I NBS • PUBLICATIONS i A 111055T33bfl

Technical Activities 1986

Molecular Spectroscopy Division

A. Weber, Chief

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Chemical Physics

Molecular Spectroscopy Division

Gaithersburg, MD 20899

November 1986

PARTMENT OF COMMERCE

I Bureau of Standards

sburg, MD 20899

RESEARCH

INFORMATION CENTER

AJ 'SSc

NBSIR 86-3483

TECHNICAL ACTIVITIES 1986

MOLECULAR SPECTROSCOPY DIVISION

7%

A. Weber, Chief

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Chemical Physics

Molecular Spectroscopy Division

Gaithersburg, MD 20899

November 1986

Prepared for: U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
; /
ABSTRACT

This report summarizes the technical activities of the NBS Molecular

Spectroscopy Division during the Fiscal Year 1986. The activities span

experimental and theoretical research in high resolution molecular

spectroscopy, quantum chemistry and laser photochemistry, and include the

development of frequency standards, critically evaluated spectral data,

applications of spectroscopy to important scientific and technological

problems, and the advancement of spectroscopic measurement methods and

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

staff.

i

FOREWORD

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

Molecular Spectroscopy Division for the period of October 1, 1985 to

September 30, 1986. The report was prepared as part of the Annual Report

of the Center for Chemical Physics within the National Measurement

Laboratory of NBS. The Molecular Spectroscopy Division is organized into

three working groups comprised of permanent staff, post-doctoral fellows,

guest scientists, and support personnel. An organizational chart of the

Division is given at the end of this Foreword.

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

its technical programs are described in the Introduction, Section 1

.

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

Resolution Spectroscopy, Laser Photochemistry, and Quantum Chemistry

Groups. Each group report gives project objectives, details of results

obtained during the past year, and plans for future work. A mini-table of

contents precedes each group report to afford a quick overview of the

group's activities.

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

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

lists the seminars hosted by the Division. The Division has

had a number of visiting scientists during the past year; these are

identified in Section 8.

This report was prepared during the period July-August, 1986. While

anticipating some of the accomplishments that resulted from the work done

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

iii
do justice to the full range of activities and accomplishments that

actually did take place. Some of these will be presented in the report

for the following year.

Further information on the activities of the Division can be obtained

by contacting the scientists identified in each project report, or by

writing to Dr. Alfons Weber, Chief, Molecular Spectroscopy Division,

Physics Building, B268, National Bureau of Standards, Gaithersburg, MD

20899.

IV
Staff of the Molecular Spectroscopy Division

A. Weber, Chief

A. Flynt, Secretary

C. V. Kurtz, Division Technician

Laser Photochemistry Quantum Chemistry

Stephenson, J. C., Group Leader

Elwell, L. B.

King, D. S.

Casassa, M. P.

Heilweil, E. J.

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

Bodaness, R. (G. S.)

Cavanagh, R. R. * Burgess, D. (P.D.)*

Krauss, M. , Group Leader

Goldgaber, N. I.

Julienne, P. S.

Mies, F. H.

Stevens, W. J.

Jasien, P. (P.D.)

Basch, H. (G.S.)

Beveridge, S. (G.S.)

Fink, W. (G.S.)

Lengsfield, B. H. (G.S.)

Konowalow, D. D. (G.S.)

Mezei, M. (G.S.)

Miller, K. J. (G.S.)

Osman, R. (G.S.)

Vahala, L. (G.S.)

High Resolution Spectroscopy

Maki, A. G. , Group Leader

Fraser, G. T. (P.D.)

Hougen, J. T. Jacox, M. E.,

Lafferty, W. J. Lovas, F. J.

Olson, W. B.

Pine, A. S.

Rotter, G. L.

Suenram, R. D.

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

Weber, A. Craig, N. (G.S.)

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

Gillies, C. W. (G.S.)

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

Ohashi, N. (G.S.)

Sattler , N. (G.S.)

Zozom, J. (G.S.)

P. D. - Post Doc

G. S. - Guest Scientist

* - Surface Science Division

v
do justice to the full range of activities and accomplishments that

actually did take place. Some of these will be presented in the report

for the following year.

Further information on the activities of the Division can be obtained

by contacting the scientists identified in each project report, or by

writing to Dr. Alfons Weber, Chief, Molecular Spectroscopy Division,

Physics Building, B268, National Bureau of Standards, Gaithersburg, MD

20899.

IV
Staff of the Molecular Spectroscopy Division

A. Weber, Chief

A. Flynt, Secretary

C. V. Kurtz, Division Technician

Laser Photochemistry Quantum Chemistry

Stephenson, J. C., Group Leader

Elwell, L. B. King, D. S.

Casassa, M. P.

Heilweil, E. J.

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

Bodaness, R. (G. S.)

Cavanagh, R. R. * Burgess, D. (P.D.)*

Krauss, M. , Group Leader

Goldgaber, N. I.

Julienne, P. S.

Mies, F. H.

Stevens, W. J.

Jasien, P. (P.D.)

Basch, H. (G.S.)

Beveridge, S. (G.S.)

Fink, W. (G.S.)

Lengsfield, B. H. (G.S.)

Konowalow, D. D. (G.S.)

Mezei, M. (G.S.)

Miller, K. J. (G.S.)

Osman, R. (G.S.)

Vahala, L. (G.S.)

High Resolution Spectroscopy

Maki, A. G. , Group Leader

Fraser, G. T. (P.D.)

Hougen, J. T.

Jacox, M. E.,

Lafferty, W. J.

Lovas, F. J.

Olson, W. B.

Pine, A. S.

Rotter, G. L.

Suenram, R. D.

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

Weber, A.

Craig, N. (G.S.)

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

Gillies, C. W. (G.S.)

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

Ohashi, N. (G.S.)

Sattler, N. (G.S.)

Zozom, J. (G.S.)

P. D. - Post Doc

G. S. - Guest Scientist

* - Surface Science Division

v

TABLE OF CONTENTS

Abstract . .

Foreword . .

Staff of the Molecular Spectroscopy Division . . .

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

A. Goal ...................

B. Organization ...............

C. Division Programs. ............

D. Collaborations ..............

E. Other Agency Support ...........

2. High Resolution Spectroscopy Group. . . . . .

A. Introduction ........

B. Calibration and Data Center Activities . .

C. Spectroscopy for Environmental Studies . .

D. Spectroscopy of Hydrogen-bonded and

van der Waals Complexes .........

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

F. General Spectroscopy and Theory. . . . . .

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

A. Introduction ...............

B. Condensed Phase Energy Transfer. . . . . .

C. Laser Diagnostics of Surface Dynamics. . .

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

E. Photophysical Stuides of Photosensitizers.

i

iii

v

1

2

2

5

5

7

9

9

12

22

32

36

43

ijij

44

52

57

61

vii
4. Quantum Chemistry Group ..........

A. Introduction ..............

B. Theoretical Calculations on Biomolecules

C. Quantum Chemistry of Small Molecules . .

D. Scattering in the Presence of Radiation.

E. Atomic and Molecular Scattering Theory .

5. Publications. ...............

6. Talks ...................

7. Molecular Spectroscopy Division Seminars. .

8. Visiting Scientists ............

63

65

66

73

76

79

83

91

97

101

viii
MOLECULAR SPECTROSCOPY DIVISION

A. Weber, Chief

SUMMARY OF ACTIVITIES

Fiscal Year 1986

1 . INTRODUCTION

A. Goal

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

forefront of modern theoretical and experimental spectroscopy in order to:

Advance spectroscopic measurement methods and techniques

Develop models, theoretical concepts and quantum theoretical

methods for predicting molecular properties and interactions

Obtain reliable reference spectroscopic data for NBS, government,

research community and industrial needs

Understand and model fundamental chemical processes

Provide state-of-the-art frequency standards

Transfer forefront spectroscopic expertise to other NBS Divisions,

other government agencies, industry and the research community.

Within the framework of these general goals the Division also follows

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

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

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

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

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

conformation, thermodynamics, and reactivity of such systems. The objectives

of our thrust in biotechnology are to develop and implement methods for the

study of metal interactions in proteins and nucleic acids using primarily

quantum mechanical techniques, while the major effort of the third thrust is

directed to the study of vibrational energy transfer.

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

groups within the Division during the past year and demonstrates how they are meeting the above objectives.

1
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 scientists.

During the past year we have had fourteen guest scientists and four post

doctoral fellows engaged in Division projects. In addition, two members of

the Surface Science Division participated in the work of the LPC group. A

list of the guest scientists, their permanent affiliations, and their

activities is given in Section 8 of this report.

C. Division Programs

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 is 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. 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 chemistry, and combustion diagnostics are a few of the many

important high technology areas that depend on modern spectroscopic

techniques

.

As described in Section 2, the work of the HR 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 weakly bound molecular species.

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

atmosphere, especially the role played by trace constituents in the

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

group's activities is devoted to providing critically needed data to help

solve some of these problems. In this effort our activities are guided by

the needs of the environmental research community, and are conducted with the

support of NASA and AFGL. These efforts will continue to play an important

role in the work of the HR group during the coming years.

2
Weakly bound molecular complexes, i.e., van der Waals and hydrogen bonded

molecules have received considerable attention in our laboratory during the

past year and will continue to do so in the future. The Division is in a unique position of having available personnel and instrumentation that

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

the microwave to the near infrared region of the spectrum in a concerted

attack on the problems posed by these weakly bound systems. These

experimental efforts are buttressed by the analytical and computational

theoretical expertise available in the Division.

The Division is the home of the NBS Molecular Spectra Data Center. This

Center, supported by the Office of Scientific Reference Data and staffed by

members of the HR group provides evaluated data on molecular rotational

spectra in the microwave region and develops evaluated infrared spectra to be

used as standards in calibrating diode lasers and infrared laser

spectrometers. This work is based on the needs of the user community. Thus

the work on infrared standards is a response to requests from the

instrumentation industry, industrial research laboratories, applied

technological user groups, and government agencies and laboratories.

Improvements in the stability of diode lasers have emphasized the need for

accurate calibration standards. The absorption frequencies of ^0 that

involve the lower energy levels are now being tied to the cesium frequency

standard. This work has been made possible through the joining of the unique

facilities of the Time and Frequency Division of NBS/Boulder with our spectroscopic expertise. Other collaborations, with non-U. S. research groups

exist as well in an effort to develop the most extensive and accurate set of

frequency calibration data. A recent activity is the critical evaluation and

tabulation of the vibrational and electronic energy levels of transient

molecules. Section 2.B of the HR group report describes the various

activities of the Data Center.

The LPC group primarily does quantum-state specific studies of molecular

dynamics. A common factor in the research of this group is the role of

energy transfer in the spectroscopy and kinetics of molecules. Three major

activities are pursued: Condensed Phase Energy Transfer, Laser Diagnostics

of Surface Dynamics, and Dynamics of van der Waals Molecules. As described

in Section 3.B, in the first of these the direction taken is the study of

vibrational energy transfer. This research was extensively discussed in the

Annual Report for FY85 and the research outlined and proposed there has been

pursued successfully during the past year in the determination of the rates

of energy transfer from vibrationally excited chemical bonds in liquids,

solicjs, and on molecules bound to surfaces.

In the second major activity the goal is to obtain an understanding of

the energy transfer and chemical dynamics occurring at metal surfaces at a fundamental level. This work is done in close collaboration with staff

members of the Surface Science Division. This collaboration has resulted in

the creation of an experimental facility to perform state-resolved studies of

3
thermally and laser desorbed molecular processes at well characterized

surfaces. Details about this year’s accomplishments are presented in Section

3.C.

The capability to perform time resolved spectroscopic measurements at

the picosecond level has been turned to good advantage in the study of

vibrational predissociation of van der Waals molecules. The experiments are performed in the time domain and thereby yield a direct measure of the

lifetimes of the predissociating states. This work is described in Section

3.D of the LPC group report.

The field of Quantum Chemistry continues to experience new developments

which extend the range of reliable and useful calculations to complex

polyatomic systems. The stabilities, conformations, spectroscopy and

reactivity of complex molecules, clusters, ions, radicals, and transition

states, among others, are successfully investigated by the Quantum Chemistry

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

problems using ab-initio rather than semi -empirical methods is significantly

enhanced by the dedicated IBM 4381 superminicomputer located in the Molecular

Structure and Modeling Facility of the Molecular Spectroscopy Division.

A major effort of the QC group described in Section 4.B is in the field

of theoretical calculations on biomolecules. The goal of this program is the

theoretical calculation of the energetics, spectroscopy, and reaction

behavior of biomolecules with emphasis on the role of metal interactions in

biomolecules. Ab initio methods are used to the exclusion of semi-empirical

methods to insure predictability. The program is developed on three levels.

At the first and fundamental level is the development of an ab initio

electronic structure, reaction field method and code which will allow quantum

mechanical calculations on systems of the order of 1000 atoms. Molecular and

statistical mechanics is used to provide conformation for quantum models and

to determine the environment for a reaction field approach. At the second

level accurate ab initio calculations of small prototype systems provide data

on a wide range of properties which also contributes to the large theoretical

data base required in the reaction field code. At the third and applied

level, model calculations that relate to specific problems of current

interest are undertaken. The reaction field code development is the major

thrust of this program.

During the last few years the three groups have been engaged in the study

of the properties of van der Waals and hydrogen bonded complexes. This

activity is continued using the experimental and theoretical expertise

represented in each group and the individual group reports give detailed

description of the work done. Only free complexes (i.e., gas phase) are investigated in order to obtain clearly interpretable data on molecules which

are unperturbed by molecular interactions that exist in the condensed phase.

Thus the HR group reports on its studies of the resolved microwave rotation

and infrared rotation-vibration spectra while the LPC focusses its attention

on the dynamics of vibrational pre-dissociation. In many cases the

interpretation of the experimental results is aided by the analytical and

4
computational theoretical studies performed in the HR and QC groups. The

work done by the three groups on weakly bound complexes constitutes therefore

a Division wide effort in which the groups are participating in a mutually

supportive and beneficial manner. The study of weak molecular interactions

as exemplified by the van der Waals and hydrogen bonded molecules is a field

of considerable present and future interest and our Division is expected to

fully participate in these investigations during the coming years (cf. the

Pimentel Report "Opportunities in Chemistry," National Academy Press,

Washington, D.C., 1985, and the more recent Brinkman report on "Physics

Through the 1990s," National Academy Press, 1986).

D. Collaborations

The Division staff conducts its research in collaboration with

postdoctoral associates, members of the Surface Science and Time and Frequency Divisions as well as with many non-NBS visiting scientists. Some of

these use the facilities of the Division and are appointed Guest Scientists

at NBS. Section 8 of this reports lists the names, affiliations, and

activities of the guest scientists who collaborated with us during the past

year. Further collaborations exist with other scientists associated with

universities from the U.S. and abroad whose names are given in the various

project descriptions and appear as co-authors of many of the publications and

talks listed in Sections 5 and 6 of this report.

E. Other Agency Support

Among the several goals of the Division are the services provided to

various agencies of the government and private industry which in turn support

some of the work done in the Division. The various agencies of the

Department of Defense (ONR, ARO, AFGL, AFOSR), NASA, DOE, as well as the

National Foundation for Cancer Research are among those that sponsored some

of the research done during the past year.

In the remainder of this Annual Report are the individual group reports

(Sections 2, 3, 4), the listings of publications and talks (Section 5, 6),

Division seminars (Section 7), and guest scientists (Section 8).

5

2. HIGH RESOLUTION SPECTROSCOPY GROUP

N. Craig, G. T. Fraser, C. W. Gillies, J. T. Hougen, M. E. Jacox,

W. J. Lafferty, F. J. Lovas, A. G. Maki, W. B. Olson, A. S. Pine,

G. Rotter, R. D. Suenram, G. A. Thompson, A. Weber, and J. Zozom.

A. Introduction

B. Calibration and Data Center Activities

1 . NpO Frequency Measurements Extending Calibration Data into the

2800 cm~1 Region

2. OCS Heterodyne Measurements for Calibration from 1866 to 1920 cm

3. Recommended Rest Frequency List

44. Microwave Spectral Tables

5. Update of Landolt-Bornstein Structural Tables of Free Polyatomic

Molecules

6. Vibrational and Electronic Energy Levels of Transient Molecules

7. Future Plans

C. Spectroscopy for Environmental Studies

1 • N

2

- and Air-Broadening in the Fundamental Bands of HF and HC1

2. Measurements of Collisional Interference in Nitrous Oxide

3. Heterodyne Frequency Measurements for Nitric Oxide (NO)

^4. New Measurements and Analysis for Nitric Acid (HNO^)

5. Rotational Intensity Dependence Measured for CIO

6. Analysis of the Spectrum of H

2 02

7. Future Plans

7
D. Spectroscopy of Hydrogen-bonded and van der Waals Complexes

1 . Rotational Spectrum and Structure of CF

3H-NH 3

2. Rotational Spectrum of the Hydrogen-bonded Formamide-water Dimer

3. Microwave Spectrum and Properties of the Methanol-Formamide Dimer

4. Microwave Spectra and Structures of the van der Waals Complexes

Ar-Formamide and Ar-Methanol

5. Nearly Free Internal Rotation in Ar-CH^Cl Determined from its

Microwave Spectrum

6 . The Microwave Spectrum of the K=0 States of Ar-NH^

7. Van der Waals Potentials from the Infrared Spectra of Rare Gas-HF

Complexes

8 . NATO- Advanced Research Workshop

9. Future Plans

E. Matrix Isolation Spectroscopy

1 . Fluorine-Atom Reaction Study

2. Excited Argon-Atom Reaction Studies

3. Reaction Intermediates in the Decomposition of Energetic

Materials

4. Future Plans

F. General Spectroscopy and Theory

1. Fourier Transform Measurements of High Temperature Species;

Infrared Gas Phase Spectra of LiCl, LiH, and LiD

2. Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene C

2H 2

+

3. The Torsional-Wagging Tunneling Problem and the Torsional-Wagging

Rotational Problem in Methylamine

4. Far-Infrared Spectrum of Methylamine

5. Transfer Optics System Design

6. Analysis of Infrared Spectra of SiH^D

7. Future Plans

8
A. Introduction

The work of this group spans a variety of spectroscopic topics many of

which are a part of programmatic efforts. Accordingly the activity

reports are grouped into five separate sections: Calibration and Data

Center Activities, Spectroscopy for Environmental Studies, Spectroscopy of

Hydrogen-bonded and van der Waals Complexes, Matrix Isolation

Spectroscopy, as well as General Spectroscopy and Theory. Each section is

concluded with a brief statement of work planned for the coming year. The

descriptions list the scientists active on the projects. These include

NBS staff, guest scientists, as well as, outside collaborators.

B. Calibration and Data Center Activities

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

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

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

molecular constants for use in other disciplines. Occasionally, spectral

fitting programs, for use in reducing data in other laboratories are also

proceeded. The group devotes a significant portion of its effort each

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

laboratories. Outputs of the program serve needs spanning the microwave

to the ultraviolet spectral regions.

1 • NpO Frequency Measurements Extending Calibration Data into the

2o00 cm

-1 Region

(A. G. Maki, J. S. Wells, and A. Hinz)

In a continuation of our collaborative work with Joe Wells (NBS,

Boulder) we have this year measured the frequency of several important hot

band transitions in the 1257 to 1335 cm

-1 region. These data were used to

provide a finer comb of calibration features in this region, but their

most important contribution has been to tie more energy levels of N

2

0 to

the cesium frequency standard. From these accurate energy level

determinations, it is possible to obtain accurate transition frequencies

outside the area of direct measurement. In particular, these new frequency measurements allow us to extend our calibration tables to the

2800 cm

-

' region, which was the primary goal of this year's work.

This work has also given us the opportunity to compare

our frequency measurements with frequencies derived from wavelength

measurements coming from various other laboratories. It appears that the

recent work reported by Toth (JPL) using the Brault interferometer at Kitt

Peak gives wavenumber values that are consistently close to those we measure. Toth's accuracy seems to be largely due to the calibration

procedure he has been using based on frequency measurements of several

bands.

9
Some of the hot band measurements that were made this past year will assume greater importance next year if we are able to measure some of the

other transitions that are on our agenda. These measurements will provide

frequency values for new energy levels and this will lead to new calibration bands in the 2300 to 3000 cm

-

^ region. This work is supported

by NASA.

2. 0CS Heterodyne Measurements for Calibration from 1866 to 1920 cm ^ (A. G. Maki, J. Wells, and A. Hinz)

Another advance in the development of 0CS absorption lines as convenient frequency standards was made by the heterodyne measurement of a number of transition frequencies in the 1866 to 1915 cm“I region. These

measurements have allowed us to prepare an improved calibration table for

this region and also provide a good measure of the energy levels for the

12°0 state from which other transition frequencies can be calculated.

This work is supported by NASA.

3. Recommended Rest Frequency List

(F. J. Lovas and G. Rotter)

The first revision of the paper originally entitled "Recommended

Rest Frequencies for Observed Interstellar Molecular Microwave Transitions

- 1984 Revision" was published in the January 1986 issue of J. Phys. Chem.

Ref. Data. While the manuscript was in the publication process, we continued to update the spectral entries through 1985 so the title was modified to "- 1985 Revision" to reflect the content more accurately.

The next revision of this paper is planned for 1988, so the

literature search of radio astronomy papers and data evaluation of new microwave studies of these species will continue. Last year preprints were widely distributed and very favorable responses were received. Since

the manuscript was published in January, several requests for magnetic

tape copies have been received and very recently Professor K. R. Lang of

Tufts University requested permission to reproduce the frequency list in a forthcoming book Astrophysi cal Formulae .

4. Microwave Spectral Tables

(F. J. Lovas, G. Rotter, and R. S. Suenram)

The general search for all papers dealing with microwave spectral

data continues. Reprints or photocopies of all pertinent reports are obtained and filed according to empirical formula. These files are the source of data for spectral evaluation reported in the various publication

series which the data center produces.

Work continues on the Part III of the Microwave Spectral Table

series. This involves the spectra of simple hydrocarbon species C

xH

y

. During the current year we have been carrying out evaluations on species

exhibiting internal rotation from methyl groups. This project is slated

for completion in September 1986 and appears to be on schedule. This is

10
part of the long range plan to develop a microwave spectral atlas, so care

is taken to keep all the coding compatible with previous compilations in

this series.

We continue to up-date our computer files on diatomic and triatomic

species. Since the publication of Part I. Diatomic Molecules, 21 new species have been added and the spectra for 3^ species have been augmented.

Similarly for Part II. Triatomic Molecules, spectra for 15 new species

have been added and the data on 30 other species have been updated.

5. Update of Landolt-Bornstein Structural Tables of Free Polyatomic

Molecules

(J. H. Callomon, E. Hirota, K. Kuchitsu, and W. J. Lafferty)

A project to update the Landolt-BQrnstein compilation,

"Structure Data of Free Polyatomic Molecules," has just been completed.

This effort includes all structural data of gas phase molecules reported

in the literature since the last volume was published in 1976 up to the

end of 1985. It includes structures obtained by microwave, infrared, Raman,

and visible spectroscopy as well as electron diffraction techniques. About

1500 new or improved structures are included in this work which has been a collaboration between workers in England, Japan, and the United States.

The new volume is expected to be in press in early 1987.

6. Vibrational and Electronic Energy Levels of Transient Molecules

(M. E. Jacox)

All of the data previously compiled on the ground-state

vibrational energy levels of polyatomic transient molecules have been

transferred to a new IBM PC-AT computer system. This system not only is

much more efficient, but also will facilitate possible future development

of a version of the tables amenable to digital search. The tables of

ground-state vibrational energy levels have been kept current; new or revised tables have resulted for approximately 75 molecules per year.

Matrix effects on the electronic spectra of diatomic molecules

have been surveyed in a paper entitled, "Comparison of the Electronic

Energy Levels of Diatomic Molecules in the Gas Phase and in Inert Solid

Matrices," to be published in a forthcoming issue of Journal of Molecular

Structure honoring Professor George C. Pimentel. Electronic band origins

and vibrational band spacings in excited electronic states have been

compared for the approximately 230 electronic transitions of diatomic

molecules which have been observed both in the gas phase and in rare-gas

or nitrogen matrices. With few exceptions, valence transitions and the

associated vibrational band spacings are shifted by less than about 1% in neon matrices. Somewhat larger shifts, often to longer wavelengths,

result from isolation of the molecule in a heavier rare gas or a nitrogen

matrix. The perturbation of Rydberg transitions by the matrix, the

effects of charge-transfer interaction with the heavier rare-gas matrices,

and the behavior of the potential curve of the matrix-isolated molecule in

the vicinity of the dissociation continuum are discussed.

11
With partial support from the NBS Office of Standard Reference

Data, experimental data are being critically evaluated and compiled for

the electronic energy levels of transient molecules possessing from 3 to

6 atoms. Both gas-phase and inert matrix data will be included. A wide

variety of observation techniques, including high resolution absorption

spectroscopy, laser-excited fluorescence, multiphoton ionization, and

photoelectron spectroscopy, are being surveyed. Energies of band origins,

fundamental vibration frequencies, rotational constants (A, B, C), and

radiative lifetimes are among the properties now being entered. When

complete, the tables will include well over 150 molecules.

7. Future Plans

Further work in our frequency standards effort is to

systematically develop the N

2 0 spectrum and energy levels for use as frequency calibration bench marks by heterodyne frequency measurements of

the lO^O-OB^O transitions near 1060 cm~^ and the 10^0-01^0 transitions

near 1635 cm

- ”' . When combined with other frequency measurements made

earlier in this program, these new measurements will provide the first

frequency data on the 02°0-00^0 transitions near 1168 cm

-

^ and thereby

will provide improved data for the 10^0 and 01^0 levels of N

2

o.

Evaluation of microwave spectra data will continue to focus on the

two major projects in progress. The radio astronomy literature is

continually searched for new observations to be included in the next

revision of the Recommended Rest Frequency compilation. When the current

work on the Microwave Spectral Tables - Part III on simple

hydrocarbons is finished, we will begin evaluation of the next group,

hydrocarbons containing oxygen.

The compilation of the electronic spectral data tables is nearing

completion. It is planned to submit these tables to the Journal of

Physical and Chemical Reference Data late in FY87.

Revisions and additions to the tables of ground-state vibrational

energy levels will continue to be kept current. It is also planned to

submit a supplement to these tables to the Journal of Physical and

Chemical Reference Data late in FY87. This supplement will include a master index, in order to facilitate a complete search of the experimental

literature on the vibrational and electronic spectra of polyatomic

transient molecules.

C. Spectroscopy for Environmental Studies

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

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

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

interest of these agencies generally focusses on molecular spectroscopy

as a remote sensing tool and for the determination of concentration

profiles. Consequently, the emphasis in this section is on band analyses

as well as the important information on line intensities, line widths,

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

12
branches of spectroscopy, the very narrow band width of infrared laser

sources now permits quite reliable studies of phenomena involved in

spectroscopic concentration determinations.

1 • N

2

- and Air-Broadening in the Fundamental Bands of HF and HC1

(A. S. Pine)

N

2

- and air-broadened lineshapes of HF and HC1 transitions in the

v=1-K> bands have been measured at T=295 and 202 K with a high-resolution

difference-frequency laser spectrometer. Pressure broadening and shifts

and collisional narrowing parameters have been extracted by least squares

fitting of several collisional profiles to the spectra. Figure 2.1 shows

the observed lineshape of the R(0) line of HF as a function of N

2 pressure

fit to Voigt and Galatry (soft collision model) profiles. At low

pressures, the collisional, or Dicke, narrowing effect causes deviations

from the Voigt profile having a Doppler-fixed Gaussian component and

yields a measure of the diffusion constants for the hydrogen halides in

the buffer gases. The pressure broadening parameters, r, were effectively

linear with pressure for the Galatry collisional narrowing model as they

should be, whereas the Voigt fits were highly nonlinear as shown by the

ratios of f for the two models in Fig. 2.1. At high J, where the pressure

shifts are comparable to or larger than the broadenings, a slight

asymmetry is observed in the lineshapes which is attributed to statistical

correlation between velocity- and state-changing collisions. Fig. 2.2

indicates this asymmetry for the P ( 9 ) line of HF at P(N 2

) - 54 Torr fit

to the formally symmetric Voigt, soft and hard collisional narrowing

profiles

.

The ratio of air-to-N2 broadening varies systematically with

rotational levels and is always somewhat larger than that given by the

relative quadrupolar contributions of 0

2 and N 2

. The temperature

variations in the broadening coefficients are also J dependent, deviating

significantly from T

- ^^ ancj exhibiting opposite behavior for HF and HC1.

These effects are summarized in Fig. 2.3 as a function of the rotational

index m. This work is supported by NASA.

13
INTENSITY

(ABSORBANCE)

INTENSITY

(ABSORBANCE)

Fig. 2.1 Lineshape of HF as a function of N

2 pressure fit to Voigt and

soft collision models.

14
o

oLDCD

OJ ocn O CD O

CO CD

O

OONvgaosav) aiisnbini

O

CO •oc

cti

"O

s_KJ x

4-> bO •Ho>

cd x

4Jo

c

3oX

CO

[i-l X

Cmo

CDC

CD X

tCM

CM

bO

•rH

15

collisional

narrowing

models.
10 8 6 A 2 0 -2 -A -6 -8 -10

Fig. 2.3 Ratio of the air-to-N

2 broadening of HF and HC1 as a function of

temperature and rotational index, M.

2. Measurements of Collisional Interference in Nitrous Oxide

(A. S. Pine)

We have been making a number of colls ional lineshape studies in

support of a NASA program to monitor the upper atmosphere for trace

contaminants using sensitive infrared techniques. One of the major

uncertainties is how to interpret the observed profile of a dense

unresolved spectrum, such as occur for the strong, characteristic Q-branch

and band-head features used for monitoring various species. When spectral

lines overlap due to collisional broadening, the resulting envelope may

not be the simple addition of individual component lineshapes if the

collisions actually transfer excitation between energy levels involved in

the unresolved transitions. This nonadditivity is a collisional

interference effect, akin to motional narrowing, variously known as line

coupling, line mixing or rotational collapse, and is well known for

densities approaching the condensed phase. However, in the range of

pressures below lO^Pa, pertinent to atmospheric and stratospheric

conditions, the deviations from additivity are typically small and subtle

16
and require exceptionally high resolution, good signal-to-noise ratio and

wide linear dynamic range as well as precise scanning control in order to

be reliably measured.

Collisional interference at subatmospheric pressures has been observed

recently using high resolution coherent Raman gain techniques for the Q

branches of N

2 , CO and NO and in the infrared for Q branches of CO2 using

diode lasers and Fourier transform interferometers . In the present work

we have performed measurements on N

2

o with a precision tunable

difference-frequency laser system. Preliminary transmission spectra of

the Q branch of the v-| + V 2 combination band of N2

O as a function of

pressure are shown in Fig. 2.4 illustrating the high quality of the data.

Fig. 2.4 Transmission spectra of the Q branch of the Vi + \>2 combination

band of N

2

0 as a function of pressure.

The wavenumber scale is linear and reproducible to S 5 x 10

-

^ cm

-1 using

digital scanning servo control; the signal-to-noise ratio and the linear

dynamic range are £ 1000:1 using real-time division of the transmitted

light by the incident light. The 0% transmission baseline is stable to

within 0.1% and the 1 00% transmission baseline is reproducible to within

Mo. This latter baseline uncertainty is the principal source of error in

these measurements, particularly for the higher pressure traces where the

troughs between the lines exhibit strong absorption and there is no clue

as to the position of the baseline over an extended spectral range. The

baseline is then determined from a sequential empty cell trace. However,

.« 17
traCe-to-trace variations in the relative sample and reference beam

channels arise predominantly due to drifts in the interference fringes of

the two detectors windows and filters (the sample cell windows are mounted

at Brewster's angle to minimize reflections) and to beam wander from

pressure-induced displacements and from thermals drifts of the

phasematching oven for the LiNb0

3 nonlinear optical mixing crystal. The

phasematching fluctuations are peculiar to the difference-frequency laser,

but the other problems are common to any high-resolution transmission

measurements. We are now developing a digitally-tuned F-center laser

system with enough power to do direct optoacoustic absorption (rather than

transmission) spectroscopy in order to improve the baseline

reproducibility for collisional interference and for wing measurement.

This work is supported by NASA.

3. Heterodyne Frequency Measurements for Nitric Oxide (NO)

(A. G. Maki, J. S. Wells, and A. Hinz)

For certain measurement applications, especially those involving

laser techniques, it is often important to know quite accurately the

frequencies of the absorption lines of NO. By using heterodyne frequency

counting techniques we have measured 28 transitions of NO from 1750 to

1930 cm

-1

. With the help of other measurements in the literature,

especially microwave and radio frequency measurements, we have determined

an improved set of constants to describe the infrared spectrum of NO.

With these constants, very accurate tables of energy levels and transition

frequencies have been prepared for NO. This work is supported by NASA.

H. New Measurements and Analysis for Nitric Acid (HNO^)

(A. G. Maki, A. Fayt, J. Wells, Wm. B. Olson, and A. Goldman)

Over the past several years we have been involved in the

measurement and analysis of the spectrum of nitric acid with the goal of

producing an atlas or compilation of transition frequencies and

intensities. Such a compilation is very useful for the analysis of remote

sensing data being accumulated in various programs run by NASA and other

agencies. As an example, Dr. Linda Brown (JPL) has taken our latest

line-list for the v

2 band of HNO

3 (near 1723 cm

-1

) and added it to the

data bank she has accumulated to help her in the analysis of ATMOS data.

She was very pleased with the accuracy and completeness of our data. Dr.

A. Goldmann (University of Denver) has also been very anxious to use our data on nitric acid and from time to time requests an updated copy of our latest data.

This past year new diode measurements were completed on the high

rotational transitions of the vr band of HNO

3

. These measurements, when

combined with microwave and earlier infrared measurements from this

laboratory, provided a more accurate set of ro-vibrational constants for

the v

2 band that enabled us to calculate an improved line-list. This

improved line-list, consisting of about 12,000 entries, has been

18
circulated to various interested parties and will be included in the next

version of the AFGL-trace gases tape, which is the major data source for

most atmospheric workers.

The analysis of the highly perturbed v

5 and 2vg bands has been

sucessfully accomplished by using a new computer program which fits data

for two asymmetric rotor bands that are coupled through Fermi resonance

and certain other resonances. This analysis allows us to fit even the

most highly perturbed transitions with an overall standard deviation of

0.0007 cm

- 1 which is the accuracy expected since a number of transitions

used in the fit are known to be overlapped even though a diode laser with

nearly Doppler limited resolution was used for the measurements. Fig. 2.5

shows a particularly clean portion of the P-branch region of Vr in which the

transitions are identified. In this figure it is easy to see how the

perturbation shifts the K

a < i 3 transitions to high frequencies while it

shifts the K

a > 13 transitions to low frequency.

hno3 v*

869.4 869.6 869.6

WAVENUMBERS (cm- 1 )

Fig. 2.5 Region of the P-branch in the band of nitric acid.

Because of a high degree of correlation between some constants, this

work will not be complete until a few more carefully chosen transitions

are measured to try to reduce the correlation as much as possible. Then

it will be necessary to test a new computer program which has been written

to prepare an atlas of line positions and intensities for these two bands.

We can anticipate that a complete line-list will be ready for the 850 - 930 cm

-

^ region of HNO^ within the next year. An important part of this

work will be the determination of the relative signs and magnitudes of the

transition moments for and 2vg. This work is supported by NASA.

19
5. Rotational Intensity Dependence Measured for CIO

(A. G. Maki, J. B. Burkholder, P. D. Hammer, C. J. Howard,

G. Thompson, and C. Chackerian)

New frequency and relative intensity data on CIO have been

determined using some diode laser measurements from this laboratory and some

FTS measurements from the NOAA-Environmental Research Laboratory in

Boulder, CO. Last year we described the reasons for this work, namely the

need to verify the line positions and intensities in a search for the reason for discrepancies between infrared and microwave measurements of

atmospheric CIO. After the work was begun here, it was learned that NOAA

was making similar measurements and we joined forces.

One of the goals of this work was to measure the rotational

dependence of the intensities of the fundamental infrared band of CIO.

The rotational dependence is related to the ratio of the dipole moment of

CIO to the vibrational transition moment. Since the transition moment

seems to be small and the dipole moment is known to be fairly large, it was expected that the rotational dependence would be significant and

could, in fact, be used to determine the transition moment. Our

measurements did not require a knowledge of the concentration of CIO

whereas all previous techniques had involved a measurement of the

concentration of CIO in the absorption cell; a difficult and controversial

measurement

.

This work is now completed and a manuscript has been prepared but

the results can be summarized as follows:

o The CIO line frequencies are now known with greater accuracy,

especially for the P-branch lines. While the new line positions are different

from the old by as much as 20 MHz, the difference is not enough to

reconcile the problems with the field measurements.

o The rotational dependence of the line intensities is now known,

but it only affects the derived concentration of CIO by 10 to 1 5% whereas

the discrepancy is larger than that.

o From the rotational dependence, the transition moment has been

derived and it agrees with the previous value of 11.8 cm

-

^ atm

-1

, although

the present results are not as accurate as we had hoped for.

o By several arguments, including a determination of the

temperature from intensity measurements, it has been shown that the

laboratory measurements involve CIO that is in thermodynamic equilibrium

at room temperature.

o It has been shown that both the infrared and microwave field

measurements involve CIO transitions that originate from the same vibronic

state which is the lowest vibronic state.

20
The net results of this work is that we are unable to explain the

problems with the field measurements, but the problems do not come from

the laboratory data. This work is supported by NASA.

6 . Analysis of the Spectrum of H

2

O 2

(W. B. Olson, R. H. Hunt, A. G. Maki, and J. W. Brault)

Rotational constants of the OG and 1G torsional states and the

matrix element coupling these states have been determined by fitting to a model of two coupled asymmetric rotors. The fitted data were primarily

combination differences. Those for the OG torsional state were formed

from published values of submillimeter wave transitions for low values of

K

a , and those for high Ka (up to Ka

= 10) were obtained from assigned

transitions in the OH stretching band. There are as yet no assigned

submillimeter transitions involving the 1G torsional state, and all of the

combination differences for this torsional state were obtained from the

assigned transitions in the torsional hotband + vij(IU) - (1G).

Combination differences alone do not supply enough information to

determine all the relevant parameters, i.e., they do not give direct

information on the energy difference between the origins of the OG and 1G

torsional states. The rotational-torsional energies of the OG state for

K

a

= 8 are only about 3 cm

-

^ above those for K

a

= 6 in the 1G torsional

state at low J, but because of the perturbation the levels of the two

states are pushed apart, the separation increasing with increasing J to

about 7 cm

-1 at J = 25. Mixing of the wavefunctions of the OG, K

a

= 8

levels with those of 1G, K

a

= 6 through the perturbation leads to

perturbation enhanced transitions from 1 G, Ka

= 6 to the V 5 state with Ka

= 7. Although these could be considered normally allowed transitions of a difference band, no transitions can be observed at J values lower than 15.

It is only at this and higher values of J (to 25) that the wavefunction

mixing is sufficiently large so as to provide sufficient intensity that

the transitions may be observed. Differences between the normal

transitions from OG, Ka

= 8 to Ka

= 7 in v^, and the perturbation enhanced

transitions from 1G, K

a

= 6 to the same upper state levels provides direct

information on the separation of the lower torsional state origins.

We obtained valuable supplementary information by assignment of

the weak spectrum of the sum band from the OG state to + vij(IU).

Direct differences between levels of the OG and 1G torsional states were

obtained by differences of transitions with the same J and K

a in the OG

and 1G torsional states to the same upper state level in the vr + vjj(IU)

vibrational-torsional state. Final constants are not yet available, but

the fit is quite satisfactory at the present stage of refinement. The

origin of the 1G torsional state is 254.550 cm~^ , about 0.35 cm

-1 above

the value obtained from the medium resolution far infrared spectrum. Our

experimental value for the matrix element for the interaction of the OG

and 1G torsional states proves to be only 1555 higher than the value

calculated by Hunt, Leacock, Peters, and Hecht [J. Chem. Phys. 42, 1931

21
(1965)]. Considering the complexity of their model and uncertainties in

parameters required for input for their calculations, the agreement with

our experimental value may be considered excellent.

7 . Future Plans

In the coming year an atlas of absorption line frequencies and

intensities will be completed for the j-V q bands of HNO 3 near 880

cm

-1

. This atlas will include all the transitions to these two states

from the ground state including those transitions that are strongly

perturbed by the effects of the Fermi resonance between these two states.

We also expect to complete a paper describing the analysis of the spectrum

of the and 2vg bands and also to complete a paper on the analysis of the V

3 and vjj bands near 1 300 cm

-1

. This work is needed by various laboratories

that are studying the concentration of nitric acid and nitrogen oxides in

the atmosphere in connection with the ozone problem as well as the

greenhouse effect.

By using a long path cell we should also like to make some heterodyne measurements on some of the weaker bands of OCS such as

11 1 0

- 00°0 (1372 cm

-1

) 03 ^ - 00°0 (1573 cm" 1 ) and 00 ° 1 - 02°0 (1015 cm" 1 ).

These measurements would locate more energy levels with respect to the

ground state and would give us frequency data for the 520 cm

" 1 and 2100

cm

"

1 regions, as well as other regions.

We also expect to have new Fourier transform spectra of the lowest

frequency fundamental band of S0 2 near 500 cm

-1

. Both frequency and

relative intensity measurements will be made on this band and the goal

will be to produce a set of band constants that will enable us and others

to accurately calculate the infrared spectrum of S0 2 in the ^00-600 cm

-1 region for any temperature appropriate to an atmospheric path.

Understanding atmospheric S0 2

, is, of course, important for understanding

the acid rain problem.

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

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

initiated several years ago now represents a significant fraction of the

group's total effort. Continuning studies using a difference frequency

laser spectrometer in the 3 micron region, the BOMEM Fourier transform

spectrometer in both the mid- and far- infrared, and studies using the

pulsed beam Fourier transform microwave spectrometer are reported. The

major efforts this year deal with structural determinations of weakly

bound dimers from microwave spectra and potentials derived from extensive

infrared spectra. This program has benefitted greatly this past year from

collaborations with guest workers. We certainly plan to continue work in

this direction for the forseeable future.

22
1. Rotational Spectrum and Structure of CF^h-NH^

(G. T. Fraser, F. J. Lovas, R. D. Suenram, D. D. Nelson, Jr.,

and W. Klemperer)

The rotational spectrum of CF

3

h-NH

3 has been obtained using a pulsed nozzle Fourier transform microwave spectrometer. A symmetric top

spectrum is observed that is consistent with free internal rotation of the

NH

3 subunit against the CF3H subunit. Rotational transitions have been

measured for both the ground and first excited internal rotor state of the

complex. The spectroscopic constants which have been obtained include: Bq

= 1996.903(2) MHz, Dj = 3.46(12) kHz, and eQ

q

N = - 3-186(8) MHz. From the

quadrupole coupling constant of the nitrogen nucleus, eQ

q

^, the bending

amplitude of the NH? unit is determined to be 22.57(10)°. The hydrogen

bond length is 2.314(5) A and the weak bond stretching force constant is

0.066(2) mdyn/A. The bond length and stretching force constant for

CF

3 h-NH 3 are similar in value to those determined for HCCH-NH 3 (2.33 A and

0.070 mayn/A, respectively).

2. Rotational Spectrum of the Hydrogen-bonded Formamide-water Dimer

(R. D. Suenram, F. J. Lovas, G. T. Fraser, J. Zozom, and C. W.

Gillies)

The characterization of hydrogen bonding between peptides and water

molecules is one of the most challenging problems in modern science. The

fundamental peptide-water hydrogen bonding may be exemplified by the

prototypical formamide-water species. To date most of the information

available on these systems has been provided by ab initio calculations.

Now for the first time the complex of these two species has been

investigated experimentally by employing a pulsed molecular beam

Fabry-Perot microwave spectrometer with a heated pulsed nozzle.

Based on the ab initio calculations of Jasien and Stevens, [J.

Chem. Phys. 84_, 3271 (1986)], we have investigated the rotational spectrum

of the lower energy form of the formamide-water complex. A planar cyclic

double hydrogen bonded structure was calculated to be the lowest energy

form, and the observed spectrum is in excellent agreement with this

conclusion. The rotational constants obtained are listed in Table 2.1.

Comparison of the "* nuclear electronic quadrupole coupling constants

between the complex and formamide indicates that the complex is slightly

non-planar with an angle of 16.8° between the planes of the subunits. The

structure of the complex is illustrated in Fig. 2.6.

23
Fig. 2.6 Structure of the formamide-water dimer.

Table 2.1 Molecular constants determined from the microwave spectra of

NH 2 CH0--*H2

0 and NH 2CH0 • • ‘HOCK

3

Parameter NHpCHO • • *H 2 0

Value (MHz)

NH2 CH0- • *H0CH3

Value (MHz)

A 1 1227.91 3(34) 10186.69(15)

B 4586.997(2) 2088.86(8)

C 3258.872(3) 1764.18(6)

eQclaa 1.330(5) 1 .287(7)

eQ%b 2.038(5) 2.026(6)

eQ<Joc -3.3685(5) -3.313(6)

-0.1095 uA

2 -5.05 uA 2

24
3. Microwave Spectrum and Properties of the Methanol-Formamide Dimer

(F. J. Lovas, R. D. Suenram, G. T. Fraser, J. Zozom, and C. W. Gillies)

Recent a_b initio calculations on formamide-water and

formamide-methanol performed by Jasien and Stevens of the Quantum

Chemistry Group [J. Chem. Phys. 8M , 3271 (1986)] indicated that the most

stable form of each is a double hydrogen bonded planar ring structure.

The results of the formamide-water study discussed above are quite

consistent with the theoretical structure, although evidence for slight

non-planarity is given.

Since the methanol complex is expected to have a structure similar

to that of the water complex, we have examined formamide-methanol. The

spectra were obtained in a pulsed beam Fabry-Perot microwave spectrometer

with a heated nozzle containing liquid formamide. Rotational assignments

were considerably aided by the unique hyperfine structure. The rotational

analysis provided the rotational and hyperfine constants shown in Table

2.1. The rotational constants are consistent with the ring structure, but

the out-of-plane moment and hyperfine constants indicate that the complex

is non-planar (disregarding the methyl group).

With the monomer geometries fixed, a rotation of 15° about the HO

axis (moving the CH^ group out of the plane of the remaining atoms)

provides good agreement with the observed moments of inertia. The

structure of the complex is shown in two views Figs. 2.7. The internal

rotation of the methyl group has also been observed. Analysis of the A-E

splittings provides a barrier to internal rotation of 231 cm

-1

, which is

about 387 lower than in free methanol.

Fig. 2.7 Structure of the formamide-methanol dimer (a) top view (b) side

view.

25
4. Microwave Spectra and Structures of the van der Waals Complexes

Ar-Formamide and Ar-Methanol

(C. W. Gillies, J. Zozom, G. T. Fraser, R. D. Suenram, and F. J.

Lovas)

During the course of characterizing the formamide-water and

formamide-methanol complexes, we detected the spectra of two additional

complexes. One of these showed 1 nuclear quadrupole hyperfine structure

while the second exhibited no splittings. By using Ne as the carrier gas

in place of Ar in the pulsed beam expansion, it was clear that each

species contained Ar.

Assignment of the hyperfine structure on the first species

provided a unique rotational state assignment for the Ar-formamide

complex. Spectral fitting provides the distortion free rotational

constants A = 10725.32 MHz, B = 1771.58 MHz and C = 1549.53 MHz. These

constants indicate a non-planar structure as shown in Fig. 2.8 with the Ar

at 45° to the formamide plane.

Based on chemical evidence we identify the second species as Ar-methanol and have assigned the J = 3 - 2 and J = 4 - 3 a-type R-branch

transitions. The rotational constants are consistent with a T- shape

structure as illustrated in Fig. 2.9. A number of strong unassigned lines

remain which we believe is evidence for near free internal rotation of the

methanol subunit.

Fig. 2.8 Structure of the Argon formamide complex.

26
Fig. 2.9 Structure of the Argon-methanol complex.

5. Nearly Free Internal Rotation in Ar-CH-^Cl Determined from its

Microwave Spectrum

(G. T. Fraser, R. D. Suenram, and F. J. Lovas)

observed, at A kHz resolution, using a pulsed nozzle Fourier transform

microwave spectrometer. The observed spectra are consistent with a T-shaped complex in which the methyl group is undergoing nearly free

internal rotation. Analysis of the ground (A-state) internal rotor state

spectrum for Ar-CH^Cl using an asymmetrical top Hamiltonian produces the

spectroscopic constants listed in Table 2.2 for both the 35 qi anci 37qi

isotopic forms. A combined analysis of the ground and excited internal

rotor states (E-state) places an upper bound of 20 cm

-1 on the three fold

barrier to internal rotation. The Coriolis interactions in the E-state

also allow the determination of |eQqab | for Ar-CH^ 5 C1 as 13-0(3) MHz.

The symmetry axis of the CH^ci subunit is nearly perpendicular (-82°) to

the line joining the centers of mass of the two binding partners. The

isotopic data indicate that the Cl end of the methyl chloride is tilted

toward the argon. The distance between the centers of mass of the two

subunits is 3-7825 A for Ar-CH^ ^C1 and 3-7839 A for Ar-CH^ ^Cl implying

an Ar-Cl distance of 3-750 A. Centrifugal distortion analysis yields a weak bond stretching force constant of 0.0157 mdyn/A and stretching

frequency of 3A.6 cm

-1 for Ar-CH^ ^Cl.

Rotational spectra of Ar-CH^Cl, for both Cl isotopes, have been

O Cl

Ar

3.7826 A

97.5“

C

Fig. 2.10 Structure of the Argon-methylchloride complex.

27
Table 2.2 Asymmetric rotor constants (in MHz) for the A internal rotor

states of ArCH

3 35ci and ArCH^Cl.

ArCH

3

35Ci ArCH

3

37ci

A 13633- 020(l4) a 13436. 8700(56) a

B 1593-5683(79) 1565.075(12)

C 1420.4572(52) 1395.6126(36)

AJ 0.0121 6(14) 0.01280(41

)

ajk 0.1381 (48) 0.1392(13)

0.00096(17) 0.00150(1 5)

h jjk -0.00058(25) • • •

e<^aa 34.895(30) 27.432(27)

eQqbb -72.185(25) -56.799(19)

eQ<loo 37-290(35) 29.367(25)

a. Experimental uncertainties are reported as two standard deviations

from the least squares fits.

6. The Microwave Spectrum of the K=0 States of Ar-NH^

(D. D. Nelson, Jr., G. T. Fraser, K. I. Peterson, K. Zhao,

W. Klemperer, F. J. Lovas, and R. D. Suenram)

has been obtained using molecular

pulsed nozzle Fourier transform

The microwave spectrum of Ar-NH

beam electric resonance spectroscopy an microwave spectroscopy. The spectrum is complicated by nonrigidity and

most of the transitions are not yet assigned. A AJ=0, K=0 progression is

assigned, however, and from it the following spectroscopic constants are obtained for Ar-^NH^ (B + C)/2 = 2876.849(2) MHz, D, = 0.0887(2) MHz,

eqQaa = 0.350(8) MHz, and y a

= 0.2803(3) D. For Ar- 1 *NH

3 we obtain:

B + C)/2 = 2768.701(1) MHz, and Dj = 0.0822(1) MHz. The distance between

the Ar atom and the 1 ^NHH-0 center of mass. Rru. is calculated in the free CH» center of mass, R^

internal rotor limit and obtained as 3-8358 A. The weak bond stretching

force constant is 0.00840(2) mdyn/A which corresponds to a weak bond

stretching frequency of 34.6(1) cm

-

^

. The NH^ orientation in the complex

is determined primarily on the basis of the measured dipole moment

projection and the quadrupole coupling constant. It is concluded that the

Ar-NH^ intermolecular potential is nearly isotropic and that the NH

3 subunit undergoes practically free internal rotation in each of its

angular degrees of freedom. Spectroscopic evidence has been obtained

which indicates that the NH^ subunit also inverts within the complex.

These conclusions concerning the internal dynamics in the Ar-NH^ complex

28
are in complete agreement with the model initially proposed in preliminary

report concerning the microwave and infrared spectra of this species

(Fraser et al., J. Chem. Phys. 82_, 2535 ( 1985)).

7. Van der Waals Potentials from the Infrared Spectra of Rare Gas-HF

Complexes

(G. T. Fraser and A. S. Pine)

The rare gas-hydrogen halide (Rg-HX) van der Waals complexes

provide a simple, systematic series for studying weak molecular

interactions pertinent to collisional and condensed phase properties.

Last year we reported high-resolution tunable infrared laser spectra of

the rare gas-HCl species including the observations of rotational

predissociation, manifest as a sharp cutoff of the J progression within a band, and of the large-amplitude bending or librational motion from the

anomalously strong bend-stretch combination band. In the present work we have extended the study to the rare gas-HF species using a difference-frequency laser to record the HF stretching fundamentals, v-|

,

of Ar-, Kr- and Xe-HF under thermal equilibrium conditions at T = 21 1 K

for path lengths up to 80 m. The observed Xe-HF spectra are shown in Fig.

2.11 along with simulations calculated from the measured and predicted

rovibrational constants for the v

1 fundamental nd + V

3 - V3 hot band.

3937

( 10 °

1 ) - ( 00 01 )

, -^^3X1-111

3936 3934 Wavenumber/cm 1

Fig. 2.11 Spectrum of the Xe-HF van der Waals complex for an absorption

path length of 80 m and a temperature of T = 211 K; observed

(top) and calculated (bottom).

29
Rotational structure has been observed up to or approaching

rotational predissociation, permitting us to model the effective radial

van der Waals potentials for these complexes. For example, Ar-HF is found

to predissociate by tunneling through the centrifugal barrier at J=40 in

the ground vibrational state, leading to the empirical potentials shown in

Fig. 2.12. These potentials provide good estimates for the binding

energies, Dq, and the van der Waals stretching frequencies, V

3

, in the

Model Radial Potential

r/A

Fig. 2.12 Empirical potentials for Ar-HF derived from spectroscopic data.

ground (v^=o) and excited (v-|=1) states of the molecules. For v-|=0 in

Ar-HF, Kr-HF and Xe-HF, we find D

Q

= 102, 133, and 1 81 cm

-1 and V

3

= 39.2,

*11.1, and 43**1 cm~^ respectively. The modes characterized by the model

potentials aid in the assignment of the + V

3 - V3

hot bands observed in our spectra. The band centers for the v

1 fundamentals are all

down-shifted in frequency from the isolated HF monomer by Av = -9.654,

-17.518 and -29.185 cm

-

^ for the Ar, Kr, and Xe complexes respectively,

indicating that the van der Waals bonds are some 10 to 15$ stronger in the

excited vibrational state. This increased vibrational attraction also

results in a contraction of the van der Waals radial coordinate manifest

in the larger rotational constants observed for v

1

; AB/Bq = +0.35, +1.00

and +1.75 1 for Ar-, Kr~, and Xe-HF. We have also observed the Q branch of

the v-j + v

2 stretch-bend combination band in Ar-HF some 70.2 cm

1 above

the v-j fundamental with a large negative AB/Bq = -2.00$ implying a strong

anisotropy in the potential.

30
8 . NATO-Advanced Research Workshop

(A. Weber)

A NATO-Advanced Research Workshop on "Structure and Dynamics of

Weakly Bound Molecules Complexes was conducted in Acquafredda di Maratea,

Italy in September 1986. The purpose of this Workshop was to bring

together researchers active in the field of high resolution spectroscopy

and dynamics of van der Waals and hydrogen-bonded molecular complexes in

order to assess the present state of knowledge, to communicate directions

and progress of research already underway and to indicate directions,

potential and level of effort needed for future research. Seventy

scientists from nearly all NATO countries participated in this Workshop.

The topics taken up included the determination of structures by microwave,

molecular beam electric resonance, high resolution infrared,

and visible spectroscopy, rotational and vibrational predissociation

determined from spectral line widths and from time dependent

photodissociation experiments, determination of potential energy surfaces

from experimental data and from ab initio calculations, as well as stability and dissociation dynamics. The proceedings of the Workshop will be

published in the NATO-ASI series of publications.

9. Future Plans

During the past year the pulsed beam Fourier transform microwave

spectrometer ( FTMWS) has provided rotational spectra of both new dimers

and van der Waals complexes. Further work in this direction will continue

next year. A number of dimer studies have been started, but are not yet

complete due to the complexity of the observed spectra. These include

studies of CHoNi^-i^O, CH

3 OH-NH

3 and several new species observed in Ar/03

expansions. We hope to complete the assignment of these spectra during

the coming year.

We will also test several new molecular beam designs. One design

allows mixing two beams in the nozzle exit channel for the study of dimers

of species which react, e.g., ammonium halides. A second design will

provide a dc electric discharge in the beam exit channel which we hope

will generate free radical species.

One of the major thrusts during FY87 will be the development of a pulsed laser ablation system which can be used in conjunction with the

FTMWS to study molecular species that are normally non-volatile. The high

power pulsed laser has been ordered and the pumping system is in place.

All that remains is to design a vacuum chamber to allow the laser-target

to be interfaced with a pulsed nozzle and the FTMWS.

A second area of major emphasis will be to investigate the

feasibility of incorporating a cryogenically cooled, extremely low noise

mixer device in the FTMWS. Theoretically improvements in signal-to-noise

with this type of device could be two or three orders of magnitude.

31
Infrared investigations will also be performed on molecular-beam

adiabatically-cooled systems. A pulsed molecular beam apparatus is now

under construction and will be used for absorption spectroscopy with the

aid of various tunable lasers. A cw-molecular-beam laser resonance

apparatus is now under development and scheduled for completion during the

coming year for vibrational predissociation measurements. Also under

development is a high resolution tunable F-center laser system for high

power excitation of van der Waals complexes for vibrational

predissociation studies.

E. Matrix Isolation Spectroscopy

1 . Fluorine-Atom Reaction Study

(M. E. Jacox)

Some years ago, the infrared spectra of cis- and trans-HOCO were obtained in this laboratory by the reaction of photolytically generated OH

with a CO matrix. These species are of considerable interest to the

combustion scientist, since the OH + CO reaction is the principal process

by which C02 is formed in combustion systems. Because HOCO can hydrogen-bond to the CO matrix, it was desirable to obtain the spectra of

its rotamers in an argon matrix. However, molecules cannot diffuse

through solid argon under the conditions necessary for effective matrix

isolation. Therefore, it was not feasible to study the OH + CO reaction

in solid argon. Preliminary studies, outlined in the Annual Report for

FY85, have shown that the reaction of F atoms with HCOOH provides a suitable source of trans-HOCO. Early in FY86, further isotopic studies on

the F + HCOOH system were conducted using the Beckman IR-9 infrared

spectrophotometer

.

2. Excited Argon-Atom Reaction Studies

(M. E. Jacox)

As was reported in the Annual Report for FY85, a good yield of

trans-HOCO can also be obtained by codepositing the Ar: HCOOH sample with a beam of argon atoms excited in a low-power microwave discharge. This

method of preparation of trans-HOCO has the advantage of eliminating

contributions of HOCO*»*HF from the spectrum. Detailed isotopic studies

of trans-HOCO formed in this system were also performed using the Beckman

IR-9. However, the product yield on deuterium substitution was relatively

low. Therefore, completion was deferred until the BOMEM Fourier transform

system can be used. The greater frequency accuracy of this experimental

system will be valuable for the determination of carbon-13 and oxygen-18

isotopic shifts. In turn, these isotopic data will permit derivation of an improved vibrational potential function for trans-HOCO.

The accessibility of the near infrared spectral region using the

BOMEM system made possible a detailed series of experiments on the

interaction of excited argon atoms with acetylene. Previous studies in

this laboratory have shown that this sampling configuration provides a

32
consistently high yield of the HC 2 free radical, an important intermediate

in astrophysical and combustion systems. Extensive deuterium- and

carbon-isotopic substitution studies spanning the 700-7900 cm

-1 spectral

region were conducted at a resolution of 0.2 cm

-

^

.

The mid-infrared studies yielded a more precise value of 1846.2 cm

-1 for the CC-stretching frequency of HC

2

. Several weak absorptions between

2100 and 3400 cm

-1 were also assigned to HC 2

. An absorption at 3610 cm

-1 has previously been assigned (M. E. Jacox, Chem. Phys. ]_, 424 (1975)) as the CH stretching fundamental of HC 2

. Although this value is anomalously

high for a CH-stretching fundamental, the observation of Fermi resonance

with the first overtone of the CC-stretching fundamental for H

12C^3c and

for H

1 3 c

2 indicates that the energy level of HC 2 which contributes this

absorption must be of Z + vibronic symmetry.

Recently, Curl, Carrick, and Merer (R. F. Curl, P. G. Carrick, and

A. J. Merer, J. Chem. Phys. 82_, 3479 (1985)) have assigned prominent

gas-phase absorptions at 3796, 4012, and 4108 cm

-1 observed by

color-center laser spectroscopy in a discharge through polyacetylene to

combination bands arising from the ground state of HC 2

. Counterparts of

these three bands at 3806, 4022, and 4134 cm

-1 are prominent in the matrix

experiments, consistent with such an assignment. The corresponding

absorptions for the carbon-13 substituted species were also observed in

the matrix experiments.

The matrix studies have provided the first observation of the A

2

n

- A

2

Z + absorption band system of HC 2 in the 4200-7900 cm

-1 spectral

region. As is evident from Fig. 2.13, a rich spectrum is obtained. The

product absorptions shown in trace (b), the spectrum obtained in an

7000 6000 5000 <1000 0*1

Fig. 2.13 HC 2 (DC 2

) absorption spectrum between 3400 and 7200 cm

-1 observed on codeposition of Ar: C

2H 2 -d n sample with a beam of

excited argon atoms. (a) Ar: C

2H 2

= 800 + Ar*; (b) Ar : HCCD = 200 + Ar*; (c) Ar : C

2 d 2 = 150 + Ar*.

33
experiment using an HCCD sample, are the sum of those characteristic of

C

2H2 samples (trace (a)) and C 2 D2 samples (trace (c)), supporting the

assignment of all of the peaks in this spectral region to HC 2 and DC2

. There are concentrations of peaks at intervals of approximately 1500 cm

- ”' , appropriate for a progression in the upper-state CC-stretching frequency.

The superposed structure is contributed by extensive perturbation by high

vibrational levels of the ground state. Although the ground-state bending

vibration has not yet been directly observed, both it and the bending

fundamental of HC 2 in the A

2

n state are expected to occur at very low

frequencies, as has been observed (C. Fridh and L. Asbrink, J. Electron

Spectrosc. Relat. Phenom. 7_, 1 19 (1975)) for the isoelectronic species

HCN +

. Such perturbations in the spectrum of N0 2 are well known. Their

occurrence for HC 2 helps to explain the properties of the emission

spectrum observed on photolysis of acetylene in the gas phase. Depending

on the photolysis wavelength, several workers have observed unstructured

emission by HC2 at wavelengths from the near ultraviolet to the cutoff of an InSb detector in the mid-infrared. Anomalously long lifetimes, between

about 5 and 30 ys, are typical. Douglas (A. E. Douglas, J. Chem. Phys.

45 , 1007 (1966)) has considered the increase in fluorescence lifetime

which is associated with extensive interaction between two electronic

states

.

Although a detailed assignment of all of the matrix data has not

been possible, the shifts on carbon-13 substitution appear to be

sufficiently regular that correlations can be made with reasonable

certainty. The isotopic shifts indicate that most of the peaks involve

CC-stretching excitations. Of particular interest is a peak at 4622 cm

-1

, which shifts by 103 cm

-1 for H

1 ^c2

. it is difficult to explain such a large shift except by postulating that two quanta of CC-stretching

vibration are excited in the transition. Such an assignment would place

the origin of the transition some 3000 cm

-1 lower, or near 1600 cm

-

. A

definitive assignment must await rotational analysis of the gas-phase

absorption spectrum. Because all of the absorptions which are observed in

the matrix experiments must arise from the X(000) state of the molecule,

the data should be useful in the analysis of spectra of HC 2 obtained in

future gas-phase studies in other laboratories.

3. Reaction Intermediates in the Decomposition of Energetic Materials

(M. E. Jacox)

As a part of a project on the spectroscopic characterization of

reaction intermediates in the decomposition of energetic materials,

supported in part by the Army Research Office, studies of the products of

the H + HCN reaction were also conducted on the B0MEM system. This

reaction is of especial interest because H

2 CN is a potential intermediate

in the decomposition of the solid propellants HMX and RDX. Preliminary

studies had provided evidence for the formation of CN and of three CH 2X

species (presumably H

2 CN and cis- and trans-HCNH) when H atoms produced by

a microwave discharge through an Ar:H2 mixture react with HCN in an argon

matrix. B0MEM observations between 700 and 4000 cm

-1 were conducted with

34
a resolution of 0.2 cm

-1

. The peak at 1336.6 cm

-

^

, which appeared even when low-energy H atoms were formed by the mercury-arc photolysis of HI, was shifted by only 0.1 cm

-1 on carbon-13 substitution, but by 3.0 cm

-1 on nitrogen-15 substitution. This behavior is similar to that of the 1500

cm

- "' CH 2 "scissors” fundamental of H2CO. Since H 2 CN is calculated to be

the most stable isomer and since it has been identified in earlier studies

of the esr spectrum of products of the mercury-arc photolysis of Ar:HCN:HI

samples, the assignment of the 1336.6 cm

-1 peak to the CH 2 "scissors"

fundamental of H

2 cn is strongly indicated. The carbon-13 isotopic shift

of the 3104 cm

-

^ peak, tentatively attributed to a HCNH species, requires

its assignment to a CH-stretching fundamental. Carbon-13 and nitrogen-15

isotopic shifts were also observed for the 886.3 cm

-1 absorption,

contributed by a photochemically unstable HCNH species, probably the cisrotamer. Despite the greater sensitivity of the B0MEM interferometer,

other infrared absorptions which can be assigned to these three products

have not been identified. The interferometer system was also used for

survey scans of the 4000-8000- and 10 , 000- 20,000 cm

-

^ spectral regions,

but no absorptions which could be assigned to the analog of the red bands

of the isoelectronic species HC0 were detected.

In a preliminary Ebert scan of the ultraviolet spectrum which

results on 122-nm photolyis of an Ar:HCN sample, moderately intense

absorptions appeared at 34990 (overlapped by the OH A-X 1-0 absorption) and

35436 cm

-1

. These two peaks correspond closely with gas -phase absorptions

previously attributed to H

2 CN, formed on flash photolysis of several more

complicated precursor molecules. Several less intense peaks extending to

37764 cm

-1 may have been contributed by previously unidentified bands of

H

2 CN. Further studies are needed to confirm this identification.

In an earlier study in this laboratory, the nitromethyl free

radical, CH 2

MC> 2 , was stabilized by the reaction of F atoms with CH

3NO2 in an argon matrix, and the first spectroscopic data for this species were

reported. Most of the vibrational fundamentals were observed for five

isotopic species of nitromethyl, and a tentative vibrational assignment

was proposed. The wealth of data available from these experiments

suggested that a detailed normal coordinate analysis should be feasible.

Among the important questions which such an analysis would address are whether the molecule is asymmetric, as had been proposed as a result of a recent ab initio calculation, and whether the unpaired electron

contributes to partial double-bond character for the CN bond. A

least-squares force constant adjustment calculation has recently been

conducted. All of the observed frequencies can be fitted very well for a planar C2v molecular structure. The vibrational assignment proposed in

the earlier experimental paper has been somewhat revised. The magnitude

of the CN-stretching force constant is appropriate for a single bond. A

manuscript presenting the detailed results of the normal coordinate

calculations is being prepared for publication.

35
A manifold has been designed and constructed to permit matrix

isolation sampling of low vapor pressure materials by controlling the

pressure of argon with which they are in equilibrium. This manifold will

be used for studies of the spectra of products of the photolysis of phenol

and nitrobenzene, also a part of the current ARO project.

4. Future Plans

During the coming year, highest priority must be given to the

completion of a number of important projects which have already been

discussed. These projects include BOMEM studies of the infrared spectrum

of trans-HOCO and Ebert studies of the electronic spectra of the H + HCN

reaction products. Detailed studies of products of the photolysis of

benzene, phenol, and nitrobenzene be conducted using the BOMEM system.

Studies of the products of the reaction of 0 atoms with these three

molecules will also be conducted.

Studies begun in 1985 on the reaction of SiH^ with H, Cl, and 0

atoms produced in a discharge are also planned. The potential

stabilization of SiHn and SiHn

0 species is of interest in relation both to

basic chemical research and to applied studies of plasma etching.

Infrared studies of interesting and important F-atom reaction

systems- e.g., F + cyclopropane —will continue to be conducted. However,

emphasis will now be placed on UV-visible studies of F-atom reaction

products, for which there is a substantial backlog.

For some time, it has been planned to use a beam of excited neon

atoms as a source of 16.6-16.8 eV energy for the production of small

molecular ions, to be trapped in a neon matrix. This energy range should

make possible the preparation of significant concentrations of such small

molecular cations as HC0 +

, H20+, NH^, and CH ij , not accessible in the

11.5-11.8 eV energy range provided by a beam of excited argon atoms. It

is hoped that later in FY87 the excited neon atom experiments, which may

provide much previously inaccessible information on the vibrational and

electronic spectra of molecular ions, can begin.

F. General Spectroscopy and Theory

In addition to the previously described programmatic activities a variety of research problems are pursued in response to special

opportunities, and special interests of the staff and outside guest

scientists. The following are descriptions of such projects of a general

nature

.

36
1. Fourier Transform Measurements of High Temperature Species;

Infrared Gas Phase Spectra of LiCl, LiH, and LiD

(W. B. Olson, G. A. Thompson, A. G. Maki, and A. Weber)

The new transfer optics for the BOMEM described below have allowed

us to make high temperature measurements of the infrared spectrum of

several compounds produced in a high temperature oven. So far we have

measured the Av=1 transitions of the two species LiH and LiD and the four

naturally abundant isotopic species of LiCl.

A portion of the spectrum of LiCl containing 95% ^Li is shown in

Fig. 2.14. The complete spectrum contained over 2500 identified

transitions for the various isotopic species. Vibrational transitions

ranged from v=1-0 to v=8~7 and rotational transitions for J values up to

75. These transitions were fit to a set of Dunham potential constants

that included 9 isotopically invariant terms and 4 terms to allow for

isotopic shifts due to the break-down in the Born-Oppenheimer

approximation. The standard deviation of the fit was 0.00027 cm

-1 which

is what we expect considering the slightly larger than normal Doppler

linewidth of these high temperature spectra.

WAVENUMBER (cm- 1 )

Fig. 2.14 A portion of the spectrum of ^LiCl at 830°C. The locations of

a number of band heads are indicated above the spectrum.

37
A portion of the spectrum of LiD is shown in Fig. 2.15. This

spectrum is much more sparse than that of LiCl and shows the advantage of

LiD

000.0 875.0 950.0 1025.0 1100.0

WAVENUMBER (cm- 1 ) Fig. 2.15 A portion of the absorption spectrum of LiD at 750°C. The wave number scale applies to the upper spectrum.

using a broad-scanning FTS instrument for making these measurements.

Earlier measurements using a diode laser spectrometer were very slow

because the lines were so far apart, the electronic tuning range of the

spectrometer was quite small, temperature tuning was slow, and there were many gaps in the coverage.

The standard deviation of the fit of the LiH and LiD spectra

(0.0076 cm

-1

) was worse than that of LiCl because the Doppler width was larger and there seems to be more of a problem with break-down of the

Born-Oppenheimer approximation. To investigate this further, we hope to

measure the pure rotational spectrum of LiH at high J values to get better

measurements on the higher centrifugal distortion constants.

2. Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene C

2H^

+ (J. T. Hougen)

The influence of a possible hydrogen atom migration in protonated

acetylene, , on the high-resolution vibration-rotation spectrum of

this ion was investigated. The migration model, which derives from ab initio

calculations, consists of a planar structure with the three hydrogen atoms

moving on an elliptical path around the two carbon atoms. Symmetry

considerations for this ion were considered in terms of the

permutation-inversion group G

2 4

. A method for calculating energy levels

for a highly idealized model of this ion can be found in the early

38
microwave literature. In this idealized model, an equilateral triangle of

three hydrogen atoms performs internal rotation about a dumbbell of two

carbon atoms, with all atoms remaining in the same plane. This model

gives rise to a symmetric rotor top, an asymmetric rotor frame, and a six-fold barrier to the internal rotation motion. An alternative method

of calculating the energy levels of this ion, involving a more recently

proposed algebraic tunneling formalism, shows that qualitatively similar

results are obtained even when the hydrogens do not retain the form of a rigid equilateral triangle during their migration motion. Energy level

diagrams, nuclear spin statistical weights, selection rules for electric

dipole transitions, etc. were developed. It is hoped that the patterns

described in the present work will aid in identifying and interpreting any

spectrum of C

2Ho

+ obtained as a result of ongoing efforts elsewhere to

record its spectrum.

3. The Torsional-Wagging Tunneling Problem and the Torsional-Wagging

Rotational Problem in Methylamine

(N. Ohashi and J. T. Hougen)

A theoretical formalism is presented for fitting rotational energy

levels in isolated (unperturbed) vibrational states of methylamine. This

formalism is obtained by recasting and extending theoretical studies in

the earlier literature, which were undertaken to help analyze the

methylamine microwave spectrum. The present formalism is applicable when

both the NH 2 umbrella (wagging) motion and the CH^ internal rotation

(torsion) motion take place near the high-barrier limit and leads to the

usual Fourier sine and cosine series expansions for molecular energy

levels. The derivation is separated into two parts, one treating the

large-amplitude vibrational problem (the torsional-wagging problem) by

itself, the other treating the torsional-wagging-rotational problem. In

both treatments, permutation-inversion group and extended group ideas are used to determine the allowed terms in an effective rotational-tunneling

Hamiltonian operator and to block diagonalize the matrix representation of

this operator for a near-prolate symmetric top. Application of the method

in detail to the methylamine spectrum is planned for a later study.

4. Far-Infrared Spectrum of Methylamine

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

The far-infrared spectrum of CHoNH 2 has been recorded in the

region 50 to 350 cm

-1 on the NBS BOMEM Fourier transform spectrometer with

a resolution of about 0.004 cm

-

^

. The torsional band as well as pure

rotational transitions originating in higher K

a levels fall in this region

of the spectrum. This molecule is of more than passing interest since it

has two large amplitude motions, the NH 2 ~wag and the CHo-torsion, and the

spectrum is rather complex as a result. Assignment of the pure rotational

transitions is complete at this point, and the stronger B-| (B 2

) +-B 2

(B-| ) and

E.|<-Ei components of the torsional band have been assigned. A small

portion of the spectrum of the torsional band is given in Fig. 2.16 where

the line assignment of two components of the r Q^- and r Qc) - transitions is

39
indicated. Ohashi and Hougen have recently derived approximate energy

level formulas for CH0NH2, and a computer program is being written to fit

the data and extract the molecular constants. Preliminary fits show that

the torsional splitting in the torsional vibrational state is about 30

times larger than in the ground state, and, somewhat suprisingly, the

inversion splitting in the excited state is about 3 times greater than it

is in the ground state.

291. 50

H h -f- 292. 00

-f- 292. 50—

h

2 931 00

H f- *29T. 5 0 CMl

Fig. 2.16 A small portion of the torsional band of CH3NH2 taken with a resolution of 0.005 cm

-

^ showing the line assignments of the

Q-branches of two components of the K=5~4 and K=6~5 subbands.

5. Transfer Optics System Design

(W. B. Olson)

An efficient transfer optics system has been designed and

constructed to couple the BOMEM FTS to an existing heat pipe in order to

obtain infrared absorption spectra of species which can only be seen in

40
the gas phase at high temperatures. This system has enabled the high

resolution observation of several bands of LiCl and LiH leading to

improved vibrational and rotational constants for these species.

The necessity of efficient and practical transfer optics for the

heat pipe was the "mother of the invention" of a fairly general and

powerful approach to transfer optics design via a "system" concept of

first order design, through analytical equations directly coupling

required throughput or entendue to the ray transfer matrix of an optical

system. The necessary ray transfer matrix for the system is uniquely

determined (except for sign), and one can relatively rapidly calculate

required distances and focal lengths of spherical mirrors required to

implement a practical system.

With a single optical element, in our applications a concave spherical

mirror, one has no degrees of freedom with which to make the design

convenient. With each added optical element one gains two degrees of

freedom to adjust features. With three optical elements one can specify

all four distances in the system to convenient or practical values, and

from these calculate the focal lengths required for each of the three

mirrors. This is the method used for the heat pipe transfer optics system

design. The resultant optical system for the heat pipe works well, but no precise explanation can be given of the function of the individual optical

elements in simple, familiar terms.

6. Analysis of Infrared Spectra of SiH^D

(R. Schaefer, R. Lovejoy, W. B. Olson, and G. Tarrago)

An analysis of the \>2 parallel band of “^SiH^D has been nearly

completed through the collaborative efforts of Ron Schaefer and Roland

Lovejoy of Lehigh University, W. B. Olson of NBS, and Ginette Tarrago of

the University of Paris, South. For a complete analysis the interactions

of V

2 with the parallel and perpendicular components of 2vg had to be

taken into account by simultaneous solution of a secular determinant

containing all three vibrational states. Three different types of

perturbations giving readily evident displacements of absorption line

positions in v

2 were found to be due to perturbations with the

perpendicular component of 2v^.

The perpendicular component of 2vg proved difficult to assign

because it was so weak, and additional spectra were run with a pressurepathlength product a factor of ten higher than that used for the initial

runs. This also increased the intensity of many other weak lines

from unidentified hot bands and silicon isotopes. Two techniques for

getting assignments, and checking them through the quality of the least

squares fit, and the success of using molecular parameters from the fit to

calculate new line positions at higher values of J and K eventually lead

to a reasonably complete set of assignments for this weak band. Many of

41
the strongest lines in 2v^ are actually perturbation allowed transitions

which get their intensity from the parallel component of 2vg through the

1-type doubling matrix element.

The secular determinant used for this work contained all possible

matrix elements through the fourth order of the twice transformed

Hamiltonian. A number of the higher order terms were found to be

insignificant, while others of high order are significant, but did not

make major contributions to the energies of any of the levels.

The values of the major low order molecular parameters of 2vg are

in quite satisfactory agreement with those of the vg fundamental. The

origin of the v

2 band is at 1593.962 cm

-1

, and that of 2vg is at 1557.258

cm“T. An article describing this work in detail is being prepared for

publication.

7. Future Plans

In the coming year we plan to exploit the optical system used for

the high temperature Fourier transform experiments to obtain Fourier

transform measurements of other systems unstable under normal room temperature conditions. We shall try to observe the far infrared spectrum

of several systems such as the pure rotational spectrum of LiH and the

spectrum of compounds with low frequency vibrations (e.g., NaCl) . We shall

also try to observe the spectrum of compounds produced in an electric

discharge with the hope that this may enable us to look at the spectrum of some metal oxides or hydrides.

Theoretical studies will focus further on large amplitude motions.

Topics to be examined are the CH 0 NH2 microwave and far infrared spectra

both in terms of pure rotational spectra and torsional spectra. The water

dimer Hamiltonian and spectral consequences will be examined in further

detail

.

42
3. LASER PHOTOCHEMISTRY GROUP

D. F. R. Burgess, Jr., M. P. Casassa, R. R. Cavanagh, L. B. Elwell,

E. J. Heilweil, D. S. King, and J. C. Stephenson

A.

B.

C.

D.

E.

Introduction

Condensed Phase Energy Transfer

Laser Diagnostics of Surface Dynamics

1. Adsorbate-Adsorbate Interactions: (NO + NH^)/pt(111) Thermal

Desorption

2. Laser-induced Desorption: NO/Pt(foil)

3. Future Plans

Dynamics of van der Waals Molecules

Photophysical Studies of Photosensitizers

43
A. Introduction

This group primarily does quantum-state specific studies of molecular

dynamics. A common factor in the research described below is the role of

energy transfer in the spectroscopy and kinetics of molecules. Research on molecular dynamics of molecules on surfaces is done in collaboration with the

Surface Science Division.

B. Condensed Phase Energy Transfer

(M. P. Casassa, R. R. Cavanagh, L. B. Elwell, E. J. Heilweil, and J. C. Stephenson)

The major effort in the condensed phase energy transfer program has been

directed at the study of vibrational energy transfer (VET) . During the past

year we have successfully measured the rates of energy transfer from

vibrationally excited chemical bonds in liquids, solids, and in molecules

bound to surfaces.

This research area was extensively discussed in the 1985 Annual Report.

The present report does not repeat last year’s lengthly discussion of

experimental detail and program rationale. In summary, we used a picosecond

infrared pump/probe method to perform time-resolved measurements of the

vibrational energy relaxation rate (T., -1

) for the high frequency modes of

species chemisorbed on a silica (Si0 2

) surface in a variety of chemical

environments. Previously, energy transfer rates have been measured for the

vibrations of many molecules in the gas phase, and for a few molecules in

liquids and low temperature solids. However, our experiments were the first

to measure vibrational energy transfer rates for vibrations on surfaces.

A summary of some surface T

1 results is given in Table 3.1. These data were limited to vibrations with frequencies v>2500 cm

-1 since the psec ir

pulses necessary to cause transient bleaching of the surface vibrations were generated in a Nd-YAG-pumped LiNbO^ OPA (LiNbO^ only transmits v>2500 cm

-1

).

The data suggest the following conclusions. T^ times are longer than most

people expected prior to these measurements. Energy relaxation rates (T-| -1

) are much slower by factors of 100-2000, than would be deduced from spectral

bandwidths of these surface vibrations. At the vacuum interface, the MOH

(M=B, Si, Al, Zn) and MOD stretching vibrations relax via anharmonic coupling

to spatially adjacent vibrational modes, particularly the M-0-H bending mode.

One reason BOH relaxes faster than SiOH is that its local accepting modes are higher in frequency and hence a better match to the OH stretch. The faster

relaxation of NH 2 and OCHo vibrations is consistant with energy transfer from

the NH stretches to the NH 2 bend and similarly from the CH stretches to the

CH bends (with which the CH stretches are known to couple strongly via Fermi

resonance). It is reasonable that T

1 of OH in the commercial zeolite ZSM-5

44
Table 3.1 T

1 decay times at room temperature for various surface vibrations

System v( cm 1 ) T1 (ps) k(

1

0

9 s ^ Notes

SiOH/vacuum v0H = 3750 220±20 4.9 Pressed Si0 2 disk

SiOH/CCljj v0H = 3690 1 59±1

6

6.3 Dry Si0 2 dispersion

SiOH/CF2Br2

v0h=3690 140±30 7.1 It

SiOH/CH2Cl 2 vqh=3660 1 02±20 9.8 If

SiOH/C6H 6 vqh=3625 87±30 1 1 .0 It

SiOH/C

6 D6 vOH = 3625 80±30 12.0 II

SiOH/H2 o/CCln v0h=3690 56±1

0

18.0 SiOH T-| , -5H 2 0/100A2

physi sorbed

SiOD/vacuum v0D=2760 1 55±1

6

6.5 OD T-j with 67% OH re- placed by OD

BOH/vacuum

B0H/CC1],

vOH= 3700 ~70 -14.3

SiNH2 /vacuum

SiNH2 /ccln

vNH

= 3^60

3520

£20 £50 Pulsewidth limited

signal, AT/T ~5% for

both stretches

SiOCH ^ /vacuum v=2860

3000

<5(?) >280(?) No pulse saturation

observed for any

CH-stretching mode

OH on zeolite

ZSM-5/CCln

(Si/Al=3600)

vOH =3690 140±25 7.1 Pressed disk CCI14

saturated

OH on ZnO ''OH-3185 <10 £100 AT/TQ=12? pulsewidth

limited

ZnO, no OH 3600-3750 <10 £100 Electronic excitation

of free carrier

absorption AT/T

= 10?.

45
is the same as on fumed Si02 , since the high Si/Al ratio (=3600) of this

particular sample precluded any appreciable bonding of OH on aluminate

sites. The very rapid relaxation of OH on the semiconductor ZnO surface may

not be caused by anharmonic coupling to adjacent vibrational modes but by

coupling to electronic excitations of impurity states in the ZnO.

Relaxation rates for the surface vibrations increase in the presence of

solvents (solid/liquids vs. solid/vacuum interface). The increase in rate is

minor for solvents like CCl^ which have no resonant high frequency modes,

(v^_d = 770 cm

-1 is the highest fundamental). An increase in rate of about

a factor of 2.5 occurs for benzene which has many nearly resonant

( ^0H'>

' t)enzene V 1 + vij , etc.) internal vibrational states which could accept

some or all of the OH quantum. Alternatively, vibrational relaxation rates

may increase in the presence of solvents which perturb the 0H(v=0 -»• 1)

absorption through hydrogen bonding or van der Waals interactions. The

dependence of T^ for surface vibrations on chemical environment, temperature,

and other factors previously mentioned is always qualitatively in the

direction expected by analogy to T.| data in homogeneous phases. However,

standard first order perturbation theory calculations (e.g., solid state

multiphonon relaxation theory) do not quantitatively agree with these trends.

During the past year a new picosecond dye laser was constructed for the

purpose of generating tunable ir pulses for v < 2500 cm

-

^

. This enables us to pump and probe interesting surface vibrations such as CO bound to metals.

For vibrational relaxation on a metal surface, it is thought that a qualitatively different energy transfer mechanism, damping by excitation of

electron-hole pairs in the metal, may be dominant. If this is true, then

perhaps the broad spectral bandwidths (-5-50 cm

-1 FWHM) observed for high

frequency vibrations on metals are really due to T

1 broadening, as has often

been assumed. A direct time-resolved measurement of vibrational relaxation

for a molecule on a metal surface is crucial to assessing the importance of

this damping mechanism, and for understanding surface spectroscopy and

kinetics.

The new laser is a synch-pumped tunable dye laser pumped by the second

harmonic (532 nm) of the rejected trains of ps pulses from our

acti vely-passively modelocked 10 Hz ps Nd-YAG laser. A single pulse from the

dye laser is cavity-dumped, amplified to -500 yJ/pulse and mixed in a LilO^

crystal with a single 532 nm pulse derived from the same 10 Hz YAG.

Typically pulses of -25 ps duration, 20 yJ, and tunable throughout the CO

stretching region (-4 cm

-

^ FWHM), are generated by this system. These pulses

are sufficient to perform pump/probe ir saturation measurements of T-| for

C0(v=1) bound to small metal particles (e.g., Pt) on an insulator support

(e.g., Si0 2

). The Pt particle size (or other transition metal) can be

controlled so that CO will chemisorb to either isolated metal atoms or larger

crystallites. Measurement of T-j for CO on the surface of supported metal

particles should begin mid October 1986.

46
Initial experiments with the new laser system have involved measuring T

1 for stretching vibrations of CO bound to transition metal atoms (metal

carbonyl complexes) dissolved in CCl^ and other solvents. In terms of

vibrational spectroscopy, the inorganic carbonyls are analogous to

surface-bound CO (the assignment of surface CO sites as "bridge-bonded” on "terminal" frequently involves comparing the surface vibrational frequencies

with corresponding metal carbonyls). Figure 3.1 shows ir pump-probe

saturation data giving T

1 for the F-j u C0(v-1) mode of Cr(C0)5 in carbon

tetrachloride and hexane solutions. T-| is very long (Ti s 490 ps corresponds

to 20,000 vibrational periods) and is clearly unrelated to the 8 cm

-1 FWHM

infrared Lorentzian bandwidth. We expect a similar T

1 lifetime for CO bound

to isolated surface metal atoms on a dielectric support. A wide variety of

metal carbonyls and nitrosyls are available and are excellent model compounds

to answer questions about VET mechanisms, such as: is there a "heavy atom

effect"; is T

1 for bridge-bonded CO different from terminal CO?; how is

vibrational energy exchanged among modes of different ligands (adsorbates)

bound to the same or neighboring metal atoms?

Fig. 3*1 Vibrational relaxation times for the F 1u C0(v=1) mode of

Cr(C0)g in solutions of hexane and carbon tetrachloride.

Studies of related energy transfer processes at semiconductor surfaces

have also recently begun in collaboration with Dr. Claude Sandroff of AT & T

Bell Laboratories. The luminescence decay times of "exciton-like" electronic

47
states of extremely small 30A diameter) and planar Pbl

2 and PbBr 2

colloidal particles suspended in organic solvents are being measured.

Preliminary experiments for Pbl

2 and PbBr 2 in alcohol, using the third

harmonic of the picosecond Nd + 3 ; YAG (355nm for Pbl 2 ) and the second harmonic

of the synchronously pumped dye laser (310 nm for PbBr 2 ) as excitation pulses,

have yielded ca. 1100 and 900 ps lifetimes, respectively. These emission

lifetimes were measured with a streak camera system. It is believed that

measurements of this sort will reveal mechanisms for light-induced chemical

reactions occurring at the liquid-surface interface. These include, for

example, charge-transfer and oxidation-reduction reactions that are known to occur at semiconductor surfaces.

Using the LiNbO^ 0PA system, a study of the T-| relaxation times of

OH ( v= 1 ) and 0D(v=1) vibrations of silanol, R^siOH, and alcohol, R3COH,

molecules in solution was completed. The results are summarized in Table

3.2. They may be compared to the surface OH results in Table I and the OH in

crystalline mica results in Table 3*3. Most of the liquid phase results can

be understood in terms of an isolated binary collision (IBC) model frequently

used to interpret gas phase collisional VET rates. The fact that T

1 for

(CH^-^SiOH = Tj for (C^H^oSiOH, even though the latter molecule has 500 times

the total density of vibrational states, shows clearly that factors other

than the total vibrational state density are important in determining T^

.

The deactivation of hydroxyls of R

3 MOH and R

3

MOD (M=C,Si) were simply

modelled using the assumption that 0H(v=1) or 0D(v=1) energy is initially

transferred to the eleven vibrations involving the OH bond and the

neighboring four tetrahedral bonds of the central M atom. Data were analyzed

by considering the frequencies of those particular accepting modes, and by

using the propensity rules that energy transfer will be nearly resonant, and

that processes involving the fewest vibrational quantum number changes

(n=number changing) will be most probable. Because of their different

vibrational frequencies, there are many more nearly resonant relaxation

channels, involving lower values of n, for the alcohols (n £ A) than for the

silanols (n a 5). This may explain why the R

3

COH and R

3 COD molecules relax

much faster than corresponding silanols. The observation that the R

3 Si 0D

molecules relax more slowly than the R^SiOH species is consistent with energy

transfer occuring predominantly to the SiOH bending motion, and is not

consistent with transfer primarily to the Si-0 stretching vibration. Similar

analysis for the methanols (CHoOH, CH

3OD, CD

3 OH, CD

3OD) suggests that energy

transfer to the CH (CD) stretching vibrations or to the C-0 stretch are probably not dominant relaxation channels.

48
Table 3-2 T

1 lifetimes for the OH and 0D(v=1) stretching vibration in

alcohols and silanols in dilute CCl^ solution ( SO .007 mole

fraction) at 298 K. Error limits are ±o, or 1055 of T

1 , whichever

is greater.

SILAN0LSa v0H,0D( cm

1 > Tl (ps) ALCOHOL3 V0H , 0D

(

cm

1 ) T-] (ps)

Me

3 siOH 3690 205 ± 21 Me

3 C0H 3614 < 6 b

Me

3 SiOD 2722 245 ± 25

Et

3 SiOH 3689 185 ± 19 Et 3 C0H 3622 < 6 b

Et

3 SiOD 2722 224 ± 22 Et 3 C0D 2673 < 20b

4> 3 SiOH 3675 206 ± 21 <j) 3 C0H 3609 < 1 5

b

(j> 3 SiOH 2712 292 ± 29

d 2 Si(0H) 2 3610,3679 80 ± 15 ch

3 oh 3641 15 - 30b

4> 2 Si ( 0D ) 2 2665,2710 134 ± 14 ch 3od 2685 52 ± 17

cd

3

oh 3642 73 ± 7

cd

3od 2690 79 ± 17

EtOH 3625 70 ± 10

4>OH 3610 5 - 20b

<j>0D 2665 15 - 25 b

c

6 f

5 0D 2640 < 15b

aMe=-CH

3

, Et—-CH 2 CH

3

, <p=—

bLimits on T-j were deduced from computer simulation,>

49
Table 3*3 T

1 vibrational lifetimes of the 0H(v=1) stretching fundamental

mode of hydroxyl ions in natural crystalline micas at 298 K.

Uncertainties in T-j are ±1 o . Where applicable, the samples are labelled by their Smithsonian Institution identification numbers.

Muscovite v0PA

( cm“1

)

Tt (ps) Biotite v0PA

( cm

-1

)

Ti (ps)

104935 3591 88 ± 16 82063 3691 198 ± 15

105013 3648 90 ± 15 03647 3691 244 ± 15

105051 3591 83 ± 18 03675-1 3691 220 ± 18

3675 85 ± 8 3575 125 ± 11

B16862 3675 105 ± 36 Ruggles 3591 72 ± 20

Ruggles 3591 79 ± 8 Unknown 3540 64 ± 5

3675 114 ± 13

For comparison to the surface (Table 3.1) and liquid (Table 3.2) phase

results, new results were obtained for the 0H(v=1) T^ lifetime of hydroxyl

ions located within the two-dimensional crystalline layers of natural

Muscovite and Biotite micas. Briefly, these silicate crystals are composed

of a sheet of metal cations octahedrally coordinated to OH" ions which is in

turn sandwiched between two covalently linked SiO^ tetrahedral layers. This

composite layer is further stacked upon others (creating several polymorphs)

to form the mica. Excess negative charge in these stacked layers is

neutralized by intercalated K

+ ions which give micas their easily cleaved

sheet-like character. With Al +

^ ions in the octahedral layer, the chemical

composition of Muscovite is best represented as [KO^Si^^ + A1-|/2>02(0H)

Al2(0H)02<Si3/2 + Al-|/2>02] n

. Since its aluminum ions occupy two of three

possible octahedral sites (two Al +

3 and one vacancy), Muscovite is called a dioctahedral mica. Because Biotites generally have all three octahedral sites

filled (e.g., by Mg +2 in Phlogopite), their chemical composition may be

written as [K0o<Si3/ 2

+ A1-, /2>0 2 (OH)Mg^(OH)02<Si3 /o + A1 1 /2> 0 3^n and thev are called trioctanedral micas. Substitutions for Mg^° in Phlogopite (by Fe

+

^ and

Al + 3, for example) gives rise to other forms of the Biotite family.

T-| values for the micas are given in Table 3*3. At room temperature, the

average 0H(v=1) vibrational population lifetime (T-|±o) for OH

- in Muscovite

is 92 ± 1 3 ps. For Biotite samples, absorptions arising from two distinct

lattice sites yield T-j = 221 ± 23 and 87 ± 33 ps, respectively. These energy

dissipation times are comparable to those observed for the surface OH, liquid

silanols, and OH in amorphous fused silica. In the micas the MOH bending

50
frequency is in the 800-900 cm

-1

range (like surface OH) while the M-OH

stretch in the micas is lowered to 400 - 500 cm

-1 (much lower than the

stretch in silanols or for surface OH). Thus the similarity of T

1 may again

be consistent with energy transfer to the M0H bend being a dominant channel.

The observed trends in T-| (Muscovite and Biotite low frequency site vs. Biotite high frequency site) can be rationalized in terms of local site

structure and hydrogen bonding. The high frequency trioctahedral Biotite

sites have the longest T

1 (221 ps) and are least hydrogen bonded because

their out-of-plane orientation leads to little 0-H0 interaction with the

octahehral layer oxygens. The Muscovites and lower frequency Biotite hydroxyl

sites are dioctahedral sites with broader, lower frequency absorption,

reduced 0-H0 distance (3.5A) and increased H-bonding and a correspondingly

shorter T

1

.

The crystalline micas are also attractive samples for measuring

transient vibrational populations by picosecond coherent anti-Stokes Raman

scattering (CARS). Such measurements will give independent confirmation that

the ir pump-probe experiments yield correct T-, times. Also the CARS probing

may determine what vibrational modes accept the energy following deactivation

of the high frequency 0H(v=1) mode. The 10 Hz dye laser described above and

the 532 nm doubled Nd-YAG form the CARS probe pair. The 0H(v=1) level is

initially pumped by the LiNbO^ 0PA. Large CARS signals have been produced from

crystalline mica when the dye laser was tuned to make the pulse pair resonant

with the OH v=0 * v=1 transition. We do not yet (mid September 1986) have

time-resolved population data using this approach.

The preceeding successfully completed experiments and those now being

done are all steps toward answering the questions 1) What is T

1

, for a molecule on a metal surface? 2) What are the pathways of energy transfer

(i.e., where does the energy go)? 3) How is VET at surfaces related to

chemistry? With continued support from the Air Force Office of Scientific

Research, we anticipate continuing progress toward understanding these and

other important questions in condensed phase energy transfer.

The most significant questions for us to answer in the area of energy

transfer for molecules on surfaces are: 1) What is T^ , for a molecule on a metal surface? 2) What are the pathways of energy transfer (i.e., where does

the energy go)? 3) How is VET at surfaces related to chemistry? The

experiments outlined below are planned to answer these questions.

We have several different strategies for measuring T-| for a molecule

on a metal surface. One approach is to perform our usual one color IR

pump/probe time-resolved bleaching experiment for molecules on supported

metal clusters. Another approach involves IR excitation of a molecular

(e.g., pyridine) bound to the surface of a metal (e.g., Au) particle

suspended in liquid as a sol; this would be followed by anti-Stokes Raman

scattering (surface enhanced) as a time dependent probe of vibrational

population. These experiments require infrared and visible laser pulses of

51
suitable energy, bandwidth, freauencv, and duration. We have two suitable

laser systems which generate the difference freauencv, wIR , = co-] — • between two

visible laser Dulses in non-linear crystals. For freauencies a 1800 cm

-1 we use LilO^ and for lower freauencies 900 < wjr < 1800 we have tried AgGaS2

- In addition, a new hvbrid 10 Hz nulsed Nd-YAG laser, Dresentlv under

construction, should generate -8 ds, 1 . 06 micron Dulses, and hence much

shorter dve. 5R2 nm. difference freauencv ir. and 0PA ir Dulses than the

nresent aDDaratus Droduces. These shorter Dulses should imDrove the time

resolution bv more than factor of four comDared to the current 10 Hz ds laser

svstem. Ir DumD-Drobe measurements of T

1 for C0(v=1) on disDersed suDDorted

small metal Darticles are being attemDted with this new laser source. The

successful T-| exceriments on "model comDounds" like the metal carbonvls will

continue. Anti-Stokes Raman scattering measurements to determine the nathwavs

(acceDting modes) involved in the VET will be done, first for homogeneous

model comnounds such as metal carbonvls in solution and crystalline micas.

It is Dossible that the disDersed suDDorted metal samDles mav scatter so much

light as to make anv anti-Stokes exDeriments imDOssible. If so. we mav still

be able to Derform such time-resolved measurements on molecules bound to the

surface of colloidal metal Darticles in liauid susnensions.

C. Laser Diagnostics of Surface Dynamics

(D. Burgess. Jr.. R. R. Gavanagh, L. R. Elwell. and D. S. King)

This Drogram is directed at understanding energv transfer and chemical

dynamics occuring at metal surfaces at a fundamental level. The aDDroach is

to aDDlv state-resolved diagnostics, shown over the last decade in gas Dhase

reaction and beam scattering exneriments to Drovide a tremendous wealth of

information about intermolecular Dotential energv surfaces to issues in

surface chemistry. Thermally driven nrocesses that have been characterized

(i.e., kinetics, binding site occuDation, . etc . ) bv more conventional surface

science techniaues have been chosen for studv because of their ubiauitous

role in heterogeneous chemistry. From the viewDoint of chemical Dhvsics,

elucidation of the rates and mechanisms for energv flow from a molecule into

a nearbv surface (dissiDation) or vice versa (fluctuation) are kev to a fuller, fundamental knowledge of surface controlled chemical Drocesses.

A collaboration between CCP Molecular SDectroscoDv and Surface Science

Divisions has resulted in the creation of an exoerimental facility to nerform

such state-resolved studies of thermal, molecular Drocesses at well

characterized surfaces. The facility includes two ultra-high vacuum (UHV)

chambers. The ma.ior chamber, for oriented single crvstal work, includes

traditional surface sensitive analytic tools such as mass snectrometrv, ion

snuttering, low-energv electron difraction and Auger electron SDectroscoDv.

The second chamber, a test chamber for laser desomtion exDeriments. is much

smaller in scale. Two laser svstems are currently available. A near transform limited excimer-Dumned dve laser is dedicated to measurements of

vibrational, rotational, SDin-orbit. and lambda doublet distributions,

orientational effects, and kinetic energy distributions for molecules (e.g.,

NO) desorbing from surfaces. This svstem is used as the state-resolved

diagnostic both for thermal and laser-induced desorntion. For the

52
laser-induced thermal desorption (LITD) studies we are using a YAG laser

(temporarily borrowed from the Center for Radiation Research) capable of

producing either nanosecond or picosecond duration pulses to probe the time

scale required to achieve equilibrium.

1. Adsorbate-Adsorbate Interactions: (NO + NH^/ptOll) Thermal

Desorption

In FY85 measurements of the internal state distributions for NO

thermally desorbing from Pt (111) were measured both for an extensive

(10-fold) range in NO coverage and under the influence of co-adsorbed CO.

Unlike the earlier, apparently non-equilibrium dynamics exhibited by the

N0/Ru(001) system, the results of the work on the N0/Pt(111) system may be

summarized as follows: 1 ) the rotational levels and spin-orbit states are characterized by a common rotational temperature; 2 ) although the kinetic

parameters describing the desorption process change significantly with

coverage and the presence of co-adsorbed CO, the rotational degree of freedom

of the desorbing NO is always nearly fully accommodated with the surface

temperature; 3 ) there was no measurable alignment of the desorbing species.

In terms of the observable dynamics there was no manifestation of

adsorbate-adsorbate interaction, i.e., no substantial deviation from full

accommodation. In both the NO-NO and NO-CO systems the adsorbate

interactions are weak-to-moderate in strength. The co-adsorption system

( NO+NHo ) /Pt (111) was chosen for study in FY86 since it had been shown by TPD

and EELS studies that there is a stable, surface bound complex formed upon

co-adsorption of NO+NH^ on cold Pt (111). Thermal desorption of NO and of NH

3 occurs simultaneously at a temperature much higher than characteristic of

either species, implying that the rate-limiting step in the co-adsorbed

system is dissociation of the more strongly bound complex.

Although the kinetics for NO desorption from Pt (111) were significantly

altered by the interaction with co-adsorbed NH^ there was no significant

deviation from near-complete rotational accommodation with the surface.

For surface temperatures of 1 50 to 400 K and NO coverages of 5$ to

saturation, the rotational temperature of the desorbed NO was 90 ± 5% of the

instantaneous surface temperature and independent of the amount of

co-adsorbed NHo. This imples that disruption of the NO/NH

3 complex leads to

NO molecules which are able to accommodate with the surface prior to

desorption.

2. Laser-induced Desorption: N0/Pt(foil)

Thermal desorption techniques probe surface processes occuring under

conditions of thermodynamical equilibrium. This is to say that external

heating rates of 1-10 K/s are much slower than rates for energy transfer in

condensed phases. To attempt to learn the important time scales for

molecular energy transfer at surfaces we have implemented a program in

laser-induced thermal desorption. Here it is hoped that sufficiently rapid

53
temperature jumps (heating laser pulse durations of 1 0“® to 2 x 10 -11 s) will

allow desorption or other surface processes to compete favorably with energy

exchange or relaxation processes characteristic of thermal processes.

A major concern in all experiments where a laser is used to heat a surface is the accurate determination of the surface temperature. Classical

heat conduction equations have been employed to establish the surface

temperature in the past, and are currently being used to analyze the

desorption yields obtained in this laboratory. However, experimental efforts

to validate the surface temperature actually achieved due to laser heating are

sparse. A direct measure of the laser perturbation of the surface properties

would offer the most satisfactory probe of the heating effects. Attempts at

exploiting the resulting time dependent black-body radiation as a signature

of the surface temperature have not been successful. The choice of our initial system for study, NO/Pt using 3~8 nanosecond heating-laser pulses,

was based on our earlier results for NO/Pt thermal desorption wherein we have always observed near-complete rotational accommodation. Our naive

expectation was that desorbed NO would exhibit rotational and

translational accommodation and thereby give us an in situ probe of the

response of such a surface in other laser-heating experiments.

The experiments were performed on a 25 x 12 x 0.25 mm polycrystalline

Pt foil in a small UHV test chamber (base pressure 2 x 10“‘° torr). A

frequency-doubled, 3 ns FWHM YAG laser was used to heat the surface. A

spatially homogeneous portion of the collimated laser beam irradiated a 1.5 mm diameter portion of the surface with 0.5 mJ/pulse at near normal

incidence. The density of desorbed NO in specific (V, J, Q) states above the

surface was probed using a frequency-doubled dye laser (0.05 cm

-1 bandwidth,

9 ns FWHM) tuned to the (0-0) or (1-1) vibronic bands of the NO A-X

electronic transition. The 1 mm diameter probe beam was aligned parallel

to the surface at a distance of 4 mm. LEF from desorbed molecules in a 1.5 mm long portion of the probe beam was detected with f/2.5 collection

optics, spatial masks, spectral filters, and a solar-blind photomultiplier.

The platinum surface was initially flashed clean to 1300 K, cooled to

200 K, and then exposed to a saturation dose of NO. A background pressure of

4 x 10" 9 torr NO was maintained during the experiments to stabilize the

surface NO concentration, which would otherwise have been slowly depleted by

the LITD process. Time of flight (TOF) distributions were obtained by

scanning the time delay between the desorption laser and the probe laser in

0.1 ysec increments. At the 30 mJ/cm^ incident energy density used, the

surface temperature jump was calculated to be 120 K from the optical and

thermal properties of platinum and the laser pulse FWHM. This implies a maximum surface temperature of Tm=320 K.

Representative TOF distributions for desorbed NO in v=0 with low (25

cm

-1

) to moderate (662 cm

-

^ ) rotational energy are shown in Figure 3-2. The

observed LEF/T0F signals correspond to maximum molecular densities of about 5 x 10

- ® torr in the probed region as determined by comparison to LEF signals

with NO flowing through the chamber. The prompt onset and early maximum

54
exhibited by the molecules in the J= 1 9 .5 rotational state became increasingly

more apparent as higher quantum states were probed. These data and TOF

spectra for 16 other rotational states with 25 < E

R < 2162 cm

-1 suggest that

there are at least two distributions of NO desorbing from the surface: a "fast" component with moderate internal excitation and a slower component

with a "colder” internal energy distribution. To characterize the

translational and rotational energy distributions of the desorbed NO, all of

the TOF spectra were fit to the sum of two velocity distributions, each of

the form P(v) = v^exp[-(a+bv+cv2

)] , which reduces to a Maxwell-Boltzmann

distribution for b=0.

For all quantum states with 25 < E

R < 2162 cm

-1

, the slow component

was fit by a distribution with a mean kinetic energy of <ET>=52±5 meV, while

<Et

> for the fast component continuously increased from 240 ± 35 meV to 460 ± 50 meV with increasing E

R

. Maxwell-Boltzmann distributions with these

average energies are characterized by effective translational temperatures

<ET>/2k of 300 K and ranging from 1400 to 2650 K, respectively. However, the

observed distributions were not Boltzmann. Each had a slightly larger energy

spread <e|>/<Et

> 2 than would a Maxwell-Boltzmann distribution. The

relative populations for the rotational levels were determined by their

magnitudes in the TOF fits. The rotational populations for states with 25

< E

r < 662 cm

-

^ were well characterized by Boltzmann distributions with

temperatures of 410 ± 50 and 170 ± 20 K for the fast and slow components,

respectively. The energetic NO molecules contributed 21 ± 3% to the

integrated desorption flux under these experimental conditions.

TOF distributions were measured for desorbed N0(v=1,J) states with E

R

< 133 cm

-

^

. These were dominated by a single velocity distribution with <ET

> = 290 ± 25 meV. The N0(v=1) rotational distribution was estimated from an excitation spectrum taken at 3 usee delay and including transitions from 7

different rotational levels. A value of T

R (v=1)=155 ± 75 K was obtained.

Comparison of the integrated population in v=1 to the v=0 fast component

yielded a ratio of 0.054 ± 0.027, consistent with T

v=900 K.

To summarize the N0/Pt(foil) laser-induced thermal desorption results,

at near saturation coverage from a platinum foil two components were

observed. Although the mean kinetic energy of the less energetic component

is comparable to the maximum surface temperature, its rotational temperature

is significantly lower than the surface temperature. Such incomplete

rotational energy accommodation has been observed in conventional TDS

experiments for N0/Ru(001) and discussed in terms of dynamical factors in the

adsorption potentials. A dramatic departure from accommodation was exhibited

by the energetic NO, with its high translational energy. While this is

consistent with exit channel effects which have been invoked for thermally

activated H2 recombination and CO oxidation reactions at metal surfaces, it

is also consistent with a sudden (non-thermal) desorption channel.

Apparently, at heating rates of 10 10 K/s new desorption channels compete

favorably with thermalizing energy relaxation pathways even under mild

temperature jump conditions. This work is supported in part by the U.S.

Department of Energy.

55
L

E

F

S

IG

N

A

L

0 •

a) Er = 662 cm'

b) Er = 281 cm" 1

c) Er = 25 cm' 1

-i 1 1 t I " '! t“ — 4 0 4 8 12 16 20

TIME-OF-FLIGHT (psGc)

Fig. 3.2 Desorbed nitric oxide LEF signal as a function of time delay

between heating and probe laser pulses for different quantum

states. Spectra are normalized to NO at 294 K. Two-component

fits to the data are also shown, a) J=19.5, <ET

> = 450 and 60 meV;

b) J=12.5 , <Et

> = 330 and 53 meV; c) J=3-5, <ET

> = 235 and 47 meV.

3. Future Plans

This work will be directed towards exploring in more detail

laser-induced thermal desorption ( LITD) . "Non-equilibrium" behavior has been

observed for NO desorbing from Pt foils at low initial surface tempeatures

when driven by 3 ns, visible heating-laser pulses. We hope to answer speculations regarding the physical origin of this non-equilbrium dynamics

(i.e., secondary desorption channels, recombinatory desorption, coverage

dependent adsorbate interactions, binding site effects, or electronic effects)

by exploring LITD results as a function of heating-laser pulse length,

wavelength and energy, of surface morphology using single crystals, and of

initial surface temperature, coverage and co-adsorbates.

56
One of the long range goals of this program is to study recombination

reactions on surfaces. For technical reasons, this is difficult by thermal

desorption techniques. Experiments currently underway entail the use of a variable pulse length heating laser (10 ns to 25 ps on loan from Radiation

Physics for four months). These experiments should ellucidate the

contribution from recombinatory desorption to the observed N0/Pt(foil) LITD

signals (described above). It is anticipated that LITD will provide a powerful

technique for detailed study of recombination rections.

D. Dynamics of van der Waals Molecules

(M. P. Casassa, L. B. Elwell, D. S. King, and J. C. Stephenson)

There has been substantial recent theoretical interest in the vibrational

predissociation of van der Waals (vdW) molecules, but very few experiments

that have looked at either the final product state distributions or obtained

real time measurement of the dissociation lifetimes. We have recently

studied the vibrational predissociation of the nitric oxide vdW dimer,

including picosecond time-resolved measurements of the lifetime following

excitation of the symmetric (v^) and antisymmetric (v^) stretching modes, and

measurements of the complete distribution of energy in NO product fragments

following v^ excitation.

Vibrational predissociation of van der Waals molecules occurs because,

in general, the constituent molecular vibrational levels lie above the vdW

bond energy. Vibrational excitation and subsequent vibrational energy flow

lead eventually to rupture of the weak van der Waals bond. While such

systems have had great appeal as models for vibrational dynamics, the

previously available lifetime data are based on indirect measurements. In our experiments, picosecond infrared pulses excited the symmetric (v^ = 1) or antisymmetric (v^ = 1) stretching modes of the NO dimer. The formation

times of the NO fragments ejected upon vibrational predissociation were measured by laser excited fluorescence (LEF) using picosecond time-delayed

ultraviolet laser pulses. A remarkable dependence of the predissociation

lifetime on the vibrational mode initially excited was observed.

The NO dimer has C 2V symmetry with a 2.236 A N-N separation and a 99.6°

0-N-N bond angle. The dimer bond energy is 800 ± 150 cm

-

^

.

Photodissociation spectra have been reported for excitations of the symmetric

v-j (vibrational symmetry A-| ) and antisymmetric vij (B2

) N-0 stretching

fundamentals at 1870 cm

-1 and 1789 cm , respectively. The low frequency

modes associated with the vdW bond are 90 (A2

), 170 (A-|), 198 (B2

), and 263

(A.|) cm”1 . We used laser-excited fluorescence (LEF) spectroscopy (using a frequency doubled C02 laser for photolysis and a 10 ns probe dye laser) to

determine the internal state distribution and kinetic energy of the NO

photofragments. For fragments with internal energy E^ nt S 400 cm

-1 (representing £ 97% of all products), the rotational states were adequately

represented by a Boltzmann distribution with TRot = 110 K, for both spin

orbit states. The two spin orbit states, F^ = NO 2

IIi/ 2 and F

2

= NO 2

^^/ 2 >

57
were formed with equal probability. The lambda doublet states were formed

with equal probability. The average product internal energy was <E int > = 135 ±25 cm

-1

.

Doppler profile measurements were made for fragments in the F

2 (J=3.5)

state. Profiles measured with the propagation direction of the probe laser

parallel or perpendicular to the electric vector of the photodissociation

laser were indistinguishable. This indicates the angular flux of the NO

photodissociation fragments was essentially isotropic. The observed Doppler

profiles were "top hat" (rectangular) in shape with 0.165 ± 0.01 cm

-1 FWHM.

The top hat shape implies a narrow kinetic energy distribution, a condition

imposed by the photon energy and E int distribution [assuming the dimers

initially equilibrated with the beam (T - IK)]. Computer modeling (including

the fragment internal state distributions) of the observed Doppler profiles

gave an average kinetic energy <EK

> = 400 ± 50 cm

-1 for each NO.

Conservation of energy gives a bond energy D

Q = 800 ± 150 cm

-1

, which may be

compared to the value D Q = 590 ± 80 cm

-1 calculated from infrared intensity

measurements.

For the lifetime measurements, dilute mixtures of NO in He or H

2 were expanded through a pulsed valve operating at 4 Hz with a .75 mm diameter

orifice. The observed LEF signals scaled with expansion conditions as x2p2.9, for NO mole fractions 0.005 ^ X S .02 and nozzle stagnation pressures

3 S P S 10 atm. This is the same scaling behavior attributed to (N0 ) 2 previously. The conditions used for the lifetime measurements were shown by

mass-spectrometry and final-state energy distribution measurements to produce

cluster signals dominated by NO-dimer, and exhibit rotational cooling (of

uncomplexed monomer) to -1 K.

Picosecond infrared (ir) and ultraviolet (uv) laser pulses were derived

from two visible dye lasers synchronously pumped by a frequency-doubled,

modelocked cw YAG laser. One dye laser was set at m-|=l7, 3 4 1 cm

-1

. The

second dye laser was tuned to uj2

= 15,471 or 15,552 cm

- so that the

frequency difference, wo = oo-| - w2 , coincided with either the v-| or V 4 fundamental. Dye amplifiers pumped by a frequency-doubled, 2 ns Q-switched

YAG laser amplified both lasers to produce visible picosecond pulses at 20 Hz

with typical pulse energies of 0.7 mJ (co-j) and 0.3 mJ (to2

). Bandwidths of

these pulses were 3 cm

-

' FWHM with autocorrelation widths of 7.5 ps FWHM and

a cross - correlation width of 10 ps FWHM. Infrared pump pulses of 4yJ were

obtained by difference frequency mixing cu-| and cu 2 in LilO^. The ir pulses were focused to a 0.3 mm diameter beam waist to vibrationally excite the

beam-cooled (N0) 2

. Ultraviolet probe pulses were produced by frequencydoubling uj2 in KDP ayd then summing 2u> 2 with residual 1.064 nm from the

Q-switched YAG in KD P. These pulses traversed a variable optical delay and

were focused to a 0.15 mm beam waist colinear with the ir pump beam.

Probe pulses polarized at the magic angle to the ir pump pulses traversed

the beam at a time t D , relative to the pump pulses, exciting those NO

fragments formed in the J = 1 . 5—7 . 5 levels of the state (P 2 band head

region), resulting in LEF signals directly proportional to the number of NO

58
fragments formed during the time t^. Previous studies showed that these

probed states 1) are not appreciably populated by uncomplexed monomer in the

expansion, 2) are formed with high probability in the (N0) 2

V| photolysis,

and 3) are not favorably formed in the 1870 cm

-1 photolysis of other NO

complexes. The LEF signal and pump and probe pulse energies were recorded

for each shot with a computer based data acquisition system. LEF signals

were normalized to the uv probe energy. Shots with ir energy falling outside

a ±20? acceptance window were rejected. For each data point, a delay setting

was randomly selected and 100 accepted shots were averaged.

Fig. 3-3 Time-resolved appearance of NO fragments following excitations of

either the symmetric (v-j) or antisymmetric (vjj) stretching

fundamentals of the NO dimer. Note the 20-fold difference in time

axes. The solid curves are least-squares fits of a simple

exponential decay function with unimolecular decay rate k uni = 1/fyp. Although only short time measurements are included for the

rapidly rising vjj data, there was no change in signal for delays

between 150 ps and 4 ns.

Results for (N0) 2 v-j(v=1) and vjj(v=1) photodissociations are shown in Fig.

3.3. A dramatic difference in dissociation rates for these two modes of

nearly equal energy is immediately apparent (Note the 20-fold difference in

time axes). Only short-time measurements are shown for the rapidly rising vjj

data; no further increase in signal was observed for delays between 150 ps

and ns.

59
If the ir laser excited an ensemble which dissociated with a single

unimolecular rate constant, k un^ = i/ T yp, the expected fragment appearance

behavior would be:

SCt]-,) = S(tp = ») [ 1 -exp( tp/iyp) ] .

Of course non-exponential or multi-exponential decay could occur. Non-linear

least-squares fits of single exponential decays to the data, shown as solid

curves, yield lifetimes of 880 ± 260 ps for v

1 and 39 ± 8 ps for V 4

. These

lifetimes represent average values for a distribution of rotational levels.

Assuming that the ir bandwidth is *4-5 cm

-

^ (convolution of two 3 cm

-1

Gaussians), the pump laser interacts with all (N0 ) 2 rotational levels

expected to be populated in the molecular beam (If TRQT = i k, levels with

J"S3, K

a "=2 account for 95% of the population). Each vibrationally-excited

rotational level could have a distinct predissociation lifetime. However,

with the present signal-to-noise, the data at many different expansion

conditions are adequately fit by the same single exponential time constant.

Including additional exponentials with different decay times did not

significantly improve the fit to the data (i.e., reduce chi square).

That vjj (1 789 cm

-1

) decays much faster than v-| (1870 cm

-1

) is

inconsistent with standard statistical theories of unimolecular reaction

which predict unimolecular rates to increase with reactant internal energy.

On the other hand, models of vibrational predissociation involving pure V-T

energy transfer correlate increased lifetime with increased energy release,

and predissociation from releases 81 cm

-

^ more energy than vi|. However,

theory quantitatively predicts a much smaller difference (factor of S2 ) between v-| and vij than was observed. In the case of (N0 ) 2 > the

predissociation might be nonadiabatic since the 2

n configurations of the NO

fragments combine to form several low-lying electronic states in the dimer.

Nonadiabatic effects have been suggested to explain the anomalously large

collisional cross-section observed for vibrational deactivation of N0(v=1) by

N0(v=0). The different overall symmetries of the v.| and vij levels of the

dimer "'a-j ground state may constrain coupling to low lying repulsive

state(s), causing the observed difference in lifetimes. Ongoing measurements

of product energy distributions following vj, dissociation and lifetimes for

states of different vibrational symmetries (e.g., combination bands involving

the vdW modes) might elucidate the source of the difference in v-j and vi|

predissociation rates.

Future progress in this area depends critically on our ability to

obtain additional funds, either internal or from other agencies, to cover staff salaries and operations. During 1987 we hope to measure the

vibrational predissociation rates following excitation of particular

vibrational modes of several vdW dimers, and also to measure the distribution

of energy (electronic, vibrational, rotational, translational) in the products.

Candidate molecules are:

60
C

2Hh*N0

CONO

C

2H2 *C2^2

1 900 cm 1

1870 cm

-1

20H 6 cm

-1

3282 cm

' 1

C 2H 14 ( vy+vs) *N0 ‘

C

2Hi|*N0(v=1

)

C0( v=1 ) *N0

^2^2^ v

3

= 1 ) *^ 2 D2

C 2

Hij + NO (J,ft,A)

C 2

Hi| + NO (J,fl,A)

C0(0)+N0(v=1 or v=0,J,£2,A)

-* C 2H2 (v,J) + C 2 D2

( v, J)

.

Many other vibrations in C

2h2 , C 2

D2 , and C 2HD complexes can be studied. In

several systems vibrational energy transfer across the vdW bond (a fast

process according to some theoretical calculations) may occur and can be

readily detected by LEF. A high resolution (e.g., transform limited,

15 ns pulses) tunable infrared laser source, similar to the low resolution ps

ir difference frequency system, is necessary for obtaining good

photodissociation spectra on the same molecular beams used for the

time-resolved measurements.

E. Photophysical Studies of Photosensitizers

(D. S. King, in collaboration with D. F. Heller and J. Krasinski of

Allied Corporation and R. S. Bodaness of National Center for Health

Services Research, OHTA)

Many photosensitizers, such as hematoporphyrin derivative (HPD), are known

to produce excited-singlet molecular oxygen which is then responsible for

ensuing oxidation reactions. Chemically, HPD has received significant

interest because it preferentially binds to tumor tissue and can be used for

tumor localization (fluorescence) and photochemotherapy. Conventional tumor

photolocalization techniques use blue to ultraviolet excitation sources which

are severly attenuated by tissue absorptions, limiting detection and

treatment to exposed surface layers. We have persued a more universal

approach which takes advantage of the two-photon spectroscopy of HPD.

Two-photon excited fluorescence spectra were taken using either a Qswitched YAG with a 20 ns output pulse duration at 1064 nm or a Q-switched

alexandrite laser with 55 ns pulse duration, tunable in the 730 to 780 nm range. Second harmonic generation was also used to directly excite the

sample at 532 or 375 nm. The evidence for two-photon excitation consists of

both the similarity of the fluorescence spectra recorded following, for

example, excitations at 1064 nm (two-photon absorption) and at 532 nm (onephoton absorption), and the quadratic intensity dependence of the two-photon

fluorescence signals. Calibrations to determine the two-photon cross-sections

were made from single-photon fluorescence data, using the known single-photon

absorption coefficients and fluorescence quantum yields. The two-photon

excitation cross-section at 750 nm (of 1.5 x 10

-4 ^ cnr s) was approximately

100-fold greater than that at 1064 nm; we attribute this to a resonant

enhancement contribution from the HPD S-j state.

61
In the alexandrite laser excitation experiments the HPD two-photon

fluorescence was of sufficient intensity to be observed by eye in a darkened

room through a single dielectric filter. This implies potential usage of

such two-photon techniques for tumor localization at substantial tissue

depths since there is an approximately one-thousand fold increase in tissue

transmission at 750 nm as compared to 400 nm. Since the two-photon

absorption rate is proportional to the square of the laser intensity, the

application of picosecond laser pulses should greatly increase the efficiency

of the two-photon absorption process, permitting the use of substantially

reduced laser pulse energies to achieve equivalent ends. Use of 50 ps, 1 mJ

pulses available from a pulsed, mode-locked alexandrite laser oscillator will

only deliver 2 % of the energy dose used in the present two-photon

experiments, however the two-photon excitation rate should be enhanced a million-fold in relation to excitation by a 50 ns pulse of the same energy.

This should lead to complete saturation of the HPD two-photon absorption.

Such experiments are planned for Spring 1987.

62
H. QUANTUM CHEMISTRY GROUP

H. Basch, D. Beveridge, W. Fink, N. Goldgaber, P. Jasien, P. S. Julienne,

M. Krauss, B. Lengsfield, M. Mezei, F. H. Mies, K. J. Miller,

R. Osman, W. J. Stevens, and L. Vahala

A. Introduction

B. Theoretical Calculations on Biomolecules

1 . Development of an Ab Initio Reaction Field Program

2. Modeling of Fe and Mn Superoxide Dismutases

3. Frozen Fragment Studies of Hydrogen Bonding

4. Monte Carlo Studies of Solvated Biomolecules

5. Binding of Pt Complexes to DNA

6. Electronic Structure of Phosphates, Phosphoranes, and Their

Analogues

7. Rotational Barriers in Formamide and Acetamide

8. Future Plans

C. Quantum Chemistry of Small Molecules

1 . Accurate Calculations on Small Molecules

(a) CrO and CrO +

(b) Silicon Carbide, SiC

2. Interaction Energy Curves of RR', (R,R* = He through Xe)

3. Future Plans

D. Scattering in the Presence of Radiation

1 . Non-adiabatic Theory of Atomic Broadening in Weak Fields

(a) Redistribution Calculations for Sr^p-^S) + Ar

(b) Redistribution Calculations for Na + Rare Gas

(c) Lorentzian Pressure-Broadening Widths for Na

63
2. Nonadiabatic Theory of Redistribution in Strong Fields

(a) Collisional Redistribution of Resonant Radiation

(b) Analysis of Multiphoton Lineshape Theory in Intense Fields

3. Future Plans

E. Atomic and Molecular Scattering Theory

1 . Half-Collision Amplitudes

(a) Analysis of Exact Close-Coupled Codes

(b) Multichannel Semiclassical Half Collision Amplitudes

2. Multichannel Photodissociation Amplitudes

3. Future Plans

64
A. Introduction

The Quantum Chemistry Group develops and applies atomic and molecular

structure and scattering methods to chemical and spectroscopic problems where

experiments are difficult to perform or so complicated that the theory is

needed to model the results. The activities of the group encompass three

broad areas: the study of the structures and properties of large

biomolecules, accurate quantum mechanical calculations on small molecules,

and atomic and molecular scattering theory.

The biomolecular research is a result of the NBS biotechnology initiative

and represents a new direction for the group. Initial work in this area has

focussed on the development of methods for accurately predicting the

electronic structure, energetics, and properties of small pieces (eg., active

sites) of large molecules. This research has resulted in new techniques for

the decomposition of molecular electrostatic potentials and polarizabilities

into transferrable fragment components that will be used to construct

effective Hamiltonians for the small parts of the biomolecules to be studied

quantum mechanically. During the past year, several collaborations with

other groups have been initiated which will concentrate on applications of

newly developed theoretical techniques to interesting problems in

biochemistry. The Molecular Structure and Modeling Facility IBM^138l computer

is used for these large scale calculations.

The determination of accurate electronic structures and properties for

small molecules has been a central focus of the group’s activities for many

years. Our unique capability for handling heavy atoms through the use of

effective core potentials has allowed the study of a variety of interesting

molecules including transition metal oxides and heavy rare-gas dimers. The

accomplishments for this year include state-of-the-art calculations on chromium oxide and silicon carbide, and a complete set of calculations of the

van der Waals potentials for the homonuclear and mixed rare-gas dimers.

In the area of scattering theory, new methods have been developed for the

determination of cross-sections and fragment state distributions associated

with the collisonal broadening and photodissociation of small molecules in

the presence of both weak and strong radiation fields. In addition,

significant progress has been made in the area of multichannel quantum defect

theory and non-adiabatic half-collision amplitudes, with specific

applications to broad-band continuum spectroscopy.

65
B. Theoretical Calculations on Biomolecules

1 . Development of an Ab Initio Reaction Field Program

(W. J. Stevens, H. Basch, and M. Krauss)

Although the methods of modern quantum chemistry and the machines on which calculations are carried out have evolved dramatically in the last ten

years, it is still not feasible to determine the geometric and electronic

structures of very large molecules (> 20-30 atoms) by ab initio techniques.

However, in large molecules such as proteins, it may be possible to learn a great deal about chemical properties by looking at the detailed electronic

structure of a small part of the molecule (the active site), while treating

the remainder of the system as a perturbing environment that plays only an indirect role in the chemistry. During the past year we have modified the

HONDO electronic structure program to include a reaction-field Hamiltonian

that allows the study of an active molecular fragment, "A" , in the field of a molecular ’'spectator” environment, ”S”. The Hamiltonian is written

Ha - + vAS

where V ^ describes the interaction of the active electrons with the

spectator part of the system. Assuming the interaction of "A” with ”S” is

non-bonding (i.e., no covalent bond, no charge-transfer), the interaction

term in the Hamiltonian may be broken down into three pieces: electrostatic,

polarization, and orthogonality and exchange repulsion. Our desire is to

represent each of these interactions by one-electron operators in the

Hamiltonian of the active part, so that the difficulty of the calculation is

primarily dictated by the complexity of the active fragment ”A”.

Our analysis of the electrostatic potentials of the spectator molecules

is based on localized molecular orbitals (LMO), Accurate SCF calculations

are carried out for prototypical molecules containing specific functional

groups. The molecular orbitals of the prototype are localized by some appropriate method (e.g., the Boys method). The electronic contribution to

the electrostatic potential is the sum of the contributions from each LMO.

The electrostatic potentials of LMOs catagorized by functional groups are found to be reasonably transferrable from one molecule to another. The

electrostatic potential of each LMO is approximated by a multipole expansion

at its center of charge. The molecular potential is then represented by a distributed set of LMO expansions. This distributed expansion produces an accurate representation of the molecular electrostatic potential for LMO

expansions that include only the centers of charge and the first few moments.

We have modified the HONDO code to include the potentials of such

distributed charges and second moments in the Hamiltonian, and test

calculations are now being done to determine the accuracy for a variety of

prototypical systems.

The polarization of "A” by ”S” occurs naturally when the Schroedinger

equation is solved for the wavefunction of "A" using a Hamiltonian that

contains the electrostatic potential of ”S”. However, the determination of

66
accurate interaction energies requires that "S" also is polarized by the

electrostatic potential of "A". In addition, there is a self-consistent

effect in the mutual polarization of the two parts of the system.

Fortunately, the self-consistent part of the polarization has been found to

be small ( -1 0% ) . For the energy contribution due to the dipole

polarizability of "S" we evaluate the expression

< *A lf,j ?i ' ?j « ("S")| f A

>

where is the electric field due to electron i in fragment "A", and

a("S") is the dipole polarizability of fragment "S". This energy term is

evaluated as a perturbation once the wavefunction for "A” is known. Putting

the operator directly into the Hamiltonian of "A" is not possible, because

second order effects related to dispersion are improperly introduced. The

polarizability of the fragment "S" is constructed from fragment LMO

polarizabilities. We have developed a coupled Hartree-Fock scheme which

allows the total calculated polarizability of a molecule to be rigorously

represented as a sum of LMO polarizabilities. A series of calculations has

shown the LMO polarizabilities for functional groups to be reasonably

transferrable from one molecule to another. Distributing the LMO

polarizabilities in the fragment "S" helps take into account the

inhomogeneous nature of the electronic field due to "A". Higher order

polarizabilities may be added in the same fashion if necessary. Currently,

the HONDO code inccorporates spherically averaged LMO polarizabilities in the

Hamiltonian of "A", and work is proceeding to incorporate tensor

representations.

Exchange repulsion and orthogonality between "A" and "S” must be

included in the Hamiltonian of "A" in order to prevent the collapse of the

"A" wavefunction into the "S" region. A rigorous evaluation of the exchange

interaction requires two-electron operators and a knowledge of the wavefunctions of "A" and "S" . In the case of atoms, it has been shown that a one-electron effective potential can be constructed which accurately mimics

the exchange repulsion between the valence electrons and the frozen core. Our group has used such potentials extensively in calculations on systems

containing heavy atoms. We are developing analogous procedures for molecular

systems where the fragment "S" is treated like the frozen core in the atomic

case. A prototypical all-electron calculation is done in which the fragment

"A" is optimized with the fragment n S" held frozen. The wavefunction of "S"

is taken as a product of frozen LMOs which are obtained from independent

calculations on "S" alone. Then the frozen LMO exchange operators in the

Hamiltonian of "A" are replaced by one-electron effective potentials. The

parameters defining the potentials are adjusted until the orbitals and energies of "A" match those obtained from the all-electron calculation using the

frozen "S" and the correct exchange operators. Since the derivation of the

effective potentials is based on LMOs, transferability from molecule to

molecule is expected. Calculations are underway to test this hypothesis.

67
2. Modeling of Fe and Mn Superoxide Dismutases

(R. Osman, W. J. Stevens, M. Krauss, and H. Basch)

This research is part of a long term objective to understand the

biochemical and biophysical role of metals in enzymatic mechanisms of

metallo-enzymes. Quantum mechanical calculations are carried out on models

of the enzymes active site, with and without the substrate, in order to

ascertain whether proposed mechanisms are reasonable from an energetic point

of view. The complexity of the model and the sophistication of the quantum

mechanical approach are chosen to produce energetics that are reliable enough

to determine which of several proposed pathways may be energetically the most

favorable. The electronic structure of the model active site is analyzed to

provide insight into the interactions that favor one pathway over another.

Three different superoxide dismutases have been chosen for study. They

are Cu,Zn-S0D, Fe-SOD, and Mn-SOD. The availability of their crystal

structures enables the construction of models of their active sites. Osman

and Basch have previously studied Cu,Zn-SOD (J. Amer. Chem. Soc. 1 06 , 5710

(1984)) and have suggested a new mechanism of action that is quite different

from the ones proposed previously. Their quantum mechanical calculations

indicate that a single superoxide (0^) cannot reduce the cupric ion because

of an arginine group (Arg-141), positioned near the copper, that forms a strong hydrogen bond with the superoxide. Thus, superoxide forms a relatively stable complex with Cu,Zn-S0D that is subsequently reduced by

another superoxide in solution. The reduced complex undergoes an internal

oxidation-reduction coupled to a proton transfer that results in regeneration

of the original enzyme and the formation of the products of the dismutation

reaction. The proposed mechanism is in agreement with all known kinetic

measurements of the enzymatic reaction. The mechanism is based on the

special poperties of the copper ion in the active site and its surroundings

in the protein (particulary Arg-141).

Fe-SOD and Mn-SOD, while similar in structure to each other, are quite

different in structure from the Cu,Zn-S0D. The proposed mechanism of action

of these enzymes is also based on the reduction-oxidation cycle which is

consistent with the kinetic studies of the dismutation reaction. However,

the details of the reaction are not understood, and analogy to the Cu,Zn

enzyme may not be warranted. The recent availability of crystal structures

for these enzymes offers an opportunity to perform quantum mechanical

calculations on models of the active sites.

We have begun the calculations by determining effective core potentials

for the Fe and Mn atoms which allow their inclusion in the model active sites

as valence-only systems. In addition, the core potentials have been modified

to reproduce accurate M

+2 -»• M

+ ^ ionization potentials. Crystal structures

indicate that the metal ion is pentacoordinate, with three hystidines, one aspartate, and one water (or OH

-

) acting as ligands. In our initial models

we have replaced the hystidines by ammonias and have considered only the five

ligands directly bonded to the metal ion. More complex models will be

68
developed using a reaction field approach to incorporate regions of the

enzyme that do not participate directly in the reaction but do influence the

energetics by virtue of their electrostatic potentials.

3. Frozen Fragment Studies of Hydrogen Bonding

(W. J. Stevens and W. Fink (UC Davis))

In a recent series of articles (J. Chem. Phys. 1983, 7j8, 4066; J.

Chem. Phys. 1985, 83, 735; J. Chem. Phys. 1986, 84, 5687) Reed and Weinhold

have proposed an energetic decomposition of hydrogen bonding interactions

based on a "natural bond orbital" analysis. The conclusions reached through

this analysis are that the electrostatic attraction and exchange repulsion

energy components of the hydrogen bond nearly cancel each other, and the

charge-transfer interaction determines the shape of the potential energy

surface and the structure of the complex. The charge-transfer analysis is

similar to the H0M0-LUM0 charge-transfer interaction proposed by Klemperer

and coworkers, but differs in that the orbitals involved are the localized

lone-pair orbital as electron donor and the unoccupied X-H antibonding

orbital as electron acceptor. The amount of charge transferred is very small

(a few thousandths of an electron), but the energy contribution is said to be

several kcals/mol. This analysis is contrary to the widely accepted proposal

of Buckingham and others that the structure and energetics of hydrogen

bonding is determined primarily by electrostatics.

One of the problems with a decomposition of the interaction energy

between two molecules is that the orthogonlity, exchange, and charge-transfer

components are difficult to unravel because they all involve the overlap of

the molecular wavefunctions. The analysis is also confused by a superposition effect that occurs when the virtual orbitals from the basis set

of one fragment help to improve the occupied orbitals of the other fragment.

Even though the analysis of Reed and Weinhold appears to be remarkably

consistent for a wide variety of hydrogen bonded complexes, it is not

impossible that the component of the energy that has been assigned to

charge-transfer is really a mixture of other effects.

In collaboration with the group at the University of California at

Davis, we have modified the HONDO program to allow for the selective freezing

of molecular fragments in an all-electron calculation. It is also possible

to selectively eliminate unoccupied virtual orbitals from the basis set. We

have used this program to study the water dimer and to assess the importance

of charge-transfer in the hydrogen bond interaction. The molecules in the

dimer are labelled "A" and "B". Using a fully optimized hydrogen bonding

geometry at the double-zeta plus polarization level, the energy is calculated

by selectively freezing the wavefunction of either "A" or "B" and optimizing

the wavefunction of the unfrozen molecule. The frozen wavefunction is

obtained from a calculation on an isolated water molecule. Charge-transfer

cannot occur from the frozen water to the variationally active water. We

have also eliminated the virtual orbitals from the frozen water basis set so that charge-transfer cannot occur from the variationally active water into the

unoccupied space of the frozen water. With all of these constraints, our

69
preliminary calculations have found that the hydrogen bond energy in the

frozen molecule calculations is within 20% of a fully variational hydrogen

bond energy. Also, a significant portion of the missing energy can be traced

to the lack of polarization in the frozen molecule. Although many more hydrogen bonded systems need to be studied, our tentative conclusion is that

charge-transfer does not determine the structure and energetics of hydrogen

bonded complexes.

4. Monte Carlo Studies of Solvated Biomolecules

(D. Beveridge, M. Mezei, W. J. Stevens, and M. Krauss)

This research project has been initiated to explore the interface

between ab initio quantum mechanical studies of large molecules and

simulation of molecules in the liquid state using the Metropolis Monte Carlo

method. The first step in this project has been to install the Hunter

College MMC program for Metropolis Monte Carlo simulations on the Molecular

Structure and Modeling Facility IBM4381 computer and the Consolidated

Scientific Computing System CYBER205 computer. The MMC program, developed by

Mezei and Beveridge, is a state-of-the-art program for simulation of a dilute

aqueous solution with a force-bias option for convergence acceleration, and a proximity analysis option for assigning the solvent molecules in each

N-particle configuration of the system to individual atoms of the solute.

The MMC program uses intermolecular potential functions based on several

current prescriptions . These functions are hardwired into the code, but the

introduction of new potentials is easily accomplished. The results of the

computer simulation include the calculated thermodynamic internal energies,

heat capacities, and various structural and energetic molecular distribution

functions. Some new methods for free energy simulations are also included.

The accuracy of the results of computer simulations of dilute aqueous

solutions is dependent on the intermolecular potential functions that are used to evaluate interaction energies and energy derivatives between

molecular constituents. The functions currently used in programs such as MMC

are based on pairwise-additive interaction potentials between points in the

molecules (usually the atoms) that are derived either empirically or from ab

initio quantum mechanical calculations. The form of the potentials is kept

simple (6-12 plus a coulombic term) because of the computer time required to

carry out the simulations. Potentials derived by fitting quantum

mechanically calculated potential energy surfaces have been restricted to

low-level SCF calculations, such as ST0-3G, due to the expense of accurate

calculations on complex molecules. Our high-speed computing resources and

recently developed methods for simplifying ab initio calculations on large

molecules (such as the reaction field approach) will enable us to examine the

dependence of the Monte Carlo simulation results on the accuracy of the

potentials. Benchmark calculations are planned for several small amides in

aqueous solution. We will also explore the use of distributed molecular

polarizabilities in the evaluation of interaction energies as a method for

incorporating many-body effects in the simulation. This will be particularly

important in the study of solvated ions.

70
Quantum mechanical calculations of the properties and reactivities of

molecules in solution or of molecular fragments imbedded in very large

molecular systems are limited by the statistical nature of the perturbing

environment at nonzero temperatures. Part of the collaboration between the

NBS and Hunter College groups will be to explore the use of snapshot

distributions from converged Monte Carlo simulations to construct models of

the solvent environment in reaction field studies of prototypical molecules.

5. Binding of Pt Complexes to DNA

(H. Basch, M. Krauss, K. J. Miller (RPI)

Preliminary calculations on the binding of cis-Pt (NH 2

r 2 ) 2 . (R=H,

methyl, and cyclopropyl) to a tetramer duplex were reported in last years

report but research has expanded on this problem because of new experimental

results reporting the binding of Pt(NH

3

) 2 to a pGpG segment (S. E. Sherman et

al., Science 230 , 412 (1985)), that suggested Pt(NH^)***0P H-bonds

significantly stabilize the complex. This research was initiated to

determine if binding energies could account for the very different anti-tumor

potencies between the methyl derivative and both Pt(NHo) 2 and the cyclopropyl

derivative. We concluded in the preliminary results which combined SCF

calculations of model binding energies with molecular mechanics

conformational energy analysis that the binding energy varies smoothly with

substitution. The constrained molecular mechanics energy minimization did

not show direct hydrogen bonding between the amine ligand and phosphate which

is a feature of the Pt-pGpG crystal structure.

Two directions are being pursued to determine if direct phosphate

H-bonding is important in the duplex structure. Full cartesian coordinate

geometry optimization is being pursued with a tetromer duplex bound to Pt.

Achieving a global minimum is an almost impossible task but a number of local

minimum structures have been examined and those which distort the H-bonding

between the bases the least were examined for the substituted amines.

Although a phosphate moiety is found to approach closer to ammonia (0***H of

the order of 3.5A), an H-bond distance of approach has not been found. The

second phase of this calculation is to examine SCF H-binding energies for

Pt(NHo) (R) 2 to phosphate with associated waters in order to examine the

possibilities for water either externally bonded or as a bridged water.

These calculations are still in progress. The results to date do not alter

the earlier conclusion that potency and binding energy are not correlated

if no chemical reaction other than ligand replacement is induced by the

Pt interaction with DNA.

6. Electronic Structure of Phosphates, Phosphoranes , and Their

Analogues

(H. Basch, M. Krauss and W. J. Stevens)

Is the stable five coordinate vanadium complex formed in the binding

of uridine vanadate to ribonuclease an analogue for a five corrdinate phosphate

transition state? By examining the comparative electronic structure and the

proton affinities (PA) the concept of transition state analogues can be made

71
more precise. The relative amount of d orbital contribution and the PA of

different sites will determine whether proton transfer from the protein to

the site as observed for the vanadium complex reflects the phosphorane

situation. At the SCF level there is a substantial difference between the

electronic structure and PA of PO^ and VO

3

. Electron correlation has only a small effect on PO^ or H 2

P 0 lj but the comparable vanadium calculation has not

yet been done.

Whether phosphorothioate anions will interact with proteins at either

the S or 0 site will, in part, be determined by the electrostatic potential

at each site. Although considerable experimental effort has been devoted to

determining bond orders and ionicities for these molecules, the simplest way

to determine the potential is to calculate it from a high quality

wavefunction. Electrostatic potential maps are compared in Fig. 1 for

H

2

> P OlJ and i^PC^S

- in a plane determined by O-P-O or 0-P-S. The outer

potential contour which determines binding behavior is shifted for S relative

to 0 by approximately in proportion to the longer bond in P-S, reflecting the

comparable ionicity of 0 and S. This suggests zwitterionic or H-bonds will

be made initially with S which would be reinforced by the larger

polarizability of S. On the other hand, there is much less difference in the

outer contours of PO^ and P 02S

-

, reflecting the smaller fractional charge

found for S in this case. Optimized geometries and PA's have been determined

for all these species. Calculations have been initiated on the NMR chemical

shifts for 31 p to correlate shifts with electronic structure and

conformation.

H2P0<1 ELECTROSTATIC POTENTIAL II2P03S ELECTROSTATIC POTENTIAL

S I ZE=3B, XC0N=-13B. -123 , -IBB, -75 , IB SIZE;3B, XC0N=-13B,-123.-1BB,-?S,1B

Fig. 4.1 A comparison of the electrostatic potentials of HpPOjj and HpPO^S.

The dashed contours represent potentials of -150 (innermost), -125,

- 100 , and -50 kcal.

72
7. Rotational Barriers in Formamide and Acetamide

(P. G. Jasien, W. J. Stevens, and M. Krauss)

The energy barrier to rotation about an amide bond is important in

the modeling of protein conformation. There have been a number of ab initio

attempts to calculate the inherent barrier, but a review reveals that the

accuracy was significantly limited by a lack of a complete polarization

basis. The present results indicate that the inclusion of polarization

functions in the basis set leads to a substantial decrease of 5 kcal/mole in

the barrier height at the SCF level. Considering electron correlation and

the zero point energy correction, the in vacuo rotational barriers for

formamide and acetamide are predicted to be 14.2 and 12.5 kcal/mole,

respectively.

8. Future Plans

The major thrust in the biotechnology area is the development of the

reaction field method. The reaction field method will be applied to the

effect of solvation on the stability of amides, reactions of phosphates and

their analogues, the binding of metals to DNA and protein model compounds,

and the mechanism of reaction of Fe and Mn superoxide dismut ase. A

continuing effort is needed in evaluating the parameters and assessing the

data (centroids, local polarizabilities, local quadrupoles, ...) required for

the reaction field. The use of both molecular mechanics and statistical

mechanics methods to determine the conformation of the environment region in

the reaction field will be investigated.

C. Quantum Chemistry of Small Molecules

1 . Accurate Calculations on Small Molecules

(W. J. Stevens, P. G. Jasien, and B. Lengsfield)

(a) CrO and CrO +

Understanding the electronic structure of the transition metal

oxides is a challenge for experimentalists and theoreticians alike. The

multiple open-shell nature of the electronic wavefunction, highly degenerate

constituent atoms, extreme covalent/charge-transfer mixing, and large

spin-orbit splitting make the electronic states difficult to characterize

theoretically and the spectra and thermodynamic behavior difficult to

analyze. The CrO, CrO

+ system is interesting from a theoretical point of

view because there are a number of experimental analyses available including

high-resolution spectroscopy of the ground state, photoelectron spectroscopy

determining the ionization potential, and several estimates of the

dissociation energies. The ground state of CrO is a state with the

dominant valence configuration

73
1 2 2 4 12

0 0 0 IT IT 6 Cr 0 b b a Cr.

The molecule is best described as Cr + 0

-

. The singly occupied o orbital on chromium is strongly polarized away from both the CT and the double occupied

bonding orbital. The bonding orbital is quite polarized toward oxygen,

and the ir a orbital is only weakly antibonding. Ionization to form the

positive ion will remove an electron from either the singly occupied sigma

orbital or the singly occupied pi orbital. Dyke, et al. (J. Chem. Soc.,

Faraday Trans. 1983, 79_, 1083) have analyzed the photoelectron spectrum of

CrO in terms of a single ^2“ state of the ion, while our calculations show a state only 0.1 eV above the ^2“ ion ground state. Our calculated

ionization potential of 7.43 eV is in reasonable agreement with the

experimentally deduced value of 7.85 eV, but our ion spectroscopic constants

disagree with the experimental numbers. Our best calculations (singles and

doubles Cl from a multiconfiguration wavefunction) predict R

0

= 1.64A(^£~),

1.62A(^n) and aje

= 801 cm

-1 (^Z

-

), 895 cm

-1

(**n) as opposed to 1.7 9A and 640

cm

-1 quoted by Dyke, et al. Our values for the ground state of CrO of

R

e

= 1 . 6 47A and toe

= 850 cm

-1

agree reasonably well with the values of

R

e

= 1.6179A and we= 898.50 cm

-1 obtained from the analysis of rotationally

resolved laser-induced fluorescence spectra by Hocking, et al. (Can. J. Phys.

1980, 58_, 516). Since the theoretical calculations for the neutral and ion

states were carried out at the same level of accuracy, we believe that the

analysis of the photoelectron spectrum is in error.

(b) Silicon Carbide, SiC

The silicon carbide molecule, SiC, is of fundamental importance in

astrophysics because of the role it may play in the formation of interstellar

grains. Although the triatomic molecule, SiC 2 , has been observed in the

laboratory and in interstellar space, no observations of the SiC molecule

have been reported. Recently, there have been a number of theoretical

calculations on SiC which predict the infrared and electronic transition

frequencies, but no accurate prediction of the microwave absorption

frequency has been made. Radiotelescope observations of interstellar

spectra are made in 128 MHz spectral windows which require several hours of

data accumulation to develop a reasonable signal-to-noise ratio. At the

absorption frequency of SiC (-18 GHz), the R

Q must be known to within ±0.004A

in order to predict the correct window for observation. Such a prediction is

within the capability of modern quantum mechanical calculations.

We have performed singles and doubles Cl calculations from

multi-reference wavef unctions for the ground state of SiC using a very

large Slater-type basis set consisting of s,p,d, and f functions on each

center. In addition to the usual d polarization functions, d and f functions

were added for correlation, and their exponents were optimized in large-scale

Cl calculations. Five points around the bottom of the potential energy curve were calculated and fit to a Morse potential. The internuclear distances for

74
the calculations were chosen to symmetrically surround the minimum and to cover an energy range surrounding the lowest vibrational level. Additional

points were added to ensure that the predicted Morse parameters were not

sensitive to the point distribution. In all cases, the Morse fit reproduced

the calculated data points to within a few wavenumbers. Several curves were

generated in this way using a variety of reference wavef unctions and Cl

selection criteria. In the largest calculations the wavefunction consisted

of more than 300,000 configurations. The several largest calculations

produced R

g values that were consistent to within 0.003A and vibrational

frequencies that agreed to within a few wavenumbers.

Spin-orbit coupling splits the ground state of SiC into Q = 2,1, and

0 components. The component is lowest in energy. Using ab initio

effective spin-orbit operators developed in this laboratory, we have

determined that the spin-orbit splitting in SiC is 3^ cm

-

^ and nearly

constant across the bottom of the potential curve. Thus, the predicted Rq

value will not be affected by spin-orbit coupling. There is a possibility

that the 3]^ state could be perturbed by other low-lying Q = 2 electronic

states. In our calculations, the closest perturbing states are 3^ and

both of which are several thousand wavenumbers higher in energy.

The results of this study have been communicated to Dr. Michael

Hollis of NASA, who will compare the predictions to known interstellar

microwave lines. If a sufficiently interesting comparison is found, an application will be made for viewing time at the Kitt Peak observatory.

2. Interaction Energy Curves of RR', (R,R' = He through Xe

(D. D. Konowalow and (Dept. Chem. SUNY Binghamton) M. Krauss)

Application of an ab initio hybrid model of the calculation of the rare gas dimers has yielded binding energies within 10-20? of experiment for

these molecules. The discrepancy is attributed to the slow convergence in

the attractive dispersion energy with high order multipoles. These

calculations were performed to determine the applicability of the model to

heavier systems to allow calculation of biomolecular models. Very

recently an accurate correlated calculation of the binding energy of HeAr (W.

Meyer et al., Phys. Rev. A33 > 3807 (1986)) suggested that correlation of the

fragments is important and the hybrid model potential may be questionable.

Correlated atom calculations are now in progress to test this conclusion for

a range of molecules. Since the hybrid model is widely used, it is necessary

to examine the significance of correlation in model systems.

3. Future Plans

Accurate quantum chemistry calculations of model systems will be

carried out to determine the level of accuracy required for predictive value

in energetic conformations and spectroscopic properties. Electron correlation

behavior in model compounds containing transition elements will continue to

be studied by both ab initio and effective operator methods.

75
D. Scattering in the Presence of Radiation

(P. S. Julienne and F. H. Mies)

The general goal of this research is to provide calculations of radiative

cross-sections and fragment state distributions associated with the

collisional broadening and photodissociation of small molecules. We have

developed extensive experience in calculating various nonadiabatic effects in

the conventional weak-field limit, and are expanding our codes to treat the

saturation of continuum transitions in intense, multiphoton fields.

Theoretical input into this program is provided by extensive sets of

computer codes available to the quantum chemistry group. State of the art

calculations of any necessary molecular potentials, couplings and radiative

transition dipoles can be obtained. Our scattering programs have been

expanded to allow for direct radiative interactions among many coupled

channels. Preliminary calculations have been performed to study the pressure

broadening of atomic lineshapes in both weak and strong laser fields. Our

scattering codes presently provide the continuum-continuum scattering matrix

elements needed in lineshape theory, and are being modified to provide the

bound-continuum matrix elements required to describe photodissociation. In

addition to these numerical procedures, the program involves extensive

analysis of many new and exciting theoretical features introduced by very

intense laser fields.

1. Non-adiabatic Theory of Atomic Broadening in Weak Fields

(a) Redistribution Calculations for Sr^P-^S) + Ar

(P. S. Julienne and F. H. Mies)

We completed our close-coupled calculations of polarized light

redistribution in the Sr + Ar system. The calculations predict the

polarization ratios of Sr^P) fluorescence following line wing excitation by

either linear or circular polarized light. Ab initio calculations were used

to obtain the ground and excited SrAr molecular potentials, which were adjusted modestly to give improved agreement with experiment. The radiative

scattering theory gives a unified description of the absorption coefficient

and subsequent fluorescence from the small detuning impact limit region to

the far wings. The cross sections for depolarizing collisions of SrC^P) + Ar were also calculated. The calculated absorption coefficient, impact

broadening rate, linear and circular polarization ratios, and depolarization

rate coefficients are for the most part in good agreement with a diverse set

of experimental data.

(b) Redistribution Calculations for Na + Rare Gas

(P. S. Julienne and L. Vahala)

We have continued to calculate final state branching ratios for

fine structure states and alignment/orientation for Na( 2 P^S) + He,Ne,Ar

systems, and have obtained good agreement with experimental measurements. In

76
analysing the exact close-coupled data we uncovered a useful spin-uncoupling

approximation of the collision dynamics and its dependence on collision

velocity.

If the separation velocity of two colliding atoms is sufficiently

large, the final state dynamics can be well approximated by the same recoil

limit that has been successfully developed for photofragmentation. Our

calculations show how the recoil limit for fine-structure branching ratios is

approached for both red and blue detuning the separation velocity increases.

The recoil approximation for orientation/alignment only applies for

collisions with small angular momentum (small impact parameter). The recoil

limit applies when the separation time of the fragments is short compared to

the characteristic times of axis rotation and spin-orbit coupling. However,

by comparison to a pure atom we can develop a spin-recoil approximation

which treats the spin projection as fixed in space during the rapid

separation to fragments while accounting for the rotation of electronic

angular momentum projection A with the internuclear axis. Comparison of the

spin-recoil model with exact close coupling results for Na + rare gas shows

the applicability of the approximation.

(c) Lorentzian Pressure-Broadened Widths for Na

(P. S. Julienne and L. Vahala)

We have calculated the Lorentzian pressure-broadened widths for

both the D1 and D2 lines of Na broadened by collisions with He, Ne, and Ar.

In addition we have performed the first calculation of the so-called asymmetry

parameter which has been widely observed in the impact regions of atomic

lineshapes

.

The absorption profile within the impact limit for one of the

p isolated ^Pj fine structure lines may be written as I(M = UriSj ) 2 -v?(0)

where Aj and 6j are the respective detunings and shift of the line. The

detuning-depenaent quantity vj(Aj) may be shown to have a finite limit as Aj

-* 0 and exactly equals the usual impact limit broadening coefficient,

including the effects of both elastic and inelastic collisions. The

dependence on detuning give rise to the Lorentzian dispersion asymmetry which

has been observed for Na + rare gas systems. We have calculated the

radiative scattering cross section for detunings near the D1 and D2 lines for

Na broadened by He, Ne, and Ar. The widths and asymmetry parameters are qualitatively in agreement with measured values. Deficiencies in the

existing molecular potentials limit the expected accuracy of the scattering

calculations.

77
2. Nonadiabatic Theory of Redistribution in Strong Fields

(a) Collisional Redistribution of Resonant Radiation

(F. H. Mies)

Close-coupling codes developed for scattering in the presence

of radiation fields can be used to calculate the pressure broadening and

collisional depolarization of the Sr resonance transition in both the weak

field and the strong field limit,

Sr(1s) + Ar + ha)^ -> Sr*(^P) + Ar -» Sr(^S) + Ar + hui.

As we increase the Rabi frequency and decrease the detuning of

the incident laser three observable properties associated with optical

collision processes are obtained from such calculations, 1) Impact Limit

Lineshapes. As -> o we can demonstrate that proper pressure broadened half

widths, which include both elastic and inelastic collisional effects, can be

rigorously extracted from close-coupled radiative scattering matrix element.

The first manifestations of a.c. Stark shifts associated with the dressing of

fragments by the radiation field are extracted from the numerical results,

and confirm the anticipated power broadening of the Lorenztian absorption

profile. This suggests that normal perturbation theory as applied to

radiative interactions must be carefully scrutinized. 2) Inelastic

transitions involving dressed atomic states. As increases and eventually

exceeds A^ the asymptotic Sr atomic states become dressed. Our fully quantal

inelastic cross-sections can be used to calculate population redistribution

rates in the presence of intense fields. 3) Collisional redistribution

spectra in saturating fields. In strong fields we obtain the redistribution

spectra for hidpj in the impact limit and find the typical three-peaked Mollow

lineshape due to fluorescence between dressed atomic states. Both the

half-width and the polarization of the emission peaks are calculated.

(b) Analysis of Multiphoton Lineshape Theory in Intense Fields

(F. H. Mies and A. Ben-Reuven)

The theory of line broadening, collisional redistribution and

collision-induced multiphoton phenomena is an interdisciplinary area of

research. Particularly when adding the new ingredient of strong laser fields

and the resultant non-linear effects induced by saturation of resonant

transitions, the analysis of such phenomena requires the integration of

several diverse fields of research.

Two complementary theoretical approaches are used to describe these

phenomena. The first is represented by our ’’optical collision” approach here

at NBS where we calculate the radiative scattering matrix elements for

atom-atom scattering in the presence of intense monochromatic laser fields.

The calculations incorporate many subtle and sophisticated non-adiabati c and

inelastic effects which are often neglected in conventional lineshape theory.

However, these data that we generate must then be incorporated into very

78
subtle and complicated theoretical expressions for the density matrix of the

interacting molecules in the field of the pumping laser fields. Only then do

our calculations yield the proper description of the redistributed radiation.

The second theoretical approach, is exemplefied by the unique and

outstanding theoretical approach developed by Professor Ben-Reuven at Tel

Aviv University. The redistribution of radiation is expressed in terms of a tetradic formalism using a Liouvillian analysis of the molecular density

matrix in the presence of collisions. Fortunatley, the formal ingredients of

this sophisticated theory are the very scattering matrix elements that are generated by our optical collision approach.

We have begun a collaboration with Professor Ben-Reuven to explore

the relationship between the tetradic description of redistribution and the

close-coupled theory of scattering in strong multi photon fields. So far we

have succeeded in showing the explicit relationships required to analyze

resonance fluorescence and resonant Raman lineshapes.

3. Future Plans

Our long range plans in the radiative scattering program require

extensive code development combined with a careful theoretical analysis of

strong field effects and multiphoton effects in the presence of collisions.

In the immediate future we intend to complete the calculation of the

pressure-broadening Mollow lineshape for Sr + Ar. At the completion of this

relatively simple, near-resonant one-photon study we intend to explore the

application of our close-coupled codes to multi photon phenomena in strongly

resonant laser fields with hopes of calculating collisional effects in

four-wave mixing (PIERS-M) in Na. This will be studied in collaboration with

A. Ben-Reuven at Tel Aviv University using his tetradic analysis of

lineshapes

.

E. Atomic and Molecular Scattering Theory

1 . Half-Collision Amplitudes

(a) Analysis of Exact Close-Coupled Codes

(F. H. Mies)

We have previously used a multichannel quantum defect analysis

(MCQDA) of exact close-coupled radiative matrix elements to show that these

can often be factored as follows:

rad

S

fi ff' f . i

. N

+i'i

where the ’incoming' half-collision matrix tracks the non-adiabatic

scattering of the incident collision complex, and N^ f summarizes the

inelastic behavior of the 'outgoing', radiatively excited collision complex.

79
We have speculated on the unitarity of these matrices which would justify the

persistent use of the pure adiabatic Born-Oppenheimer matrix element

S®9^i in evaluating total radiative cross-sections for continuum

transitions. Based on a convincing but admittedly indirect numerical

analysis of several previous studies we have been very confident in advancing

the merits of this approximation. We have now modified our exact

close-coupled codes to extract the exact half collision amplitudes without

any commitment to unitarity. This new general capability of obtaining N

+ and

N“ directly from close-coupled codes will enjoy a wide range of applicability

in continuum spectroscopy and allow us to judge the validity of the unitarity

condition.

(b) Multichannel Semiclassical Half Collision Amplitudes

(F. H. Mies and Y. Band)

In addition to solving the close-coupled equations exactly, we have developed a new simplified numerical procedures designed to obtain the

half collision amplitude whenever the non-adiabatic couplings are restricted

to classically accessible regions and the unitarity of N

+ is insured. This is

equivalent to solving a set of coupled semiclassical equations. We begin by

choosing a set of diagonal reference potentials tailored to the nature of the

exact interaction matrix. These are used to generate a set of elastically

scattered WKB reference wavefunctions. Two obvious choices that we have

explored in some detail are either pure diabatic, or pure adiabatic

reference potentials, although in any given situation the ’preferred' choice

is usually self evident. By using a random phase approximation to neglect

quickly oscillating contributions to the close coupled equations we can develop a set of first order differential equations for the half collision

amplitudes which are perfectly unitary, and, in appropriate and well

understood circumstances, reproduce the exact close-coupled amplitudes. This

multichannel semiclassical approximation is presently being applied to a number of interest situations, including multichannel curve crossings, and

asymptotic frame transformation among fine-structure states.

2. Multichannel Photodissociation Amplitudes

(P. S. Julienne)

The expansion of our multichannel close-coupling codes to calculate

photodissociation cross-section is nearly complete. This gives very stable

procedures for evaluating bound-continuum transitions, including all possible

subtleties associated with non-adiabatic effects in molecular dissociation.

A renormalized Numerov algorithm has been incorporated into the codes for the

calculation of the usual continuum-continuum S-matrix elements needed for

collisional problems, but more specifically for calculation of

bound-continuum transition matrix elements needed for photodissociation.

This algorithm is still being tested. In particular the code is designed to

calculate the final state distribution of the photofragments. Ultimately

these exact matrix elements can be compared to the predictions of our half

collision amplitudes based on MCQDA.

80
3. Future Plans

Our first priority is to complete the development of the

non-adiabatic photodissociation codes. The implementation of the

photodissociation (and closely associated predissociation) codes will be

stimulated by forthcoming studies of the Schumann-Runge predissociation of 0

2

in collaboration with L. Vahala at Old Dominion University. The half

collision amplitude studies will be continued and applied to an analysis of

fine-structure distriutions in both continuum-continuum and bound-continuum

transitions. Using our new semiclassical algorithm we intend to examine its

application to polyatomic curve-crossing phenomena in collaboration with Y. Ban at Ben-Gurion University. In addition, we will continue to pursue our NCQDA studies of molecular dissociation in conjunction with Professor S-h.

Pan from the Chinese Academy of Science in Beijing who will be spending a year with us at NBS. In particular we will analyse the charge transfer

states of alkali-halide molecules.

81

5. PUBLICATIONS

(a) Publications of Past Year

Basch, H., Krauss, M. , and Stevens, W. J., "Cation Binding Effect on H-Bonding,"

J. Am. Chem. Soc. 107, 7267 (1985).

Basch, H., Krauss, M. , Stevens, W. J. , and Cohen, D., "Electronic and Geometric

Structures of Pt(NH

3 )|+ Pt(NH 2 ) 3 Cl2 , Pt(NH 3

) 3 X and Pt(NH 3) 2 XY

( X , Y = H

2 o,OH)," J. Inorg. Chem. 2_4, 331 3 ( 1985).

Basch, H., Krauss, M. , Stevens, W. J., and Cohen, D., "Binding of Pt(NH

3 )^

+ to Nucleic Acid Bases," J. Inorg. Chem. 25_, 684 (1986).

Bodaness, R. S. and King, D. S., "The Two-Photon Induced Fluorescence of the

Tumor Localizing Photosensitizer Hematoporphyrin Derivative via

1 064nm Photons from a 20ns Q-switched Nd-YAG Laser," Biochem. and

Biophys. Research Commun. 126, 346 (1985).

Casassa, M. P., Heilweil, E. J., Stephenson, J. C. , and Cavanagh, R. R., "TimeResolved Measurements of Vibrational Relaxation at Surfaces," J.

Electron. Spec., _38, 257 (1986).

Casassa, M. P., Heilweil, E. J. , Stephenson, J. C. , and Cavanagh, R. R., "TimeResolved Measurements of 0H(v=1) Vibrational Relaxation on Si0 2

Surface: Isotope and Temperature Dependence," J. Chem. Phys., 84 , 2361 ( 1986 ).

Casassa, M. P., Stephenson, J. C., and King, D. S., "Vibrational

Predissociation of Nitrix Oxide Dimer: Total Energy Distribution in

the Fragments," J. Chem. Phys., 86_, 2333 (1986).

Chackerian, C., Jr., Bus, E. S. , Olson, W. B., and Guelachvili, G.

,

"Determination of Aq - Bq of CH^D from Perturbation-Allowed

Transitions," J. Mol. Spectrosc., 117, 355 - 360 (1986).

Dang-Nhu, M. and Pine, A. S., "Intensities des Raies d'Absorption dans la

Bande Chaude vo + vii - v-t i de l'Allene," Can. J. Phys. 64, 289-296

(1986). —

G. T., Nelson, D. D., Jr., Peterson, K. I., and Klemperer, W. , "The

Rotational Spectra of NH^-CO and NHo-N 2", The Journal of Chemical

Physics, ^4_, 2472 (1986).

G. T., Nelson, D. D., Jr., Gerfen, G. J., and Klemperer, W. , "The

Rotational Spectrum, Barrier to Internal Rotation, and Structure of

NH

3 -N 2 0," The Journal of Chemical Physics, 83_, 5442, ( 1985).

G. T. and Coy, S. L. , "Absorber Speed Dependence of the Coherence

Relaxation Rate of the J=0-1 Transition of N

2 o," The Journal of

Chemical Physics, 83, 5687 (1985).

Fraser

,

Fraser

,

Fraser

83
Fraser, G. T. , Lovas, F. J., Suenram, R. D. , Nelson, D. D. , Jr., and

Klemperer, "Rotational Spectrum and Structure of CF^h-NHo," The

Journal of Chemical Physics, 84, 5983 (1986).

" J

Heilweil, E. J., Casassa, M. P., Cavanagh, R. R., and Stephenson, J. C.,

"Time-Resolved Vibrational Energy Relaxation of Surface Adsorbates,"

J. Vac. Sci. Technol . B3, 1471 (1985).

Heilweil, E. J., Casassa, M. P. Cavanagh, R. R., and Stephenson, J. C.,

"Vibrational Energy Decay of Surface Adsorbates," Proceedings of

Time-Resolved Vibrational Spectroscopy Conference (Spr inger-Verlag:

Proceedings in Physics, A. Laubereau and M. Stockburger, eds., Vol.

4, pg. 71 , 1985).

Hillman, J. J., Jennings, D. E., Olson, W. B., and Goldman, A., "High

Resolution Infrared Spectrum of Hydrogen Peroxide: The Fundamental

Band," J. Mol. Spectrosc., 117, 46-59 (1986).

Hougen, J. T. , "A Generalized Internal Axis Method for High Barrier Tunneling

Problems, as Applied to the Water Dimer," J. Mol. Spectrosc., 114,

395-426 (1985).

Hougen, J. T., "Symmetry beyond Point Groups in Molecular Spectroscopy," J. Phys. Chem. 90, 562-568 (1986).

Howard, B. J. and Pine, A. S., "Rotational Predissociation and Libration in

the Infrared Spectrum of Ar-HCl," Chem. Phys. Lett. 112, 1-8 (1986).

Jacox, M. E., "Comparsion of the Ground-State Vibrational Fundamentals of

Diatomic Molecules in the Gas Phase and in Inert Solid Matrices," J.

Mol. Spectrosc., 113, 286 (1985).

Jacox, M. E. , "Reaction of F Atoms with the Methyl Haildes. Vibrational

Spectra of CH^XF and of H2 CX —HF Trapped in Solid Argon," J. Chem.

Phys. 83, 3255 (1985).

Jasien, P. G. and Stevens, W. J., "Ab Initio Study of the Hydrogen Bonding

Interactions of Formamide with Water and Methanol," J. Chem. Phys.

84, 3271 (1986).

Jasien, P. G. and Stevens, W. J., "Calculated Proton Affinities for Some

Molecules Containing Group VIA Atoms," J. Chem. Phys. _83, 2984

(1985).

Jennings, D. E., Weber, A., and Brault, J. W. , "Raman Spectroscopy of Gases

with a Fourier Transform Spectrometer: The Spectrum of D

2

," Appl.

Opt. 25, 284 (1986).

Julienne, P. S. and Mies, F. H., "Analytic Multichannel Theory of Molecular

Dissociation," in Electron and Atomic Collisions , ed. D. C. Lorents,

W. E. Meyerhof, and J. R. Petersen (North Holland, Amsterdam, 1986),

p. 725.

84
Karpas, Z., Stevens, W. J., Buckley, T. J., and Metz, R., "Proton Affinity

and Gas-phase Ion Chemistry of Methyl Isocyanate, Methyl

Isothiocyanate, and Methyl Thiocyanate," J. Phys. Chem. 89_, 5274

(1985).

Krauss, M. , Stevens, W. J., and Basch, H., "Relativistic Effective Potential

SCF Calculations of AgH and AuH," J. Comp. Chem. 6_, 287 ( 1985).

Lovas, F. J., "Recommended Rest Frequencies for Observed Inter sellar

Molecular Microwave Transitions - 1985 Revision," J. Phys. Chem. Ref.

Data, J_5, 251-303 (1986).

Maki, A. G. , Wells, J. S., and Hinz, A., "Heterodyne Frequency Measurements

on the 12°0-00°0 Band of OCS," Int. Journal of Infrared and

Millimeter Waves, ]_, 909-917 (1986).

Mantell, D. A., Cavanagh, R. R., and King, D. S., "Internal States

IDistributions of NO Thermally Desorbed from Pt (111): Dependence on Coverage and Co-Adsorbed CO," J. Chem. Phys. 8*1 , 5131 (1986).

Mialocq, J.-C. and Stephenson, J. C., "Picosecond Laser Study of the

Collisionless Photodissociation of Dimethylnitramine at 266 nm,"

Chem. Phys. Lett. 123, 390 (1986).

Mialocq, J.-C. and Stephenson, J. C., "Picosecond Laser Induced Fluorescence

Study of the Collisionless Photodissociation of Nitrocompounds at 266

nm," Chem. Phys (1986).

Miller, K. J., Taylor, E. R., Basch, H., Krauss, M. , and Stevens, W. J., "A

Theoretical Model for the Binding of Cis-Pt(NHo)| + to DNA," J.

Bio. Struct. Dynam. 2_ , 1 157 (1985).

Nelson, D. D., Jr., Fraser, G. T. , and Klemperer, W. , "Ammonia Dimer: A Suprising Structure," The Journal of Chemical Physics, 83 , 6201

(1985).

Ohashi, N., Lafferty, W. J., and Olson, W. B., "Fourier Transform Spectrum of

the Torsional Band of Hydrazine," J. Mol. Spectrosc., 117, 119-133

( 1986 ).

Perry, J. W. , Woodward, A. M. , and Stephenson, J. C. , "Picosecond Coherent

Anti-Stokes Raman Scattering (CARS) Study of Vibrational Dephasing of

Carbon Disulfide and Benzene in Solution," SPIE Proceedings 620,

(1986).

Pine, A. S. and Howard, B. J., "Hydrogen-Bond Energies of the HF and HC1

Dimers from Absolute Infrared Intensities," J. Chem. Phys. 84 , 590-596 (1986).

Rosenkrantz, M. E. and Krauss, M. , "Damped Dispersion Interaction Energies

for He-H 2 , Ne-H 2 , and Ar-H 2

," Phys. Rev. A32, 1 402 ( 1985).

85
Suenram, R. D., Lovas, F. J., and Pickett, H. M. , "The Microwave Spectrum and

Molecular Conformation of Peroxynitric Acid (H00N0 2 )," J. Mol.

Spectrosc., 116, 406-421 (1986).

Suenram, R. D., Lovas, F. J. , and Pickett, H. M. , "Fluoromethanol : Synthesis,

Microwave Spectrum and Dipole Moment," J. Mol. Spectrosc. 119 ,

( 1986 ).

Thompson, G. A., Maki, A. G. , and Weber, A., "Infrared Tunable Diode Laser

Spectroscopy of the Av = 2 Band of j. of Mol. Spectrosc.

118 , 540-543 (1986).

Thompson, G. A., Maki, A. G. , and Weber, A., "The Infrared Spectrum of GeO,"

J. Mol. Spectrosc., 116, 136-142 (1986).

Thompson, G. A., Maki, A. G. , and Weber, A., "Infrared Spectrum of Lil," J. Mol. Spectrosc., 118, (1986).

Weber, A. (ed.), "Technical Activities 1985 - Molecular Spectroscopy

Division," NBSIR 86-3313 (1986).

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

,

"Rovibrational Analysis of v, HCN HF Using Fourier Transform

Spectroscopy," J. Mol. Spectrosc., _83_, 6188-6192 ( 1985).

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

"Rovibrational Analysis and Vibrational Predissociation in the v

2

Band of HCN Dimer," J. Chem. Phys. 85, 105-108 (1986).

Wofford, B. A., Jackson, M. W. , Bevan, J. W. , Olson, W. B., and Lafferty, W.

J., "Rovibrational Analysis of an Intermolecular Hydrogen-bonded

Vibration: The v

1 5 Band of HCN—HF," J. Chem. Phys. 84, 6115-6118

(1986).

Vahala, L. L., Julienne, P. S., and Havey, M. D., "Nonadiabatic Theory of

Fine Structure Branching Cross-Sections for NaHe, NaNe, and NaAr

Optical Collisions," Phys. Rev. A 34, 1856 (1986).

86
(b) Publications in Progress

Basch, H., Krauss, M. , and Stevens, W. J., "Cation Binding Effect on Imidazole Tautomerism, Intern. J. Quant. Chem. (in press).

Bodaness, R. S., Heller, D. F., Krasinski, J., and King, D. S., "Two-photon

Laser-Induced Fluorescence of the Tumor Localizing Photosensitizer

Hematoporphyrin Derivative: Resonance Enchanged 750nm Two-photon

Excitation into the Near UV-Soret Band," J. Biol. Chem. (in press).

Burgess, D.,Jr., King, D. S., and Cavanagh, R. R., "Kinetics and Dynamics of the

Ammonia/Nitric Oxide Coadsorbate System on Pt (111)," in preparation

for submission to Surface Science.

Burgess, D. , Jr., Mantell, D. A., Cavanagh, R. R., and King, D. S. , "Dynamics

of the Laser-induced Thermal Desorption of Nitric Oxide from a Platinum Foil," J. Chem. Phys. (in press).

Burkholder, J. B., Hammer, P. D., Howard, C. J., Maki, A. G. , Thompson, G. A., and Chackerian, C. , Jr., "Infrared Frequency and Intensity

Measurements of the CIO Radical," (manuscript in preparation)

.

Casassa, M. P., Stephenson, J. C., and King, D. S., "Vibrational

Predissociation of the Nitric Oxide Dimer," Faraday Discuss. Chem.

Soc. 82_, (1986) (in press).

Casassa, M. P., Stephenson, J. C., and King, D. S., "Picosecond Measurements

of the Dissociation Rates of the Nitric Oxide Dimer v-|(v=1) and

v/j(v=1) Levels," J. Chem. Phys. (submitted).

Fraser, G. T. , and Pine, A. S. , "Van der Waals Potentials from the Infrared

Spectra of Rare Gas-HF Complexes," The Journal of Chemical Physics,

( in press)

.

Fraser, G. T., Suenram, R. D., and Lovas, F. J., "Nearly Free Internal

Rotation in Ar-CH^Cl," The Journal of Chemical Physics (in press).

Heilweil, E. J. , Casassa, M. P., Cavanagh, R. R. , and Stephenson, J. C.,

"Population Lifetimes of 0H(v=1) and 0D(v=1) Stretching Vibrations of

Alcohols and Silanols in Dilute Solution," J. Chem. Phys., (in

press )

.

Heilweil, E. J., "Vibrational Population Lifetimes of 0H(v=1) in Natural

Crystalline Micas" Chem. Phys. Letters (in press).

Hinz, A., Wells, J. S., and Maki, A. G. , "Heterodyne Frequency Measurements

on the Nitric Oxide Fundamental Band," J. Mol. Spectrosc.

(submitted)

.

Hinz, A., Wells, J. S., and Maki, A. G. , "Heterodyne Measurements of Hot

Bands and Isotopic Transitions of N

2 0 Near 7.8pm, " Zeitschrift fur

Physik D, (submitted).

87
Hougen, J. T., "Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene C

2H3 + ," J. Mol.

Spectrosc. (submitted).

Jackson, M. W. , Wofford, B. A., Bevan, J. W. , Olson, W. B., and Lafferty, W.

J., "Infrared Spectrum of the Overtone Band 2 v^o in the Hydrogen

Bonded Complex HCN-HF," J. Chem. Phys. (in press).

Jackson, M. W. , Wofford, B. A., Bevan, J. W. , Olson, W. B., and Lafferty, W.

J., "Rovibrational Analysis of the C-H stretching Band and its

Overtone in the Hydrogen-bonded Heterodimer HCN HF," J. Chem. Phys.

(in press).

Jacox, M. E., "Comparison of the Electronic Energy Levels of Diatomic

Molecules in the Gas Phase and in Inert Solid Matrices," J. Mol.

Struct . ( in press)

.

Jasien, P. G. , Stevens, W. J., and Krauss, M. , "Ab Initio Calculations of the

Rotational Barriers in Formamide and Acetamide: The Effects of

Polarization Functions and Correlation," J. Mol. Struct. Theochem.

(in press).

Jasien, P. G. and Stevens, W. J., "Theoretical Studies of Potential Gas-phase

Charge Transfer Complexes: NH3 + HX (X = Cl, Br, I)," Chem. Phys.

Letters (in press).

Jennings, D. E., Weber, A., and Brault, J. W. , "FTS-Raman Flame Spectroscopy

of High-J Lines in H

2 and D2

," (submitted).

Julienne, P. S. and Mies, F. H, "Nonadiabatic Theory of Atomic Line

Broadening: Redistribution Calculations for Sr('P *• 'S) + Ar , " Phys.

Rev. A ( in press)

.

Julienne, P. S. and Vahala, L. L., "Close Coupled Theory of Fine Structure

Transitions in Collisional Redistribution," in Spectral Lineshapes , Vol. 4 , ed., by R. Exton (in press).

Lafferty, W. J. and Olson, W. B., "The High Resolution Infrared Spectra of

the v

2 and V3 Bands of H0 C 1 ," J. Mol. Spectrosc. (in press).

Lafferty, W. J., Suenram, R. D. , and Lovas, F. J. , "The Microwave Spectra of

the (HF) 2 , (DF ) 2 » HFDF and DFHF Hydrogen Bonded Complexes," J. Mol.

Spectrosc. (in press).

Lindsay, D. M. , Thompson, G. A., and Wang, Y. , "ESR Spectra of in an

N

2 Matrix, J. Phys. Chem., (in press).

Lovas, F. J., Suenram, R. D. , Ross, S. , and Klobukowski, M. , "Rotational

Structural Ab Initio and Semirigid Bender Analysis of the Millimeter

Wave Spectrum of HjCO-HF," J. Mol. Spectrosc. (in press).

Lovas, F. J. and Suenram, R. D., "Design and Operation of a Pulsed Beam

Fourier Transform Microwave Spectrometer , Measurements on OCS and

Rare Gas-OCS Complexes," J. Chem. Phys. (in press).

88
Maki, A. G. , "High Resolution Measurements of the v

2 Band of HNO

3 and the

Band of trans-HONO," (submitted).

Mies, F. H., Julienne, P. S., Band, Y. B., and Singer, S. J., "A Converged

Analysis of Radiative Matrix Elements," J. Phys. B., (in press).

Monten, K. M., Walmskay, C. M. , Henkel, C., Wilson, T. L., Snyder, L. E.,

Hollis, J. M. , and Lovas, F. J, "Torsionally Excited Methanol in Hot

Molecular Cloud Cores," Astron. Astrophys. (in press).

Nelson, D. D., Jr., Fraser, G. T. , Peterson, K. I., Zhao, K. , Klemperer, W.

,

Lovas, F. J., and Suenram, R. D., "The Microwave Spectrum of the K=0

States of Ar-NH^," The Journal of Chemical Physics (submitted).

Ohashi, N. and Hougen, J. T. , "The Torsional-Wagging Tunneling Problem and

the Torsional-Wagging-Rotational Problem in Methylamine, " J. Mol.

Spectrosc. (in press).

Olson, W. B., Hunt, R. A., Maki, A. G. , and Brault, J. W. , "Hydrogen

Peroxide: Rotational Constants of the Ground State and the Lowest

Torsional Component of the First Excited Torsional State,"

( submitted)

.

Pine, A. S. and Looney, J. P., "N 2

- and Air-Broadening in the Fundamental

Bands of HF and HC1," J. Mol. Spectrosc. (in press).

Schaeffer, R. D., Lovejoy, R. W., Olson, W. B., and Tarrago, G. , "High

Resolution Spectrum of 2 °SiH^D from 1450 to 1710 cm

-1 ," (submitted).

Suenram, R. D. and Lovas, F. J. , "The Microwave Spectrum and Structure of the

Argon Acrylonitrile van der Waals Complex," (submitted).

Thompson, G. A., Olson, W. B., Maki, A. G. , and Weber, A., "Fourier Transform

Infrared Spectrum of the Fundamental Band of LiCl at 830°C,"

( submitted)

.

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

"Rovibrational Analysis of the Vc Vibrational Band in the HCN-HF

Hydrogen Bonded Cluster," (submitted).

89
'

.

i
6. TALKS

Casassa, M. P., "Picosecond Measurements of Vibrational Relaxation of

Chemisorbed Molecules," Laser Users Group Seminar, NBS, February 1986.

Casassa, M. P., "Time- and State-Resolved Studies of van der Waals Molecule

Predissociation," Gordon Conference, August 1986.

Casassa, M. P., "Picosecond Measurements of Vibrational Predissociation of

van der Waals Dimers," Symposium on Clusters and Cluster Ions, Baltimore,

MD, September 1986.

Casassa, M. P., "Picosecond Studies of van der Waals Molecule Vibrational

Predissociation," Surface Science Lunch Bunch Seminar, NBS, September 1986.

Casassa, M. P., "Picosecond Studies of Vibrational Predissociation in van der

Waals Molecules," Symposium on State-to-state Chemistry, ACS National

Meeting, Anaheim, CA, September 1986.

Fraser, G. T., "van der Waals Potentials from the Infrared Spectra of Rare

Gas-HF Complexes," 41 st Symposium on Molecular Spectroscopy, Columbus,

OH, June 1986.

Fraser, G. T., "Rotational Spectrum, Internal Rotation, and Structure of

Ar-CHoCl," 41 st Symposium on Molecular Spectroscopy, Columbus, OH, June

1986.

Fraser, G. T., "van der Waals Potentials from the Infrared Spectra of

Rare-Gas-HF Complexes," 4lst Symposium on Molecular Spectroscopy,

Columbus, OH, June 1986.

Fraser, G. T., "Infrared and Microwave Spectroscopy of Weakly Bound

Complexes," 20th Mid-Altantic Regional Meeting of the American Chemical

Society, Symposium on Clusters and Clusters Ions, Baltimore, MD,

September 1986.

Fraser, G. T., "Nearly Free Internal Rotation in Ar-CHoCl," NATO-Advanced

Research Workshop on Structure and Dynamcis of Weakly Bound Molecular

Complexes, Maratea, Italy, September 1986.

Fraser, G. T., "Rotational Spectrum and Structure of CFoH-NH^," NATO-Advanced

Research Workshop on Structure and Dynamics of Weakly Bound Molecular

Complexes, Maratea, Italy, September 1986.

Heilweil, E. J., "Picosecond Vibrational Dynamics of Hydroxyl Groups in

Condensed Phase Systems," National Institutes of Health, January 1986.

Heilweil, E. J., "Picosecond Vibrational Dynamics of Hydroxyl Groups in

Condensed Phase Systems," Chemistry Department, Physical Chemistry

Seminar, University of Illinois, Urbana, IL, February 1986.

91
Heilweil, E. J., "Population Lifetimes of 0H(v=1) and 0D(v=1) Vibrations in

Alcohols, Silanols and Crystalline Micas," Ultrafast Phenomena

Conference, Snowmass, CO, June 1986.

Heilweil, E. J., "Picosecond Vibrational Dynamics of Hydroxyl Groups in

Condensed Phase Systems," Gordon Conference on Vibrational Spectroscopy,

Wolfeboro, NH, August 1986.

Hougen, J. T., "Symmetry Beyond Point Groups in Molecular Spectroscopy," Duke

University, Durham, NC, November 1985.

Hougen, J. T., "Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene," University of California

at Irvine, March 1986.

Hougen, J. T., "Overview of Molecular Spectroscopy at NBS," Jet Propulsion

Laboratories, Pasadena, CA, March 1986.

Hougen, J. T., "Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene," NBS, Boulder, CO, March

1986 .

Hougen, J. T., "Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene, C

2Ho

+ ," ill st Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, June 1986.

Hougen, J. T., "Far-Infrared Spectrum of Methylamine and the Group

Theoretical Treatment of Its Large Amplitude Motions," 41 st Symposium on Molecular Spectroscopy , Ohio State University, Columbus, OH, June 1986.

Hougen, J. T., "Hydrogen Migration Tunneling Effects in the Rotational and

Vibrational Spectrum of Protonated Acetylene, C

2H

3

+ ," 9th International

Conference on High Resolution Infrared Spectroscopy , Liblice (near

Prague), Czechoslovakia, September 1986.

Hougen, J. T., "The Role of Tunneling Models in Analyzing High-Resolution

Spectra of Weakly Bound Molecular Complexes," NATO-Advanced Research

Workshop on Structure and Dynamics of Weakly Bound Molecular Complexes,

Maratea, Italy, September 1986.

Jacox, M. E., "Vibrational Spectra of Free Radicals Formed by the Reaction of

F Atoms with Small Molecules," Department of Chemistry, Bryn Mawr

College, Bryn Mawr, PA, November 1985.

Jacox, M. E., "Vibrational Spectra of Free Radicals Formed by the Reaction of

F Atoms with Sipall Molecules," AT & T Bell Laboratories, Murray Hill,

NJ, December 1985.

Jacox, M. E., "Vibrational Spectra of Free Radicals Formed by the Reaction of

F Atoms with Small Molecules," Delaware Valley Section, Society for

Applied Spectroscopy, Philadelphia, PA, February 1986.

Jacox, M. E., "Vibrational Spectra of Free Radicals Formed by the Reaction of

F Atoms with Small Molecules," Department of Chemistry, University of

Colorado, Boulder, CO, February 1986.

92
Jacox, M. E., "Matrix Isolation Spectroscopic Studies of the Reaction of F Atoms with Small Molecules," Symposium on Matrix Isolated Species,

Central Regional American Chemical Society Meeting, Bowling Green, OH,

June 1986.

Jacox, M. E., "Spectroscopy of Free Radicals Formed in the Early Stages of

Nitramine Decomposition," U.S. Army Research Office Workshop on Combustion Probes for the Solid Nitramines, Combustion Research Facility,

Sandia National Laboratory, Livermore, CA, June 1986.

Jacox, M. E., "Comparison of the Electronic Energy Levels of Diatomic

Molecules in the Gas Phase and in Inert Solid Matrices," 41 st Symposium

on Molecular Spectroscopy, Columbus, OH, June 1986.

Jacox, M. E., "The A

2

n - X

2

Z + Near Infrared Absorption Spectrum of HC 2

Isolated in Solid Argon," 41 st Symposium on Molecular Spectroscopy,

Columbus, OH, June 1986.

Julienne, P. S., "Close Coupled Theory of Fine Structure Transitions in

Collisional Redistribution," 8th International Conference on Spectral

Line Shapes, College of William and Mary, Williamsburg, VA, June 1986.

Julienne, P. S., "Close Coupled Theory and Analytic Models for Collisional

Redistribution," Department of Atomic Physics, University of Newcastle,

Newcastle-upon-Tyne, England, July 1986.

Julienne, P. S., "Close Coupled Calculations on Laser Assisted Collisions,"

Max Planck Institute for Quantum Optics, Garching, West Germany, July

1986.

Julienne, P. S., "Scattering Theory of Photoassisted Collisions: Application

to Alkali and Alkaline Earth Atoms Colliding with Rare Gas Atoms,"

and "Half Collision Analysis of Photoassisted Collisions: Development

of Physical Insight," Department of Physics, University of

Kaiserslautern, Kaiserslautern, West Germany, August 1986.

Julienne, P. S., "Close Coupling Calculations of Atomic Line Profiles and

Collisional Redistribution for "*P and 2

P Transitions," Observatoire de

Paris, Meudon, France, September 1986.

King, D. S., "Molecule-Surface Dynamics," Rice University, Department of

Chemistry and Rice Quantum Institute, Rice University, Houston, TX,

November 1985.

King, D. S., "Two-Photon Excitations of the Tumor-Localizing Photosensitizer

HPD," International Laser Science Conference, Dallas, TX, November 1985.

King, D. S., "Dynamics of the Molecular-Surface Interaction," JILA Symposium,

Boulder, CO, March 1986.

93
King, D. S., "Thermal and Laser-Induced Desorption Dynamics," Cornell

University, Department of Chemistry, Cornell University, Ithaca, NY,

March 1986.

King, D. S., "Dynamics of the Photodissociation of van der Waals Dimers,"

Virginia Commonwealth University, Chemistry Department, Richmond, VA,

March 1986.

King, D. S., "Vibrational Predissociation of NO-containing van der Waals

Dimers," ACS Symposium on Dynamics of Clusters, National Meeting, NY,

April 1986.

King, D. S., "Photodissociation of van der Waals Dimers," Brookhaven National

Laboratory, Chemistry Department, Long Island, NY, April 1986.

King, D. S., "NO/Pt Interactions: Thermal- and Laser-Induced Desorption,"

IBM, Yorktown Heights, NY, April 1986.

King, D. S., "Vibrational Predissociation Dynamics of the Nitric Oxide

Dimer," NBS Staff Research Seminar Series, NBS-Gaithersburg, May 1986.

King, D. S., "Dynamics of the Vibrational Predissociation of the Nitric Oxide

Dimer," Mid-Atlantic Regional Meeting of ACS, Baltimore, MD, September

1986.

King, D. S., "Vibrational Predissociation Dynamics of van der Waals Dimers,"

Faraday Soc. Disc, on Photodissociation Dynamics, Bristol, England,

September 1986.

King, D. S. , "Vibrational Predissociation Dynamics of van der Waals Dimers,"

NATO- Advanced Research Workshop on Structure and Dynamics of Weakly Bound

Molecular Complexes, Maratea, Italy, September 1986.

Krauss, M., "Pt Binding to DNA," Chemistry Department, Drexel University,

Philadelphia, PA, May 1986.

Krauss, M. , "Pt(NH 2

R )2 Binding to a tetramer Duplex of DNA," Meeting of the

American Society of Biological Chemists, Washington, D.C., June 1986.

Lafferty, W. J., "Sub-Doppler Resolution Infrared Spectroscopy of the Carbon

Dioxide Dimer," Mid-Atlantic Regional Meeting of ACS, Baltimore, MD,

September 1986.

Lovas, F. J., "Microwave Spectrum and Properties of the Methanol-Formamide

Complex," Mid-Atlantic Regional Meeting of ACS, Baltimore, MD, September

1986 .

Maki, A. G. , "High Resolution Measurements and Analysis of the v

2 , v^, vjj,

v^, and 2

vg

Bands of Nitric Acid," 4lst Symposium on Molecular

Spectroscopy, Ohio State University, Columbus, OH, June 1986.

94
Maki, A. G. , "New Infrared Frequency and Intensity Measurements on Chlorine

Monoxide (CIO)," 41 st Symposium on Molecular Spectroscopy, Ohio State

University, Columbus, OH, June 1986.

Mies, F. H., "Collisional Redistribution of Intense Laser Radiation,"

Department of Chemistry, Tel Aviv University, Tel Aviv, Israel, February

1986.

Mies, F. H., "Collisional Redistribution of Intense Laser Radiation,"

Department of Chemical Physics, Weizmann Institute, Rohovot, Israel,

February 1986.

Mies, F. H., "Collisional Redistribution of Intense Laser Radiation,"

Department of Physics, Bar Ilan University, Ramat-Gan, Israel, March

1986.

Mies, F. H., "Collisional Redistribution of Intense Laser Radiation,"

Department of Chemistry, Ben Gurion University, Beer Sheva, Israel,

March 1986.

Olson, W. B. , "Experimental Values for the Rotational Constants of the OG and

1G Torsional Components of the Vibrational Group State of ^

2

^2 * and the

Matrix Element Coupling Them," 41 st Symposium on Molecular Spectroscopy,

Ohio State University, Columbus, OH, June 1986.

Pine, A. S., "High-Resolution Infrared Spectroscopy of Hydrogen-Bonded and van der Waals Complexes," Department of Chemistry, Johns Hopkins

University Baltimore, MD, November 1985.

Pine, A. S., "High-Resolution Spectroscopy of van der Waals and

Hydrogen-Bonded Complexes," NBS Staff Research Seminar, NBS

Gaithersburg, MD, May 1986.

Pine, A. S., "N 2 and Air Broadening in the Fundamental Bands of HF and HC1,"

4lst Symposium on Molecular Spectroscopy, Ohio State University,

Columbus, OH, June 1986.

Pine, A. S., "Vibrational Anomalies and Dynamic Coupling in Hydrogen-Bonded

van der Waals Molecules," NATO- Advanced Research Workshop on Structure

and Dynamics of Weakly Bound Molecular Complexes," Maratea, Italy,

September 1986.

Stephenson, J. C. , "Picosecond Studies of Vibrational Energy Transfer:

Surfaces, Solids, and Liquids," AFOSR Contractors Meeting, Dayton, OH,

November 1985.

Stephenson, J. C. , "Picosecond Studies of Vibrational Energy Transfer:

Surfaces, Solids, and Liquids," Department of Chemistry, University of

Pittsburgh, PA, March 1986.

Stephenson, J. C. , "Picosecond Studies of Vibrational Energy Transfer:

Surfaces, Solids, and Liquids," Department of Chemistry, Cornell

University, April 1986.

95
Stephenson, J. C. , "Picosecond Studies of Vibrational Energy Transfer:

Surfaces, Solids, and Liquids," Department of Chemistry, U. C. Berkeley,

Berkeley, CA, June 1986.

Stephenson, J. C., "Picosecond Studies of Vibrational Energy Transfer:

Surfaces, Solids, and Liquids," International Quantum Electrons

Conference, San Francisco, CA, June 1986.

Suenram, R. D., "Rotational Spectra and Structural Determinations of Several

van der Waals Complexes," Rice University, Department of Chemistry

Colloquium, Houston, TX, March 1986.

Suenram, R. D., "Rotational Spectra and Structural Determinations of Several

van der Waals Complexes," 11th Annual Austin Symposium on Molecular

Structure, Austin, TX, March 1986.

Suenram, R. D., "Rotational Spectrum of the Hydrogen Bonded Formamide-Water

Complex," Mid-Atlantic Regional Meeting of ACS, Baltimore, MD, September

1986 .

Suenram, R. D., "Structures of the Ar •• •Acrylonitrile and Ar • • *Formamide

van der Waals Complexes," NATO- Advanced Research Workshop on Structure

and Dynamcis of Weakly Bound Molecular Complexes, Maratea, Italy,

September 1986.

Suenram, R. D., "Rotational Spectrum of the Hydrogen Bonded Formamide-Water

and Formamide-Methanol Complexes," NATO-Advanced Research Workshop on Structure and Dynamics of Weakly Bound Molecular Complexes, Maratea,

Italy, September 1986.

Thompson, G. A., "Infrared Spectroscopy of High Temperature Diatomics,"

Rome Air Develpment Center, Griff isse Air Force Base, NY, May 1986.

Thompson, G. A., "High Temperature Spectroscopy of Alkali Halides and

Hydrides at High Temperature," Advanced Fuel Research, Inc., East

Hartford, CT, May 1986.

Thompson, G. A., "Infrared Spectroscopy of High Temperature Diatomics,"

Photometries, Inc., Boston, MA, May 1986.

Thompson, G. A., "High Resolution Infrared Spectroscopy of LiCl at 800°C," ^llst

Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH,

June 1986.

Weber, A., "Research in High Resolution Molecular Spectroscopy at NBS,"

ENEA, Centre Ricerche Energie Frascati, Frascati, Rome, Italy,

September 1986.

Weber, A., "Research in High Resolution Molecular Spectroscopy at NBS,"

Physics Deparment, University of Florence, Florence, Italy, September

1986 .

Weber, A., "Research at NBS in High Resolution Spectroscopy of Weakly Bound

Molecular Complexes," Physics Department, University of Giessen, Giessen,

Germany, October 1986.

96
7. MOLECULAR SPECTROSCOPY DIVISION SEMINARS

Balasubramanian, K. , Department of Chemistry, Arizona State University,

Tempe, NJ, "Electronic Structure of Moelcules Containing Heavy Atoms"

May 1986.

Ben-Reuven, A., School of Chemistry, Tel-Aviv University, Tel-Aviv, Israel,

"New Challenges in the Study of Spectral Line Shapes," September 1986.

Bevan, J., Chemistry Department, Texas A&M University, College Station, TX,

"Dynamical Behavior in Linear Hydrogen-Bonded Interactions," November

1985.

Beveridge, D., Department of Chemistry, Hunter College, New York, NY,

"Aqueous Hydration of Biological Molecules: Free Energy Simulations,"

February 1986.

Clough, S. A., Air Force Geophysics Laboratory, Hanscom AFB, MA, "Line Shapes

and Molecular Collisional Broadenings with Applications to Atmospheric

Problems," February 1986.

Cook, M. , Naval Research Laboratory, "Electronic Structure of the

MoFeoSij(SH) 5

^ _ Ion: A Model for the Nitrogenase Active Site,"

October 1985.

DeFrees, D., Molecular Research Institute, San Jose, CA, "Theoretical Studies

of Interstellar Chemistry," November 1985.

Fink, W. H., Chemistry Department, National Science Foundation, Washington,

D.C., "Influence of External Potentials of H-Bonding," December 1985.

Graner, G., Universite de Paris-Sud, Orsay, France, "Recent Work in Molecular

Spectroscopy at Laboratoire d' Infrarouge, " June 1986.

Harris, D., Chemistry Department, University of California, Santa Barbara,

CA, "Spectroscopy and Structure of MgOH: A Quasi-Linear Molecule,"

December 1985.

Harrison, J., Chemistry Department, Michigan State University, East Lansing,

MI, "Electronic and Geometric Structure of Transition Metal-Containing

Cations," August 1986.

Herman, R., Department of Physics, Penn State University, PA, "Velocitychanging Effects on Collisional Lineshapes," May 1986.

Kok, R., University of Georgia, Athens, GA, "Molecular Mechanics and Ab

Initio Calculations on Some Organic and Organo-metallic Compounds,"

October 1985.

Konowalow, D. D., Ballistics Research Laboratory, Aberdeen, MD, "High Spin

States of He 2

," July 1986.

97
Lester, W. , Department of Chemistry, University of California, Berkeley, CA,

"Quantum Monte Carlo of Electronic Structure," August 1986.

Lisy, J., Chemistry Department, University of Illinois, IL, "Vibrational

Predissociation Spectroscopy of Molecular Clusters," March 1986.

Malvezzi, M. , Gordon McKay Laboratory, Harvard University, MA, "Picosecond

Laser Excitation, Melting, and Vaporization of Solid Surfaces,"

April 1986.

Mautner, M. , Chemical Kinetics Division, NBS, Gaithersburg, MD, "Cluster

Ions and The Ionic Hydrogen-Bond," January 1986.

Mialocq, J.-C., Centre d'Etudes Nucleaires de Saclay, Department de

Physico-Chimie, France, "Ultrafast Molecular Dynamics Studied with

Picosecond Lasers," June 1986.

Miller, A. E. S., Department of Chemistry, Bryn Mawr College, Bryn Mawr, PA,

"Electronic Structure of Transition-Metal Hydrides," March 1986.

Mollendal, H., Universiteteti Oslo, Oslo, Norway, "Microwave Spectra of

1,4 defluoro -2-butyne and Several Molecules with Intra-molecular

Hydrogen Bonds," March 1986.

Osman, R., Department of Physiology and Biophysics, Mt. Sinai School of

Medicine, New York, NY, "A New Mechanism of Action of Superoxide

Dismutase," November 1985.

Rappe, A. K., Department of Chemistry, Colorado State University, CO,

"Studies on 2^ + 2ir Reactions: Allowed Reactions Involving Transition

Metal Complexes," October 1985.

Rosenkrantz, M. , Ballistic Research Laboratory, Aberdeen, MD, "Application of

Quantum Chemistry to Molecules of Atmospheric Interest," October 1985.

Sandroff, C., AT & T Bell Communications, "Layered Semiconductor Colloids and

Their Uses: Interfacial Science, Cluster Chemistry and New Solid

Materials," December 1985.

Saykally, R., Chemistry Department, University of California, Berkeley, CA,

"Laser Spectroscopy of Molecular Ions and van der Waals Molecules,"

May 1986.

Schrotter, H.-W. , Sekton Physik, Universitat Munchen, West Germany, "High

Resolution CARS and Inverse Raman Spectroscopy," August 1986.

Scoles, G. , Department of Chemistry, University of Waterloo, Waterloo,

Canada, "Infrared Laser Molecular Beam Spectroscopy of Molecule-Noble

Gas Clusters," April 1986.

98
Shapiro, M. , Chemical Physics Department, Weizmann Institute of Science,

Rehovot, Israel, "Coherent Control of Unimolecular Reactions," January

1986.

Shuker, R., Physics Department, Ben Gurion University of the Negev, Beer

Shene, Israel, "Laser Induced Cherenkov and Non-linear Effects in Sodium

Vapor," June 1986.

Smalley, R., Chemistry Department, Rice University, Houston, TX, "New Results

with Supersonic Cluster Beams," November 1985.

Sutter, D. H., Institut fur Physikalische Chemie, Universitat Kiel, Kiel,

Olshausenstrasse, Germany, "Microwave Zeeman and Fourier Transform

Studies at Kiel," February 1986.

Toennies, J. P., Max-Planck-Institut fur Stromungesfor chung , Gottingen,

West Germany, "State Resolved Molecular Beam Scattering Experiments on Atom-molecule Inelastic Collisions," March 1986.

Vidal, C. R., Institut fur Extraterrestr ische Physik, Max-Planck-Institut

fur Physik und Astrophysik, Bayern, West Germany, "Laser Intracavity

Absorption," April 1986.

Watts, R. 0., The Australian National University, Canberra, Australia,

"Molecular Structure and Dynamics from Molecular Beam Experiments,"

April 1986.

99

8. VISITING SCIENTISTS

The Molecular Spectroscopy Division has been host in the past year to the

following scientists who have worked with NBS scientists on problems of

mutual interest.

Adams, George, A., Ballistics Research Laboratory, works with M. Krauss on electronic structure codes for the chemistry and spectroscopy of

large molecules.

Basch, Harold, Bar I lan University, Israel, is engaged in research on metal

interactions with biomolecules. He works with M. Krauss.

Bodaness, Robert S., National Institutes of Health, works with D. S. King

on laser photosensitization processes in biological samples.

Craig, N., Oberlin College, works with W. J. Lafferty on the Fourier

transform Infrared spectrum of 1 ,2-difluorethylene.

Fink, W. H., National Science Foundation, works with W. J. Stevens on the

development of model potentials and their application to large

molecules.

Gillies, C. W. , Chemistry Department, Rensselaer Polytechnic Institute, works

with F. J. Lovas and R. D. Suenram on pulsed beam Fourier transform

microwave spectroscopy of hydrogen bonded molecules.

Hollis, Jan Michael, NASA/Goddard Space Flight Center, works with F. J. Lovas

and R. Suenram on software development for automated microwave

spectrometer and collaborates on astronomical studies.

Jennings, Donald E. , NASA/Goddard Space Flight Center, works with A. Weber

on a project to do Raman spectroscopy of gases with the Fourier

transform spectrometer of the National Solar Observatory (Kitt Peak).

Konowalow, Daniel D., Professor at SUNY-Binghamton, New York, works with M.

Krauss on studies of electronic structure of alkali and alkaline

earth molecules.

Lengsfield, Byron H., Ballistic Research Laboratory, works with M. Krauss

on electronic structure codes for the chemistry and spectroscopy of

large molecules.

Ohashi, Nobukimi, Kanazawa University, Physics Department, Japan, works with

J. T. Hougen, W. J. Lafferty, and W. B. Olson on high resolution

infrared spectroscopy and theory of methylamine.

Osman, R., Mt. Sinai School of Medicine, New York, works with W. J. Stevens

on the application of ab initio quantum chemistry to biochemical

modeling.

101
Sattler, Joseph L., Harry Diamond Laboratories , U.S. Army, collaborates with

W. J. Lafferty and A. Maki on precise line frequency measurements

in the infrared.

Vahala, Linda L., Physics Department, Old Dominion University, Norfolk, VA,

works with P. S. Julienne on the theory of molecular line broadening

and final state branching ratios for the resonance transitions of

alkali atoms perturbed by rare gas collision partnes.

Zozom, Jennifer, Chemistry Department, Rensselaer Polytechnic Institute works

with Drs. F. J. Lovas and R. D. Suenram on pulsed beam Fourier

transform microwave spectroscopy of hydrogen bonded molecules.

102
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 86-3483 NOVEMBER 1986

4. TITLE AND SUBTITLE

Technical Activities 1986

Molecular Spectroscopy Division

5. AUTHORS, A_ ffeber( ed _

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

10. SUPPLEMENTARY NOTES

j 2] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report summarizes the technical activities of the NBS Molecular Spectroscopy

Division during the Fiscal Year 1986. The activities span experimental and theoretical research in high resolution molecular spectroscopy, quantum chemistry and

laser photochemistry, and include the development of frequency standards, critically

evaluated spectral data, applications of spectroscopy to important scientific and

technological problems, and the advancement of spectroscopic measurement nethods

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

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) biomolecules; energy transfer; high resolution molecular spectroscopy; hydrogen bonded

molecules; laser photo chemistry; picosecond spectroscopy; quantum chemistry; spectral

frequency standards; trace atmospheric species; van der Waals molecules.

13. AVAILABILITY

X^Unl imited

| | For Official Distribution. Do Not Release to NTIS

| | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

KXl Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

113

15. Price

$16.95

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