, S, AfS , K).

(b) A least sums - least squares solution to the network of

reactions which comprise the network of the Krebs cycle

reactions

D. Standards for Calorimetry

Proposals for research in the development of standards for calorimetry were submitted to the Office of Standard Reference Materials in May

1 984 and two projects were awarded funds for work in FY 1985. Both were

continuations of on-going projects in the following areas: (1) development of a standard reference material (SRM) for the combustion calorimetry

of organic phosphorus compounds and (2) continuous testing of SRM coal

samples for chemical stability and integrity on standing or in storage.

The latter is monitored through determinations for moisture, ash, and

calorific value of the SRM coals.

Our calorimetric standards activity on an SRM for organic phosphorus

compounds is complemented by the collaborative research program which has

begun with the U.S. Department of the Army, Aberdeen, MD to carry out

calorimetric mesurements and develop estimation methods for organic

phosphorus compounds.

133
E Other Agency Research

Other agency research accounts for about 25 percent of the Division's

resources and consists of both experimental and data programs. Some have

been renewals or add-on programs to those already in place while others

are new research efforts. The Division's other agency research programs

are summarized below:

Funding Agency NBS Program Manager/

Investigator

Program Title/

Description

Department of Defense

Air Force Office of

Scientific Research

( AFOSR)

S. Abramowitz

D. A. Ditmars

High temperature thermc

dynamic measurements on refractory substances,

pure elements and

compounds

Department of Energy

Pittsburgh Energy

Technology Center

(PETC)

D. Garvin

R. N. Goldberg

V. B. Parker

D. Smith-Magowan

Thermodynamic data for

flue gas clean-up in

fossil fuel technology

Department of Energy

Energy from Municipal

Waste (EMW)

E. S. Domalski

K. L. Churney

A. E. Ledford

S. S. Bruce

T. J. Buckley

Chlorine content of

municipal solid waste;

Threshold condition for

the formation/ destruction of chlorinated

organic species.

Design Institute for

Physical Property

Data (DIPPR)

D. Garvin

R. N. Goldberg

T. L. Jobe, Jr.

Bibliographies of

thermodynamic data on

sour water and various

salt systems.

U.S. Department of the

Army, Aberdeen Proving

E. S. Domalski

D. R. Xirklin

Combustion calorimetry

and estimation methods

Ground for organic phosphorus

compounds

.

Solar Energy Research

Institute (SERI)

E. S Domalski

T. L. Jobe, Jr.

Revision and up-date of

previous publication on thermodynamic data for

waste incineration.

124
2 . CHEMICAL THERMODYNAMIC MEASUREMENTS

A. Thermodynamics of Enzyme-Catalyzed Reactions

(E. Gajewski, R. N. Goldberg, D. Steckler, and Y. B. Tewari)

Accurate thermodynamic data on biochemical reactions are needed to

facilitate a better understanding of the reactions themselves and for the

optimization of product yields for reactions which have industrial

significance. The goal of this effort is to provide reliable thermodynamic data (equilibrium constants, enthalpies, and heat capacities) for

biochemically and industrially important reactions. The accomplishments

this past year are in several areas: (1) measurements on specific

enzyme-catalyzed reactions, (2) expansion of laboratory capability and

method development, and (3) computational procedures (see section on Modelling of Complex Aqueous Solutions).

Specific enzyme-catalyzed reactions which have been investigated are:

(1) The conversion of glucose to fructose, (2) xylose to xylulose, (3)

fumarate to malate, and (4) aspartate to fumarate and ammonia. A summary

of the results of these investigations follows.

Process (A): D-glucose (aq) * D-fructose(aq) , in the presence of

glucose isomerase.

Using a combination of high-pressure liquid chromatography (HPLC) and

microcalorimetry , the following values, which refer to a reference

temperature of 298.15 K, were determined for process (A): AG° = 345 ± 53 U

mol* 1 , AH 0 = 2730 ± 200 J mol" 1

, and AC° = 76 ± 30 J mol'V. The HPLC

measurements yielded values of X

A from 298 to 358 K. The values of KA

were found to be independent of the reaction being carried out either in

phosphate or TRI3/HC2, buffer and also independent of the use of either

solubilized or immobilized enzymes. Micro-calorimetric measurements

yielded values of AH£ and AC£,A determined from the temperature

derivatives of the equilibrium constants which were in good agreement with

the values of AH£ obtained from the microcalorimetri c investigation. The

results of this investigation are of particular interest since large

quantities ( = 2x1 0° kg/year) of fructose-rich corn syrup are being

produced each year using this reaction. Using the above thermodynamic

parameters, the maximum product yield for this process was calculated from

298 to 373 X.

Process(B): xylose(aq) xylulose (aq) , in the presence of glucose

isomerase

The above reaction is an intermediate step in the conversion of hemicelulose, a major constituent of biomass, to ethanol; this process may

have significant industrial applications. The approach adopted in this

investigation was the same as was used in the glucose to fructose study,
namely a combination of HPLC and microcalorimetry. To date, HPLC measurements have been performed from 298 to 343 K and microcalorimetric measurements from 313 to 332 K. Tentative values for process (B) at a reference

temperature of 298.15 K are AG° = 4,380 J mol -1

, AH 0 = 16,300 J mol -1

, and

ACg = 39 J mol -1 K

-1

.

Process(C): fumarate(aq) > malate(aq), in the presence of fumarase

enzyme.

This is a fundamental biochemical reaction which occurs in the Kreb's

cycle. In the food industry, L-malic acid is used as an acidulent in

fruit and vegetable juices, carbonated soft drinks, jams, and other food

products; in the pharmaceutical industry, L-malate salts of basic amino

acids are used in amino acid infusions. One of the aims of this investigation was to test a quantitative derivatization/gas chromatographic (GC)

procedure (see below) which was introduced in this laboratory for the

measurement of equilibrium constants for process (C) from 298 to 320 K.

Microcalorimetry was used to determine values of AHg from 298 to 320 K.

Tentative values for process (C) at 298.15 K are: AGg = -3,540 J mol -1

, AHg = -15,700 J mol

-1

, and AC° = 41 J raol

-1 K-1

. The value of AGg at

298.15 K corresponds to a value of K

c

= 4.17 and is in agreement with the

value of K r = 4.22 determined by Bock and Alberty (J. Am. Chem. Soc. 75 , 1921 ( 1 95 3 T ) using a spectrophotometric procedure. The effects of

variations of the ionic strength were examined in both the GC and in the

microcalorimetric measurements.

Process(D): aspartate(aq) fumarate(aq) + NHij +

, in the presence of

aspartase enzyme.

The literature contains several values for X

D which range from 0.008 to

0.04 mol Kg

-1

. The measurement of Xq and of AHg is complicated by the

fact that samples of aspartase are generally contaminated with fumarase

which catalyzes process (C). We are currently investigating several

techniques which may serve to eliminate or to significantly lessen this

problem. These techniques include: (1) carrying out the reaction at a temperature where fumarase is inactive, (2) introducing a chemical

inhibitor of fumarase into the reaction mixture, and (3) separating the

aspartase from the fumarase using electrophoresis and/or HPLC.

The microcalorimetry laboratory has been modernized this past year.

Features wnich have been introduced include the completion and installation of a third microcalorimeter, installation of skid-state temperatures

controllers which utilize proportional and reset modes of operation, and

the interfacing of all those microcalorimeters to a microcomputer. This

latter feature permits us to operate all three microcalorimeters simultaneously and to perform a reasonable number of measurements (9 to 15) in one day. All of the calculations are also performed on the microcomputer.

The measurements of the equilibrium constants of the conversion of

glucose to fructose and of xylose to xylulose have required the development of the proper separation of the reactants and products from each

136
other and from the buffer, enzyme, and cofactors. This time consuming and

difficult procedure should be significantly aided by the recent installation of a modern HPLC in this laboratory.

An alternative to HPLC and to spectrophotometry is lyophilization

This is a procedure in which a reaction mixture is rapidly frozen in

liquid nitrogen, lyophilized or freeze-dried, derivatized (e.g., with

N,0-bis(trimethylsilyl)trifluoroacetanide or BSTFA) and then analyzed in a capillary-column gas chromatograph. This type of methodology is of very

general application to many biochemical reactions and has been demonstrated to work satisfactorily for the measurement of the equilibrium

constant for the conversion of fumarate to malate. We plan to use it for

the determination of the equilibrium constant for the conversion of

fumarate to aspartate and other related enzyme-catalyzed systems.

Our immediate plans include the completion of the xylose/xylulose

equilibrium measurements. The development of finding a method to eliminate the fumarase catalyzed side-reaction from the aspartate/fumarate

equilibrium is in progress and will facilitate the measurement of the

equilibrium constants and enthalpy changes for process (D) above as a function of temperature. A study of the conversion of ribose to ribulose

is also planned as well as an investigation of the conversion of glucose

6-phosphate (G6P) to fructose 6-phosphate (F6P). This investigation would

serve to connect the following reaction equilibria in the thermodynamic

cycle: (H) = (F) + (G) - (E).

Glucose + ATP - G6P + ADP (E)

Fructose + ATP =* F6P + ADP (F)

Glucose = Fructose (G)

G6? = F6P (H)

A study of the conversion of glucose to glucose 6-phosphate will

follow. This study will complete the thermochemical cycle: ( 2 ) = (I) + (J).

Glucose + H3PO4 = G6P + H2 0 (I)

ATP + H

2

0 = ADP + H3PO4 (J)

We plan also to study transaminase catalyzed reactions of the type:

nh

3 + RC0C~ = RCH(NH2 )COO" (K)

This generic type of reaction is relevant to the synthesis of amino acids,

such as phenylalanine and tyrosine. In addition, we plan to use penicillin acylose to carrying out the reaction:
(or benzyl penicillin) Penicillin G -* 6-amino-penicillanic acid (L)

(6-APA)

+ phenyl acetic acid

The above reaction is a direct route to the synthesis of 6-APA which in

turn is used for the synthesis of many different kinds of antibiotics.

B. Weak Interactions in Model Enzyme-Substrate Systems

(S. P. Wasik, H. DeVoe, R. H. Wood, M. M. Miller, and J. Owens)

Intra- and intermoiecular interactions between functional groups and

other moieties of molecules dissolved in an aqueous medium play crucial

roles in a variety of properties and reactions. For example, the tertiary

structure of a globular protein determined by non-bonded intramolecular

interactions, as well as by hydrogen bonding. The binding of a substrate

molecule to an enzyme also depends on these interactions, the objective of

this research is to measure the free energy, enthalpy and entropy components of the interaction of several hydrophobic solutes with a variety of

molecules dissolved in water, and to analyze these quantities in terms of

either group-group interactions or solute dimerization. These data are needed for understanding the physical basis of hydrophobic effects, for

predicting various solubility and binding properties, and for refining

empirical potential functions for the pairwise interaction of molecular

groups immersed in water. The information is of great interest to

technologies which deal with stability and binding effects involving

macromolecules, such as enzyme-substrate specificity, to researchers in

the area of molecular recognition and host/guest chemistry when hydrophobic groups are involved, and to those who calculate aqueous properties by

Monte Carlo and molecular dynamics computations.

The proposed research, already begun, is based upon measurements of

the solubility of a solid hydrophobic compound in aqueous solutions

containing a second solute (the additive). At a given temperature and

pressure, the chemical potential of the dissolved hydrophobic solute in

the saturated solution is constant regardless of the concentration of the

additive. Therefore, the effect on the solubility of changing the

additive concentration can be interpreted as an activity coefficient

effect; or, alternatively, as a free energy of interaction between the

hydrophobic molecule and the additive species. The data (solubility versus molality of additive) from measurements with additives containing

different proportions of the same functional groups is treated by the

method of R. H. Wood to provide the free energies of group-group interactions. The temperature dependence of these free energy parameters yields

the enthalpy and entropy components of the interactions.

The aqueous solubilities are measured using a coupled-column

liquid chromatography (CCLC) method developed at NBS. A generator column

is packed with crystals of the solute, or with a chromatographic solid

support coated with the solute. Pure water or an aqueous solution of the

additive is pumped through the column to saturate the solution with the

solute. The total solute concentration in the aqueous phase is found by

138
extracting the solute from a known volume onto an extractor column, and

eluting this solute onto a reverse phase analytical column for determination by HPLC.

An automated version of the CCLC apparatus was used to measure the

aqueous solubilities of five n-alkylbenzenes (ethylbenzene through

n-hexylbenzene) over the temperature range 7 - 45°C. The automated system

includes control by a microcomputer of temperature, valves, the HPLC pump,

and the integrator. From the data, values of AG°, AH 0

, and AS 0 at 25°C

for the process solute(liquid) = sol ute( aqueous ) were calculated using the

statistical model of Clarke and Glew [Trans. Faraday Soc . 62_, 539 (1966)].

The measured values were compared with literature values, where available,

and agreement was excellent. This work is being prepared for publication.

The automated CCLC apparatus has been modified to allow it to change

the aqueous solutions pumped through the generator column for studies of

solute-additive interactions. The solubility of naphthalene, pyrene, and

other hydrophobic solutes will be determined in aqueous solutions of a number of alcohols, amines, and heterocyclic species buffered when

necessary to prevent ionization. The studies will be made over a range of

additive concentration and at three or more temperatures. The results

will be analyzed in terms of free energy, enthalpy, and entropy contributions to group-group interactions.

Hydrophobicity scales for the side chains of the naturally occurring

amino acids will be constructed from these contributions according to the

Savage-Wood additivity principle [J. Solution Chem. 5, 733 (1976)]. This

principle assumes that the total interaction between two solute molecules

is the sum of all possible group-group interactions. One such scale, for

instance, would be for pyrene and would characterize the interaction

(hydrophobic bonding) of pyrene with different amino acids in the aqueous

environment. This scale, because it is based on actual hydrophobic

interactions, would be an improvement over existing hydrophobicity scales

which were calculated from data for the transfer of amino acid 3ide chains

from water to the vapor phase or a nonaqueous solvent.

A second study will be undertaken to search for measurable interactions of benzene with polycyclic aromatic hydrocarbons in water. Tucker

and Christian [J. Phys. Chem. 83 , 426 (1979)] have observed activity coefficient effects in aqueous solutions of benzene which they interpret as benzene dimerization. It would be of great interest for the understanding

of hydrophobic effects to determine the thermodynamics of the formation cf

heterodimers of benzene + pyrene, for instance.

During FY 1984, Michele M. Miller transfered from the Chemical

Thermodynamics Division to the Organic Analytical Research Division.

139
C. Bomb Calorimetry of Key Biochemical Compounds

(D. R. Kirklin and J. C. Colbert)

Precision bomb calorimetry has been used to measure the enthalpies of

combustion of thermodynamically important molecules to derive precise

values for their enthalpies of formation. There are many biochemical

molecules which play an active role in biologically important processes

for which the enthalpy of formation has not been determined. It is

necessary to have precise values for enthalpies of formation to understand

the stability or reactivity of such molecules in important bio-processes.

1 . Enthalpy of Combustion of Anhydrous Guanosine

The enthalpy of combustion of an anhydrous sample of guanosine

was measured in the NBS aneroid adiabatic bomb calorimeter. Guanosine is a nucleoside of one of the two purine bases found in DNA and RNA . Purine,

adenine, adenosine, guanine and the dihydrate of guanosine were studied in

previous years in this bomb calorimeter. The calorimetric data of

anhydrous guanosine was correlated with that of guanine and also that of

the adenine/adenosine base/nucleoside pair.

Considerable sample preparation was necessary to obtain meaningful

calorimetric data on anhydrous guanosine. Anhydrous guanosine readily

absorbs moisture from the atmosphere to form the dihydrate. The internal

combustion bomb atmosphere must be saturated with water vapor during a combustion experiment. Therefore, the commercial dihydrate of guanosine

was dehydrated in a vacuum oven and encapsulated in paraffin wax before

each calorimetric experiment. The "waxed" anhydrous guanosine samples

were exposed to a water saturated bomb atmosphere of 3 MPa for 24 hours

to determine whether or not the anhydrous samples would remain anhydrous

during the equilibration period of the calorimetric experiment. The

results proved that the anhydrous samples can be successfully encapsulated

in paraffin wax.

The energy of combustion of the paraffin wax utilized was measured to

determine the energy correction for its combustion in the calorimetric

experiments of "waxed" anhydrous guanosine. The measured energy of

combustion for paraffin wax was -46 689.21 ± 5.84 J*g

_1

.

The enthalpy of formation of anhydrous guanosine was calculated from

the measured energy of combustion and then correlated with that of our previous measurements on purine bases and their corresponding nucleosides.

The enthalpies of formation of guanosine and guanine (-950.10 kj^mol -1 and

-183.89 kJ*mol

-1

) were correlated with that of adenosine and adenine

(-651.58 kJ*mol

- and +96.92 kJ*mol _1

). The mean difference between a nucleoside and its corresponding purine base was calculated to be -757.36

kJ*mol

-1

. This mean difference allows the calculation of the enthalpies

of formation for any purine nucleoside from a measured or literature value

of that of the purine base with an accuracy of the order of 1 percent.
2 . Characterization of D-Ribose

Combustion bomb calorimetric measurements were completed on a sample

of D-ribose in FY 1983. The molar enthalpy of combustion at 303*15 K for seven measurements is A

C H = -2349.26 ± 0.68 kJ*mol _1 (2sdm). Studies were

carried out in FY 1984 to characterize the D-ribose sample using DSC and

NMR methods. DSC measurements did not show sharp melting for the sample

and estimates of purity from such tests were calculated to be - 97

percent. NMR studies suggest the presence of four anomeric forms in the

solid phase, two being major and two extremely small. From an examination

of the literature, we suspect that the most predominant form which

comprises - 65 percent of the solid phase is the 0-pyranose form and that

the next most prevalent for which amounts to - 35 percent is the

a-pyranose anomer . The a- and 0- furanose anomers probably constitute the

remainder, however, additional NMR studies are planned for FY 1985.

D. DSC Measurements and Standards

Efforts in fiscal year 1984 were directed toward expanding our instrumental capabilities in differential scanning calorimetry (DSC). The

Division has had a Perkin-Elmer DSC-II system which is useful for studying

the heat capacities, melting points, and phase transitions of pure solid

substances. The need to study biochemical systems in aqueous solution

required procurement of a DSC instrument which has (1) a larger cell

volume (1 to 2 ml) than the Perkin-Elmer DSC-II (0.06 to 0.10 ml),

(2) capability for scanning in both a heating and cooling mode,

(3) versatility with sample cell removal and cleaning, (4) availability

with a suitable data acquisition system and software package,

(5) temperature scan rates as low as 1°C/hour, and (6) scan rate

reproducibility of -0.01 percent. After examining the commercially

available instruments, the Hart Scientific Model 7707 DSC was selected.

Delivery is expected in September 1984.

Plans for research in FY 1985 will follow three areas of interest:

(1) determination of the suitability of lipids as standard materials for

identifying temperature and enthalpies of transition for biochemical

systems in aqueous solution, (2) establishing the thermal character

of enzymes, (i.e., denaturation , complexation, reversible transitions),

and (3) determination of the thermal character of glucose isomerase

through interactions with monosaccharide substrates and co-factors in

aqueous solution.

Personnel assignments to the DSC studies will include two full-time

scientists and a guest worker on a part-time basis.

E. Calorimetric Standards/OSRM

(D, R. Kirklin and J. C. Colbert)

The NBS effort in precision bomb calorimetry permits the certification of primary and secondary calorimetric standards that are used

worldwide to calibrate and check calorimetric procedures. The primary

calorimetric standard is benzoic acid which has been available from NBS
since the 1920's. Secondary calorimetric standards are needed for

compounds which contain elements other than carbon, hydrogen and oxygen

and therefore require slightly modified calorimetric procedures to obtain

precise and accurate data.

Three projects were performed to determine secondary calorimetric

standards. Sulfur is an important element in many organic compounds, and

therefore, a secondary combustion standard is necessary for sulfur-containing compounds. In consumer oriented areas, fuel laboratories need

secondary calorimetric standards for the types of fuels which are being

utilized. The QSRM distributes SRM coals which are certified for trace

elements and sulfur content. Their certification has been extended to

include a certified calorific value. Since the long term (> 1 year)

stability of coal samples is not known, these SRM coal samples are recertified annually. In addition, the development of a refuse-derived

fuel (RDF) as an article of commerce requires a calorimetric standard

which is free of variability due to geographical location and ambient

conditions. Ref use- derived fuels are primarily cellulose with varying

amounts of non-combustible materials and water. Therefore, a synthetic

RDF was developed with fixed cellulose and non-combustible components for

certification as a standard reference material (SRM).

1 . Certification of the Enthalpy of Combustion of Thianthrene

Primary and secondary standards are necessary to calibrate

calorimeters and to check calorimetric procedures. The primary standard,

benzoic acid, is useful for the calibration of oxygen bomb calorimeters

but cannot be used to check the procedures necessary for organic sulfur

compounds. A secondary standard containing sulfur is needed. W. H.

Johnson (a former NBS employee) examined thianthrene (diphenyl disulfide)

and found it suitable for a secondary sulfur standard. The Office of

Standard Reference Materials (OSRM) contracted with the Aldrich Chemical

Company, Inc. of Milwaukee, WI to prepare a thianthrene sample of suitable

purity to be used as a SRM. Calorimetric measurements were performed on

the Aldrich thianthrene sample for certification as a secondary combustion

calorimetry standard.

The NBS aneroid adiabatic calorimeter was used to make measurements

of the enthalpy of combustion because a combustion product of organic

sulfur compounds is sulfur dioxide. Inside the sealed combustion bomb the

sulfur dioxide is oxidized to sulfur trioxide which reacts with the water

in the bomb to form aqueous sulfuric acid. To adequately account for the

energy produced from the formation of sulfuric acid instead of sulfur

dioxide, the sulfuric acid must be made homogeneous by rotation of the

bomb after the ccmbustion is complete before the equilibration afterperiod. Hence, the unique NBS rotating adiabatic bomb calorimeter was necessary for this project. Three sets of measurements were made using

different amounts of water in the bomb for the formation of sulfuric acid.

It was found that the minimum amount of water required is three times the

volume of the bomb in cubic decimeters.
The results of the bomb calorimetric measurements yielded a certified

value of 33 511.56 J*g _1 with a standard deviation of the mean of 2.39

J*g _1 when the sample is burned under certificate conditions using a quantity of water in the bomb equal to or greater than three times the

volume of the bomb in dm3.

2. Certification of a Synthetic Refuse-Derived Fuel

A synthetic ref use-derived fuel (RDF) was prepared from 80

percent microcrystalline cellulose, 15 percent silica, and 5 percent

alumina. The mixture was blended and 3tored in 13 SRM gross containers.

During the container filling process, samples were taken from the bottom

and top of each container to determine the homogeneity of the blended

material. The material was tested for ash content and residual moisture.

The samples were randomly placed in seven sets of six analysis

samples. Six analysis samples per set were chosen because of the geometry

of our moisture oven. This allowed duplicate measurements on all but six

samples. Robert Paule, a NML statistician, analyzed the results to

determine the sample variability. The sample variability was found to be

less than 1.3 percent for moisture (which is affected by the ambient

relative humidity) and less than 0.35 percent for dry ash. The sample was judged to be suitable for a standard reference material (SRM) at these

levels of variability.

The gross samples were bottled with a sample size suitable for

issuance as MBS SRM 1657. A similar protocol has been devised to determine the certified calorific value. Moisture and ash will be redetermined

to verify that no sample segregation has occurred during the bottling

process. These measurements are currently underway.

3. Coal Combustion Calorimetry Standards

The heats of combustion of SRM 1632a (a bituminous coal) and SRM

1635 (a sub-bituminous coal) are being measured annually 30 that tney can

be used as secondary combustion calorimetry standards. Secondary combustion calorimetry standards are necessary to check calorimetric and

analytical procedures on materials that have combustion characteristics

similar to those materials encountered in laboratory analyses. Since

fossil fuels are routinely encountered in commercial laboratories there

was a great need for certified heats of combustion of coal samples.

However, since coal samples are oxidized slightly upon storage under

laboratory conditions, the certified heats of combustion are remeasured.

SRM coal specimens are taken from a sample bottle that is being stored

under laboratory conditions and calorimetric measurements are performed

annually to check the certified heats of combustion.

SRM coals 2682, 2683 , 2684, and 2685 which were certified for sulfur

content, calorific value and ash content during the previous fiscal year

have been included in this variability study. The SRM sample bottles that

were used in the original certification are also being stored under
laboratory conditions. Specimens are taken from the sample bottles and

determinations are made annually for moisture, ash content, and calorific

value

.

F. Large Sample Combustion Flow Calorimetry

(E.S. Domalski, D. R. Kirklin, J. C. Colbert, K. L. Churney, M. L.

Reilly, A. E. Ledford, Jr., S. S. Bruce, and T. J. Buckley.)

Financial support for this activity has come from the U.S. Department

of Energy, Energy from Municipal Waste Division, Office of Renewable

Technology.

1 . The Calorific Value Problem

Incinerator-boiler systems which are used to dispose of municipal solid waste (MSW) are bought and sold on the basis of thermal specifications. To determine whether a system meets its thermal specifications,

the calorific value of the input waste stream must be known. At present,

the calorific value is determined at commercial test laboratories by a bomb calorimetric procedure. This procedure involves using gram-size

samples derived from MSW that has been processed down to a particle size

of 2 mm or less. Critics of the procedure argue that gram-size samples

are too small to be representative of such a large quantity of so heterogeneous a material, and that processing MSW may also alter its composition. The latter might occur because of excess local heating in some of

the milling operations or because of sampling and handling of a material

which is inhomogeneous and segregates easily.

The general question of whether or not the calorific value of a multi -ton quantity of MSW can be obtained by sampling to one or two grams

can be broken into two parts. The first part is: how should one sample a multi-ton pile or stream of raw MSW to obtain increments of sufficient

number and size to characterize the calorific value with a given precision? The second part is: how can one obtain representati ve gram-size

samples that characterize these measurements? The dispute about the

validity for determining the calorific value of MSW is primarily concerned

with the second of these questions.

To test the bomb calorimetric procedure, a combustion flow calorimeter that can accommodate a 2.5 kg sample of MSW was designed and con- structed during 1981 through early 1983- This size of sample was selected as a convenient function of a sampling increment for the waste

stream on the basis of extensive conversations with combustion engineers

and statisticians.

During FY 1983, experimental calorimetric work was initiated to

address two problems. The first was the sampling problem: how does one obtain a representative gram-size sample from a kilogram-size quantity of

highly processed MSW (i.e., < 2 mm particle size)? The second was the

processing problem: Does processing MSW down to a 2 mm particle size

alter its calorific value significantly (i.e., one percent or more)?

Ihh
During the course of this study 22 flow calorimetric measurements

using 2.5 kilogram-size samples and 16 bomb calorimetric measurements

using 2.5 gram-size samples were carried out. In FY 1984, results were

calculated, analyzed, and presented in four talks and two publications.

The results in brief are as follows:

The sampling problem : Calorimetric data on processed MSW were

obtained using both the kilogram-size flow and a gram-size bomb calorimeter . The sampling division (i.e., 2.5 kg to 2 g) was done with a Brinkmann riffler because the method is insensitive to the effects of

segregation. The calorific values (on a dry basis) of the gram-size test

samples and other kilogram-size parents agree within the uncertainty of our experiments. The average difference of the parent minus gram-size

test samples was -0.1 percent with an imprecision (95 percent confidence

level) of 1.1 percent of the mean calorific values.

The processing problem : The effects of processing on sample composition were determined by intercomparison of processed and minimally

processed MSW (150 mm or less particle size) that were nominally identical. Reduction of the particle size of the sample was carried out in a Wiley Mill. Powdered dry ice was added to the sample to prevent excessive

local heating. The difference of the unprocessed calorific values on a dry basis was -0.5 percent with an imprecision of 2.9 percent of the mean

calorific value. Most of the imprecision is due to the sample preparation

procedure; it did not yield identical processed and unprocessed samples.

These results show that representative gram-size test samples can be

obtained from 2.5 kg increments provided excessive local heating is

avoided in reducing the sample particle size and the sample division

technique is insensitive to segregation. Our overall results are that the

difference in the calorific value (on a dry basis) of the unprocessed

parent 2.5 kg sample minus the gram size processed test sample is -0.6

percent with an imprecision of 3.1 percent of the mean calorific value.

During FY 1985, we plan to perform combustion calorimetric studies on refuse samples under exces3-air, stoichiometric-air, and starved-air

conditions. No data are available for validating methods for measurement

of the degree of incomplete combustion and the calculation of corrections

to calorific values obtained with test samples in 100 percent oxygen as compared to air-combustion conditions in an incinerator. Thi3 work is

needed to reduce the gap between data from laboratory tests and operation

of real-world combustion systems.

2. Organo-chicrine Compounds from MSW Combustion

Widespread reports of the presence of dioxins and related

compounds in emissions from municipal waste combustors have proven to be a major impediment in the use of MSW as a fuel in energy recovery units. The

reported dioxin levels vary greatly between combustion facilities. Sinct

there seem to be no intrinsic thermodynamic or kinetic (e.g., at least in

the gas phase) barriers to dioxin destruction, modifications in operation

or design of a facility should lead to minimization of emissions of
dioxins. The empirical data-base needed for developing a strategy for

this minimization (e.g., studies related dioxin levels to incinerator

operating variables) is just beginning to appear. Unfortunately, the

growth of this data-base is hindered by high costs and long time scales

for the necessary research to be conducted.

Two problems are being studied at NBS. The first problem is to

determine the quantity and distribution of chlorine in municipal solid

waste. Very little data exists (one study of the chlorine content on five

days of sampling MSW in Wayne County, Michigan, in 1979). The second

problem is to determine the threshold conditions for the formation and/or

destruction of organo-chlorine compounds in the combustion of MSW. This

problem is being studied in conjunction with the threshold conditions for

the formation of other products of incomplete combustion (e.g., CO,

300t-carbon, various hydrocarbons , sulfur containing molecules) to see if

these last mentioned species, which are easier and cheaper to monitor, can serve as indicators of dioxin formation and/or destruction. The threshold

problem is closely related to work on the calorific value determinations

to be performed in 0

2 /N 2 mixtures in FY 1985.

Sampling of MSW was carried out at two sites, each for periods of

five days. Five days samples were obtained from Baltimore County,

Maryland, and five days samples from Brooklyn, New York to provide sane

idea of differences between MSW from different areas. About 1 3~20 kg of

refuse was sampled daily from the output of the primary shredder of the

Baltimore County Resource Recovery Facility in Cockeysville , Maryland.

The New York samples were obtained by removing five to seven increments of

MSW on each of five days from the receiving pit of the Green Point

Incinerator in Brooklyn, New York. All samples were dried and hand sorted

into combustible components (paper, hard and soft plastics, textiles,

wood/ vegetable matter, and fines/sweepings) and non-combustible components

(glass, metals). Each of the combustible components has been processed

to < 2 mm particle size, blended (i.e., homogenized), and sampled for

analysis to determine water-soluble chlorine and total chlorine content.

A reconstituted sample of each of the ten days waste was prepared to

determine the chlorine and the total sulfur content of the original (i.e.,

unsorted) MSW. Analytical determinations are now complete and being

evaluated

.

3. Combustion Studies to Establish Threshold Conditions

The 2.5 kilogram capacity flow calorimeter will be used to

pursue combustion measurements to establish the threshold conditions for

the formation/destruction of chlorinated species. Although the combustion

studies will be carried out in the calorimeter, no heat measurements will

be made to calculate a calorific value since this datum is not applicable

to the proposed study. Threshold conditions will be established from the

measurements of the following parameters:

• Variation in the MSW composition, including synthetic and

real-world RDF samples doped with chlorine compounds.
• Variation of the input flow rate (O2 concentration) under

excess-air, stoichiometric-air, and starved-air conditions.

• Temperature profile of the reaction zone and post-reaction zone

of the combustor.

• Determination of the chlorine content of the MSW stream,

particulates (fly ash when formed), bottom ash, and gaseous

effluent

The detection of chlorine-containing species in the gaseous effluent

will be carried out using IR and GC-MS methods. Information regarding

the limits of on-line detection of gaseous chlorine species will be

derived from these studies. The results of the combustion studies will

link the threshold conditions for the formation/ destruction of chlorinecontaining species to the degree of decomposition of the MSW sample. The

influence of cool-zones in the combustor will be investigated to find out

what effect their presence has on the kind and quantity of chlorinated

species which can be found in the gaseous effluent and ash residue.

Trial combustions in air have begun. Analytical characterization of

organo-chlorine compounds is being carried out by the Organic Analytical

Research Division of MBS.

G. Drop Calorimetry of Non-Reacting Systems Below 2800 K

(D. A. Ditmars)

Financial support for this work has come from the U.S. Department of

Defense, Air Force Office of Scientific Research.

This activity represents a continuation of a calorimetric research

program whose goals include:

1. to obtain accurate thermodynamic data on non-reacting refractory

materials in either the solid or liquid state up to 2800 K.

2. to measure specific heat at temperatures above 273 K of wellcharacterized materials of the highest attainable purity for which

existing C

p

data are insufficient or in conflict and which are:

1) critical to energy- and space-related technology, or 2) especially

suited as models on which to test refined lattice-dynamic calculation of

thermodynamic properties.

3. to produce and support high- temperature calorimetric standard

reference materials (SRM's) for heat capacity and for differential thermal

analysis (DTA) calorimetric measurements

The number of facilities, nationally, which can sustain programs of

high- temperature calorimetric research is seriously diminishing at a time

of increasing needs for just the data produced through this type of

investigation. There is a constant demand for thermodynamic data on new materials capable of functioning acceptably in the harsh environments
associated with space propulsion systems or upon re-entry, new energy

technologies and defense-generated needs for materials to enable space

systems to withstand overt aggression. Also, the existing thermodynamic

data on some elements, for which it is now possible due to theoretical

advances to calculate directly some thermodynamic functions, is often not

accurate enough to allow a meaningful test of theory.

The Chemical Thermodynamics Division has available two operative

precision drop calorimeters for the ranges 273-1200 K (isothermal,

phase-change calorimeter) and 1200-2800 K (adiabatic receiving calorimeter). These are being used in research intended to provide calorimetric

measurements at the highest accuracy on materials related to the hightemperature applications mentioned above. Every effort is made to

coordinate this research with that of another NBS facility (very-hightemperature pulse calorimeter) in order to provide the capacity for

continuous coverage of high- temperature thermodynamic studies up to 5000 K

or above on especially important systems. We have in addition an operative high- temperature discrete-heating adiabatic calorimeter (300-800 K)

and we are installing a 1 ow- tamperature (4-300 K) adiabatic calorimeter

for future research needs.

In order that NBS thermal data and that of other high- tamper at ure investigations can be fairly intercompared and assessed for accuracy, we maintain a continuing effort to update or extend the range of coverage of

high- temperat ure standard reference materials. In recognition of the

growing need for accurate DTA temperature and enthalpy standards we attempt to identify systems whose thermal properties can be measured by

both conventional calorimetric and DTA techniques.

Fiscal year 1984 was devoted to completion of heat-capacity measurements on a-and 0 -SioN4 in the range 273-1200 K and on hexagonal

(" graphitic") BN ana cubic ("diamond-like") BN in the same temperature

range. Both sets of data for the two BN polymorphs are consistent with

available low- temperat ure data and conform more reasonably to anticipated

behavior of these substances at high temperatures than the literature data

would indicate.

These materials (SioN4 and BN) are widely used in the fabrication of

components for service at high temperatures in chemically aggressive

environments and especially where high resistance to mechanical and

thermal shock are important. Accurate high- temperature thermodynamic

functions for Si^i^ and BN (each with two polymorphs) should provide

useful engineering data and assist in theoretical interpretations of the

Si-N and B-N systems.

In addition in FY 1984, we measured the heat capacity of fee (face

centered cubic) calcium of high chemical purity in the range 273~71 6 K.

Calcium is a CODATA "key substance”. Accurate knowledge of the thermodynamic properties of this element is essential to a correct thermochemical

description of a large number of technologically important calcium

compounds. The thermodynamic data on calcium up to 200 K rests on a tenuous base. Above 200 K, to the fcc-bcc transition temperature (716 K),
reliable data was entirely lacking. The heat capacity from 200 to 716 K was formerly calculated by judicious extrapolation and constraint of the

extrapolation to behave in "an anticipated fashion" relative to similar Mg

and Sr data. Our measurements in the range 273~7l6 K have yielded

improved thermodynamic functions for calcium (with a caveat related to the

Cp data below 200 K) and will place future CODATA evaluation of Ca

compounds on a more secure footing.

In fiscal year 1985, we will extend the heat capacity measurements on

one or more of the nitride polymorphs described above to temperatures

above 1200 K. Highest service temperatures are not yet well-defined for

these materials. We plan as well to investigate the fcc-bcc transition of

calcium at 716 K and perhaps begin measurements on the bcc (body centered

cubic) phase. If guest-worker participation can be realized, we will

initiate heat capacity studies on very pure copper with the view in mind

of establishing copper as a substitute heat-capacity standard and of

providing the most accurate heat-capacity data available for comparing to

theoretically calculated values for Cp.

H. Raman Characterization of the Main Phase Transition in Lipid Bilayers

(W. H. Kirchhoff, Research performed at the National Institutes of

Health)

Within the past ten years, Raman spectroscopy has been used with

increasing popularity to characterize the main phase transition (gel to

liquid crystal) in lipid bilayers. In spite of the growing use of this

technique, no attempt has yet been made to investigate quantitatively the

shapes of intensity-ratio versus temperature profiles and to interpret

those shapes in terms of the thermodynamic properties of the phase

transition.

A simple, two-state model in which the distribution of domain sizes

does not vary as a function of temperature has been developed for expressing the intensity ratio of Raman spectral features as a function of

temperature. The model provides the following analytical expression for

the ratio of intensities at two scattering frequencies:

R

1 2 = I ( v-j ) /I ( V2

) A*a(T-T 0

) B+3(T-T0)

, T e tn

+

1 +e~

tn

where T

Q is the midpoint of the phase transition and i3 defined as the

temperature at which the chemical potentials of the two states are equally

shifted by a small but determinable amount due to the mathematics of

monitoring intensity ratios rather than the intensities themselves; t is a reduced temperature = (T-T Q )/D and where D i,s related to the enthalpy

difference between states A and 3 by D = RT /AH ; A and 3 are functions

of the intensities of the states A and 3 atthe^two different scattering

frequencies v-j and v'2 and a and 3 are empirical slopes in the asymptotes

of the sigmoidal profile introduced to account for domains of small size;

and n is an effective number of molecules per domain. Depending on the
density of data points in the phase transition interval, n, as determined

by a least squares fit of the analytical expression to the Raman intensity

ratios, varies between the root mean square of the domain size to

<v2 >/<n>. For broad domain size distributions, <n2 >/<n> can be an order

of magnitude greater than <n>.

From the least squares fit of the analytical expression to the

spectroscopic data, the simple model reproduces the temperature profiles

to within experimental error. Improvements in the accuracy of intensity

measurements, in the precision of temperature control, and in the accuracy

of temperature measurement, all of which are achievable with existing

instrumentation, should allow for examination of somewhat more complex

models

.

At present, the computer program which has been written allows for a systematic analysis of Raman spectroscopic temperature profiles and

provides an independent estimate of van't Hoff enthalpies of phase

transitions. This work has been performed in cooperation with scientists

at the National Institutes of Health and the computer program which has

been developed is now used for the routine analysis of Raman spectroscopic

temperature profiles.

I. Future Plans

In biothermodynamics, on-going programs will continue beyond FY 1986

with the likelihood of expansion such as: biomass conversion to

saccharides, measurement of activities and activity coefficients of dilute

and concentrated sugar solutions, and measurements of redox potentials,

biochemical couples, and low temperature heat capacity measurement on key

compounds of interest in industrial biochemistry. Research in combustion

bomb calorimetry will pursue measurements on oligopeptides in order to

test the constancy of the energy of the peptide bond. Similar measurements are applicable to oligosaccharides in which the constancy of the

energy of the oxygen or ether linkage between repeating saccharide units

can be examined.

The rationale for pursuing seme of the new research tasks in

biotherraodynamics in FY 1985 and beyond is given below:

1. Heat Capacities and Absolute Entropies of Biomolecules and

Biopolyraers

An understanding of the conformational changes which occur in

biomolecules and biopolymers as a function of temperature provides

important thermal data needed to explain the behavior of proteins, lipids,

and carbohydrates in industrial biochemical prcceses. Absolute entropies

are needed along wiht enthalpy data to calculate equilibrium properties

over temperature intervals of interest. Heat capacities provide

information regarding the nature of intramolecular transitions (e.g., 1st

order vs. 2nd order).

150
Entropy data are derived from accurate measurement of the heat

capacity over a temperature range extending from the liquid helium range

eup to room temperature. The specific technique applicable to these

measurements, low temperature adiabatic calorimetry, has been demonstrated

in the literature to be also a useful technique for identifying

conformational changes of biomolecules and for probing the interaction of

biopolymers with are molecules and ions of dissolved salts.

2. 3iomass Conversion to Saccharides

Conversion of biomass (cellulosic materials) to saccharides, such as glucose, can be accomplished by either acid hydrolysis or enzymatic

hydrolysis. Several advantages are possible with the enzymatic approach

because it allows for lower operating costs and does not require

corrosion-resistant equipment. High temperatures are required in acid

hydrolysis of cellulosic materials because of the need to break down

crystalline structure. However, high temperatures cause decomposition of

the resulting surgars. Enzymes can convert cellulosic materials to

various oligosaccharides at moderate temperatures, have high specificity,

and do not react with other non-cellulosic materials or impurities.

We plan to study the enzymatic conversion of cellulose to glucose and

hemicellulose to xylose as initial systems. Pretreatment methodologies

will be examined after simple cellulosic materials have been measurd for

their ability to undergo saccharification. Later, attention would be

given to the bioconversion of lignocellulosic materials.

3. Measurements of Activities and Activity Coefficients

The activity of biomolecules in solution and an understanding of

their departure from ideal solution behavior are of importance from both a theoretical and practical standpoint. Occassionally , equilibrium

constants disguise or cancel the impact of non-ideality in solution

processes. Interpretation of equilibrium data in which several reaction

products are formed requires a knowledge of activities and activity

coefficients to understand the observed yields.

4. Redox Potentials and Biochemical Couples

There are several important biochemical processes which "couple" with

other biochemical processes to bring about chemical reactions which

otherwise would not occur. For example, the phosphorylation of gluocose

to glucose-6 -phosphate is not thermodyanmically favorable:

Glucose + H^POjj = C-lucose-6-phosphate

However, when adenosine 5’ -triphosphate (ATP) is present, the following

reaction is favorable:

Glucose + AT? = Glucose-6-phosphate + ADP
The difference between the two processes above is the hydrolysis of ATP to

adenosine 5* -diphosphate (ADP):

ATP + H

2 0 * ADP + phosphate

The above process couples not only with glucose but also with other

carbohydrates and with several hundred other substrates. Other important

biochemical couples and redox potentials involve nictotinamide adenine

dinucleotides (NAD+ /NADH) , nicotinamide adenine dinucleotide phosphate

(NA-DP + /NADPH) , and flavin adenine FAD (FAD/FADH2

) . Because of the

ubiquitous nature of these redox potentials and couples in biochemical

reactions, a precise understanding of the their thermodynamics is of both

fundamental and practical importance.

The recertification of the energy of combustion of benzoic acid will

take place during FY 1986 - FY 1990 and should be accompanied by efforts

to improve the degree of precision and accuracy by one order of magnitude.

Similar efforts toward improving the precision and accuracy of the

enthalpy of combustion of methane will be considered. These improvements

in the precision and accuracy of combustion calorimetric measurements are partly due to recent advances in the development of large sample capacity

combustion bomb and flow calorimetry.

We anticipate that the U.S. Department of the Army will support

research in the Division on the combustion calorimetry and estimation

procedures for organic phosphorus compounds. The development of standard

state corrections to calorimetric data for organic phosphorus compounds

should evolve naturally from this work.

In large sample combustion flow calorimetry, we expect that

collaborative research between NBS and Warren Spring Laboratory in the U.K.

will be carried out in which a comparison of combustion results on samples

of municipal solid waste will be made in kilogram-capacity calorimeters.

High temperature heat capacity using drop calorimetry will be pursued

for calcium and strontium metals from their melting points to room temperatures. These measurements will assist theoretical calculations on

the lattice dynamics of bcc and fee systems. In addition, high temperature measurements will be performed on high purity zirconia from 1200 K to room temperature.

152
3. THERMODYNAMIC DATA COMPILATIONS

A. Chemical Thermodynamics Data Center

The Chemical Thermodynamics Date Center has as its mission the

preparation and distribution of reliable thermo chemical data suitable for

use throughout science and industry. The work is supported by the Office

of Standard Reference Data (OSRD), the CCP biotechnology program, the

Department of Energy (DoE) , and the Design Institute for Physical Property

Research (DIPPR). Closely related work is carried on in the Electrolyte

Data Center and the Ion Energetics Data Center.

These recommended data are prepared by reassessment, selection and

synthesis from published experimental measurements and theoretical

calculations, and, where necessary, prediction of values. The principal

types of data contained in our data bases of selected values are enthalpies of formation, Gibbs energies of formation, entropies, enthalpy

increments and heat capacities. The major classes of input data are

enthalpies of combustion, reaction, solution and phase change; equilibria

in reactions, emf cells, solubility and vaporization; heat capacities and

enthalpy changes and molecule parameters from spectral data. Gases,

liquids, solids, solutions, and ions (gaseous and in solution) are

considered.

The applications of the recommended thermodynamic data are in the

calculation of heat balances in industrial processes, the calculation of

equilibrium yields in chemical reactions, calibration and testing of

experimental equipment, correlation and prediction of properties of

closely related compounds and interpretation of measurements. The

activities during the past year are reported here under three headings:

Evaluation and Numerical Data Bases; Accessions, Information and Support;

and Automation Activities.

1. Evaluation and Numerical Data Bases

(W. H. Evans, D. Garvin, V. B. Parker, R. L. Nuttall, R. H. Schumm,

and D. D. Wagman)

In biothermodynamic data efforts, work has been performed in two

areas: (1) the compilation of tables of thermodynamic properties of

important biological compounds, and (2) the estimation of thermodynamic

properties by group contribution value techniques.

A search of a major organic data-base provided to the Chemical

Thermodynamics Data Center by J. B. Pedley, University of Sussex, was carried out to extract and evaluate the enthalpies of formation at 298 K

of amino acids, peptides, and heterocyclic compounds. Additions to

this collection on enthalpies of formation data were made from the recent

literature

A major compilative effort of the Chemical Thermodynamics Data Center

has appeared in the literature during fiscal year 1984 and is entitled"

Heat Capacities and Entropies of Organic Compounds in the Condensed Phase"

153
by E. S. Domalski , W. H. Evans, and E. D. Hearing, (J. Phys. & Chem.

Ref. Data 13, supplement no. 1, 1-286 (1984)). Data on the heat capacities and entropies of over 1400 organic compounds in the liquid and solid

phases are provided. Values for the enthalpies and entropies of phase

transitions (solid/solid, fusion, vaporization, and sublimation) which

were encountered as part of the evaluation are included in the tables.

Over 800 references were examined and cover the period from 1881 to 1982.

The search of the literature, data evaluation, and computerized- tabulation

of the data and bibliography were carried out primarily through contracts

with local vendors.

Group contribution values are being generated for use in the estimation of the following properties at 298 K on organic compounds: enthalpy

of formation, heat capacity, and entropy in the gas, liquid, and solid

phases. Past efforts in this area have provided about 450 group contribution values for estimation of the enthalpy of formation of liquids and

solids at 298 K. During the current year, work was completed on group

contribution value calculation for the heat capacity and entropy in the

liquid and solid phases at 298 K. Evaluation of group contribution values

is underway in order to bring about consistency among given groups as estimates are made to calculate values for a compound in the gas, liquid,

and solid phases

.

A major activity has been the preparation of the prototype edition of

the CODATA Thermodynamic Tables. These are the first of a new international set of thermodynamic tables. These comprehensive tables contain

contributions from groups in four countries: thermochemistry (MBS and the

Institute for High Temperatures (IVTAN), Moscow), thermal functions (the

JANAF group at Dow Chemical, Midland, Michigan, and IVTAN), alloys

( ENSEEG , University of Grenoble, France), fused salts (AERE, Harwell,

United Kingdom), and aqueous solutions (NBS). This edition is devoted to

compounds of calcium. This Data Center has analyzed some 375 measurements

on calcium species and provided recommended values for their thermochemistry. In parallel the Electrolyte Data Center has prepared data on

the aqueous solutions of calcium chloride. In addition all of the

technical coordination, editorial work, and production of the tables has

been done here. The tables were submitted in August 1984 to the CODATA

Task Group on Chemical Thermodynamic Tables for final review and subsequent publication.

A second major evaluation activity has been the correlation of data

on aqueous solutions of sulfur dioxide, a joint activity with the

Electrolyte Data Center. A set of recommended values has been prepared

that covers vapor-liquid equilibria and ionization for processes in

sol ution.

15b
S02 (g) * S02 (aq)

S02 (aq) + H2

0 = H

+ (aq) + HSO^aq)

HSO^Caq) = H

+ (aq) + SO^aq)

HSO^Caq) = H2

0 + S2 05 (aq)

The temperature range 25-1 00°C is covered. Recommendations are made for

enthalpies of the processes, equilibrium constants, and activity coefficients. New, more precise measurements and analysis using our weak

electrolyte modelling codes have led to substantially improved recommendations. One result is an appreciable change in the A^H and AfG of SO^Caq),

a situation that may have an impact on tabulated data for many metal

sulfides. These new evaluations were prepared for the DoE flue gas

desulfurization program, the DIPPR Electrolyte Handbook, and OSRD.

Additional detail is given in the section on electrolyte data.

New recommendations for thermochemical properties of Sr +2 (aq)

and several strontium salts have been made, jointly with researchers at

the U.S. Geological Survey. New recommended values for the enthalpy

of combustion of components of natural gas have been prepared, jointly

with workers of the Thermodynamic Research Center, Texas A&M University.

The estimation of enthalpies of formation, heat capacities, and

entropies of liquid and solid organic compounds continues, is being

coordinated with work at the University of Sussex, U.K.

The preparation of numerical data bases, in machine readable form, is

a major task in the modern technique for evaluating thermochemical data.

These data-bases of measurements are used as input to programs that

calculate enthalpies and Gibbs energies of formation and entropies and are also used to document recommended values. Three data-bases were prepared

or extended during the year. One is the set of measurements on calcium

compounds for the CODATA Tables, mentioned above. The second is a catalog

of measurements documenting the CODATA Key Values for Thermodynamics. This

very important data base covers the substances upon which many data

evaluations hang and will be the basis for future CODATA Key Values

activities. The third is a data-base covering compounds of sulfur with

hydrogen or oxygen (or both) that are of interest in flue gas desulfurization problems. It includes measurements published through 1983 and is a start on updating the CODATA recommended values into the M NBS Tables of

Chemical Thermodynamic Properties.”

2. Accessions, Information and Support

(T. L. Jobe, E. D. Hearing, and C. Jackson)

These are ongoing activities that make possible the systematic

evaluation of data. During the year 2800 articles have been located in

the current literature, abstracted and entered in our index to thermodynamic measurements. This work produced 14,000 index entries and will be

the basis for the Inorganic Substances section of volume 27 of the

Bulletin of Chemical Thermodynamics. Information activities during the

ce
year have emphasized a joint project with the Electrolyte Data Center,

preparation of a series of bibliographies on the thermodynamics of

aqueous solutions.

During FY 1984, Joyce J. Grimes transfered from the Chemical

Thermodynamics Data Center to the NBS Equal Employment Opportunity Support

Division.

3. Automation Activities

(D. Bickham, R. Coyne, P. Fagan, and D. B. Neumann)

Four data centers share the use of a mini-computer (HP-1000) via

terminals and interfaced microcomputers. The principal use of the HP-1000

is word processing and data manipulation. Calculations are a very small

fraction of the work load. Individual researchers reduce data, model

systems and prepare graphic output. All of these users are supported by

the automation group of the Chemical Thermodynamics Data Center which is

responsible for systems maintenance and development. The group is aided by

individuals who have particular needs for special utilities.

An exciting, new activity is the development of a chemically oriented

data base management system suitable for use by four CCP data centers

(Chemical Thermodynamics, Electrolyte, Chemical Kinetics, Ion Energetics).

This is a multi-year effort, part of which has been completed. The

entire system will contain packages that accept, check, edit, store and

retrieve chemical nomenclature (in a broad sense), chemical reactions and

associated quantitative data, bibliography, subject abstracts, recommended

physiochemical data and interfaces to calculated programs.

This year our work focused on the bibliographic and abstracts

portions of the system. The bibliographic system, originally designed

for use in thermochemistry, has been expanded to meet the demands of other

data centers and also to include the ANSI standards on bibliographic

format. This work is being done jointly with the programming staff of

the Office of Standard Reference Data. The resultant programs will be

tested and applied widely in the OSRD system. The present system has been

applied successfully in the preparation of the CODATA Tables and the DIPPR

bibliographies.

Subject abstracts have been prepared in automated form since 1970 in

the Chemical Thermodynamics Data Center. The files have been processed to

give hardcopy for our index to thermodynamic measurements and for the

Bulletin of Chemical Thermodynamics. During this year the automation

effort has been devoted to implementing a system to retrieve the accumu- lated abstracts in a form suitable for loading into a data base management

system. The first phase is complete. Retrieval has been achieved and the

output form specified. Programming is proceeding to provide for loading

into the DBMS. One spin-off from this work is the preparation of a transferable program for processing the current subject abstracts. It is

being provided to the editor of the Bulletin of Chemical Thermodynamics

for use in the preparation of copy and in the development of derivative

daca-bases

.

156
3. Electrolyte Data Center

(R. N. Goldberg and D. Smith-Magowan

)

The Electrolyte Data Center serves as a resource for correlated data

on properties of solutions of aqueous electrolytes and for compilations of

data. The Data Center provides, on a continuing basis, information

services and standard reference data for selected thermodyanmic properties of aqueous solutions of electrolytes. It provides methodologies for

correlation and estimation of the properties of classes of electrolytes.

Three problem areas that are addressed directly by this electrolyte

evalution program are environmental quality, energy, and health. Data are needed in laboratory research, water pollution studies, advanced battery

design, water and sewage processing, corrosion control, flue-gas washing

processes, geothermal energy systems, biotechnology, oceanography and

specialized electrolytic applications such as analyses using ion-selective

electrodes

There are four areas into which recent activities can be grouped:

(1) a correlation of the thermodynamics of the SO2 +H2 O system. (2)

development of procedures and algorithms for the modeling of complex

aqueous solutions, (3) development of a data base of activity and osmotic

coefficients and of programs for the manipulation and use of the data

base, and (4) completion of specific bibliographies of industrial

interest

1 . Thermodynamics of the SO2 +H2 O System

A manuscript written in collaboration with V. Parker which summarizes

the thermodynamics of two SO2 +H2 O System Ions has been prepared and is

presently in review. The results of this manuscript are summarized herein.

A consistant set of thermochemical property values, AfH°, A^G 0

, S°, and

C ° at 298.15_K were selected for the constituents of aqueous S02

(S0|(aq), HSCL(aq), SCL (aq), H

+ (aq), and S

2 0- (aq)). AI 30 tabulated

are

- values of^MO meunibnic activity coefficients, osmotic coefficients,

partial pressures of S 0P

2 (g), and the relative apparent molar enthalpy as

a function of the stoichimetric concentration of S02 (aq) at 298.15 X.

The analysis of the data considered a wide variety of measurement

techniques: calorimetric enthalpies of solution and reaction, heat

capacities equilibrium constants, solubilities, and vapor pressure

measurements, both partial and total, over the temperature range 273 to

393 X. All auxiliary data was taken from the most recent set of CCDATA

values which were converted to a standard state pressure of 1 bar (0.

MPa). For the process SC^Cg) = SO^Caq), the selected '’best" values are:

K=1.22 ± 0.05 mol kg

-

] bar

-

', AG° = -0.50 ± 0.10 kj mol -1

, and AH 0 =* -27.18 ± 0.20„kJ mol

-1

at 298.15 K. The standard state partial molar

entropy of S0^ (a) is -15.30 ± 0.70 J mol

-

K

-1

at 298.15 X. This value

"• 157
was obtained by the analysis of data via two independent thermodyanmic

pathways. Parameters are given which extend the predictions to

temperatures up to 373 K.

2. Data Base of Activity and Osmotic Coefficients

A data-base of activity and osmotic coefficients and excess Gibbs

energies for approximately 400 aqueous binary electrolyte solutions has

been assembled. The data base contains, for each electrolyte solution,

the coefficients of one or more of ten different correlating equations.

The data base has been assembled into FORTRAN "DATA” statements so that it can be compiled as a subroutine and then linked with other codes for

efficient use. Programs have also been written which can be used to

calculate values of the activity and osmotic coefficients at mobelities

specified by the user. The programs can be used either interactively or

in batch modes of operation. Before this package (which will be made

available on magnetic tape) can be released, there must be final assembly

of the data base and the algorithms accompanying it, a thorough check of

its operation, and documentation must be written.

3. Modeling of Complex Aqueous Solutions

This activity was described in detail in our last two panel reports.

A lengthy manuscript describing the procedures is currently in press.

These same procedures have also been applied to the modelling of two

SO2 +H2 O systems (see above). Progress this past year on the development

of the codes and procedures have included the following: (1) a very

careful structuring of the 23 subroutines used in the calculations which

permits one to modify or to extend the codes in an efficient manner, (2) an extension of the codes to allow for a much larger number of species to

be treated in two equilibrium calculations (The actual number is now

machine and not algorithm dependent), and (3) a demonstration that

hydration effects coupled with the introduction of the species CuSO,.,

2 + 2

-

4 Cu2S0^ , and Cu(S0i})

p in the equilibrium model can be used to represent

the thermodynamic behavior of aqueous copper sulfate solutions up to a mobility of at least one mol kg

-1

.

While the algorithms can in principal handle very large numbers of

species, the actual solution of the simultaneous non-linear equations

which describe the equilibria in solution has been found to be highly

dependent upon providing good initial estimates of the sought after

parameters when treating systems containing more than about ten different

species. There are several approaches which can be taken to solve this

problem. They are: (1) improved numerical scaling of the parameters, (2)

heuristic search procedures, and (3) a brute-force approach in which the

n-dimensional space containing all possible solutions is systematically

searched for the correct answers. In any case, a reasonable solution to

this problem must be found before we are able to deal with the kinds of

larger scale systems which we would like to treat in the future.

158
This kind of procedure is particularly applicable to solutions which

display ion-pairing, complexation , and hydrolysis behavior. These are the

kind of phenomena which become more significant at high temperatures. A

logical application and next step in the use of these codes is to regress

the parameters of the model itself from measurement data (Gibbs energies,

enthalpies, and heat capacities) on solutions over a wide range of

temperature

4. Bibliographies of Industrial Interest

Eight bibliographies of thermodynamic data pertinent to aqueous

solutions have been completed. They are: C0 2 , H 2S, H 2SO4 , NH3

, CuCl2 > H^Oij, ZnCl 2 » and (H 2S+C0 2 , H 2S+NH

3

, C02 +NH3, C02 +H2S+NH

3

) . Papers

included in these bibliographies pertain to vapor-liquid-equilibria, the

thermal and excess properties of these solutions, and equilibria in

solution. These bibliographies were sponsored by the Design Institute for

Physical Property Data of the American Institute of Chemical Engineers.

C. Future Plans

This next year we will begin development of a machine data-base of

the evaluated information gathered on A^H, Cp, and S at 298 K for amino

acids, peptides and heterocyclic compounds so that tables of data can be

derived from an appropriate algorithm. We will prepare a manuscript

containng group contribution values for estimating A^H, C0 and S of

organic compounds in the gaseous, liquid, and solid phases at 298 K.

Our longer range plans include the supplement of the CODATA Key

Values for Thermodynamics with data on substances commonly used in science

and industry and for which there are high-quality thermochemical

measurements. Addition of data on a thousand substances is the goal. As

a necessary and much needed compliment to the evaluation work we will

develop new methods for predicting values of properties based on molecular

and crystal structure, lattice energies, similarities within classes

inorganic of compounds and their position in the periodic tables. Prepare

reaction catalogs needed by CODATA and documenting "the NBS tables" and

publish these.

Next year our automation activities will deliver a fully operational

bibliographic retrieval system. We should a30 be in a position to implement a prototype search and retrieval system for subject abstracts and

reaction catalog data. We will also onvert an existing organic

thermochemistry retrieval system to operation on the local HP-1000 system.

We also plan to add selected calculation and graphics to the HP-1000

system in support of thermochemical data evaluation and upgrade the

minicomputer system to the HP-1000 A 900 series. During the year we will

expand the NBS index to thermodynamic measurements by selecting and

abstracting papers published in 1984-85, prepare the inorganic

thermochemistry section for volume 28 of the Bulletin of Chemical Thermodynamics, and provide information services in response to request.

15Q -*•/>
In the Division's efforts to make the Chemical Thermodynamics Data

Center preeminent source for thermochemical data, world-wide five activities are being undertaken. They are:

1. Maximize the amount of evaluated thermochemical data compatible

with the CODATA Key Values and at the same time document the NBS

Tables of Chemical Thermodynamic Properties. The principal

mechanism is the construction and use of catalogs of measurements used by data evaluators in preparing tables.

2. Define a key network of thermo chemical data and keep current the

evaluated data for it. This network will include industrially

important processes, those essential for thermodynamic data

evaluation, and those of major scientific concern, e.g.,

geochemistry, molecular structure studies, biochemistry, etc.

3. Participate in the joint preparation of tables of thermodynamic

data by international and national groups. This is our preferred route to rapid expansion of the stock of up-to-date

evaluated thermodynamic data. The principal mechanism will be

participation in the work of the CODATA Task Group on Chemical

Thermodynamic Tables (and development of a leadership role

there), and bilateral exchange agreements or collaborations with

institutions in the U.S.S.R., the U.K. and in the U.S.A.

4. Design and implement a thermochemical information system which

includes a rapid reply system, (based on our files), updates to

CIS, addition of the JANAF Tables to our files and CIS, and,

very important, a suite of programs to calculate properties of

reactions

.

5. Develop predictive methods for thermodynamic properties. This

is an activity to extend existing data-bases, and also to let us select among divergent data. Methods are needed for both

organic and inorganic compounds.
PUBLICATIONS

(a) Publications of Past Year

Abramowitz, S. , "Barriers to Internal Rotation in Inorganic Species,"

Thermochemistry and Its Applications to Chemical and Biochemical Systems,

MATO Advanced Study Institute, Series C, Mathematical and Physical

Sciences, Vol. 119 (M.A.V. Ribeiro da Silva, Editor; D. Reidel Publishing

Co.), 789-802 (19811).

Abramowitz, S., Domalski, E.S., Churney, K.L., Ledford, A.E., Ryan, R.V., and

Reilly, M.L., "An Oxygen Flow Calorimeter for Determining the Heating

Values of Kilogram-3ize Samples of Municipal Solid Waste," Thermochemistry

and Its Applications to Chemical and Biochemical Systems, MATO Advanced

Study Institute, Series C, Mathematical and Physical Sciences, Vol. 119

(M. A. V. Ribeiro da Silva, editor; D. Reidel Publishing Co.), 61-76, (1984).

Abramowitz, S. , Wagman, D.D., Parker, V.B., and Garvin, D., "Critical Evaluation of Thermodynamic Data - A Research Activity," Thermochemistry and Its

Applications to Chemical and Biochemical Systems, NATO Advanced Study

Institute, Series C, Mathematical and Physical Sciences, Vol. 119 ( M.A.V.

Ribeiro da Silva editor; D. Reidel Publishing Co.), 803-814, (1984).

Abramowitz, S. , "Barriers to Internal Rotation in Inorganic Species,” Proceedings of High Temperature Chemistry Symposium, Bombay, India, January

28-30, 789-802, (1983).

Armstrong, G.T., and Jobe, T.L., Jr., "Heating Values of Natural Gas and Its

Components: Conversion of Values to Measurement Bases and Calculation of

Mixtures," Stationary Gas Turbine Alternative Fuels, ASTM ST? 809, (J.C.

Clark and S.M. DeCorso, Eds., American Society for Testing and Materials,

Philadelphia), 314-334, (1983).

Churney, K.L., Domal3ki , E.S., Ledford, Jr., A.E., Colbert, J.C., Bruce, S.S.,

Buckley, T.J., Paule, R.C., Reilly; M.L., "Assessing the Credibility of

the Calorific Value of Municipal Solid Wa3te," NBSIR 34-2825 (DoE),

February, ( 1 984)

DeVoe, H. and Wasik, S.P., Aqueous Solubilities and Enthalpies of Solution of

Adenine and Guanine," J. Solution Chem. 1

3

> 51-60 (1984).

Ditmars, D.A., "Heat-capacity Calorimetry by the Method of Mixtures," Chap 13,

Compendium of Thermophysical Property Measurement Methods, 1_, (K.D.

Maglic, Editor; Plenum Publishing Corporation. ) 527-553 (1984).

Dizdaroglu, M. , Gajewski, E., Simic, M.G., "Enzymatic Digestibility of

Peptides Crosslinked by Ionizing Radiation," Int. J. Radiat. 3iol.

45 (3), 283-295, (1984).
Domalski, E.S., Evans, W.H., and Hearing, E.D., "Heat Capacities, Entropies

and Some Phase Transition Properties of Organic Compounds in the

Liquid and Solid Phases," J. Phys. Chem. Ref. Data J_3, Supplement No. 1,

1-26 (1984).

Fuger, J., Parker, V.B., Hubbard, W., Oetting, F., "The Chemical Thermodynamics of Actinide Elements and Compounds. Part 8. The Actinide Halides,"

IAEA, Vienna, Austria 1-267 (1983).

Goldberg, R.N., "An Equilibrium Model for the Calculation of Activity and

Osmotic Coefficients in Aqueous Solutions," J. Research NBS, 89_, No. 3.

251-263 (1984).

Kirklin, D.R., and Domalski, E.S., "A 25 Gram-Capacity Oxygen Bomb Calorimeter," Resources & Conservation J_0. 177-187 (1983).

Kirklin, D.R., and Domalski, E.S., "Enthalpy of Combustion of Adenine,"

J. Chem. Thermodynam. J_5_ 941—947 (1983).

Kirklin, D.R., and Domalski, E.S., "Enthalpy of Combustion of Purine,"

J. Chem. Thermodynam. J_6^ 633~64l (1984).

Miller, M.M., Ghodbane, S. , Wasik, S., Tewari, Y.B., Martire, D.E., "Aqueous

Solubilities, Octanol/Water Partition Coefficients, and Entropies of

Melting of Chlorinated Benzenes and Biphenyls," J. Chem. Eng. Data 29 , 184-190 (1984).

Tewari,. Y.B., and Goldberg, R.N., "Thermodynamics of the Conversion of Aqueous

Glucose to Fructose," J. Solution Chem. 13, 523-547 (1984).
(b) Publications in Progress

Abramowitz, S., "Biotechnology and Bioseparations, Thermodynamic Meeds of the

Decade Ahead," National Science Foundation, Workshop Proceedings, 1983, (in

press )

Busenberg, E. , Plummer, L.N., Parker, V.B., "The Solubility of Strontianite

(SrCO^) in C02_ H

2 0 Solutions Between 2 and 91 °C, the Association Constants of SrHCO^ and SrCO^ Between 5 and 80 °C and an Evaluation of the

Thermodynamic Properties of Sr

2+

(aq) at 25 °C and 1 Atmosphere," Acta Geochiraica, (in press).

Doraalski, E.S., Churney, K.L., Ledford, Jr., A.E., Colbert, J.C., Bruce, S.S.,

Buckley, T.J., Paule, R.C., Reilly; M.L., "Determining the Calorific Value

of Municipal Solid Waste: ' Experimental Results From a 2.5 Kilogramcapacity Flow Calorimeter," Proceedings of Energy from Municipal Waste

Research Workshop, Kissimmee, Florida. February 22-24, 1984, (in press).

Gajewski, E., Dizdaroglu, M., Krutzsch, C., and Simic, M.C-., "OH RadicalInduced Dimerization and Racemization of Methionine in Peptides." Int. J.

Radiat. Biol., (in press).

Garvin, D., Goldberg, R.N., and Staples, B.R., "Evaluated Data and Data Bases:

Chemical Thermodynamics in the Steam Power Cycle,” Proceedings of the

Workshop in Steam Power Cycles Data Requirements, H. White and 0. Jonas,

Editors, (in press).

Garvin, D., Staples, B.R., and Jobe, Jr., T.L., "A Bibliography on the

Equilibria in the CUCI 2-H2 O System," Handbook of Aqueous Thermodynamics,

Design Institute for Physical Property Data, (DIPPR), (submitted).

Goldberg, R.N. and Parker, V.B., "Thermodynamics of Solution of S02 (g) in

Water and of Aqueous Sulfur Dioxide Solutions," (submitted).

Goldberg, R.N. and Steckler, D.,"A Bibliography of Sources of Thermodynamic

Data for the System: C0 2

+ NH^ + H

2 0, C02

+ H

2 3 + H90, H

2 S -* NHo + H2

0,

CO 2 + NH^ + H

2S + H2O", Handbook of Aqueous Thermodynamics, Design

Institute for Physical Property Data (DIPPR), (submitted).

Goldberg, R.N., "Compiled Thermodynamic Data Sources for Aqueous and Biochemical Systems: An Annotated Bibliography (1930 to 1983)," Handbook of

Aqueous Thermodynamics , Design Institute for Physical Property Data

(DIPPR), (submitted).

Jobe, Jr., T.L., and Goldberg, R.N., "A Bibliography of Sources of Thermodynamic Data for Aqueous Zinc Chloride Solutions," Handbook of Aqueous

Thermodynamics, Design Institute for Physical Property Data, (DIPPR),

(submitted)
Jobe, Jr., T.L., and Goldberg, R.N., "A Bibliography of Sources of

Thermodynamic Data for Aqueous Phosphoric Acid Solutions,” Handbook of

Aqueous Thermodynamics, Design Institue for Physical Property Data,

(DIPPR), (submitted).

Kirklin, D.R., Decker, P.H., and Domalski, E.S., "Characterization of Refuse-Derived Fuel at Various Stages of Processing,” Resources and Conservation, (submitted).

Simic, M.G., Gajewski, E. , and Dizdaroglu, M. , "Kinetics and Mechanism of

Radiation-induced Dimerization of Phenylalanine Peptides,” J. Phys. Chera.

(in press).

Tewari, Y.B., and Goldberg, R.N., "A Note on the Thermodynamics of the

Conversion of Aqueous Glucose to Fructose,” Appl. Biochem. & Biotech.,

( in press)

.

Wasik, S.P., Schwarz, F.P., Tewari, Y.B., Miller, M.M., Purnell, J.H.,

"A Head-Space Method for Measuring Activity Coefficients, Partition

Coefficients, and Solubilities of Hydrocarbons in Aqueous Solutions,” J.

Research NBS, 89 , (in press).

Miller, M.M., Wasik, S.P., Guang, G.L., Shui , W.Y., and MacKay, D.,

"Relationships Between Octanol-water Partition Coefficient, Aqueous

Solubility and Bioconcentration Factors, (submitted).

l6h
MOLECULAR SPECTROSCOPY DIVISION

A. Weber, Chief

SUMMARY OF ACTIVITIES

Fiscal Year 1984

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 Picosecond Spectroscopy. Properties of weakly bonded systems are obtained for small atomic and molecular complexes held together by van der

Waals, hydrogen bond, and other weak forces in order to develop a data

base to understand conformation, thermodynamics, and reactivity of

biochemical 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 picosecond thrust has been directed to the

study of vibrational energy transfer.

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

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

are meeting the above objectives.

165
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 twelve guest workers and six postdoctoral

fellows engaged in Division projects. A list of the guest workers, their

permanent affiliations, and their activities is given in part 10 of this

report

.

C. Needs of the User Community

With the technological advances of micro- and main frame computers

and the development of tunable dye and tunable 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

.

D. Interactions with the User Community

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. Details about our Calibration Standards work are given in

Sections B and H of the HRS group report.

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

1 66
our continuing involvement in problems of national concern. Details about

these and other activities may be found in Section C of the HRS group

report

Our Division has significant other agency contracts with ONR, ARO,

AFOSR , DoE f NASA, 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 lists of consultations,

talks, publications, and collaborative activities show our strong coupling

with industry, other government agencies, and the research community.

The above constitutes only a sample of our involvement and response

to the needs of the user community. Our Division is host to several guest

workers from universities, other government agencies, and from overseas

countries (see section 10). The numerous and varied interactions and

collaborations of the Division's staff with non-NBS scientists and user groups are detailed in part 9 - Professional Interactions, Consulting and

Advisory Services.

E. Interactions within NBS

One of our principal objectives is to provide major spectroscopic

expertise to other NBS divisions and programs.

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. These activities are described in sections 3 and H of

the HRS group report.

We have completed the second year of a new program in the spectroscopy of condensed phase systems. This effort is part of an overall

program of the Center for Chemical Physics' new emphasis on condensed

phase chemistry, including selective areas of biotechnology. The

Division's participation is in the areas of picosecond studies, quantum

chemical investigations of the interaction of metal atoms with biomolecules, molecule-surface interactions, high resolution spectroscopy of

hydrogen-bonded molecules, and metal clusters. These activities are described in the individual group reports. To augment the experimental

facilities for microwave spectroscopy in order to study the spectra of

weakly bonded complexes, a new pulsed beam, Fabry-Perot cavity, Fourier

transform microwave spectrometer was constructed during the past year.

Details about this instrument and its performance are given in section G.3

of the HRS group report.

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 3tates, amcr.g

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

Chemistry group.

167
2. High Resolution Spectroscopy Group

J. T. Hougen, B. J. Howard, W. J. Lafferty, R. W. Lovejoy, F. J. Lovas,

A. G. Maki, N. Ohashi, W. 3. Olson, A. S. Pine, M. Raab, G. Rotter,

R. D. Suenram, A. Weber

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.

These investigations are made possible through the diverse instrumentation

(difference frequency laser spectrometer, high resolution Fourier transform spectrometer, pulsed beam Fabry-Perot cavity Fourier transform

microwave spectrometer) available in the division.

Molecular radio astronomy in recent years has generated a very

pronounced demand for microwave laboratory data and spectroscopic

expertise. A portion of the current measurement and analysis effort is

aimed at providing the important data needed to complete the reviews of

spectra of known interstellar molecules described in Section H of this

report. Our collaborations with radio-astronomers provide the contacts

and experience necessary to insure that these reviews are both practical

and useful.

The major activity of the data center 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

.

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 ncn-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, the

National Research Council of Canada, the Harry Diamond Laboratories of the

U.S. Army, the NASA-Greenbelt , NASA-Langley , NASA-Goddard , and NASA-Ames

Laboratories, and the Kitt Peak National Observatory.

163
Our group's scientists are also aware of the need to serve the

scientific community at large. This they do in a variety of ways,

including membership and active participation in national and international committees, workshops and review panels. This activity is listed

elsewhere in this report (see Sections 8 and 9).

The various technical activities of the High Resolution Spectroscopy

group for the period covered by this review are described under the

following seven, broad categories (B-H) , each of which lists several

projects. Names of group members active in a project are listed at the

beginning of each project's description.

3. Development of Calibration Standards for the Infrared Spectrum

The development of frequency standards for the infrared spectral

region is a long term project of the group. Until this project was undertaken most of the available frequency standards had uncertainties

between ±0.002 and ±0.005 cm

-1

. Since this is an order of magnitude worse

than their measurement capability, many users of modern high resolution

tunable laser systems and other high-performance spectroscopic devices

have asked us for advice on calibrating their measurements. In many cases

we have been able to help them to our mutual satisfaction, but in some

cases the available calibration sources have been unsatisfactory and the

inquiries have only served to spur our efforts by demonstrating the need.

Our strategy in developing calibration standards has been to follow

the precedent already set by IUPAC (International Union of Pure and

Applied Chemistry) in providing calibration for lower resolution devices.

These standards consist of tables of frequencies for absorption line

spectra of inexpensive, easily handled, and readily available gases. Such

standards do not require expensive additional equipment, are easily ana

accurately reproduced, and can be quickly utilized by relatively untrained

personnel

The best standards are provided by substances that give single line

spectra when measured under conditions giving Doppler limited resolution.

Ideal ^ candidates have at least one calibration quality feature at every

1 cm

-1 interval and preferably more. We prefer to use substances that

have a well behaved energy level pattern so that unmeasured transition

frequencies and their uncertainties may be calculated with confidence from

a minimum set of physical parameters that have been very accurately measured. We also prefer to use substances for which the necessary physical

parameters have been measured by several laboratories. Frequency measurement techniques for determining the physical parameters are preferred over

length or wavelength measurement techniques because frequency measurements

are generally less susceptible to unexpected systematic errors.

So far our efforts have been concentrated on the molecular species

CO, 0CS , N-O^, C 2 H2

, and C0 2

. Others have used HCl, HF, NH~ , HCN, and

H2 0. We have paid particular attention to systematically developing a set

of molecular parameters that would enable us to calculate unmeasured

transitions with a reliably predictable uncertainty. The difficulty of
reliably estimating the uncertainty of certain transitions has caused us

to avoid using more complex molecular species until we have exhausted all

calibration possibilities using the simpler species.

During the past year our efforts have concentrated on a) preparing a review article on calibration data available for tunable laser devices,

b) completing publication of the measurements on DBr (calibration 50-200

cm

-

' and 1595-1990 cm

-1

) and N2 0 (calibration 4340-4755 cm

-1

), c) measuring the pressure induced frequency shift of N0^ and d) continuing the

analysis of the FTS measurements on 0CS, N

2 o, and CO2 from 450 to 720 cm

-1 •

1 . New Measurements on N

2 o

( A . G . Maki)

In our continuing collaboration with Joe Wells and others in

the Time and Frequency Division at N3S-Boulder , new heterodyne measurements have been made on the 01 1 1 - 00°0 band of N2 0 near 1880 cm

- "' using a CO laser as a frequency transfer standard. These measurements, coupled

with frequency measurements on rotational transitions in both the upper

and lower states of N^Q, give us an excellent grid of frequency standards

from 1830 to 1920 cm .

This is a particularly valuable band from which to obtain frequency

measurements as it can be used as a test on the accuracy of the Fourier

transform spectra which we have obtained from Dr. Kauppinen of Oulu

University, Finland. That is to say, the sum of the frequencies 01 1

1 -00^1, 00°1 - 01 1 0 , and 01 1 0 - 00^0 must equal the frequency of 01 1

1 -OO^O. It also provides a test of the Fourier transform measurements

published by Dr. Guelachvili near 1290 cm

-1

(by means of the frequencies

01 1 1 - 0^0 and 01 1 0 - 00°0).

New heterodyne measurements have also been made on N

2 0 in the region

around 1100 cm

-

"

1 but due to a regrettable lack of coincidences, these

measurements will not be very useful for improving the N2 0 calibration in

the 1115 to 1340 cm

-1 region. These measurements are primarily on hot

bands and only include a few high J levels of the F-branch.

New Measurements of Pressure Induced Frequency Shifts

(A. G. Maki and A. S. Pine)

There are very few measurements of the pressure-induced

frequency shifts of absorption features in the infrared. Because of this,

it is difficult to estimate with confidence the magnitude of the frequency

shift to be expected for a frequency calibration gas at a particular

pressure. While making lineshape measurements on NO (described in section

C below), we were also able to measure the pressure-induced frequency

shift of the lines.. The shift of -1.1 3±0 . 23 MHz/kPa of NO (-0.15±0.03

MKz(Torr) for the 2 ^/2 transitions and 1.05±0.23 MHz/kPa of NO

(-0.14±0.03 MHz/Torrj for the 2

II-| / 2 transitions agrees in both sign and

magnitude with the more poorly measured shift of CO, a similar molecule

170
with nearly the same mass and dipole moment. The shift is also similar to

that measured for N

2 o although the shift of N2 O was also rather poorly

measured

The present state of development of infrared frequency standards is shown in summary form in Figs. 2.1a and 2.1b, special attention

being given to the accuracies to which the various standards are known.

Care must, however, be exercised in using these figures, for (1) in many

cases there are gaps of 2 or 3 cm

-1 between usable frequency standards and

(2) it is difficult to find the appropriate papers in the literature since

there is no complete compilation that would make it easy to find calibration data. A goal of this work is, of course, to make a compilation of

calibration data available to everyone. In the meantime Art Maki is

preparing a review article on frequency calibration standards to be

published in the Journal of the Optical Society. This review includes a bibliography of all papers presenting data that might be used for calibration.

It will be noticed in Fig. 2.1b that there are no adequate frequency

standards from 2000 cm

-1 to 4120 cm-1

. In fact, from 2250 cm

-1 to 4120 cm

-1 there are no calibration standards (except a few isolated lines) with

an absolute uncertainty of better than ± 50 MHz. Calibration in this

important region obviously needs attention. At the moment there seems to

be little interest in making heterodyne measurements in this region

although we would certainly encourage such measurements. It may be

possible to use N

2 0 hot band transitions with the same upper state as the

N

2 0 bands between 4320 and 4755 cm

-1

of the N

2 0 bands from 3200 cm

-1

to

4000 cm

-1

. Carefully calibrated Fourier transform measurements- may be our principal hope for improving the available calibration data in the 2250 to

4120 cm* 1 region.

It would be very valuable if some high rotational transitions of HC2,

and HF and of vibrationally excited states of 0CS and H?0 could be

measured using the new heterodyne techniques being developed for the

far-infrared by Evenson and Jennings at NBS-Boulder. These measurements

would materially improve our ability to extend the infrared calibration

tables to higher rotational levels and reduce our need to make heterodyne

measurements on a wide range of ro-vibrational states.

In the coming year we plan to

1

2 .

Complete^the compilation of calibration standards in the 480

-700 cm

-

' region.

Use Pine’s difference-frequency laser spectrometer to measure

the 1 0° 1 -00°0 and 1 4°0 - 00 Q

0 bands of 0CS between 2870 and

2980 cm

-1 using the CH^ P ( 7 ) line for calibration and Pine's

carefully stabilized etalon to measure the small frequency

differences involved.

r
ACCURACY

OF

INFRARED

FREQUENCY

CALIBRATION

STANDARDS

L- CQ

CN O

CM XCM CO

o oCM Q o o o z

IT 2

WAVENUMBER

(cm-

)1

Fig.

2.1a.

1

984

status

of

infrared

frequency

calibration

program,

0-2000

ein
o

o

o

10

-» 7*3

-L I ~J

•

lb.

status

of

infrared

frequency

calibration

program

2000-5000

cm
3. Add recently published microwave and laser-Stark measurements

on OCS to our data base and recalculate some of the calibration

tables for OCS.

4. Collaborate with NBS-Boulder on new heterodyne measurements of

OCS and N2O including some far-infrared measurements.

C. Spectroscopy in Environmental and Space Sciences

Part of the Molecular Spectroscopy Division's task is to provide

spectroscopic data and standards needed for the modeling of the earth's

atmosphere, other planetary atmospheres, and interstellar matter. This

work continues to receive some support from NASA’s upper atmospheric

research office because of our unique combination of spectroscopic

expertise and measurement capabilities.

One of the missions of NASA is to understand the physics, chemistry,

and transport properties of the earth’s upper atmosphere in order to

reliably predict the long term effects of anthropogenic perturbations of

the atmosphere on the weather and on the protective ozone layer.

For years it has been known that the chemistry of the upper atmosphere is driven by a large array of interlocking chemical reactions.

3ecause of the complex interdependencies of these reactions, the range of

temperatures, and the uncertainties in circulation models, it has been

difficult to develop and test realistic models of the chemistry of the

atmosphere, and particularly to extrapolate these models to hypothetical

future atmospheric changes.

A key test of the validity of these models is to verify their

predictions of the concentration of certain reaction intermediates.

Another test is to verify predictions of diurnal variations, or the lack

thereof, in the concentrations of certain trace species. For these

tests, and to provide important input parameters for these models, it is

essential to reliably measure the temporal and spatial distribution of

many chemical constituents of the atmosphere. Many of the key atmospheric

constituents are present in minute quantities, with mixing ratios of 1 0

-

- or less, yet their role in the chemical chain is so important that NASA

has given their measurement a high priority.

Spectroscopy is the obvious answer to NASA's remote measurements

requirements. Many different types of spectroscopic devices have been

designed and/or proposed for making atmospheric measurements from

balloons, aircraft, and spacecraft, as well as from the ground. The

design of these devices and experiments and the interpretation of data

they produce require a knowledge of such spectroscopic properties as absorption frequencies, linewidths and shapes, and absorption intensities.

In many cases the interfering effects of other atmospheric species must be

understood in order to design a measurement technique and evaluate the

results

.
The High Resolution Spectroscopy group is involved in assuring the

integrity of atmospheric measurements in several ways. Our work in

providing very accurate frequency calibration standards aids in comparisons between field and laboratory measurements as well as in inter-laboratory comparisons. Cur studies of lineshapes are providing a better

understanding of the role collisions play in the shapes of absorption

lines in the upper atmosphere. Our work also provides the accurate

spectroscopic data needed for the field measurements.

The Chemical Manufacturers Association (CMA) is also quite involved

in supporting and coordinating atmospheric and laboratory measurements

related to the ozone problem. This past year NBS hosted a workshop on atmospheric spectra sponsored by CMA and heavily attended by NASA re- searchers. This workshop concentrated on issues related to the measurement of C2,0 in the atmosphere, but many other topics related to molecular

spectroscopy and both microwave and infrared measurements were also

covered. In the coming year we expect to continue to work closely with

CMA.

Such studies of atmospheric molecules also aid the Defense Department

which needs to be able to predict the transmission properties of the

atmosphere under various conditions of altitude, pathlength, temperature,

etc. for various applications ranging from laser communication to infrared

surveillance to various homing devices. We collaborate closely with

Dr. L. Rothman of the Air Force Geophysical Laboratory (AFGL) who is

involved in extending and improving the AFGL compilation of atmospheric

absorption line parameters.

In the past, most of the work of the High Resolution Spectroscopy

Group has involved the measurement and study of frequency data and, to a lesser extent, intensity data. Since Alan Pine's difference frequency

laser spectrometer is uniquely suited to the study of lineshapes, we have

this year undertaken several projects for NASA to study the shapes of NO,

HC2, , and HF absorption lines. This work has resulted in new insights in

the role of collisional narrowing for determining the lineshapes of these

species in the atmosphere. The effects of pressure broadening have also

been described with better accuracy than was hitherto available. In the case of the NO absorption lines a new phenomenon was observed giving rise

to a pressure induced asymmetry in the linewidths of the closely spaced

doublets observed in the “H.

2

H-|

/

2 transitions. In addition to shese

lineshape measurements we have also measured the intensities of the

absorption lines with unprecedented accuracy.

We also have continued our efforts to provide and interpret high

resolution infrared spectra of HNO^ and H^O?. This past year our constants for the band of HNO^ were used by Dr. Goldman of the

University of Denver to generated linelist, including thousands of

transitions of HNO^, which he is using to aid in the interpretation of n:s

atmospheric spectra.

175
Pressure Broadening, Lineshapes , and Intensity Measurements on

NO

(A. G. Maki and A. S. Pine)

1 .

NO was chosen for lineshape measurements using Pine's difference frequency laser spectrometer because NO is a key molecule in atmospheric chemistry, and yet it has a simple enough spectrum to allow us to

measure the lineshape over a wide range of pressures.

This work has shown that the collisional narrowing coefficient for

the 2-0 band is about -1.7 MHz/kPa (-0.23 MHz/Torr) for both NO/N 2

collisions and NO/NO collisions. This collisional narrowing coefficient

is so small that it only results in a reduction of the effective Doppler

width by about 9 percent at 50 Torr. Such a small reduction is almost

negligible for most atmospheric applications.

One unexpected result of these measurements was the observation that

the doublet structure of the lines of the 2

II -| /2 - transition

acquires an asymmetry, at pressures of 50 Torr, that is not present at

very low pressures. This asymmetry has been interpreted as a sign that

the pressure broadening coefficient of one of the lines of the doublet

pair is greater than the coefficient of the other line.

This work has also verified the observation of other workers that the

p pressure broadening coefficient of the II 3/2 state is greater than that of

the 2

n-, /2 state by about 6 percent. Fig. 2.2 shows the self-broadening

coefficients measured for the 2-0 band of NO.

- 1 -

1

cm atm

r

a tr

0.07-1

0.05

0.05

0 . 0 4H

0.03

- 0

N0/H0 Pressure Broadening

w o o

X *

x X ° O

O O o O 0

*X****X XX *X

I uncertainty

o 2

IT.

2

TT.

3/2

1/2

5.5 10.5

Iml

15.5

Fig. 2.2. Self-broadening coefficients for the 2-0 band of NO at 296 K.
The present measurements give the best line intensity values yet

obtained for NO. In addition to providing an improved value for the

transition moment, these measurements give for the first time an accurate

experimental value for the Herman-Wallis effect. This Herman-Wallis term

is in agreement with only one set of relative signs for the coefficients

of the dipole-moment expansion of NO, and the agreement between the

observed and calculated values is very good, [a(cbs) = 0 . 00357 ±0.0001

a(calc) = 0.00361]. This research has been supported in part by

NASA-Langley Research Center and NASA-Headquarters

2. Spectral Intensities in the Fundamental Bands of HF and HC2,

(A. S. Pine, A. Fried (CAC) and J. W. Elkins (CAC))

In collaboration with Drs. A. Fried and J. W. Elkins of the

Center for Analytical Chemistry the transition intensities of the fundamental bands of natural isotopic HF and HC2, have been measured with

Doppler-limited resolution using a tunable difference-frequency laser

spectrometer . Measurements were carried out in support of the NASA Upper

Atmosphere Project HALOE to monitor ozone depletion caused by reactions

with halogen-containing molecules. Precise values for the integrated band

intensities, S^ , the vibrational transition moments, <iiy> , and the

Herman-Wallis ^-factors, (

1 +C 2M + C 2M

2

), have been determined. Particular

care was taken with sample handling of these reactive vapors to ensure

minimum impurities and accurate pressure measurements. Detailed lineshapes were examined for deviations from predicted Doppler-Gaussian

profiles induced by unresolved hyperfine splittings and from instrumental

distortions due to scan rate, baseline uncertainties, system nonlinearities, noise and laser jitter. This research has been supported in

part by NASA-Headquarters.

3. New Developments in the Study of Nitric Acid (HNO^)

(A. G. Maki)

Continuing a project briefly described last year, we have this

year measured and assigned the spectrum of the 2 vg band of nitric acid

(HNO^) near 890 cm

-1

. This band is in resonance with the V

5 band, which

was studied last year, and an understanding of the spectrum of nitric acid

requires that we understand this interaction. The analysis of the vc band

was very important for assigning the transitions of the 2 vg band because

it told us which transitions of 2vg would be displaced due to tne interaction with vc and it also greatly simplified the spectrum where the two

bands overlap.

We now have a good set of band constants that describes quite well

the nitric acid spectrum from 850 cm

-1

to 925 cm

-1 with the exception of

the most strongly perturbed transitions. In the coming year we plan to use our new perturbed asymmetric rotor fitting program to obtain an analysis that will even include the badly perturbed transitions.

$

I
This work is currently being carried on in collaboration with

Dr. Aaron Goldman of the University of Denver. We had hoped to obtain

Fourier transform spectra from Oulu, Finland, but the spectra were not

very good due to the reaction of the nitric acid with the absorption cell

windows. Dr. Goldman was able to provide us with spectra taken with a resolution of 0.004 cm

-

^ which was adequate to resolve many of the

transitions.

In addition to using the Fourier transform data, we have been able to

measure many portions of the spectrum with our tunable diode laser

spectrometer. Of special importance was the measurement and analysis of

the very crowded Q-branch regions for both the and 2vg bands. The

spectrum of a part of the v

5 Q-branch is shown in Fig. 2.3.

HN03 V5 Q-BRANCH

10 5 Kg=1

87S.9 879.0 879.1 879.2 379.3

WAVENUMBERS (cm- 1 )

Figure 2.3- The first-derivative spectrum of the Q-branch region of of

HNOo. The K a = J transitions are identified above the

j a. tt spectrum and the I< a

= J-1 transitions for K

c =2 are identified below the spectrum. All identified transitions

are A-type Q-branch transitions.

One result of this work that is of scientific interest, although it

has little interest for atmospheric studies, was the discovery that some

of the 2vg transitions are doublets with a splitting of about 50 MHz. The

vg vibration is an OH torsional mode and the splitting of the 2vg

transitions is due to a torsional tunneling of the hydrogen atom from one

side of the planar molecule to the equivalent opposite side. The

178
splitting gives us a rough idea of the height of the torsional barrier,

but the spectra show that there is a strong variation of the tunneling

splitting with rotational state.

The best way to study the torsional barrier would be to look at the

vg band at 450 cm

-1 and particularly at the hot band 3vg - 2vo which

should be easy to analyze since we already have the rotational constants

for the 2vg state and we know which levels are perturbed by the interaction with vc. Such a study of the vg band would also help in the analysis

of the hot band connected with the and 2vg bands. These hot band

transitions are obviously contributing to the atmospheric spectra and

Goldman has already attempted to take them into account. This work has

been supported in part by NASA-Langley Research Center.

4. Analysis of the Spectrum of H

2 02

(W. B. Olson)

Considerable progress has been made in the assignment of

hydrogen peroxide lines in the region of the bending combination around

2650 cm

-1

. About forty subbands have been assigned in the bending

combination, and about seventeen have been assigned in each of the two

overtone bands.

Torsional separations have been determined in all three bands to

about 0.001 cm

-1

. The center of the strongest band, the a-plus b-type

hybrid band v

2

+ vg, is at 2649.014 cm

-1 and the torsional separation is

11.712 cm

-1

, only slightly above the ground state value of 11.437 cm

-1

. For 2vg, a c-type band, with different torsional selection rules from the

combination band mentioned above, the band origin is at 2494.315 cm

-1

, but

the vibrational origin is actually higher by 11.437 cm

-1

, at 2506.252

cm

-1

. The upper state torsional splitting is very much larger than that in

the ground state, being 33-029 cm . For 2v2 , also a c-type band, the

vibrational origin is again 1 1 .437 cm

-

"

1 above the observed origin of

2754.555 cnT 1

, but the torsional splitting is here only 3-533 cm

-1

.

We now have available an asymmetric rotor fitting program, written by

Art Maki which will fit two vibrational states with an interaction

between them, by least squares. We will pool data with Prof. R. Hunt,

of Florida State University in order to fit the ground state perturbation.

This work was supported in part by NASA-Goddard Space Flight Center.

D. Spectroscopy of Hydrogen-Bonded Systems and Other Clusters

1. Vibrational Predissociation in the HF and DF Diners

(A. S. Pine and W. J. Lafferty)

We previously reported a mode-specific vibrational predissociation rate in the HF dimer based on different ro-vibrational transition

linewidths observed for the "free-H" stretching band, v1 , and the

"bound-H" stretching band, v2 , recorded with a high-resolution difference-frequency laser spectrometer. Predissociation occurs because the

excited H-F stretching bands are metastable since they have roughly twice

179
the frequency of the hydrogen bonding energy of the complex. Linewidths

in excess of the known Doppler and pressure-broadening contributions by

about 200 MHz (HWHM) were observed for v2 lines, corresponding to a lifetime of about 0.8 nsec. No excess widths were resolvable for

lines. Recently, DeLeon and Muenter, using subDoppler molecular beam

techniques, have established a lower limit of >30 nsec for the "free-H"

predissociation lifetime. This mode specificity indicates that direct

coupling to the hydrogen-bond stretching coordinate is the dominant

predissociation channel rather than a rapid intramolecular energy randomization as invoked by statistical models.

We have now examined the high resolution spectrum of the DF dimer for

clues to the predissociation mechanism provided by the isotopic variation.

Here the binding energy of the complex and the total mass of the fragments

are changed only marginally, but the D-F stretching frequencies are reduced by - /2 and the rotational constants of the fragments by - 2. In

the spectrum of the DF dimer, we observe no excess broadening for either

the v-| band "free-D" stretch or the \>2 band "bound-D" stretch. This

result is in clear distinction to "momentum gap" predissociation models

based on exponentially increased overlap with continuum translational

wavefunctions for the reduced energy above dissociation for the D-F

stretches. These models predict (DF)p predissociation rates, and hence

linewidths - 10^ greater than for (HF/ 2

. At present we have neither a quantitative nor a qualitative explanation for these results, but they are clearly important for understanding mode-selective chemistry and intramolecular energy transfer.

2. High Resolution Spectrum of the HC& Dimer

(N. Ohashi and A. S. Pine)

Rotational structure in the spectrum of the H-C2, stretching

bands of the HC2, dimer has been fully resolved using a tunable difference-frequency laser. The spectrum of a natural isotopic sample was recorded under thermal equilibrium conditions near the condensation point

(T - 130 K) of HC2, at low pressures (3~4 Torr) and long pathlengths (64-80

m) . A portion of the spectrum in the null gap of the monomer is shown in

Figure 2.4. Rotational assignments have been made for K" = 0. 1 and 2

3Ubbands of a perpendicular-type band centered near 2880 cm~ and a parallel-type band near 2857 cm

-1 sharing a^common vibrational ground

state. For the ground state of the H^CZ-H^cfl, dimer, the K" = 0 rotational constants, B= 1944.29(7)MHz, and the K" = 1 asymmetry splitting,

B-C = 19.5(3)MHz, yield a zero-point center-of-mass separation of the two

monomer constituents of 3 .797(1 ) A. Another difference band for the ^Cl

isotope has also been observed as well as a similar set of bands for the

mixed H^C2,-H-^C£ species. The spectra suggest an extremely rapid

tunneling motion between equivalent forms of nearly orthogonally oriented

monomer units. The single bandhead spike shown on the r QQ subbranch

contour for the mixed dimer species in Figure 2.4 is a manifestation of

tunneling-averaging of the two inequivalent equilibrium isotopic conformations expected.

180
HCI DIMER

h

35 ci—

35 ci U'

rQ0

Fig. 2.4. Transitions in the Q-branch region of the HC2, dimer.

3. Resolved Infrared Structure in the Ar-HC2. van der Waals Complex

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

We have resolved the rotational structure associated with the H-Ci

stretching vibration of the Ar-HCi van der Waals complex using a difference-frequency laser spectrometer. A portion of the spectrum exhibiting a regular series of lines attributed to the R-branch of the fundamental band

is shown in Figure 2.5. The spacing between the lines yields information

about the rotational constant, 3, and the size of the complex is in

excellent agreement with earlier molecular- be am electric-resonance studies

of the ground state. The convergence of the lines at higher wavenumber

yields the vibrational dependence of the 3 value implying a dominant

repulsive wall for the van der Waals potential. The sharp cut off of

intensity above 2888.7 cm

-1

is a unique feature indicative of rotational

dissociation as the rotational energy exceeds the binding energy and the

centrifugal barrier. The sharp, nearly Doppler-limited, linewidths are evidence for a relatively slow vibrational predissociation rate. Work is

proceeding to assign hot bands and combination bands in this region which

should reveal direct information about the low frequency van der Waals

bond bending and stretching motions.

181
.15

.12

.09

.06

.03

0.00

A 1 0 1

Fig. 2.5.

4. Metal Clusters

(A. Weber and M. Raab)

Using the apparatus completed during the previous year experiments were performed to obtain rotationally resolved spectra of sodium and

lithium trimers produced in a supersonic expansion. A two step photoionization method was used employing a tunable single frequency cw dye

laser (line width less than 1MHz) for excitation and a cw fixed frequency

ionizing laser. Signal detection was accomplished by a quadrupole mass spectrometer tuned to the mass peak of the appropriate trimer. Though

strong trimer signals were observed no resolved rotational structure was seen. It is not established whether this failure results from bound-free

transitions, pre-dissociation, or congestion of rotational transitions.

Further work on other systems is planned.

E. Fourier Transform Spectroscopy

1 . Fourier Transform Infrared Spectroscopy

(A. Weber, W. 3. Olson, and W. J. Lafferty)

The BOMEM DA3.002 high resolution Fourier transform spectrometer

delivered last year has, since the last report, undergone extensive tests.

As presently equipped the instrument spans the spectrum between 10 and

132
3000 cm -1 and additional optics for use in the visible to 4000 A have been

ordered. With a maximum optical path difference of 250 cm the maximum

unapodized resolution is 0.0025 cm

-

^

.

In addition to performing the usual tests with gases such as CO, 0CS,

HC2, and others, all of which have simple and uncrowded spectra, we decided

to do additional performance testing with cyanogen (C 2N2

). This is

because cyanogen has a fairly dense spectrum, which more closely

approximates the line density of spectra that are to be studied, and also

because the spectrum can be fully understood on the basis of the

Hamiltonian of a linear molecule (for a report on this work see section

E.4). Tests for the relative and absolute frequency accuracy showed that

in the region of 2100 cm

-1 the measured frequencies are offset from the

accepted values by +0.003 cm

-1 on the average, while the precision of the

measurements is about 10 times better. For high accuracy work the

frequency scale must be calibrated using independently determined

molecular absorption frequencies.

In addition to the work on cyanogen mentioned above, the instrument

has also been used to generate spectra of hydrazine and deuterosilane

Descriptions of these activities are given in sections E.5 and E.6.

The range of use of the instrument has been enlarged through the

design and construction of special transfer optical systems to permit its

employ in matrix isolation studies [see section 3-E2], and for use with a 20m multipass White cell. An additional transfer optical system is being

designed for use with a coolable 100m multipass White cell. An

independent study of the optical system of a White cell for use with

Fourier transform spectrometers was undertaken. These activities are reported in sections E2 and E3.

2. Transfer Optics Systems

(W. S. Olson)

Two transfer optics systems have been designed and constructed in

order to utilize the 30MEM FTS for specialized experiment studies. The

first of these is for interfacing to Marilyn Jacox's apparatus for

matrix isolation reflection spectroscopy (See Section 3. 22), and the secona

for coupling to a commercial long path multiple reflection absorption

cell. For both optical transfer systems, the maximum possible throughput

was determined from unavoidable geometrical and optical constraints, and

ray transfer matrices were used to design the system to obtain the maximum

throughput and maximum spectral wavelength coverage. In particular, we

are able to work in the very far infrared region with the limiting

throughput being that of the 30MEM instrument, and not that of the

external apparatus.

For the interface to Dr. Jacox’s apparatus the system is of a modified Czerny-Turner design to minimize aberrations. Furthermore the

cola reflection mirror of the matrix isolation apparatus i3 a field mirror

in order to conserve all of the available light. While it is true that

with longer wavelength optical perfection requirements are relaxed, ;ne

183

»
cannot be too sloppy about design if one wishes to optimize performance.

An aberrated image requires a larger than necessary detector area to

collect all of the radiation, and this larger detector area leads to an increase in the noise level, resulting in a loss of information. Also,

for a system that is to have wide spectral coverage, the optical

perfection should be based upon the shortest wavelength to be observed.

For the second system, the transfer optics to the multiple reflection

absorption cell, we have used relatively long focal length imaging

mirrors, which require several flat mirrors for beam folding. These flat

mirrors lead to some reflection loss, but the long focal length and low

off-axis angles at the reimaging mirrors lead to a good image at all foci,

and to a near ideal image diameter. One discovery about the utility of

ray transfer matrix design was made during the design process.

In order to conserve all available light the windows of a multiple

reflection absorption cell are usually field lenses. It is usual to assume that these lenses are at the front focal plane of the multiple pass

cell. In fact, they are not. We found that with a short base path cell one could get little or even no effect by using field lenses calculated

for the focal plane position at the position actually required by the real

geometry, i.e., at about an inch in front of the focal plane. It was

found that a proper field lens focal length can be found for use at the

actual required physical position, that leads to the same optical transfer

function at the front focal plane of the multiple pass cell, and at the

back mirrors of it, as for the ideal case.

3. White Cell Design

(W. B. Olson)

We have put together some new ideas on multi passing , throughput

of a multipass cell, and astigmatism to give a new system of design for

multipass absorption cells for use with circular or square apertures.

This treatment provides exact criteria for the geometry required to

minimize the volume of an absorption cell and astigmatism of the absorption cell (the major optical aberration limiting performance), given only

initial conditions required by physics. A paper giving the derivation and

results of this investigation has been published in Applied Optics.

Calculations show that it should be possible to reduce the volume by

a factor of two to four below most "good" existing multipass cells and

reduce the effect of astigmatism at the same time. A conservative

estimate is that one should be able to make a multipass cell with six rows

and seven columns of images on the front mirror with low reflectivity and

astigmatism losses. This cell would have 78 total transits. Somewhat

more optimistic would be the possibility of eight rows and nine columns of

images on the front mirror for 136 transits. With a 2.5 meter base path

one could match the throughput of the EOMEM FTS instrument (which is

relatively large) down to 55 cm

-1

(a worst case) with a calculated cell

transmission of 0.25, which is not unreasonably low.
4. Fourier Transform IR Spectrum of Cyanogen

(A. Weber, W. J. Laf f erty , and W. 5. Olson)

Work was started on the analysis of the spectrum of cyanogen,

C

2 N2 , as part of our testing program of the 30MEM Fourier transform

spectrometer (see section E.1). Since previous work done on cyanogen was

done at a resolution inferior than that available with the BOMEM instrument a new analysis of the infrared spectrum of C

2N2 was undertaken. Of

the five fundamental vibrations only two are infrared active, the

anti-symmetric stretching vibration (parallel band) and the doubly

degenerate anti-symmetric bending vibration (perpendicular band).

Spectra were obtained with gas pressures of 1 and 5 torr and an absorbing

path length of 20 cm. Figures 2.6 and 2.7 show the v^-parallel band

resulting from the coaddition of 25 interf erograms recorded in a total

time of 125 minutes. Band analyses have been performed for vg and the

associated identified hot band^. Twenty coadded interf erograms of the low

frequency perpendicular band at 235 cm

-1

were also recorded at full

resolution in 8 1/2 hrs. Since this band falls in a region where the

black body emission from the globar source is low and where the bolometer

detector is less sensitive, much more time was required for the recording

of this band than was needed for v^>. Both band systems have been analyzed

and preliminary results have been reported. Further work is in progress.

5. A Study of Inversion and Torsional Splittings in the Low

Frequency Torsional Band of Hydrazine

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

From a spectroscopic viewpoint hydrazine (NH2 -NH2 ) is a fascinating molecule. It has two large amplitude vibrations. One, a NH2 wagging motion, is analogous to the umbrella motion in NH^ but with a sufficiently small barrier to inversion to produce a large splitting in

the ground state energy levels. The other is a torsional vibration, with

a small enough barrier to internal rotation to permit tunneling through a trans barrier; a small but observable splitting from this source has also

been observed in the pure rotational spectrum. Only very recently has the

microwave spectrum been completely understood (1) with the aid of a theoretical formalism based cn double group theory developed in this

laboratory by Hougen (2)

The spectrum of the torsional band of N

2

Hij, which is centered at 376 cm

-1

, has been recorded on the MBS BOMEM Fourier transform spectrometer

This band was chosen for study for a number of reasons. The torsional

splitting in this band is much larger than in the ground state, and the

determination of this parameter will lead to a more accurate estimate of

the barrier to internal rotation. In addition, this band is the lowest

frequency vibration in N

2 H4 and is sufficiently isolated from other

vibrations so that the assignment will not be complicated by the presence

of accidental resonances. Also, this band fortunately is a 3-type band

185
Fig. 2.6. Overview of the v^-band of C 2N2 recorded at a resolution of

0.004 cm

-1

.

MAX= 3.01 E*!0

1 1 1 1 1 1 1 : 1 1 L 2I42.C 2142.4 2142.8 2143.2 2143.6 2144.0 2144.4 21448 2145.2 2145.6 2146.0

WAVENUMBER (cm"')

2.7. Detail of v^-band of C 2 N2 with partial assignment.

136
which very closely resembles a perpendicular band of a symmetric top

molecule. Such bands spread over a large frequency range and line

overlapping is reduced.

Line assignment has been started and, at this point, a number of

' ( J ) and series have been assigned. Because of the internal

motions of N

2 H14 , these transitions appear as well-resolved quartets or sextets, depending on whether K" is even or odd. Except for the K" = 2

lines, which are broadened, asymmetric rotor splitting is not observed.

Although the results are very preliminary, approximate values of both

the torsional and inversion splittings have been derived. The torsional

splitting is about 2050 MHz compared to 5.796 MHz in the ground state.

The inversion splitting shows a surprising vibrational dependence, being

roughly 7580 MHz, about half that of the ground state splitting (16040

MHz)

(1) Tsunekawa, T. Kojima, and J. T. Hougen, J. Mol. Spectrosc. 95_, 133

(1982).

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

Fourier Transform IR Spectra of Mono- and Tri-Deuterosilane

(R. W. Lovejoy and W. B. Olson)

Previously reported work (1,2) on SiH-D and SiHD^ was limited

to those vibration-rotation bands above 2000 cm

-

' because or spectrometer

considerations. With the availability of the 30MEM spectrometer, the

entire infrared spectrum of these molecules can now be studied at 10 times

the previously available resolution. Thus far a spectrum of the vj, V

5

, and vg bands of SiH^D has been obtained at 0.005 cm

-1 resolution by adding

54 interf erograms . The spectrum of V2 has also been obtained at a resolution of 0.004 cm

-1

. For all these spectra the pressure was 5 Torr and the

oathlength 20 cm. These four bands, in addition to the two Si-H stretching fundamentals studied previously, complete the spectra of ail of the

fundamentals of this molecule.

At present most subbands of vg have been assigned. A preliminary fit

places the band origin at 784.323 cm

-1

. In the parallel component of the

overtone 2 vg at 1557.258 cm

1

, ten subbands have been assigned, but the

perpendicular component has not yet been identified. The v? and bands

are strongly Coriolis coupled and will be difficult to assign at high J,

however the band origin of the former has been determined to be 912.993

cm

-1

. Six subbands of have been assigned. These assessments only

permit an estimate of ^Qq at about 952. 7. cm

-1

. The origin of j 0 , the Si-D

stretching frequency, is at 1593.967 cm

-1

.

(1) Olson. W. B. and Lovejoy, R.W., J. Mo. Spectrosc. 66 , 31 ^“3 30 (1977).

'"-137
(2) Fromer, C., Lovejoy, R. W., Sams, R. L. , and Olson, W.B., J. Mol.

Spectrosc. 89, 261-267 (1981).

7. Fourier Transform Raman Spectroscopy of Gases

(A. Weber and D. E. Jennings)

Spontaneous, incoherent Raman spectroscopy of gases at high

resolution has been successfully performed in the past using specially

built grating spectrographs or spectrometers. The techniques based on spectrographs have been fully developed (1,2) with the best resolution

attained being in the neighborhood of 0.05 cm

-1

. Very much higher

resolution (-0.001 cm

-1

) is achieved using coherent Raman scattering

techniques among which the CARS (Coherent Antistokes Raman Scattering) and

SRGS (Stimulated Raman Gain Spectroscopy) methods are the best known.

Since the limiting resolution is in principle governed by the Doppler

effect the use of high resolution Fourier transform spectrometers suggests

an alternative to the coherent laser techniques for obtaining high

resolution Raman spectra. Since there has been no report of the successful

use of Fourier transform (FT) spectrometers in Raman spectroscopy a pilot

study was undertaken in collaboration with J. W. Brault of the Kitt Peak

National Observatory.

The FT spectrometer of the McMath telescope at the Kitt Peak National

Observatory was used in the present work. Its instrumental, single pass,

unapodized resolution is 0.005 cm

-

"

1 . The detector was an FW-130 photomuliplier. A brief description of the instrument is given in (3)- The

Raman scattering apparatus consisted of a cell designed for a slit

spectrograph and used in the arrangement described in (4). The cell was placed both inside and outside the laser cavity, both laser polarizations

were used, and the exciting radiation was the (multimode) argon 4880 A

line. Pure rotational and rotation-vibrational spectra of H

2 , D2 , N2 , O2 . CO, C0 2

» C 2 H2, and c 2^2 were obtained. An example is shown in Figures 2.8

which shows the spectrum of Dg with a resolution of 0.10 cm~ obtained in

3 hours at a gas pressure of 600 Torr . The signal to noise ratio in the

pure rotation spectrum (top) is 150:1.

The present pilot study shows that, contrary to earlier expectations

(5), Raman spectra of gases can be obtained with good signal to noise

using Fourier transform spectrometers. Work is under way to improve the

technique

.

(1) A. Weber, "High Resolution Raman Studies of Gases" in The Raman

Effect , A. Anderson, Editor (M. Dekker, Inc., New York, 1973). Vol.

II, Chapter 9 , pp . 543-757.

(2) Raman Spectroscopy of Gases and Liquids , Topics in Current Physics,

Vol. II, A. Weber, Editor (Springer Verlag, Berlin-Heidelberg-New

York, 1979).

.38
S(2)

5 "

* a.

Ui>£ oy * ...

RAYLEIGH i 0.02

20 4*9 204*7 204*3

S(0)

2030* 20307 20303 20 IW 201*9 201*7 20074 2C072 20070

S(5) 5(6)

'9939 9937 -9933 9*4* 19944 9*42 973* 9734 .9732 9*29 9*27 -9*23

0( 2)

0 ( 1 )

0(3)

0(4)

0(0)

S(0)

|S(I)

S(2)

VviviV

i

! \ VfAj y

I7»7 :T503 17455 17.53 17.5* 7373 7370 73)5 73t0 7 705 7 700 710 7055 7057

Fig. 2.8. Raman spectrum of D

2 at 600 torr (Stokes side). Pure

rotational spectrum (top) and fundamental band (bottom). The

frequency scale is in units of cm

-1

; vQ

= 20486.648 cm

-1

.

(3) J. W. Brault, "A High Precision Fourier Spectrometer for the Visible” , in Proc . ESQ/CERN Conference on Auxiliary Instrumentation for

Large Telescopes , S. Laustsen and A. Reiz, Editors (Geneva, 1972).

(4) A. Weber and J. Schlupf, J. Opt. Soc. Am. 62 , 428 (1972).

(5) S. R. Schildkraut and T. B. Hirschfeld, "Fourier Transform Raman

Spectroscopy", in Laser Raman Gas Diagnostics , M. Lapp and C. M

Penney, Editors (Plenum Press, New York, 1974) pp . 379-388.

F. Development and Applications of Theories of Molecular Spectra.

1 . New Computer Programs for the Study of Asymmetric Rotor Spectra

(A. G. Maki)

As a follow-up to the changes we made last year on the program

that fits asymmetric rotor spectra, we have had to modify our program tha

calculates synthetic spectra in order to make it compatible with the

various theoretical equations used in the fitting program. To make tr. .j--

modifications many of the algorithms developed for the fitting program

were introduced into the structure of the spectral synthesis program.

Other modifications also were made in order to make the program mere

139
compatible with other computer systems. This program is essential to our studies of atmospheric molecules as it is used to calculate spectra from

preliminary constants in order to extend the assignments of spectral

details

.

One task of our studies of atmospheric molecules has been to

understand the spectrum of nitric acid between 850 cm

-1 and 920 cm

-1

. This region includes absorption features due to two bands, and 2V9,

which are coupled through a resonance. In order to more completely

understand the spectrum we have written a computer program which can be

used to fit these two interacting bands. In the coming year this program

will be used to help in the analysis of the spectrum of nitric acid and

will also be used to help produce an atlas of observed and calculated

transition frequencies and intensities for use by laboratories studying

atmospheric field measurements.

2 . Spin-Orbit Interaction in XY ^ Molecules

(J. T. Hougen)

The rotational energy levels of electronic states of open shell

XY molecules can be expected to exhibit effects due to: (i) tetrahedral

splittings, (ii) spin-orbit interaction, and (iii) the Jahn-Teller effect.

The effects of spin orbit interaction form the subject of the present

work, which is summarized below.

Rotational energy levels in vibronic ground states of 2 A,

and. 2 F electronic states of open shell XY^ molecules, as well as rotational line intensities for allowed transitions between such states,

are discussed, including the effects of spin-orbit interaction and

tetrahedral splittings. Jahn-Teller effects are assumed to be small, and

are only taken into account implicitly, through their contributions to

various parameters in the effective Hamiltonian. Qualitative information

is obtained by considering several limiting-case coupling schemes among

the electron spin angular momentum S, the electron orbital angular

momentum L, and the pure rotational angular momentum R. These limiting

cases are similar in spirit to Hand's coupling cases in diatomic

molecules, but differ sufficiently from the latter to make detailed

correspondences unhelpful. Quantitative information on rotational energy

levels and line intensities is obtained numerically by diagonalizing a Hamiltonian matrix set up in a oasis set characterized by uncoupled

molecule-fixed projections of S, L and the total angular momentum J, and

symmetrized so that all basis set functions belong to a definite species

in the subgroup D 2d of the true point group T d

. Hamiltonian matrix

elements are determined by ladder operator techniques. Three sample

spectra, corresponding to P( ^F2 )-s( 2

A-| ) ,d( 2 E)-p( 2 F

2

) and d( 2 F

2 )-p( 2 F

2

) were calculated. As one might expect, when the spin-orbit constant A is

set equal to zero, then both qualitative and quantitative aspects of the

rotational-electronic problem in open-shell XY^ molecules can be mapped

easily onco discussions of the rotation-vibration problem from the CH^

literature.

190
3 . Group Theoretical Treatment of Internal Rotation Tunneling in the HFDimer

(J. T. Hougen and N. Ohashi

)

The first phase of the work on the (HF) 2 molecule, which

consists primarily of group theoretical considerations, has now been

completed. It was finished in collaboration with Dr. N. Ohashi, a guest

worker from Kanazawa University in Japan, and the principal results are summarized below. Further theoretical work on the spectra of (HX) 2

species is envisaged, but the precise choice of research problem and

method of attack has not yet been decided upon.

The (HF) 2 dimer exhibits an internal rotation tunneling

process between two planar but nonlinear equilibrium configurations,

during which tunneling the roles of the hydrogen-bonded and the free

hydrogen atom are interchanged. This process can be represented

schematically with labeled atoms as H

1 FaH2Ft) t F aH-jFt,H2, and gives rise

to a permutation-inversion group G^ containing four operations. In the

present work the vibration-rotation-tunneling problem in (HF) 2 is treated

group theoretically in three ways: (i) by allowing tunneling only through

a trans planar C 2h intermediate, (ii) by allowing tunneling only through a cis planar C 2v intermediate, and (iii) by considering the trans and cis

tunneling processes both to occur, though not necessarily with the same probability. The molecular symmetry groups used for these treatments are:

(i) the point group C 2h , (ii) the point group C2V , and (iii) a double

group, which might be thought of as G^t = C2ftt = ^Zv^ ’ Nonplanar

tunneling paths are not considered, since the internal axis method (IAM)

coordinate system used here cannot easily be adapted to nonplanar internal

rotation motions in this molecule. Various details of energy level

diagrams, symmetry species for operators, selection rules for

spectroscopic transitions, and statistical weights are presented for the

(HF) 2 tunneling problem, as well as some speculation on the general

question of when point groups, permutation-inversion groups, or double

groups are preferable for treating large-amplitude vibrational motion

problems

4 . Summary of Group Theoretical Results for Microwave and Infrared

Studies of H2 02*

(J. T. Hougen)

Because of renewed interest in the microwave and infrared

spectra of the atmospherically important, chemically reactive species

H9O2, ana because of the study of the v2

+ v^ band of this molecule by W. 3 . Olson of our laboratories, it seemed useful to collect some results from

the theoretical literature on related molecules which are applicable with

only minor changes to H

2 C 2

. These results are summarized below.

Group theoretical treatments found in the literature for other

molecules, based on double groups of appropriate permutation-inversion

groups, were modified slightly and applied to H

2 02- This modification

permits a more unified view of many theoretical results derived in various

earlier studies of the molecule. Briefly, if no effects aue to internal

191
rotation tunneling were observed in the H

2

C>2 spectrum, the molecule could

be treated using the C 2 point group of its equilibrium geometry. If

effects due to internal rotation tunneling through only the trans barrier

are observed (as is presently the case), the molecule should be treated

using the C 2h point group of its trans planar conformation at the top of

the tunneling barrier. If effects due to internal rotation through both

the trans and cis barrier can be detected, it will be necessary to treat

the molecule using a double group C 2h t of the C 2h point group. Vibrational, torsional and rotational basis function, symmetry species, energy

level diagrams, selection rules for electric dipole transitions and

perturbations, etc. are derived here for the C 2h t double group, but these

results can easily be converted to those for the C 2h point group by

dropping all s and d subscripts. An empirically discovered successful

fitting procedure for the ground state rotational levels, reported in a treatment of microwave and millimeter wave measurements on H

2 o2 , can be

rationalized on the basis of the present theoretical results.

G. Microwave Spectra

The general aim of this effort is to develop experimental and

theoretical methods which can be used by other laboratories, and to make

use of our unique capabilities to provide specific data on substances of

unusual interest. The current emphasis is on a) providing laboratory rest

frequencies for radio telescope investigations of molecules in stellar and

interstellar environments, b) generating and investigating the spectra of

weakly bound molecules to determine their structure and molecular properties, and c) developing techniques which employ microwave spectroscopy as

a diagnostic tool to confirm the synthesis (existence) and structural

properties of unusual (and in some cases unstable) molecular species.

1. Millimeterwave Study of Peroxynitric Acid (H00N02

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

The peroxynitric acid molecule is thought to be a temporary

sink for odd nitrogen species in the complex stratospheric chemistry.

Until now no high resolution rotational or vibration-rotation spectrum has

been obtained on H00N02

, thus preventing any atmospheric measurement to

verify its presence and concentration. Earlier this year we began a microwave study of this species in order to characterize the rotational

spectrum and determine the molecular structure.

The spectrometer employed was a Stark modulated millimeterwave

spectrometer with a parallel plate absorption cell. Small samples of

H00N02 were produced by adding -0.2 gm 3FqN02 to about 1 ml of 50 percent

H 2 0?. This mixture was cooled to 0°C and was admitted directly into the

absorption cell. The vapor produced in the reaction was then pumped

slowly through the cell.

We expected HO0N02 to exhibit a typical a- and b-type asymmetric

rotor spectrum. However, we found it to be more complicated as illustrated in Fig. 2.9. For each a- or b-type transition we observed three

lines, a closely spaced doublet and a single line displaced to higher

192
frequency. We have assigned the a- and b-type doublet structure to

tunneling levels of the gauche conformation of HOONO2

. In addition c-type

transitions which occur between rotational levels of the two tunneling

states were assigned. These allowed the determination of the differences

in energy of the two states. The energy difference obtained is 1779.8

MHz. The singlet spectrum has been assigned to a low energy vibrationally

excited state, most likely the N-0-0 in-plane bend, which lies about 100

cm

-1 higher in energy than the ground state. The tunneling splitting for

the excited state transitions is too small to be resolved, however we have

assigned c-type transitions for the vibration state and have determined an energy difference of 475.6 MHz for the tunneling states.

Fig. 2.9. Spectral scan of the J=1 3 — 1 2 , K=10 a-type transition of

peroxynitric acid. The lower frequency doublet is assigned to

the ground state and the singlet to a low energy vibration.

- 153
The qualitative molecular structure is illustrated in Fig. 2.10. All

the 0 and N atoms lie in the a,b-plane and the proton tunnels between a position above or below the a,b-plane, giving rise to the doubling of the

rotational transitions. Studies of enriched isotopic substituted forms

will be needed to obtain a precise structure.

Fig. 2.10. Approximate structure of H00NCU.

2. Millimeterwave Study of Hydrogen Bonded H

2 C0-HF; The Deuterated

Isotopes

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

Recently Baiocchi and Klemperer (1) have reported the microwave

spectrum of K2 C0-HF from a molecular beam electric resonance study. Last

year we have examined this system with a conventional waveguide Starkmodulated absorption cell under thermal equilibrium conditions and

obtained further information about higher energy states which are not

accessible in a cooled beam experiment. With the aid of the molecular

beam data we have identified numerous a-type R-branch transitions up to

J=1 3 and K=10. A combined analysis of the molecular beam spectrum and the

present data requires inclusion of several P , P° and p”1

*3 terms in the

Watson Hamiltonian in order to fit the higher K transitions.

A similar study of the isotopic forms H

2 C0**DF and DqC0**HF, which

were not studied in the beam experiment, has been carried out. After

correcting the rotational constants for the large changes due to the 0**HF

stretching and bending vibrations, we have derived the structure parameters given below: The geometry of the monomer units (H2 C0 and HF) were fixed at the values reported in the literature and only the 0**H distance

and C0**HF angle were fitted to the corrected moments cf inertia. The

values obtained are:

19^
r (0 • *H) = 1 .780±. 035 A

< CO • *F = 104.2° + 2.5°

These may be compared to the values reported by Baiocchi and

Klemperer derived from only ground state H2C0**HF: r(0**H) = 2.794 A

<C0 •

• F - 109.5°

(1) F. A. Baiocohi and W. Klemperer, J. Chem. Phys. 78, 3509 (1983).

3. Pulsed Beam Fabry-Perot Cavity Fourier Transform Microwave

Spectrometry

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

During the past year much of our efforts have been directed at

completing the construction of a Fourier Transform Microwave Spectrometer

similiar to the original design by Balle and Flygare (1). Our spectrometer

consists of 36 cm diameter spherical mirrors for the Fabry-Perot cavity

which provide a loaded Q = 1-3x10^ over the range of 4-18GHZ. The cavity

is placed in a cylindrical vacuum chamber 56 cm in diameter and 1m long.

High speed pumping i3 attained with an 8000 1/s diffusion pump packed by a 90 cfm roughing pump. The microwave pulse is coupled into and out of the

cavity with co-axial loop antennae. All microwave components are of the

co-axial type, thus eliminating the need for waveguide to co-axial

transitions

Initial tests of the spectrometer are now in progress. A time domain

trace of the J = 1->-0 emission signal of the 4 percent 0CS in Ar carrier

through a 50 in diameter nozzle (continuous beam) is shown in Fig. 2.11.

The spectrum was produced from 100 microwave pulses of -0.5 jjs duration

and digitized at 0.5 us per point for each pulse. The Fourier transformed

power spectrum of the data in Fig. 2.11 is shown in Fig. 2.12. It is

typical of this type of spectrometer that each transition appears as a doublet due to Doppler effects of an unskimmed beam. The true transition

frequency occurs at the minimum between the two peaks. Due to slow

delivery from the vendor we have not yet used the pulsed nozzle beam

source

The major direction of the studies to be carried out with this

instrument will be measurement of weakly bound species, with major

emphasis on hydrogen bonded molecular species. We are therefore also

carrying out tests of the instrument on the van der Waals molecule Ar**HCi

with the same nozzle employed for the 0CS measurement. The Ar h'C2 complex

was first studied by Novick et al . (2,3). It has a linear equilibrium

geometry dipole moment of 0.8 Debye and dissociation energy of about 180 cm

-

(-0.5 kcal/mole). We have examined the hyperfine structure of the

J=2~3 transition near 10068 MHz.

The frequency agreement of both the 0CS and Ar HC2. measurements with

previous molecular beam electric resonance measurements is 2kHz.

(1) T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52, 33 (1981).

195
PAUSE — <cr> to continue

3.00 r

» i.00

0.00

-i.00

0.00 0.20 0.40 0.60 0.80 1.00

Time-uaec- Ch« .2us E-2

1.20

Fig, 2.11. Time domain spectrum of the J=10 transition of 1 percent OCS

in Argon carrier gas.

Fig, 2.12. Fourier transform of data in Fig. 2.12, the J=1-0 line of OCS,

196
(2) S. E. Novick, P. Davies, S. J. Harris and W. Klemperer, J. Chem.

Phys. 59, 2273 (1973).

(3) S. E. Novick, K. C. Janda, S. L. Holmgren, M. Waldman and W.

Klemperer, J. Chem. Phys. 65_, 1 114 (1976).

4. Laboratory Microwave Studies

Several manuscripts describing recently completed experiments

on H00N02 , H2 C0**HF and CH 2 F0H will be finished in FY 35. Further

microwave studies of isotopic forms of HOONO? will be started and measurements of its vibrational spectrum in the far-IR region will be attempted.

Most of our laboratory effort will be directed at studies of new hydrogen bonded complexes with the Fourier transform spectrometer after

completing tests with previously analyzed complexes.

H. Molecular Spectra Data Center

(F. J. Lovas , A. G. Maki , W. J. Lafferty, W. B. Olson, G. Rotter, R. D. Suenram)

The objective of the Molecular Spectra Data Center is to collect,

critically evaluate, and disseminate molecular spectra frequencies and

constants obtained from molecular spectra in the microwave and infrared

spectral regions. Publications of critically evaluated spectral data are designed to facilitate applications in kinetics, chemical analysis,

atmospheric constituent measurement, radio astronomy and spectroscopic

research. Publications of critically evaluated spectral data of both broad

and specialized interests are produced in this project.

Since this project was initiated, two major compilations of

critically evaluated microwave spectra data have been published. These

are parts I and II of the Microwave Spectral Tables on diatomic and

triatomic molecules, respectively. Other specialized critical reviews

have been completed for the series entitled "Microwave Spectra of

Molecules of Astrophysical Interest." To date, 21 papers on 26 of the

interstellar molecules have been published; approximately half cf these

investigations were carried out at NBS and the remainder completed 'under

the guidance of the project. Gas-phase molecular structures have been

critically evaluated. A list cf recommended rest frequencies for observed

interstellar molecular lines has been prepared.

The development of frequency standards in the infrared spectral

region involves both precise new laboratory measurements and critical

analysis of both microwave and infrared spectra. High resolution infrared

spectroscopy is now employed in diverse technological areas, sucn as,

industrial process monitoring, stack emission measurements, atmospheric

constituent measurements, combustion studies and gas dynamics.

19T"
1. Revision of Recommended Rest Frequency List.

In 1979 we published a paper in the Astrophysical Journal

Supplement Series entitled, "Recommended Rest Frequencies for Observed

Interstellar Molecular Transitions." This was received and used with

enthusiasm by radio astronomers. During the past five years a dozen new species and numerous new transitions (>700) of previously known species

have been reported. Since this nearly tripled the previous information, an update of this information was deemed appropriate. This effort began with

the selection of recommended frequencies and coding all the pertinent information on the new transitions. The data in the prior publication was checked for revisions, coded and added to this new data file during the

current year. An extensive reference file of both laboratory measurements

and astronomical reports has also been coded for eventual computer

typesetting

.

The final editing of these files is now in progress. In order to

insure both complete coverage and accuracy, we have sent preliminary

copies of the list to all the major radio astronomy groups and facilities

to solicit unpublished observations to be incorporated.

2. IR Calibration Atlas 500 to 720 cm

-1

.

This project is part of our effort to develop infrared frequency standards compilations for use in calibrating tunable IR laser

spectrometers. Seme very accurate data on 0CS, N

2 0, and C0 2 have been

obtained from literature publications and via new measurements in the

longer wavelength region. Art Maki spent several weeks at the University

of Oulu in Finland with Dr. Kauppinen to obtain some of the new measurements on these species with the long wavelength Fourier transform spectrometer at Oulu. The analysis of both these IR data and accurate

microwave measurements is underway and progressing well.

3. Microwave Spectral Tables-Hydrocarbons

The publication of a third part of the microwave spectral

tables is planned. The fitting of the species which can be classified as normal asymmetric rotors is progressing well and should be completed by

the end of the current fiscal year. We have also written a fitting

program to analyze symmetric top species of which there are several in

this group. About 15-20 hydrocarbon species exhibit internal rotation

splittings and thus, are more difficult to analyze. We are currently

implementing fitting codes to treat asymmetric rotors with internal

rotation. Approximately 70 hydrocarbon species will be included in the

review

.

I. Future Plans

As the foregoing activity report of the High Resolution Spectroscopy

Group indicates it is engaged in providing frequency standards and

critically evaluated reference spectroscopic data, and in transferring

forefront spectroscopic expertise to the user community. It also is

198
engaged in advancing spectroscopic measurement methods and techniques, in

developing models, theoretical concepts and methods for predicting

molecular properties and interactions and in conducting research to

understand and model fundamental chemical processes. We will continue in

these endeavors during the coming years.

Specifically, the substance of the group's activities is in the areas

of development of infrared frequency standards for the calibration of

laser spectrometers, the critical evaluation of spectroscopic data,

spectroscopy of atmospheric trace gases, spectroscopy of weakly bound

molecular complexes, and development of experimental techniques and

instrumentation.

The development of frequency standards suitable for the calibration

of infrared laser spectrometers and the critical evaluation of spectral

data will continue to play a significant role. The needs for infrared

frequency standards are well known and the philosophy of our frequency

standards development program has been repeatedly stated. Briefly, this

is to furnish to the user community accurate absorption standards based on

readily available and easy to handle molecular gases so as to obviate on

the part of the user any specialized instrumentation or procedures for the

calibration of his spectrometer . Such standards guarantee their most

widespread use. A very detailed assessment of the present state of

affairs and plans for the immediate future has been given in section B.3

of this report.

The Molecular Spectra Data Center is part of the National Standards

Reference Data System and operates under the aegis of the Bureau's

separate Office of Standard Reference Data. The activities of the Data

Center are focussed on microwave spectral data and infrared frequency

standards compilations.

A manuscript on the revised "Recommended Rest Frequencies for

Observed Interstellar Molecular Transitions in the Microwave Region" will

be completed near the beginning of FY 85. Efforts to incorporate new

observations will continue up to production of the publication in order to

keep the information content as current as possible.

The final analyses and manuscript of the "Microwave Spectral Tables.

Part III Hydrocarbons" will be completed in early FY 85. In parallel with

this publication a magnetic tape of the measured and calculated spectra of

the most abundant isotopic form will be produced for distribution to

interested users on request. This is part of the long range plan to

develop a microwave spectral line atlas, so care is being taken to keep

all the coding compatible with the previous files developed. It should be

noted that we continue to maintain and update the computer files on the

groups of species previously published, i.e., diatomics and triatcmics.

The infrared frequency standards compilation will benefit frctn new microwave measurements that have just become available. These will be

used to improve seme of the tables. We plan to include in the compi^ati:n

figures showing the spectrum in each frequency interval covered by the

199
tables. Many workers feel that such figures will make it much easier to use the tables. Because of difficulties encountered in obtaining real

spectra suitable for this purpose, the atlas will probably include figures

that are generated from the tables by a computer plotter. Since the

tables will contain all of the strong lines and most of the weak lines,

such a synthetic spectrum will closely match a real spectrum. Of course,

we have real spectra which will be used to verify the accuracy of the

synthetic spectrum. One advantage of plotting a synthetic spectrum is the

flexibility in specifying several effective spectral intensities (or

pressure pathlength products) in order to illustrate the wide range of

intensities that we shall have in the tables. Completion of the manuscript "IR Calibration Atlas from 500-720 cm

-1 " will occur in FY 85.

Our activities in the area of trace atmospheric species are in

support of other agency programs, notably those of NASA and CMA. The

thrust of this work is to provide spectroscopic data for those trace

atmospheric species that play a significant role in controlling and

modifying the properties of the earth's atmosphere. The needs of these

programs are of long duration and we expect not only to continue to

provide accurate data and expertise to than but also hope to somewhat

expand our efforts through additional other agency contracts.

Two years ago the group initiated a new phase of research in the high

resolution spectroscopy of weakly bound molecular complexes. Our initial

study on the hydrogen bonded HF-dimer molecule was the first such work

ever reported in which a rotation-vibration band of a hydrogen bonded

complex was fully resolved. This work has been followed by the resolution

of the HCJl-dimer spectrum and of the spectrum of the Argon-HCil van der

Waals complex as reported in sections D.2 and 3. The availability of the

high resolution Fourier transform spectrometer and the pulsed molecular

beam Fa’ory-Perot microwave spectrometer permits us to expand our activities in the study of weakly bound complexes to the far infrared and

microwave region of the spectrum. Based on this total spectroscopic

capability, which spans the region from 4000 cm

-

' to 10 cm

-1

, we will

continue in the coming years to work on the spectra of hydrogen-bonded and

van der Waals molecular complexes. As part of these efforts we will

continue to improve and advance our instrumental capabilities and techniques .

200
3. LASER PHOTOCHEMISTRY GROUP

M. P. Casassa, R. R. Cavanagh, L. B. Elwell, E. J. Heilweil,

M. E. Jacox, D. S. Xing, D. A. Mantell , J. Perry, J. C. Stephenson,

and A. Woodward

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.

We have been fortunate in having five post-doctoral fellows (Casassa,

Heilweil, Mantell, Perry, Woodward) join the group in the last few months.

Because of the difficulty of the picosecond studies of energy transfer,

and the ultra high vacuum thermal desorption experiments, the new postdoctoral scientists are particularly important to the success of this

research

B. Picosecond Studies of Molecular Energy Transfer

(M. P. Casassa, R. R. Cavanagh, L. B. Elwell, E. J. Heilweil,

J. Perry, J. C. Stephenson, A. Woodward)

The major effort in the picosecond chemistry 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.

We used an infrared pump/probe method to do time-resolved measurements of the vibrational energy relaxation race (T-j” 1 ) for hydroxyl groups

chemisorbed on a silica (SiC 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, there have been no previous

vibrational energy transfer measurements 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) suspended in iiauid^CCI^ at room temperature. Infrared absorption in the 3000-^000 cm”' region is due to

isolated surface hydroxyl groups (v=0-»-v=1 at 3691 cm”

1 ), and physisorted

molecular water (=3^00 cm” 1 ).

The picosecond apparatus used in this investigation was based on a modelocked Nd T 3 : YAG laser. Single 1.06 um pulses of -35 mJ and 10 Hz

repetition rate pumped a chain of three LiNbO- crystals to parametrically

generate tunable infrared picosecond radiation, which had a spectral

bandwidth of 10 cm” 1 FWHM, duration of 1 4 ps FWHM, and delivered about

200 pJ of pump energy to the sample.

201
The surface hydroxyl v=0-»-1 transition was excited with a strong 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; j£n(T/To) is proportional to OH (v= 1 ) population.

Recovery of sample transmission was monitored by a weak, time-delayed

probe pulse (-5 pJ) derived from the saturating pulse and a variable

optical delay. At each delay (t D ), the probe pulse transmission was measured to 0.5 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.

Fig. 3-1. Vibrational population relaxation (v=1) of surface hydroxyl

groups on silica dispersed in CC 1I 4 at 291 K. Typical data for

(a) dried sample and (b) sample containing adsorbed water

exhibit similar lifetimes of 170 ps from least squares fits.

202
The surface hydroxyl v=1 population for silica in decayed

exponentially with a characteristic time constant T-j = 150±20 ps. For

silica suspended in the solvent CF2 Br2, T-| = 120 ps, and for the solvent

CH 2

C2. 2 the surface hydroxyl population decayed with T-j = 100 ps.

A pressed disc (same surface area but no solvent) of the material

gave a relaxation time T 1 = 220 ps for the OH vibration (3750 cm

-1

) at the

surface-vacuum interface. This same T-j was observed for the sample at room temperature and cooled to T = -185C.

A sample was prepared in which 70 percent of the surface OH groups

were deuterated (converted to 0D). T-| for the 0H(v = 1) species in this

deuterated sample suspended in CC£]^ was the same as for the ordinary

non-deuterated sample. A sample with substantial coadsorbed H

2

0 also gave

this same T-j .

Although we have not yet directly determined where the OH vibrational

energy goes, we may draw some conclusions from the preceding observations.

Most of the 3700 cm

-1 of energy must be deposited in the Si02 lattice. If

this energy were to be transferred to the internal modes of the solvents,

a five quantum number change (Av£5) would be required in CCJl^, av^ 4 in

CF 2 Br 2 , or Av>2 in CH 2 C22

. Intermolecular VET processes requiring large

Av are expected to be improbable. However, collisions with the solvent

molecules may induce energy transfer from the 0H(v=1) to the Si02

substrate (intramolecular energy transfer). The presence (or absence, in

the deuterated sample) of neighboring surface OK groups or coadsorbed H

2 0

is apparently not important in determining T-j . The absence of a temperature dependence of T-j at the surface-vacuum interface is consistent with

theories for an intramolecular vibrational relaxation T-| process in an isolated molecule.

Pump/probe experiments on the bulk OH absorption feature of a 1mm

thick quartz window produced a similar degree of saturation and a 150 ps

lifetime. This result is consistent with the idea that the 3700 cm

-1

of

0H(v=1) energy goes to the Si02 lattice for the surface hydroxyls.

Spectral linewidths observed in surface vibrational spectroscopy have

sometimes been interpreted as T-j uncertainty broadening. However, the

width of the OH absorption feature must arise from site inhomogeneities or dephasing processes, since the linewidth corresponding to T-j for surface

OH of -0.03 cm~ ' is 2000 times smaller than observed.

The long T-j for these surface vibrations may have chemical

importance. Vibrational excitation of certain bonds greatly increases the

rates of specific chemical reactions, and apparently vibrational lifetimes

can be comparable to or longer than the timescales for an adsorbate to

reorient, hop to an adjacent site, or diffuse many Angstroms. The energy

released in a reaction, rather than dissipating instantaneously into the

substrate, may remain available for energizing bonds of other reactants.

•203

'
There are many experiments underway to elucidate the pathways of

energy transfer in molecules on surfaces, and to identify the important

physical parameters of the substrate and adsorbate which determine the

rates of YET. On theoretical grounds, and by analogy to gas phase

results, we expect several factors will be important, including the match

between the frequencies of the substrate and adsorbate, the distance from

the surface and the orientation of the excited bond, its permanent and

transition dipole moment, and the presence of coadsorbed molecules. The

published IR spectra of a number of high area metal oxides suggest that we may do similar experiments on substrates such as Ti02 , ZnO, AlpOo, etc.

Likewise, many adsorbates such as Si-NH2 (vjjh = 3525, 3447 cm

-

') or Si-O-CH^ (vqh = 3000, 2958, 2853 cm

-1

) have frequencies in the v ^ 2800 cm range accessible to our present apparatus.

The VET rate for molecules on the surface of conducting metals is a subject of much current debate. The mechanism of relaxation via electron-hole pair formation has been predicted to be effective, and the

rather large spectroscopic linewidths of vibrational transitions of

molecules on metals have been interpreted as T^ broadening. We will try

to measure T-] for molecules on small metal particles in several systems

where the known IR and Raman spectroscopy suggests that our approach may be

successful

.

In other experiments, the relaxation times of HC2, (v=1) molecules

dilute in room temperature CCI 4 and CCI^F solvents were determined. As in

the surface experiments, the vibrations were excited by tunable IR pulses.

However, the time dependent population in the HC2. (v=1) was monitored by

spontaneous anti-Stokes Raman scattering (SPARS) using frequency doubled

Nd-Yag probe pulses. The IR/SPARS method is preferable in several ways to

the IR/IR saturation method: for instance, it permits study of samples

where the v=1-»-v=2 excited state absorptions would make the one-color IR/IR

experiment difficult to interpret. For HC2, (v=1) in CCl^, T^ was measured

as 4.7±.4 ns; for CCl^F, T-j = 6 . 4± . 9 ns.

These results may be compared to the relaxation of the surface OH

( v = 1 ) groups in CCI4 solvent. The HC2, (v=1) transition in CC 2,4 is at 2840

cm

-1

, while the OH (v=1) transition in is at 3690 cm

-1

. We expect

the rate of VET from surface-bound OH ( v =1 ) to CCS,^ to be even slower than

for HC2, ( v = 1 ) to CCI4 , since the OH quantum is significantly larger. ( AVS5

in CC2,i| is required to accept 3690 cm

-1

; AV = 4 to accept 2840 cm ').

The deactivation rate T

^ -1

of HC2, (v = 1) by CC£~F in the liquid is

just what one would obtain by the simplest possible'extrapolation of the

dilute gas phase bimolecular deactivation rate constant k(cm^/molec*sec)

to the liquid phase number density p (molec/cm^) via T-|

-1 (liquid) = k

(gas) *p(liquid) . The apparent success of this "isolated binary collision"

(IBC) model to explain liquid phase data has been observed previously in

studies of a few other diatomic molecules (hydrogen, HX) which have high

frequency vibrations. Nevertheless , the use of gas phase rate constants

to predict rates in condensed phases is difficult to justify or to

understand. We hope to do several additional experiments in liquids to

determine the limits of applicability of the IBC approximation.

204
This research has been supported in part by the Air Force Office of

Scientific Research.

C. Desorption of Molecules from Surfaces

(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 sputtering. Auger electron

analyzer), computer controlled data acquisition and sub-doppler (150 MHz)

laser resolution is being applied to the study of the thermal desorption

of molecules chemisorbed to well characterized surfaces. Results for NO

desorbing from clean Ru(001) and pre-oxidized Ru, including rotational-,

spin-, and lambda doublet state population distributions, molecular orientation anisotropy, and Doppler profiles have been measured and published.

Work is currently underway on NO/P t (111). Our choice of platinum was based, in part, on the following two considerations. First, NO beam

scattering experiments have been performed on Pt (117) and Pt foils giving

an indication for T pot (trapping/desorbing NO) = T sur f ace . Second, unlike

the NO/Ru system, there is little or no dissociation of the surface bound

NO during the desorption event. In the NO thermal desorption spectrum

from Pt (111) there are three prominent desorption features. We have

sufficient signal-to-noise to compare the energy distribution in the

desorbed NO corresponding to each distinct desorption feature. The

NO/P t (111) thermal desorption spectra have been shown to exhibit dramatic

effects due to changes in coverage and the presence of co-adsorbates such

as CO. We shall follow the dynamics over a factor of 10 change in

coverage and in the presence of co-adsorbed CO or oxygen.

Thermal desorption probes processes occurring under "equilibrium"

conditions. Thi3 is to say that a heating rate of 1 0K/sec is much slower

than energy transfer processes. We hope in the coming year to study

direct desorption following very rapid laser heating, where the heating

rate may exceed 10 12K/sec and compete with other important energy exchange

processes

This work is supported in part by the U.S. Department of Energy.

D. Photochemistry in Molecular Beams

(L. 3. Elwell, D. S. King and J. C. Stephenson)

These experiments represent, in part, an outgrowth of work supported

by the U.S. Army Research Office and the U.S. Office of Naval Research to

characterize the decomposition of energetic materials. The experiments

resulted in the total characterization of energy disposal in tne NO

product fragment formed by infrared multiphotcn dissociation of CH-ONC

both gas bulb and molecular beam environments.

For methyl nitrite, CH^CNO, dissociated in a pulsed molecular beam

under collisionless conditions by CO? laser pulses of well defined

intensity, the only reaction was CH^CNO+CHjOtX^) + NO(X-n) , i.e. both

products being in the ground electronic state. In a typical experiment,
the CC>2 photolysis pulse was a 50ns temporal square wave of intensity

1=400 MW/cm2 . Ultraviolet laser excited fluorescence of the gamma (A*- X)

bands of the NO fragment was carried out at 0.2 cm

-1 resolution to

determine the vibrational, rotational, spin, and lambda doublet-state

distributions of the nascent NO. The ratio of NO (v=1) to NO (v=0)

corresponds to a vibrational temperature T

y = 460 ± 50 K. The rotational

states of NO must be considered simultaneously with the two electronic

spin-orbit states, 2

II-|/ 2

( F-j ) and 2

IIo/ 2 (F2 ), which are separated by A=123 cm

-1

. The low lying (v=0) rotational states (ERot <i000 cm

-1

) are populated in Boltzmann-like distributions, characterized by rotational

temperatures T Rot ( F-j ) = 399 ± 9 K and TRot (F2 ) = 529 ± 97 K (See Figure

3.2). The rotations of the NO (v=1 ) level are a little colder than for

the v=0 : T Ro^. (F-j , v=1 ) = 220 K vs. TRot

(F-j , v=0) = 399 K. The ratio of

total population in all F-| nuclear rotation states to population in F 2 is

2.6, corresponding to a spin-orbit temperature of T so = 180 K. These

values for product energy do not depend strongly on the intensity of the

500 IOOO 1500 2000

INTERNAL ENERGY (cm 1 )

Fig. 3-2. Distribution of rotational sates in the NO product formed by

C0 2 laser induced dissociation of CH^ONO under collisionless

conditions in a molecular beam. Open circles, 2n-|/ 2 (J) states:

closed circles, 2

II-3 / 2 (J) states.

206
CO 2 laser. Unlike the spin-orbit states, the lambda doublet states of NO

are formed equally (classically, the two lambda doublets correspond to the

electron density in the singly occupied pi orbital localized in, or out,

of the plane of molecular rotation) . Rotation of the probe laser polarization indicated an isotopic distribution of NO J vectors.

Experiments were also performed, in the molecular beam, at much

higher resolution (-0.01 cm

-1

) to determine the Doppler widths, and hence

the kinetic energy of NO initially formed in well defined internal states.

The observed spectral widths were Gaussian over two e-foldings, and when

converted to center of mass translational temperatures correspond to T cm = 330 ± 50 K, the same for different rotational states with energy in the

range 0<E Rot < 1300 cm

-1

.

These results are consistent with the usual RRKM picture of a simple

bond-breaking reaction from states of internal excitation for which the

laser excitation rate equals the microscopic rate of unimolecular

reaction. We estimate a total of only 4 kcal of energy (internal and

kinetic) to be in the CH^O and NO photofragments. This is the average

energy, in excess of the enthalpy required to break the CH^O-NO bond, of

the molecules which dissociated. A calculation consistent with the

experimental Arrhenius A factor suggests an RRKM lifetime of 0.5 to 1.0

nsec for CH^ONO molecules with this excess energy. The observed statistical distribution of lambda doublets is expected for a (free rotor)

transition state in which there is no distinction between cis and trans

CH^ONQ. The observed spatially isotropic J distribution is expected for

this process where the excitation and dissociation times greatly exceed

molecular rotational times. These results contrast with many recent

observations of NO in beam scattering and ultraviolet (prompt) photolysis

experiments, where polarization and non-statistical state distributions

have been observed.

These results were for photolysis of CH^ONO in a seeded beam where

the initial internal energy of the reactant molecules was very low. The

rotational temperature in the beam was £ 6 K, and the CH-ONO was also

significantly cooled vibrationally . However, despite this cooling, the

product energy distribution was the same for molecules photolyzed in the

beam and for CH^ONO photolyzed in a room temperature bulb, where the

initial reactant rotational and vibrational energy was significantly

nigner. This is again in contrast to many one-photon direct dissociations

where product distributions reflect initial reactant distributions. Cur

results suggest that the broad Bcitzmann-like product energy distributions

we have observed in many IRMPD reactions are not related to the initial

thermal energy of the reactant.

During the coming year this apparatus, incorporating 0.01 cm

- laser

resolution and molecular beam source, will be employed to explore tne

infrared photodisscciation dynamics of low lying vibrational levels of van aer Waais molecules such as ethylene-nitric oxide.

207
E. Matrix Isolation Spectroscopy

(M. E. Jacox)

1. Fluorine-Atom Reactions

Studies of the vibrational spectra of the primary products of

the reaction of F atoms with small molecules, trapped in solid argon at 14

K, have continued, yielding not only the spectra of reaction intermediates

but also previously inaccessible information on the mechanisms of these

chemical reactions. A review paper entitled "Spectroscopy and Photochemistry of Free Radicals Formed by the Reaction of F Atoms with Small

Molecules" and summarizing the results of these and other recent F-atom

reaction studies is being prepared for submission to Reviews of Chemical

Intermediates

.

A series of experiments on the reaction of F atoms with CH 2 f2 and

HCF

3 has been completed. Prominent infrared absorptions of HF, HCF2 , and

of the HCF2 ***HF hydrogen-bonded complex resulted in the CH 2 F2

+ F

studies. The absorptions of DCF 2 «**DF grew substantially in intensity

when CD 2 F2

+ F samples were exposed to radiation of wavelength shorter

than 300 nm, but the spectra characteristic of CH 2 F2

+ F samples were unchanged on sample irradiation. This observation suggests the formation

of a weak CH2 F2 ***F complex which decomposes by a tunneling mechanism.

Moderate yields of CF^ and HF resulted in the HCF3

+ F experiments, but

there was very little evidence for the reaction of F atoms with DCF^. Such

isotopic selectivity is consistent with the occurrence of tunneling

through a small potential barrier to the reaction. A paper presenting the

results of these experiments is in preparation.

A detailed series of experiments on the F + CHoCN system, including

isotopic substitution, concentration dependence, and photodecomposition

threshold studies, has been completed. Reaction has been found to occur much more rapidly than had been believed on the basis of the detection of

the chemiluminescence of the HF product. Both H-atom abstraction and

F-atom addition at the central carbon atom have been found to be important. Evidence for the addition of the F atom at the terminal nitrogen

atom in the primary reaction was inconclusive, although the infrared

spectrum of a product which possesses an NF bond was identified. Although

infrared absorptions of isolated HF implied the stabilization of H

2 CCN,

the infrared absorptions of this radical product could not be positively

assigned. The diffusion of F atoms through the solid also leads to the

formation of the hydrogen-bonded complex of HF with H

2 CCN. The behavior

of the infrared absorptions of this hydrogen-bonded HF demonstrated that

H

2 CCN is photolytically stable out to 250 nm and that the strength of the

hydrogen bond is similar to that of the CH

3

CN***HF complex. The infrared

absorptions of the addition product, CH

3 CF=N, were sufficiently prominent

to permit an almost complete vibrational assignment for this species,

which was found to have a CF bond of typical strength and to have a photodecomposition threshold near 280 nm. A manuscript presenting these

results is in preparation.

208
During the coming year, it is planned to complete a series of

experiments on the reaction of F atoms with CH^I, in which evidence has

been obtained for both the stabilization of CHolF and of H2 CI + HF.

Another important system to be studied is the F + HCOOH reaction, in which

infrared spectroscopic evidence may be obtained for the H-CO2

intermediates, as well as for cis- and trans-HOCO, previously identified

in this laboratory in studies of the reaction of OH with a CO matrix.

Other systems which will be of particular interest during the coming year

are F + NH^ and F + SiHij.

With the help of Thomas Buckley, Chemical Thermodynamics Division,

hardware interfacing of a microcomputer to the Ebert scanning monochromator used for ultraviolet absorption spectral observations has been

completed, and software development is in progress. Priority will be

given to studies of the electronic spectra of the products of the reaction

of F atoms with CH

3 N02 , CH 3 CHO, CH 3 CN, and CH 3 SH.

2. Excited Rare Gas Atom Reactions

Preliminary studies have suggested that a small concentration

of CH 2 may be stabilized when the products of collisional interaction

between excited argon atoms and methyl iodide are frozen in an argon

matrix. The availability of partially deuterium-substituted methyl iodide

may permit the first infrared spectroscopic identification of CHD. Because

of the weakness of the product absorptions, the high sensitivity of the

Bomem interferometer (see Section E) will be important to the completion

of these studies, deferred from the previous year because of the pressure

of other commitments and the need to complete tests of the performance of

the 3omem interferometer under high resolution conditions.

In experiments reported during the past year in which a beam of

discharged neon was codeposited with an Ar:QF

3 NNCF 3 sample, the most

prominent product absorption was that of CF_, with weaker absorptions of

CF^ and of an unidentified product, possibly the requisite negatively

charged species which maintains overall charge neutrality of the sample.

This study provides an important test of the feasibility of detecting the

infrared and electronic spectra of molecular ions produced by codepositing

a beam of excited neon atoms with a suitable precursor molecule isolated

in solid neon. 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 an earlier series of experiments

in this laboratory using exci ted

+argon at^ms, permitting a search for such

important ions as HC0 +

, NH_, and CH^. The transition to a neon

matrix will permit the use of a larger pinhole, minimizing wall

deactivation of excited neon atoms as they exit from the discharge tube

and enhancing the product yield. The use of a neon matrix will also

eliminate spectral perturbations due to charge transfer interaction

between molecular cations and the matrix, important in solid argon. In

order to pursue neon matrix studies, a continuous-transfer liquid helium

cell (Heiitran) has been acquired, and construction of a system to permit,

its use with the Bomem interferometer is in progress. The high

sensitivity of this instrument will further enhance the possibility of
detecting vibrational and low-lying electronic transitions of molecular

ions. During the coming year, experiments should proceed, with high

priority to be given to the search for the molecular ions already cited.

3. Vibrational Energy Levels of Transient Molecules

Several months of intensive work were necessary in order to

complete and prepare for publication the comprehensive compilation of the

vibrational energy levels of ground-state transient molecules possessing

from 3 to 1 6 atoms. This compilation, which includes almost 500 molecules, will appear in the Journal of Physical and Chemical Reference Data

later in 1984. It is planned to maintain an up-to-date digital record of

this material and to publish periodic supplements, in order to assist

studies of the chemistry and spectroscopy of transient molecules throughout the scientific community.

4. Vibrational Fundamentals of Ground-State Diatomic Molecules

Although the spectra of diatomic molecules have been more exhaustively studied than have been those of more complicated molecules, even

for diatomics there are many gaps in our knowledge of the ground-state

vibrational frequencies. These have frequently been reported in inert

matrices but not in the gas phase. A systematic comparison of gas-phase

and inert matrix data is needed. Under conditions in which the matrix

shifts are relatively small, matrix data can aid in the assignment of

emission spectra and in the development of infrared laser probes, as well

as provide a test for the validity of various approximations in ab initio

calculations. Approximately 225 pairs of gas-phase and rare gas or nitrogen matrix observations of ground-state vibrational frequencies of

diatomic molecules spanning the entire Periodic Table have provided a consistent sample for such a comparison. Conclusions of this study are:

(1)

Matrix shifts tend to increase in the order Ne < Ar < Kr < Xe <

(2) Group la and Ilia halides have large red shifts, which may be

correlated with the relatively low ionization potential of the

Group la and Ilia metals. (Very few data are available for

compounds of calcium, strontium, and barium, which also have

relatively low ionization potentials.)

(3) The one intra-icnic vibration observed, that of Co, behaves

normally

.

(4) van der Waals molecules have large blue shifts.

(5) Most other diatomic molecules—even heavy-metal oxides and

fluorides —have argon matrix shifts of less than 2 percent.

The results of this survey are being prepared for publication.

210
5. Reaction Intermediates in the Decomposition of Energetic

Materials

With support from the Army Research Office and from the Office

of Naval Research, a series of experiments has been conducted to determine

the role of free radicals in the first stage of the decomposition of

energetic materials (e.g., propellants, explosives). The objectives of

these studies, for which support terminated at the end of March, included

the identification of free radicals formed in the decomposition of such

materials, a task to which matrix isolation observations are especially

well suited, and the probing of their energy distributions using the

techniques of gas-phase laser spectroscopy.

During the three-year period of this support, emphasis was placed on determining processes which are important in the initial stages of the

decomposition of nitromethane and methyl nitrite. A key study in this

project has been that of the photodecomposition of isotopically substituted nitromethane in an argon matrix, completed and published during this

past year. An earlier study (1958) of this system, with much less

extensive isotopic substitution, suggested that the photodecomposition

products in a matrix environment might differ from the gas-phase products.

However, several prominent absorptions in that study remained unassigned,

and two peaks which were initially believed to have been contributed by

HNO have since been found to be contributed by other, unidentified

products. In the early stages of the photolysis, infrared absorptions of

cis- and trans-CH^ONO grew in intensity. On somewhat longer photolysis,

these absorptions diminished in intensity, and there was continued growth

in the infrared absorptions now assigned to the H2 C 0 ***HN0 hydrogen-bonded

complex. Most of the previously unidentified peaks, as well as the two

peaks previously attributed to HNO, continued to grow at this stage of the

experiment. These peaks can now be assigned to the cis- and transrotamers of the nitrosomethanol , recently discovered by Dr. R. P. Muller

and Prof. J. R. Huber at the University of Zurich. (In October, Dr. M. E. Jacox visited their laboratory to discuss this reassignment and other

topics of mutual scientific interest.) In the later stages of the

photolysis, absorptions of CO, NO, and HNCO and of the H2 0***HNC0 hydrogen-bonded complex became increasingly prominent. When the enhanced role

of cage recombination in the argon matrix is taken into account, the

overall decomposition mechanism is compatible with that determined from

gas-phase studies of the photolysis of nitromethane. Because the high

probability of cage recombination and of secondary decomposition characteristic of the argon-matrix photolysis experiments is common to the high

pressure and condensed phase decomposition of nitromethane, the matrix

results are of more general importance.

Support has been received from the Army Research Office for continuation and extension of the matrix isolation observations. During the term

of this new support, emphasis will be placed on the infrared and ultraviolet spectroscopic characterization of reaction intermediates derived frcm

the decomposition of benzene and nitrobenzene, in order to provide a firm

base for later studies of more complicated aromatic molecules. Spectroscopic identifications will be sought for a number of other free radicals,

211
including H

2 CN, CH 3 N, (CH 3

) 2

N, and HCN2 , which also play important roles

in the decomposition of explosive or propellant molecules. During the

coming year, attempts will be made to obtain a more complete infrared and

ultraviolet spectroscopic characterization of the phenyl and phenoxy

radicals and to obtain the infrared and ultraviolet absorption spectra of

H

2 CN, a potential intermediate in the decomposition of RDX and HMX, and of

HCN2> believed to be important not only in the decomposition and combustion of energetic materials but also in systems as diverse as the chemistry of the Titan atmosphere and the formation of "prompt" NO in

combustion.

F. Future Plans

There are many experiments planned in the area of vibrational

relaxation of molecules in condensed phases. The rates and pathways of

energy transfer in molecules on surfaces and in ordinary liquids are not

well known. Through our experiments, we hope to understand how the rates

are related to structural molecular parameters such as vibrational

frequencies, bond lengths and orientations, permanent and transition

multipole moments, etc. VET rates for many molecules on metal oxide

surfaces will be measured. For instance, on Si02 surfaces the relaxation

of groups such as OD, OCH 3

, NH2 bound to surface Si atoms will be studied.

Other metal oxide substrates such as Ti02 , ZnO, BeO, and A 2, 2 03 appear to

be suitable for study with our present IR/IR or IR/SPARS methods.

Crystalline systems like 0H~ in the conduction plane of 8 - aluminas may

permit better definition of bond lengths, angles, and substrate vibrations

than the amorphous materials studied so far. We will try to measure VET

rates for molecules bound to conducting metals. Metal sols (e.g., Ag, Ar,

Rh) adsorb certain molecules (e.g., pyridine) so that infrared and Raman

spectra characteristic of surface bound (as opposed to bulk/solution)

molecules are prominent, and such samples may permit VET rates at the

metal surfaces to be measured with our present apparatus. Alternatively,

metal clusters (e.g., 10-30A diameter) supported on dielectrics (e.g., Pt on n alumina) adsorb many simple species, and offer the high surface area required for our present experimental apparatus to work. However, success

in any of these experiments cannot be guaranteed. We note our earlier

failures to measure VET rates in molecules bound to dispersed supported

metals (i.e., CH^NC bound to isolated Rh atoms on AJI 2 O

3

) • We will try to

directly determine where the vibrational energy goes during the relaxation

process by measuring the time dependent excitation in different vibrational modes. Either IR absorption spectroscopy using a second independently

tunable psec probe pulse (e.g., pump one of the NH bonds in Si-NH2

, probe

the other), or IR/SPARS may be used.

A better understanding of the relation between energy transfer and

the observed vibrational spectra of molecules in condensed phases (e.g., on surfaces) is desirable. Our initial result for the relaxation of OH

( v=1 ) on Si0 2 showed that one would have underestimated the T-j time by

2000 if the observed linewiath had been identified as T-j broadening. For

all our samples we would like to measure the spectrum (IR, Raman), the

vibrational relaxation time, and the homogeneous linewidtn (T2

_1

). We may

212
estimate the latter through a picosecond CARS experiment using our synchronously pumped tunable dye lasers, or perhaps with an IR pump-probe

hole-burning experiment.

Although the preceding discussion has focussed on the kinetics of

surface-bound molecules, there is also very little known about energy flow

in molecules in solution, or in large isolated (gas phase) molecules. For

instance there are no obvious experiments in any phase which are comparable to our initial results for OH on Si02. We expect to measure VET rates

and pathways in certain molecules in solution with the IR/SPARS approach

that we used earlier to study the relaxation of HCJ, (v=1). Molecules that

may help us understand the surface VET results, e.g., molecules with

vibrational frequencies comparable to the substrate modes, will have top

priority

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 environment. The result of our work on N0/Ru(001) has been published and reviewed herein in past

years. We are currently performing measurements on the NO/Pt (111) system.

Our preliminary results indicate significant differences in the molecular

dynamics important in even this simple bond breaking process for the flat,

basal planes of Pt vs Ru. 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 vs Ru we shall explore thermal desorption as a means of studying the dynamics of

related systems. We shall probe NO desorption from surfaces such as Pt

where there is only simple molecular desorption, from Ru where decomposition competes with desorption and Ir which exhibits atom-atom recombination, leading to the desorption of NO. Similarly we shall look at

gaseous partners exhibiting different chemistries 3uch as CO and H?.

These studies are only now becoming practical with the advances being made

on vacuum ultraviolet laser development. Potassium has shown to be an important promoter in hydrocarbon chemistry. It has also been observed

that small amounts of K induces large shifts in the thermal desorption

spectra for CO from Ni. This is one system we hope to follow to explore

the role of reaction promoters or inhibitors.

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 cnemica^

bonds. In the thermal desorption experiment the heating rates traditionally used are much slower (e.g. 10 K/sec) than other molecular relaxation

processes which is good if one’s goal is to study the dynamics of a highly

213
energetic system under "equilibrium" conditions. However, it is impossible under these conditions to learn about energy transfer at lower levels

of molecular excitation at clean metallic surfaces. The second branch of our project will use rapid pulse laser heating to directly desorb transient species at metal surfaces. Specifically, we now have available

tunable picosecond lasers (developed through the Picosecond Competences

Program) which will enable us to vibrationally excite adsorbate species

such as NO, CO, OH, etc. to their v=l or 2 states. At some variable time

delay (i.e., 1 to 100 ps) a second intense, non-resonant picosecond pulse

will heat the surface (at a rate of -10^ K/sec) to a temperature of -1000

K from which the adsorbate will be directly desorbed. Those surviving in

the v— 1 state during the time period At between pump and heating laser

pulses will be defected by laser fluorescence or ionization techniques in

the gas phase as they leave the surface. These experiments will provide

the first direct measurements of vibrational lifetime of adsorbates on single crystal metals. Determination of the rates and mechanisms for

energy transfer at these low levels are important for understanding

heterogeneous reactions and the role of catalysts or reaction promoters/inhibitors in such processes. Although many spectroscopic studies of

adsorbates on metals exhibit linewidths consistent with sub-picosecond

uncertainty broadening our results for OH on the surface of Si02 show that

vibrational relaxation rates may be much slower than that predicted from

linewidth analysis. The proposed rapid desorption technique should allow

the successful extension of their work into the realm of ordered metallic

surfaces

.

The completion of the matrix isolation setup for use with the Bomem

interferometer will open up new vistas for studies of reaction intermediates. The much improved sensitivity of this instrument will make possible

the study of weak product absorptions, previously below the threshold for

detection. Two important classes of molecule—carbon-13 substituted

molecules present in natural abundance and products of 0-atom reactions in

discharge sampling experiments —will become accessible under these

conditions. Observations of the carbon- 1 3 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 this interferometer will both facilitate and mandate a series of studies of the reactions of 0 atoms with small molecules.

Studies of the products of the reaction 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. The interferometer also will

provide access to the 4000-12,000 cm

-1 spectral region, for the first time

permitting study of the near infrared absorption spectra of such important

free radicals as C

2 , HC 2 , and HO? in an inert matrix environment.

Emphasis will be placed on the studies of the interaction of excited

neon atoms with small molecules, to be implemented during FY 85. It is

hoped that these studies will provide much previously inaccessible

information on the vibrational and electronic spectra of molecular ions, a topic of great current scientific interest. Since the study of F-atom
reactions has proved to be an extremely versatile tool, it will continue,

but with increasing emphasis on the identification of the electronic

spectra of free radicals formed in F-atom reaction systems.

The need for more complete characterization of the energy levels of new free radicals and molecular ions stabilized in studies of the

reactions of 0 and F atoms and of excited neon atoms with small molecules

will mandate the construction of a laser-excited fluorescence setup for

matrix isolation studies. Recent improvements in laser sources of

ultraviolet radiation will make possible the detection of previously

inaccessible low-lying molecular electronic states. Fluorescence and

phosphorescence may result when these states are pumped by cage recombination of the molecular fragments formed on laser photodissociation of the

matrix-isolated molecule.

The compilation of the ground-state vibrational energy levels of

transient molecules will be kept current, and from time to time supplements will be issued. It is also planned to initiate a compilation of

excited electronic states and their vibrational energy levels for transient molecules.

215
M. QUANTUM CHEMISTRY GROUP

M. Krauss, H. Basch, P. S. Julienne, F. H. Mies,

M. Rosenkrantz, S. B. Sears, and W. J. Stevens

A. Introduction

Atomic and molecular electronic structure and scattering

calculations are 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 problems are now

drawn from biotechnology areas. Quantum chemistry methods are being

applied to energetic, conformational, and spectroscopic approaches to

biochemical problems. Applications of scattering theory have been made to areas such as high temperature materials, photodissociation of high

temperature species, and pressure broadening and depolarization of atomic

radiation. The summary of activities for the past year is divided into

two areas: electronic structure and scattering. Several collaborative

activities with other groups were undertaken during the past year. These

are described in sections B.I.b, 3~6 and C.6,7.

B. Electronic Structure

(M. Krauss, W. J. Stevens, H. Basch (NBS Research Associate)

1 . Biomolecule Calculations

Fundamental studies are underway on the electronic structure at

active sites of enzymes and for the determination of the interaction

energies and spectroscopic behavior of metal complexes interacting with

DNA and RNA by using suitably chosen model systems. Ab initio quantum

mechanical techniques are used and relativistic effective potential

methods permit the calculation of the electronic structure and properties

of metalloorganic molecules with metals from anywhere in the periodic

table

.

a. Compact Effective Potentials

Compact effective potentials (CEP) and their concomitant

expansion bases have been developed to reduce the cost of ab initio

molecular orbital calculations and make the inclusion of heavy metals

feasible. It is now possible to carry out double zeta (or better) basis

set calculations on large biomolecule fragments. This is necessary since

the interactions between such fragments are modelled from their charge

distributions which, up till now, have been obtained with small, possibly

inaccurate, basis sets. The polarization of the fragments is also found

to De important and it is possible that this effect is underrated because

the small basis set calculations are not flexible enough to account for

the magnitude of the polarization.

216
Excellent comparisons between CEP and all-electron single and

multi-configuration self-consistent-field calculations have been obtained

for a variety of molecules in the first two rows of the periodic table for

both binding energies and spectroscopic constants. Relativistic

potentials are being derived for all elements beyond the second row, and

the spin-orbit potential is also being obtained. To date only selected

metal potentials have been considered as needed, but we plan to construct

potentials for the entire periodic table. A paper describing the

algorithm for generating the potentials and tabulations for the first two

rows of the periodic table has been prepared.

b. Binding of Pt complexes to DNA

Binding energies of the PtCNHg) ^complex to the bases,

H

2 O, NH3

, imidazole, pyrimidine, 2-pyriraidone , 4-pyrimidone , guanine,

cytosine, adenine, and thymine, were calculated using the CEP at the SCF

level. The results show that the apparent binding to the different sites

in the DNA bases is likely to be determined by enthalpic and not kinetic

effects. The binding is much larger to guanine and cystosine sites than

to the other bases.

Experimentally it is known that the platinum complex forms covalent,

and often bifunctional bonds with guanine, cytosine, and adenine. The

preferred binding has been deduced experimentally to be intrastrand and

primarily to guanine bases. However, little is known about the strength

of the binding to various sites, whether the binding preference is

determined by thermodynamic or kinetic factors, or what disruption of the

DNA structure occurs in order to accommodate the complex. Large-scale

quantum mechanical self-consistent-field calculations were undertaken to

quantify the binding energies of platinum ammonia complexes to the various

base ligands listed above. Since platinum foriqs four-coordinate square

planar complexes, a triamine complex, Pt(NHo)_ , was used to examine the

binding to the individual bases. For exampie;' Figure 4.1 shows the

preferred orientation of PttNH^) interacting along the lone pair of N7 in

guanine

The stability order for binding to various sites in the free bases is

calculated to be N7(G) > N3(C) > 02(C) > 06(G) > N3(A) > N7(A) > 04(T) > 02(7). Binding N7(G) is favored by 10-15 Kcai/mole over N3(C), 02(C) and

06(G) and by mere than 30 Kcal/mole over sites in adenine and thymine.

Calculations of the optimum Pt-base ligand distances and the energy

variation as a function of bending the molecular plane of the bases were

determined in order to characterize the essential features of the

interaction energy surface.

1
Fig. 4.1. Preferred orientation of the Pt(NH

3 )_ complex interacting with

guanine. ^

The quantum mechanically determined binding energies and potential

surface features were used to develop moa<=l, DNA structures which could

accommodate intrastrand binding of Pt(NH

3

) 2

to adjacent bases. Molecular

mechanical modeling of DNA was carried out in collaboration with Prof. K.

J. Miller and E. R. Taylor of the Renssalaer Polytechnic Institute

Department of Chemistry. Principal findings include a low-energy kinked

structure of poly (dG) *poly( dC) , as shown in Figure 4.2, which can easily

accommodate binding of the platinum complex to adjacent guanines in a nearly optimal conformation in the major groove. In addition, a novel

binding site in poly (dGpdC) has been proposed in which an anti -*• syn

rotation of cytosine allows the binding of the platinum complex between

N7(G) and and N3(C) on adjacent bases. Several other structures, including

binding to non-adjacent base pairs and binding in the minor groove, were examined and found to be energetically unfavorable.

c. Energetics and Conformations of cis and trans Pt(NH3

) 2AB

( A,B = Cl or A=NH

3

, H20, OH"; 8=H20, OH

-

).

These studies were undertaken to learn more about the

drug compounds that directly interact with DNA 3ince little is known

experimentally about the H

2 0 and OH

- systems. Using energy gradient

methods, the isomeric energies and conformations were calculated for all

218
Fig. 4.2. Stereo image of poly(dG) *poly(dC) kinked by 40° to allow

Pt-complex binding in the major groove.

the molecules. The energy surface for cis-trans isomerization for the

PtCNH^^ complex was also examined. The calculated geometries were used

to quantify the 'trans influence' ligand ordering. The variation in bond

lengths as a function of trans substitutions produced an order OH

-

> Cl > NH -i > H2 G. This ordering agrees with the conventional one except for 0H~

which is usually considered to be similar to H

2 O. The discrepancy is

probably connected with the difficulty in experimentally characterizing

hydroxo complexes.

Strong hydrogen bonds are obtained with the ionic ligands for

favorable geometric configurations. When these H-bonds are formed, the

energy difference between the hydroxy containing isomers is very small.

d. Cation Effect on H-bonds and Tautomeric Energies

In the local vicinity of the cation (bare or complex),

the binding energies and conformations of nearby hydrogen bonds and the

proton or substituent positions can be substantially altered. A set of

model calculations on H bonding effects was done for a pair of

hydrogen-bonded imidazoles. Binding energies, equilibrium bond lengths,

and barrier heights are all substantially affected by the perturbat ions

Detailed analysis of the perturbations in the charge distributions due tc

cation binding is still underway.

I
2. Electronic Structure of Metal Oxides

(M. Krauss and W. J. Stevens)

This work represents the completion of research supported by

the Air Force Office of Scientific Research. Complete active space (CAS)

multi-configuration self-consistent-field (MC-SCF) calculations were performed for 12 valence electrons distributed among the sigma, two pi, and

one delta molecular orbitals for a large number of electronic states of

FeO and RuO. Spectroscopic constants and spin-orbit coupling constants

were evaluated and the results used to analyze binding in transition metal

oxides. The accurate spin-orbit coupling constants show that the

CAS-MC-SCF wave functions give a qualitatively correct description of the

electronic structure but the spectroscopic constants are far more accurate

for the RuO states than for FeO.

Calculations on the comparative behavior of Nd and U oxides is still

continuing. Of particular interest is the calculation of sf3(5j) and

sf^dC^K) configurations (states) of almost the same energy for the ground

state of UO. In the case of NdO the sf 2d configurations are considerably

higher in energy than the sf3. The comparable ion configurations have

also been calculated and intermediate spin-orbit coupling has been

included in the analysis of the ions.

3. Ab initio Study of Mn+ Reacting with H

2

(with M. Dupuis and W. Lester, Lawrence Berkeley Laboratory)

Using the ion beam mass spectrometry technique, Armentrout and

coworkers (U. of Cal. Berkeley) have measured the bond strengths of atomic

metal ions with the species H, 0, and CH

n

(n = 0 - 3) . Bond energies of

the metal hydride ions are obtained from the reaction cross sections for

M

+

+ H

2

* MH + + H ( 1

)

two peaks for

to different

Mn+

, attributed T The ‘ S spin configuration is

believed to be essentially nonreactive with H2

. The configuration is

presumed to account for MnH

+

.

A plot of the cross section vs. energy for (1) shows

unlike other metal ions studied, which is

reactivity of the ^3 and states of Mn

+

We have characterized the essential features of the potential energy

surfaces which govern the reactions of Mn

+ (^S, ^S) with H

2

. The

reactants, transition states, and products have been accurately

characterized by ab initio quantum mechanical methods to provide insight on the factors responsible for the different reactivity of the above

states of Mn

+

. Although the analysis is not yet complete, our results do

not support the proposed different reactivities of the and states of

Mn

+

.

220
The calculations have been carried out with compact effective

potentials (CEP) replacing the core electrons on manganese. Test

calculations on MnH + using all-electron methods have compared extremely

well with the CEP results.

4. Electronic Structure of Low-Lying States of NaK

(with D. Konowalow and L. Ratcliff, SUNY Binghamton)

We have calculated the potential energy curves for all

molecular states of NaK which may be obtained from the interactions

Na(3s)+K(4s) , Na(3s)+K(4p), Na(3p) +K(4s ) , Na(3s )+K(5s ) , Na(3s )+K(3d) , and

for the A states corresponding to the interactions Na(3d)+K(4s) and

Na(4p)+K(4p) by full-valence configuration interaction computations which

utilize effective core potentials to describe the core electrons, the

core-valence orthogonality constraints, and the core-valence correlation

(CVC) energy. The differences between our computed curves and those

deduced from experimental spectra are generally small and can be accounted

for by: (1) the modest size of the basis set, which is insufficiently

diffuse to describe Na

- and K~ resonances associated with excited charge

transfer interactions and related molecular Rydberg character, and (2) the

approximate way in which the CVC interaction is included.

These accurate potential curves for the 22 lowest states of NaK will

be of considerable use in the analysis of molecular spectra and molecular

beam scattering. A tabulation of dipole transition moments for all

allowed transitions among these states is now in preparation.

5. Effect of Electron Correlation on e-N 2 Scattering

(with J. Rumble, NBS and D. Truhlar, U. of Minn.)

Most electron scattering calculations are carried out using

Hartree-Fock target wavefunctions . Although this approximation may be

valid for target molecules near their equilibrium geometries, electron

correlation can be important for vibrationaliy excited targets. To test

this possibility, elastic scattering cross sections for electron scattering by N

2 have been calculated in the static-plus-local-exchange model

using two different target wavef unctions : Hartree Fock (HF) and Multiconfiguration Self-Consistent -Field First-Order-Conf igur ati on- Inter act ion

(MCSCF-FOCI) . The range of impact energy considered is 1.5 to 30 eV.

Inclusion of electron correlation in the target wavefunction changes the

position of the Kg low-energy resonance by about 0.5 eV. Nonresonant

partial cross sections typically change by 2 percent at the equilibrium

internuclear separation but sometimes more (up to 26 percent at the

equilibrium internuclear distance and even more at stretched geometries,.

6. Damped Dispersion Interaction Energies

(Ph.D thesis of M. Rosenkrantz, SUNY, Binghamton, directed by

M. Krauss)

A theory has been developed for the calculation of the

dispersion energy for overlapping polyatomic fragments. A computer code

has been written and applied to ArH ?> ArHF , and ArHCZ. The present
calculations provide the first theoretical data on the anisotropy of the

dispersion interaction. Critical analyses of the previous theoretical

treatments shows considerable differences which are attributed to the

semi-empirical nature of such treatments and suggests a caution in the use

of such empirical methods.

C. Scattering

1. Nonadiabatic Theory of Dissociative Molecular States

(P. S. Julienne and F. H. Mies)

Given current access to powerful quantum chemistry codes which

provide interaction potentials, electronic wavefunctions and coupling

matrix elements for diatomic molecules, and given the very stable

numerical close-coupling codes that ar^ available, we can generate

multichannel continuum wavef unctions 4'. for the diatom AB of almost

arbitrary accuracy. If we couple these numerical capabilities with the

rigorous analytic description of the diatomic wavef unctions provided by

multichannel quantum defect analysis (MCQDA) , especially in the vicinity

of dissociation thresholds, we are in a position to describe a wide

variety of diatom phenomena with a great deal of confidence, and with

renewed insights.

There are a number of examples of interesting phenomena associated

with the interaction of light with colliding atoms, represented by the

reaction

[A + 3]

i + n&u0

-* [A* + B*]f + (n ± 1 )4uj0 (1)

For example, if the A’«-A transition is dipole allowed and the perturber

does not change state, equation (1) represents ordinary atomic line

broadening. Such a collision has been called an optical collision. If

the A * <-A transition is forbidden, (1) represents a collision-induced

transition. If both A and B change states, (1) represents a radiative

collision, sometimes called LICET. From the viewpoint of molecular

spectroscopy these are all simply diatomic continuum-continuum transitions

with a radiation scattering matrix element given by

S fi = -2Tri|<Yf

e 0

e0 *u|4'->. (2)

Even in this weak-field, linear dipole jj, approximation, we require

proper multichannel continuum wavef unctions for the 4L incoming complex

[

A-*- 3] ^ and the 4^ outgoing collision complex [A'+B'] f in Eq. (1). These

incorporate the non-adiabatic effects which determine the internal state

distributions of the atomic fragments.

We have developed a close-coupled theory of atomic collisions in the

presence of radiation coupling which yields a direct determination of S fi

and may be used to calculate the distribution of final atomic states which

222
results from absorption of polarized light during a collision. The

radiation field is assumed to be weak enough that the transition

probabilities remain linear in field intensity e Q

. The molecular picture

is emphasized in which the wavefunction is expanded in a basis of

field-free molecular states and the Hamiltonian is blocked in accordance

with molecular quantum numbers.

In addition, using MCQDA, we have shown how Sq. (2) can often be

factored as follows,

* BO + S fi = (N J S_(N^ ) (3)

ff fi ii

the 'incoming' half-collision matrix N» . tracks the incident collision

complex from infinity, where it enters m channel state ^ , into distances

near R Q> where the system 'coalesces' into the selected electronic

rotational state we designate as such that

<P A = ^ m if -(R0

). (4)

i i'

The transformation matrix m(RQ

) is orthogonal, mm = 1 0

, and defines a molecular state which we postulate is the dominant source of absorption at

the prescribed photon frequency. Further, we assume that only a single,

photon-selected upper state is accessed at R=R0 ,

~ ^f » ' , f

• (5)

f

1

The MCQDA defined 'outgoing' half-collision matrix follows the

subsequent dissociation of this quasi -molecule as it"

- proceeds to R=* w.nere the atoms depart into channel ipf . Thus N

+ and N

_ isolate and summarize

all the inelastic and non-adiabatic coupling effects operative among the

incident and final channels respectively. These_£ actors are bridged oy

the usual Born-Oppenheimer (30) matrix element which approximates the

transient total molecular states in the vicinity of R r as simple products

of an electronic-rotational state and a radial wavef unction

We have applied the half-coliisicn theory to the analysis of the

Sr( 1 P)+Ar depolarization and find that we can reproduce much of the

close-coupled data with this simple model. First order corrections

suggested by MCQDA brings us into almost quantitative agreement with the

exact quantal results.
Many more calculations need to be performed on a variety of systems

to fully test the half-collision analysis. However, even if there are occasional quantitative failures, the qualitative insights that are offered give us good direction in choosing parameters for the full quantal

calculations. Further, we must explore the applications in other branches

of continuum spectroscopy. N^te that if we substitute a bound state for

the initial continuum state in Eq. (2) we would obtain a bound-continuum

transition matrix element in place of S

fi which would involve the same N~

half-collision matrix. Thus our analysis of N“ and the implications of

unitarity and frame transformations should find equal application to

photodissociation (and predissociation).

2. The Thermodynamic Effects of Inelastic Collisions

(F. H. Mies)

The accurate evaluation of thermodynamic properties at elevated

temperatures requires proper analysis of the dissociated states of

molecules. These continuum states make substantial contributions to the

molecular partition functions and result in significant modifications of

the equation of states and equilibrium constants for high temperature

gases. Conventional spectroscopic analysis of the partition functions, as employed in the JANAF Tables, generally tend to overestimate the density

of states for diatomic molecules at high energies, and therefore

underestimate the degree of dissociation into atomic fragments.

Alternately, explicit summations which exclude energy states which lie

above the diatom dissociation limit will exaggerate the extent of

dissociation

.

Rigorous analysis of the molecular partition function can only be

derived from the quantum mechanical treatment of the continuum states

using molecular scattering theory. These continua are synonymous with the

scattering wavefunctions which define the elastic and inelastic collisions

experienced by the departing fragments. This approach yields unambiguous

interpretations of metastable, or predissociating states, as well as proper descriptions of second virial coefficients and imperfect gas

effects at high temperatures. Great calculational simplifications can be

achieved using classical phase integrals, but such techniques must first

be certified by comparison to rigorous quantum results.

Using formal scattering theory to characterize the continuum

wavef unctions associated with molecular fragmentation we derive rigorous

expressions for the molecular partition function Q. In particular the

analysis includes the effect of inelastic collisions which are introduced

by the off-diagonal elements of the scattering matrix _S(E) at each total

energy. Expressions are reduced to a simple form using the eigenphases of

the associated Y_(E) matrix derived from our recent multichannel quantum

defect analysis (MCQDA) of molecular dissociation. The partition function

Q = Qel + Q^nei can 136 separated into distinct elastic Q eJ_ and inelastic

Qinei contributions. In addition rigorous interpretations of the

thermodynamic effects of predissociation are derived and shown to have

negligible consequences on equilibrium properties.

22b
Using previously evaluated _Y(E) matrix elements, we would have to

conclude that the neglect of inelastic coupling in the calculation of

thermodynamic functions is entirely reasonable. There are a number of

subtleties associated with thresholds that could modify the partition

functions, but these effects would probably be of limited importance,

especially at elevated temperatures

The biggest limitation on the general validity of our conclusion is

that all our numerical evaluations were devoted to simple two-channel

couplings. It is possible that there are sane fortuitous cancellations

that are not present in the more general cases, and it remains for us to

explore more complicated inelastic events.

3. Analyses of Pseudopotentials and their Applications to

Multichannel Couplings in Atoms

(F. H. Mies and S. B. Sears)

Previously we have calculated norm-conserving pseudopotentials

which accurately reproduce the elastic scattering of a Rydberg electron in

the static-exchange limit for collisions with both closed Na+

( Is 2

, 2s 2, 2p°)

and open He+ (1s) shell ion core states. We have shown that these

potentials, obtained from accurate numerical Hartree-Fock atomic

wavef unctions for the lower Rydberg states, such as the He(1s,np')

1 P states, can be scaled to obtain reliable representations of the

correlated Rydberg wavefunction throughout the higher members of the

Rydberg series and into the ionization continuum.

Our present studies are devoted to extending the use of exchange and

correlation localized potentials to the inelastic scattering effects

associated with the configuration-interaction between various open-shell

Rydberg series. Specifically we are using the well studied four channel

coupling in the He

+ + e

- system to test our development of appropriate

off-diagonal pseudopotentials,

He

+

( 1 s ) + e U=1) He

+ (2s) + e"U=1)

He

+ (2p) + e~U=0,2) .

(1)

First, we have derived new pseudopotentials from the ‘P states of

He(2s,np') He(2p,ns’) and He(2p,nd*). These, combined with our previously

determined He(1s,np’) potential gives us an accurate representation cf the

elastic scattering in each of the four channels which contribute to Eq.

(1). We have also obtained the various combinations of conf igur ation

interaction matrix elements, e.g., <1s,np' H 2p,nd’>, which are needed to

develop our off-diagonal potentials. In addition we have modif.ed our close-coupled scattering codes to handle Coulomb scattering boundary

conditions and have already performed accurate calculations of the

inelastic scattering and the autoionization cf the ccre-excited Rydberg

levels in the complete absence of exchange. Our success in developing

useful algorithms to handle off-diagonal exchange remains to be tested.

'225
4. Adiabatic Representations of Distant Perturbations in Molecular

Spectroscopy

(F. H. Mies)

We have considered the strong coupling of a given molecular

state xp 0

C

r* , R ) to an energetically distant state ip-j ( r , R ) such that the

total wavefunction at total energy E must be represented as follows,

|E> = ( r , R ) F( E, R) + ^(r,R) G(E, R) . (1)

A particular radial coordinate R in the molecular Hamiltonian H(r,R) has

been isolated for special consideration, and r is meant to represent the

composite of all remaining degrees of freedom. Thus \j> 0 and ijj-j are but two

of an infinite set of states which span the space of r. We generate a pair of coupled equations for F and G by imposing the condition (H-E) E> = 0 and operating from the left with and ^ followed by an integration

over r

,

» ’ V00 (R) - E] F + V01 (R) G = 0 (2)

—1

1

-3 + 31 - E] G + V 10 (R) F - 0. (3)

Note that the functions and i|;-j may contain an implicit dependence on R

and will not in general commute with the radial kinetic energy operator T

= -3^/3R^. Thus the 'potentials' may actually contain momentum operators

which are required to operate on the F and G functions. For the present

purposes it is adequate to assume that we can neglect such radial

Born-Oppenheimer couplings, and thus we can also assume that the

off-diagonal coupling V

01

= V-|q is both real and symmetric.

We have previously discussed the role of adiabatic electronicrotational (AER) states in diatomic spectroscopy, where and ^

represent pure electronic-rotational states constructed in some appropriate Hund's coupling scheme. Further we have already made explicit

application of AER theory to describe the effects of distant perturbations

in atomic scattering. In that case E is continuous since it exceeds the

dissociation energy limit E>Vqq(°°). Here we want to explicitly apply AER

theory to pure bound state spectroscopy where E<VqqO) and both channels

are closed. Also we have relaxed any restriction to diatomic molecules

and have applied AER theory to vibronic coupling in polyatomic systems.

Our major concern is the applicability of AER theory to strongly

perturbed systems. For this purpose we consider the strong vibronic

couplings and resultant intensity borrowings analyzed by Innes and others

for systems such as formaldehyde and propynal. As implied in Eq. (1) we are only allowing a single active vibrational mode, with displacement R,

to couple the two states. Both i^q and now contain a common set of

frozen vibrational wavefunctions with identical quantum numbers and we

226
have merely treated the polyatom as a pseudo- diatom . We could of course

unfreeze two coordinates, say R-| and R 2 , and postulate a set of

interaction potentials V00 (r 1> r2 ), V-|-|(R 1f R2 ) and Vgi(R 1 ,R2 ), together

with a pair of kinetic energy operators T-j(R-|) and T2(R2)* This would

give rise to the Duschinsky effect and we believe the AER theory could be

applied with equal force to this problem. However, we shall only consider

the simple one-mode coupling represented by Eqs. (1), (2), and (3).

In the absence of any Vq-j coupling we wou^d simply replacg Eq. (1)

with a pure Born-Oppenheimer wavefunction - f (Egn

,R) where f belongs

to the set of diabatic vibrational wavefunctions generated by Vqq at the

unperturbed eigenvalue EQn ,

If the energy is scaled such that the zero-point energy Eqq of the n=0

ground vibrational level exactly equals 1/2, and if we approximate Vqq = R

2 /4 as a pure harmonic potential the unperturbed eigenvalues are located

at E Qn = (n + 1/2). The levels, and the associated quantum numbers are displayed on curve (a) of Fig. 4.3. These levels are perturbed by a state

with an identical harmonic potential V .

= R

2 /4 + A which is merely

displaced by a constant energy

This simple model is not a necessary ingredient in the adiabatic theory

but does expedite the evaluation of the exact wavef unctions and

eigenvalues that we require to compare to the AER results. For the

propynal potential (curve b) A = 25.313, while the formaldehyde potential

(curve c) was simulated with A = 24.920. The associated off-diagonal

coupling is taken to be of the form Vqi = XR with X = 2.1197 and X = 2.7890 for curves (b) and (c) respectively. These curves represent the

lower adiabatic potential given by

o o

< T * V

00 ) f . E0n f n n

(4)

A ” V

11 " v00- (4)

( 5 )
Fig, 4.3 curve (a)

curve (b)

curve (c)

Pure harmonic potential in reduced units Vq

= R

2 /4 such that eigenvalues exist at En

= (

1 / 2 ).

Adiabatic potential for perturbed v-j g

mode of

1 A" ( S-j ) state of propynal vibronically coupled to

distant ^A'(So) state. In reduced units A = 25.313, A = 271197 in Eq. (5).

Adiabatic potential for perturbed vjj mode of

^ A9 ( S-j ) state of formaldehyde vibronically coupled

to distant 82 ( 82 ) state. In reduced units A = 24.920, A = 2.7890 in Eq. (5).

223

oa
where

,

y

- 2 Vqi / A = tan 2<J>( R )

( 6 )

This potential is associated with the adiabatic wavefunction

Ea,n > =

{ COS(P( R Ho " 3inq>( R )

}

*f* (Ea>n ,R)

where

(7)

I3 2

3R2

+ V } f a aa n

( 8 )

The eigenvalues defined by Eq. (8) are shown in Fig. 4.3 and are

found to reproduce the exact eigenvalues to a very high degree. Noticeable deviations (=±0.001) begin to occur at £ S 3*000. However this

accuracy is only attained if the adiabatic radial 3orn-0ppenheimer term

( d4>/dR

)

2 is retained in Eq. (5). Actually this correction is only

important for the lowest eigenvalues, and merely contributes about +0.008

to the n=0 eigenvalue for both systems.

Comparing Eq. (7) to Eq. (1) we see that the radial components of the

exact two state wavefunction are approximated as follows,

Fn = cos(R) f (R) (9) n

G = -sin(R) f a

(R) (10)

n These approximations have been tested numerically by evaluating the

overlap of the exact and the adiabatic functions with the set of

unperturbed harmonic functions in Eq. (4). The two sets cf overlap

integrals agree to within ± 0.003 for E n<3.00. This is mere than adequate

to insure that the adiabatic wavefunctions will reproduce any measurable

vibronic intensity borrowing in the Propynal and Formaldehyde spectra to

within 5 percent.

5. Predissociation of the Schumann-Runge bands of 0£

(P. S. Julienne)

The absorption of solar ultraviolet radiation by the

Schumann-Range bands of molecular oxygen control the transmission of tne

solar flux in the 200-175 nm range through the upper stratosphere and is a process of fundamental importance to stratospheric photochemistry.

Construction of an accurate solar transmission model has been hampered by

experimental uncertainties in the oscillator strengths and predissociaticr.

linewidths of these bands. We have therefore constructed a theoretical

model for the individual predissociation linewidths for the

229
vibrational-rotational fine structure sublevels. This represents an extension of our earlier work on this problem (Julienne and Krauss, J.

Mol. Spectrosc. 56, 270 (1975); Julienne, J. Mol. Spectrosc. 63_, 60

(1976)). The main ingredient of this new work is to recalculate the

spin-orbit coupling matrix elements between the predissociation B3 E"

state and the ^Hu, 3nu, 5 jiu> and 3x+ repulsive continuum states, since

the original work required using an atomic approximation for the necessarymatrix element. The new calculation gives very accurate spin-orbit matrix

elements based on CAS-CI ab initio wavefunctions and the AESOP technique.

The new spin-orbit matrix elements differ from the old ones by less than

25 percent. Thus, we obtain a quantitative improvement in line widths,

but no major qualitative changes. This work was supported by

NASA-Headquarters

.

6. Nonadiabatic Line Broadening Calculations

(P. S. Julienne)

A collaborative effort has been undertaken with Dr. Linda

Vahala, Old Dominion University, Norfolk, VA to use our nonadiabatic

theory of atomic line broadening and our computer codes for scattering in a radiation field to calculate the distribution of fine structure states

2 2 2 when sodium ^Pj is excited by absorption in the far wings of the P- S

resonance line perturbed by collisions with He, Ne, or Ar. The

theoretical calculations were compared with experimental data by Dr. Mark

Harvey, also at Old Dominion, and good agreement was found. Typical

results are illustrated in Figure 4.4.

7. Classical and Quantal Calculations of Radiative Decay Rates

(P. S. Julienne)

This is a collaborative effort with Dr. Joel Tellinghuisen of

Vanderbilt University. Dr. Tellinghuisen has discovered that the

expectation value of

tt 4 A(R) = [V’(R) - V"(R)] 3 u

2 (R)

for an upper state bound vibrational state is excellent approximation to

the total spontaneous radiative decay rate for the state. Here V* and V”

represent the respective upper and lower molecular potentials and p is the

transition dipole. This result bears a strong relation to earlier work by

P. Julienne on collision-induced emission, for which a similar expression

was utilised based on a classical path integral of the R-dependent

spontaneous emission rate. We have shown that the theories of total

emission rates for bound and free states are closely related, and both can

be evaluated by classical, as well as quantal, expectation values. The

approximate quantal and classical sum rule integrals were compared to the

exact sums for several test cases involving bound levels and were found to

be in excellent agreement with each other. Thus, molecular radiative

lifetimes can be accurately evaluated in terms of single expectation

values instead of difficult sums over all final states. A manuscript has

been prepared for submission to the Journal of Chemical Physics.

230
D1/D2

4.0- NaAr

E=200cm^

2 . 0

-

1 . 0

-

0

•

fl

400 300 200 ICO 0 -100 -200

DETUNING Ian'1

)

Fig. k.h Measured (+, error bars) and calculated (•, points) Na

^

l°1 /2

// ^^3/2 f’)- uorescencs branching ratios when Na 13 excited

by laser'Vadiation during a collision with an argon per*- ir:.er

atom. The abscissa is the detuning from the atomic transition frequency.
D. Future Plans

In the development of microscopic theories of biochemical processes

there is often a need for quantitative molecular data that are difficult

or impossible to elucidate experimentally. Such data can be as simple as the strength of a bond between two interacting molecules or as complex as the nature of the transition state in an enzyme-catalyzed reaction.

Interaction energies, conformational stabilities, potential energy

surfaces, reaction coordinates, electronic charge distributions, and many

other molecular properties may be determined theoretically using the

computational methods of Quantum Chemistry. Recent advances in

methodology, particularly in the algorithms for characterizing

multidimensional potential energy surfaces, and the continued advancement

of computer technology have allowed the application of accurate ab initio

Quantum Chemistry methods to biochemical problems. Although direct

quantum mechanical calculations are restricted to modest-sized fragments

of three or four dozen atoms, more complex systems may be studied by

hybrid methods which treat local sites quantum mechanically and the

environment surrounding the sites, such as solvent molecules and distant

molecular components, via semi-classical "reaction-f ield" Hamiltonians.

It is also possible to use quantum mechanical calculations of

fragment-fragment interactions to parameterize classical two-body

potentials for use in more approximate "molecular mechanics"

investigations of the secondary and tertiary structures and interactions

of larger molecules such as polypeptide and poly-nucleic acids. Such

potentials may also be used in Monte Carlo or molecular dynamic

simulations of complex solutions.

The Quantum Chemistry Group has been developing new theories and

computer programs which allow accurate quantum mechanical calculations on increasingly complex systems, including heavy metal atoms from anywhere in

the periodic table. Our goal is to develop methods that are applicable to

the study of interactions and reactions in large molecules, particularly

proteins and enzymes containing metal atoms. To this end we will initiate

a research program to develop hybrid "reaction-field" methods for

determining the influence of solvents and surroundings on fragment

structures and reactivities. We will also continue studying the influence

of metal cations on polypeptide conformations, hydrogen bondings, and

tautomer stabilities. In collaboration with other groups, we will begin

generating a library of classical atom-atom potentials for the

interactions of polypeptides and residues with the inclusion of metal

cation interactions. This will provide a database for future molecular

dynamical and Monte Carlo simulations on systems of interest.

We will continue in our development of the multichannel quantum

defect analysis (MCQDA) of molecular dissociation and explore its many

possible applications in molecular spectroscopy and atomic scattering

theory. In particular we shall exploit the half-collision analysis of

non-adiabatic effects in dissociation continua as it applies to atomic

lineshapes, photodissociation and predissociation. An important element

of our research in the coming year will be extensive code development

232
which will incorporate photodissociation and bound state capabilities into

our close-coupled scattering programs. Our initial applications will be

to diatomic systems, but we are approaching the code development with the

intent of ultimately treating polyatomic molecules, especially for loosely

bound van der Waals and hydrogen-bonded systems.

233 '

r
5. PUBLICATIONS

(a) Publications of Past Year

Aliev, M. R. and Hougen, J. T., "The Effects of Vibration-Rotation

Interaction on the Quadrupole Hyper fine Structure of Molecular

Rotational Levels," J. Mol. Spectrosc. 1 06 , 110 ( 1 984 )

.

Birnbaum, G. , Frommhold, L., and Krauss, M., "Collision Induced Dipoles of

Rare Gas Mixtures," J. Chem. Phys. 80_, 2269 (1984).

Cavanagh, R. R. and King, D. S., "Molecule-Surface Interactions and

Dynamics, " 'J. Vac. Sci. Tech. k2_ , 1036 (1984).

Dang-Nhu, M. , Pine, A. S., and Lafferty, W. J., "Les Intensities dans

les 3andes Vr, Vy, et vg + v-j -j de 1* Ethane ^ 2 C

2Hg," Can. J. Phys.,

62, 512-519 (1984).

Ho, P.-L., Lee, C. H., Stephenson, J. C. , and Cavanagh, R. R., "A Diamond

Opto-electronic Switch," Optics Communication 46, 202 (1983).

Henry, L., Valentin, A., Lafferty, W. J., Hougen, J. T., Devi Malathy, V.,

Das, P. P., and Rao Narahari, K., "Analysis of High Resolution

Fourier Transform and Diode Laser Spectra of the vg Band of Ethane,"

J, Mol. Spectrosc. 1 00 , 260-289 (1983).

Hougen, J. T., "Some Effects of Spin-Orbit Interaction and Rotational

Levels and Rotational Line Intensities in Vibrationally Unexcited

2 A,

2 E and 2 F Electronic States of XY^ Molecules," J. Mol. Spectrosc.

106, 134-154 (1984).

Jacox, M. E., "The Reaction of F Atoms with Methyl Nitrite. Infrared

Spectroscopic Evidence for the Stabilization of FON in an Argon

Matrix," J. Phys. Chem. 37, 4950-4945 (1983).

Jacox, M. E., "Infrared Spectrum of the C

2 F^ Free Radical Trapped in Solid

Argon in Discharge Sampling Experiments," J. Phys. Chem. 88, 445-448

(1984).

Jacox, M. E., "Matrix Isolation Study of the Decomposition of CF^NNCF^ by

Photons and by Excited Rare Gas Atom Bombardment of Energies Between

4.9 and 16.3 eV," Chem. Phys. 83., 171-180 (1984).

Jacox, M. E., "Photodecomposioion of Nitromethane Trapped in Solid Argon,"

J. Phys. Chem. 88, 3373-3379 (1984).

Julienne, P. S. and Mies, F. H., "Nonadiabatie Theory of Atomic Line

Broadening: Final State Distributions and the Polarization of

Redistribution Radiation," Phys. Rev. A30, (1984).

234
Knudtson, T. K. and Stephenson, J. C., "Vibrational Relaxation of KC1

in Dilute CCl^ and CCl^F Solutions," Chem. Phys. Letts. 1 07 , 385

(1984).

....

Krauss, M. and Stevens, W. J., "Effective Potentials in Molecular Quantum

Chemistry," Ann. Rev. Phys. Chem. 35_, 357 (1984).

Maki, A. G. and Sams, R. L., "Diode Laser Spectra of Cis - H0N0 Near

850 cm

-1 and trans-HONO Near 1700 cm

-

"'," J. Mol. Struct. 1 00 , 215-221

(1983).

Maki, A. G. , "Infrared Tunable Diode Laser Spectra of Lithium Fluoride at

High Temperatures," J. Mol. Spect. 1 02 , 361-367 (1983).

Mantell, D. A., Ryali, S. B., and Haller, G. L., "Time Resolved Infrared

Emission Studies at C0 2 Formed by CO Oxidation on Pt and Pd," Chem.

Phys. Lett. 102, 37 (1983).

Mies, F. H., "A Multichannel Quantum Defect Analysis of Diatomic

Predissociation and Inelastic Atomic Scattering," J. Chem. Phys.

80, 2514-2525 (1984).

Mies, F. H. and Julienne, P. S., "A Multichannel Quantum Defect Analysis

of Two-State Couplings in Diatomic Molecules," J. Chem. Phys. 80 , 2526-2536 (1984).

Nazemi, S., Javan, A., and Pine, A. S., "Collisional Effects on the Rovibrational Transitions of the HD Fundamental Band," J. Chem. Phys.

78, 4797-4805 (1983).

Ohashi, N. and Pine, A. S., "High Resolution Spectrum of the HC1 Dimer,"

J. Chem. Phys., _8, 73 (1984).

Olson, W. 3., "Minimization of Volume and Astigmatism in White Cells for use with Circular Sources and Apertures," Appl. Opt. 23, 1580 (1984).

Petersen, F. R., Wells, J. S., Siemsen, K. J., Robinson, A. M. , and Maki,

A. G. , "Heterodyne Frequency Measurements and Analysis of C02 Laser

Hot Band Transitions," J. Mol. Spectrosc. 105, 324-330 (1984;.

Pine, A. S., Coulombe, M. J., Camy-Peyret, C., and Flaud, J. M. , "Atlas

of the High Temperature Water Vapor Spectrum in the 3000 to 4000 cm”

Region," J. Phys. Chem. Ref. Data, _1_2, 41 3-465 (1983).

Pine, A. G. , Maki, A. G. , Robiette, A. G. , Krohn, 3. J., Watson, J. K. G.

,

and'Urbanek, Th. , "Tunable Laser Spectra of the Infrared-Active

Fundamentals of Cubane," J. Am. Chem. Soc. 106, 391-897 (1984).

235
Pollock, C. R., Petersen, F. R., Jennings, D. A., Wells, J. S., and

Maki, A. G., "Absolute Frequency Measurements of the 2-0 Band of CO

at 2.3u®; Calibration Standard Frequencies from High Resolution Color

Center Laser Spectroscopy," J. Mol. Spectrosc. j)9, 357-368 (1983).

Rosasco, G. J., Hurst, W. S., and Lempert, W. , "Three-beam

Phase-modulation Technique for Coherent Raman Spectroscopy," Opt.

Letters, Vol. 9, No. 1 (1983).

Singbeil, H. E. Gillis, Anderson, W. D., Davis Wellington, R., Gerry, M.

C. L., Cohen, E. A., Pickett, H. M., Lovas, F. J., and Suenram,

R. D., "The Microwave and Millimeter-wave Spectra of Hypochlorous

Acid," J. Mol. Spectrosc., 103, 466-485 (1984).

Stephenson, J. C. , Blazy, J. A., and King, D.

f S., "Spectroscopy and

Collisional Quenching Rates for A C

2H2 (v? = 0,1,2)," Chem.

Phys. 85, 31 (1984).

Stevens, W. J. and Krauss, M. , "Finite Field SCF Calculations of the

Dipole Polarizabilities of Heavy Atoms Using Relativistic Effective

Potentials," J. Phys. BJ6_, 2921 (1983).

Suenram, R. D. and F. J. Lovas, "Millimeter Wave Spectrum of Chlorine

Nitrate," J. Mol. Spectros., 105, 351*359 (1984).

Weber, A., Jennings, D. E., and Brault, J. W. , "High Resolution Raman

Spectroscopy of Gases with a Fourier Transform Spectrometer," Proc.

IXth International Conference on Raman Spectroscopy, Tokyo 1984.

236
(b) Publications in Progress

Basch, H., Stevens, W. J., and Krauss, M. , "Electronic States of Al

2

,"

Chem. Phys. Lett, (in press).

Blazy , J. A., Stephenson, J. C. , and King, D. S., "Infrared Multiphoton

Dissociation of Methyl Nitrite: Under Collisionless Conditions:

Product Yield and Energy Partitioning in the Primary Frangments,

to be published in " Lasers as Reactants and Probes in Chemistry,"

William Jackson, Editor, (Howard University Press, 1984).

Cavanagh, R..R. and King, D. S., "Laser Studies of Surface Chemical

Reactions," Chemical and Physics of Solid Surfaces , Vol. 5,

(Spr inger-Verlag) (in press).

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. (in press).

Hougen, J. T. and Ohashi, N., "Group Theoretical Treatment of the Planar

Internal Rotation Problem in (HF) 2

," J. Mol. Spectrosc. (in press).

Hougen, J. T., "Summary of Group Theoretical Results for Microwave and

Infrared Studies of H-pO?," Can. J. Phys. (in press).

Jacox, M. E., "Ground-State Vibrational Energy Levels of Polyatomic

Transient Molecules (3 to 16 Atoms)," J. Phys. Chem. Ref. Data,

( in press)

King, D. S. and Stephenson, J. C. , "Kinetic Energy Disposal in the

Unimolecular IRMPD of Methyl Nitrite, " Chem. Phys. Lett, (submitted).

King, D. S. and Stephenson, J. C. , "Infrared Multiphoton Dissociation of

Methyl Nitrite in a Molecular Beam; Internal States of the Nitric

Oxide Fragment," Chem. Phys. Letts, (submitted).

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. (in press).

Pine, A. S., Lafferty, W. J., and Howard, 3. J., "Vibrational

Predissociation, Tunneling and Rotational Saturation in the HF and

DF Dimer3," J. Chem. Phys. (in press).

Pine, A. S., "Tunable Laser Spectroscopy of Upper Atmospheric Molecules,"

CMA Workshop, edited by A. Weber.

Pine, A. S., Fried, A., and Elkins, J. W. , "Spectral Intensities in the

Fundamental Bands'of HF and HC1,"J. Mol. Spectrosc. (submitted,.

237
Pollock, C. P., Petersen, F. R., Jennings, D. E., Wells, J. S.,

and Maki, A. G. , "Absolute Frequency Measurements of tile 00°2-00°0,

20°1-00°0 Bands of N

20 by Heterodyne Spectroscopy," J. Mol. Spectrosc.

(in press).

Rumble, J. R., Stevens, W. J., and Truhlar, D. G. , "Effect of Electron

Correlation in the Target Wavefunction on Electron-Molecule

Scattering," J. Phys. B (in press).

Stevens, W. J., Basch, H., and Krauss, M., "Compact Effective Core

Potentials and Efficient Shared-Exponent Basis Sets for the

First- and Second -Row Atoms, Li-Ar," J. Chem. Phys. (submitted).

238
(C) Recent Publications of New Staff Members: (Casassa, Heilweil, Mantell,

Perry, and Woodward) Describing Work in Previous Positions

Casassa, M. P., Western, C. M. , Celii, F. G. , Brinza, D. E., and Janda, K. C. , "Infrared Photodissociation of Hydrogen-bonded Clusters: C?H 4 *HF

and C

2H

4 *HC1," J. Chem. Phys. 79, 3227 (1983).

Casassa, M. P., Western, C. M., and Janda, K. C., "Effects of

Inhomogeneity in the Infrared Photodissociation Spectra of (C2H

4

) 2

, C

2H l4 *HF and C

2

Hj

4 *HC1," J. Chem. Phys. (in press).

Celii, F. G., Casassa, M. P., and Janda, K. C., "Photodesorption of

Weakly Bound Molecules," Surface Science ' (submitted)

Chupka, W. A., Woodward, A. M., and Colson, S. D., "Electron

Photodetachment from Transient Negative Ions in the Multiphoton

Ionization of CH^I," J. Chem. Phys. (in press).

Doany, F. E., Heilweil, E. J., Moore, R., and Hochstrasser , R. M.

,

"Picosecond Study of an Intermediate in the Trans to Cis

Isomerization Pathway of Stiff Stilbene," J. Chem. Phys. _8£, 201

(1984).

Mantell, D. A., Ryali, S. 3., Haller, G. L., and Fenn, J. B., "The

Dynamics of CO Oxidation of Pt Deduced from Translational, Rotational

and Vibrational Excitation in the Product C02 ," Surface Science

(submitted).

Moll, D. J., Perry, J. W. , Kuppermann, A., and Zewail, A. H., "High-Energy

Overtone Photoacoustic Spectroscopy of Some Deuterated Methanes I.

Rotational Analysis," J. Chem. Phys., (submitted).

Perry, J. W. , Scherer, N. F., and Zewail, A. H., "Picosecond Pump-Probe

Multiphoton Ionization of Isolated Molecules: IVR and Coherence,"

Chem. Phys. Lett, 1 03 , 1 (1983).

Perry, J. W. and Zewail, A. H., "High-Energy CH-overtone Spectra of

Benzene at 1.8K," J. Chem. Phys. 80, 5333 (1984).

Perry, J. W. , Moll, D. J., Zewail, A. H., and Kuppermann, A., "High

Energy Overtone Photoacoustic Spectroscopy of Some Deuterated

Methanes II. Vibratinal Analysis," J. Chem. Phys., (submitted).

Western, C. M. , Casassa, M. ?., and Janda, K. C. , "Infrared

Photodissociation of the Hindered Internal Rotors Ne‘CpH 4 and

Ar*C2H

4 ," J. Chem. Phys. 80, 4781 (1984).

Woodward, A. M., Chupka, W. A., and Colson, S. D., "Two-Color Laser

Studies of the Multiphoton Ionization and Dissociation of trans

1 , 3-Butadience, " J. Phys. Chem. (in press).

r\ 39
.
U.S.

DEPARTMENT

OF

COMMERCE

National

Bureau

of

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N8S-U4A (REV. 2 -ao

U.S. OEPT. OP COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 84-2582 October, 1984

4. TITLE AND SUBTITLE

AMUAL REPORT - CENTER FOR CHEMICAL PHYSICS

S. AUTHOR(S)

?. J. Ausloos and R. A. Haines

S. PERFORMING ORGANIZATION (If joint or other than N3S, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 23234

7. Contract/ Grant No.

S. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10. SUPPLEMENTARY NOTES

f ! Document describes a compucer orogram; SF-185. FIPS Software Summary, is attached.

11. ABSTRACT (A 200-wora or less factual summary of most significant in formation. If document includes a significant

bibliography or literature survey, mention it here)

This renort summarises research Dro.jects, measurement method development,

testing and data evaluation activities carried out during Fiscal Year 1984 in

the N3S Center for Chemical Physics. These activities fall in the areas of

surface science, chemical kinetics, chemical Thermodynamics and molecular

snectroscony

.

12. KEY JVORDS (Six to twelve entries ; alphabetical oraer; capitalize only prooer names; and separate <ey words by semicolons)

Chemical kinetics; chemical physics; molecular spectroscopy; surface science;

t hermodvnami c s

13. availability

;

" Unlimited

' For Official Distribution. Co Not Release to NTIS

' ' Order From Suoermtencent of Documents, U.S. Government Printing Office. Wasmngton. D.C.

2G 402.

Order Prom National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. CF

PRINTED PAGES

255

15. Price

USC3VM-OC S043-P90
: