Resonances. In Electron-Molecule Scattering, van der Waals Complexes, and Reactive Chemical Dynamics

513 Pages·1984·8.625 MB·English

by  Donald G. Truhlar (Eds.)

Resonances
In Electron-Molecule Scattering,
van der Waal
Reactive Chemical Dynamics
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
ACS SYMPOSIUM SERIES 263
Resonances
In Electron-Molecule Scattering,
van der Waals Complexes, and
Reactive Chemical Dynamics
Donal
University of Minnesota
Based on a symposium sponsored by
the Division of Physical Chemistry
at the 187th Meeting
of the American Chemical Society,
St. Louis, Missouri,
April 9-12, 1984
American Chemical Society, Washington, D.C. 1984
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Library of Congress Cataloging in Publication Data
Resonances in electron-molecule scattering, van der
Waals complexes, and reactive chemical dynamics.
(ACS symposium series, ISSN 0097-6156; 263)
"Based on a symposium sponsored by the Division of
Physical Chemistry at the 187th Meeting of the
American Chemical Society, St. Louis, Missouri, April
8-13, 1984."
Includes bibliographies and indexes.
1. Excited state chemistry—Congresses
excitation—Congresses. 3. Van
Congresses.
I. Truhlar, Donald G., 1944- .II. American
Chemical Society. Division of Physical Chemistry.
III. Series.
QD461.5.R47 1984 541.2 84-16934
ISBN 0-8412-0865-4
Copyright © 1984
American Chemical Society
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In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
FOREWORD
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In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
PREFACE
COLLISIONAL RESONANCES PROVIDE a unifying framework for the inter­
pretation of phenomena in the areas of electron-molecule scattering, van der
Waals complexes, and reactive chemical dynamics. Of these three areas,
resonances have been studie
scattering. Resonances i
order of magnitude or more in vibrational excitation and electronic excita­
tion cross sections. As such, they have been widely studied for their effect on
e-beam laser initiation, their role in energy degradation in planetary atmo­
spheres and in magnetohydrodynamic energy generation in plasmas and
other electron-rich systems, and their effect on transport in electronic devices
and radiation chemistry. They also provide a mechanism for dissociative
attachment, and they are of great fundamental interest from both a structu­
ral and a scattering-theory point of view.
Structurally, resonances provide information on metastable negative
ions, negative electron affinities, orbital energies of unbound orbitals, and
doubly excited electronic states. From the scattering theory point of view, a
resonance is one of the clearest ways to test a quantum dynamical treatment,
and it generally provides more definite and more sensitive tests of theory
than nonresonant or background cross sections do. An important recent
theoretical advance is the calculation of resonances by pseudo-bound-state
techniques such as complex scaling and stabilization. This field has seen very
rapid progress in the last few years because it is the cutting edge of a new
way of thinking and calculating whereby dynamical processes are treated as
much as possible by using well established structural methods and minimiz­
ing all nonessential scattering boundary conditions.
Another recent advance in electron-molecule resonances is their role in
molecular autoionization and photoionization. Here they show up as an exit-
channel effect. The increasing availability of synchrotron sources and the
proliferation of high-resolution laser spectroscopic techniques are leading to
expanded interest in these processes because of the necessity to interpret the
resonance features for a greater variety of molecules of chemical interest.
Electron-molecule shape resonances are also responsible for structure in
inner-shell electron energy-loss spectra in the region around the core ioniza­
tion threshold; acting as a final-state interaction, the same resonances
ix
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
explain characteristic features observed in X-ray absorption spectra in the
region between the absorption edge and the EXAFS diffractive features.
Resonances in van der Waals systems has also been a very exciting area
for several years now. The resonance model provides a way to interpret
vibrational and rotational predissociation of molecular complexes such as
the well studied van der Waals systems HeT and (tetrazine) or the more
2 2
recently studied hydrogen-bonded (HF) and (HF)„. Van der Waals com­
2
plexes are being increasingly implicated in enhanced probabilities for low-
energy or low-temperature vibrational energy transfer processes in bulk
systems and in super-cold molecular beams. The now mature techniques of
supersonic molecular beams and pulsed lasers are leading to increasingly
detailed probes of the vibrational predissociation process. This process in
turn is one of our most quantitative probes of state-selected vibrational-
rotational-translational energ
features responsible for suc
Waals complexes is particularly relevant to many branches of chemistry
because, much more so than conventional photodissociation of strongly
bound species, it probes the weak attractive forces responsible for solvent
bath effects. In addition, though, the process is sensitive to the strong
repulsive forces that often dominate collisional energy transfer. Van der
Waals resonances also provide test cases for current theories of mode-
selective and statistical intramolecular energy redistribution.
Resonances in reactive dynamics is the newest field of the three, but
potentially the most significant. The possible existence of a resonance in the
F + H reaction has generated much interest because this reaction has been
2
studied by both molecular beam techniques and fully resolved IR chemilumi-
nescence, in both cases as a function of translational energy. Furthermore,
the most sophisticated and most generally applicable techniques of molecular
collision theory are being applied to this reaction. Resonances are also of
great current interest for the prototype chemical reaction H + H . Recent
2
predictions of subthreshold resonances (which are related to the new subject
of vibrationally adiabatic bound states on potential energy surfaces with no
wells) are also of special interest. In addition to these bimolecular examples,
resonances are providing an increasingly useful model for detailed interpreta­
tion of unimolecular dynamics above the dissociation threshold, as well as
the reverse associations. A particularly significant result that is emerging in
both bimolecular and unimolecular studies is the role of resonance states in
determining branching ratios and product state distributions, that is, chemi­
cal and quantum-state specificity. Resonance effects in chemical reactions are
also getting attention in fundamental studies because they are dramatic
quantal interference effects that may sometimes have significant and even
dominant implications for macroscopic chemical dynamics.
It is not possible for a book like this to present a comprehensive
overview of all activity in the fields covered. But it does, I believe, provide a
x
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
snapshot of some of the dominant strains of current research in these rapidly
growing fields.
I am grateful to James Kinsey, Al Kwiram, Sue Roethel, Robin Giroux,
Brenda Ford, and the chapter referees for their assistance with the Sympo­
sium and the volume.
DONALD G. TRUHLAR
University of Minnesota
Minneapolis, Minnesota
June 1984
xi
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
1
Roles Played by Metastable States in Chemistry
JACK SIMONS
Department of Chemistry, University of Utah, Salt Lake City, UT 84112
Metastable states are important in chemistry for
reasons which relate to the fact that such states
have finite lifetimes and finite Heisenberg energy
widths. They are observed in spectroscopy as peaks
or resonances superimpose
they are buried
time for energy transfe to occu betwee consti
tuent species which eventually become separated
fragments. It is often the rate of such intra -
fragment energy transfer which determines the
lifetimes of resonances. The theoretical explora­
tion of metastable states presents special diffi­
culties because they are not discrete bound
states. However, much of the machinery which has
proven so useful for stationary electronic and
vibrational-rotational states of molecules has been
extended to permit resonance energies and lifetimes
to be evaluated. In this contribution, examples of
electronic shape and Feshbach, rotational and
vibrational predissociation, and unimolecular
dissociation resonances will be examined. Finally,
a novel situation will be treated in which the
energy transfer dictating the decay rate of the
metastable species involves vibration-to-electronic
energy flow followed by electron ejection.
The purposes of this chapter are to provide overview and perspective
concerning the various kinds of metastable species found in chemical
systems as well as to focus attention on an interesting class of
temporary anions (1-4) whose lifetimes are governed by vibration-
electronic coupling strengths. To emphasize the importance of
metastable states in experimental chemistry, it is useful to first
analyze how they are created via collisional or photon absorption
processes. This provides a basis for discussing the signatures
which metastable states leave in the instrumental responses seen in
the laboratory. Having introduced metastable states in relation to
the experimental situations in which they arise, it is useful to
0097-6156/84/0263-0003$06.00/0
© 1984 American Chemical Society
In Resonances; Truhlar, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1984.