Dissociative substitution mechanisms abound in organometallic chemistry. For certain
systems, such processes can be isolated in the gas phase, where, as sequences of
elementary unimolecular and bimolecular reactions, their kinetics can convey information
on fundamental energetics and dynamics of metal-centered chemical transformations.
Methods of competitive kinetics, using time-resolved infrared absorption
spectrometry, provide relative and absolute rate constants for comparatively fast
reactions. Work yielding unimolecular decay and relative bimolecular production rate
constants for selected bis- and tris-ethylene complexes of iron and chromium carbonyls is
summarized together with a report of new work on the CO-for-C2H4 substitution
kinetics of (C2H4)Cr(CO)5.

Steven J. Duray

Dawn M. Becher

Edward R. Grant

Elementary Rate Processes in the
Dissociative CO for C2H4
Substitution Reactions of
Organometallic Complexes in the
Gas Phase

The method of time-resolved optoacoustics was used to measure the rate of vibrational
relaxation of CH3Cl(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BD;</mml:mi></mml:math>6) and CH3Br(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BD;</mml:mi></mml:math>6) by Ar. The pressure pulses generated by the
relaxing gas revealed that the rate of production of translational energy from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BD;</mml:mi></mml:math>6 &#61; 1 is
approximately twice the decay rate of IR fluorescence from 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BD;</mml:mi></mml:math>3 &#61; 1. No evidence was
found for a previously proposed bottleneck in rotational relaxation, which would have
resulted in an acoustic relaxation rate slower than the fluorescence decay. The faster
rates observed here can be explained qualitatively by a rapid energy release from energy levels above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BD;</mml:mi></mml:math>3 which precedes the IR fluorescence. A simple three-level model,
however, is unable to explain our observations quantitatively.

Kenneth M. Beck

Robert J. Gordon

A Test for Bottlenecks in the
Vibrational Relaxation of CH3C<small.letters>l</small.letters>
and CH3B<small.letters>r</small.letters> by A<small.letters>r</small.letters>

Laser-assisted modification of solid surfaces, e.g. thin film deposition and etching,
frequently involves a chemically reacting system whose optical and thermal properties
change continuously in time. This leads to a coupling between excitation and reaction
which has profound consequences for the cumulative effect of microscopic processes
occurring at the surface, whether thermal or photochemical in nature. An understanding
of this coupling provides an essential bridge between investigations of discrete mechanistic
steps in surface photoreactions and measurements of macroscopic rates of laser
etching and deposition. A range of situations have been found to lead to significant
interdependence between optical properties and reactivity: formation of waveguides
and gratings, chemical enhancement of absorption, coupling of optical properties and
temperature, and optical feedback in scanning. Typical examples are presented and
discussed. The real-time effects of growth of a reflecting Ni film on an absorbing
substrate, of optical interference in the laser thermal oxidation of copper and of grating
formation in the photochemical deposition of copper are described in detail.

F. A. Houle

Interdependence of Optical
Excitation and Surface Chemistry
in Laser Induced Deposition and
Etching

This review considers several applications of photochemistry to the synthesis of chemicals
by industry. Factors relevant to the use of lasers by industry in the synthesis of
specific chemicals are also considered.

Peter A. Hackett

Making Light Work&#8212;Applications
of Lasers to Chemical Production

Eric Weitz

Introduction

The technique of photofragment imaging is described, and several examples of the
power of the technique are presented. Two-dimensional images of state-selected
photofragments from the photodissociations of CD3I and H2S illustrate how photofragment
imaging reveals &#946; parameters, brancing ratios, Doppler profiles and vector
correlations. Comparisons are made with Doppler profiling and one-dimensional
time-of-flight techniques.

John W. Thoman

David W. Chandler

David H. Parker

Maurice H. M. Janssen

Two-dimensional Imaging of
Photofragments

Experiments on the effect of Selective vibrational, translational and orientations excitation
of reactants in bimolecular reactions can give important insights into the microscopic
dynamics of elementary chemical reactions. The information obtained in such
experiments can be compared with the results of theoretical calculations of the reaction
dynamics based on ab initio potential energy surfaces and is also of basic interest to
improve the kinetic data used in detailed chemical kinetic modelling.Rotational and vibrational energy transfer between H2 and H0 has been studied
directly using Raman excitation combined with time resolved CARS spectroscopy. The
competition between reactive and inelastic channels was investigated for reactions of
atoms with vibrationally excited H2 and HCl molecules. Selective vibrational excitation
was achieved by using infrared laser or Raman-pumping. The reaction products were
detected by time resolved atomic line resonance absorption mass-spectrometry and
CARS-spectroscopy. In some cases information on the contributions of adiabatic and
non-adiabatic reactive pathways could be obtained. The reaction H + <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>O</mml:mtext><mml:mn>2</mml:mn></mml:msub><mml:mo>&#x2192;</mml:mo></mml:math> OH + O has
been studied using translationally hot H atoms at various energies. Absolute total
reactive cross-sections, nascent rotational state distributions and information on the
distribution of orientations of the OH angular momentum vector using polarized
dissociation and analysis laser sources have been obtained.

J.  Wolfrum

Laser Stimulation and
Observation of Simple Gas
Phase Radical Reactions

Molecules embedded in low-temperature matrices experience conformer interconversion
processes due to IR- (or UV-)irradiation. This paper summarizes the known
IR-induced conformational changes, of which the majority has been studied by using
broad-band sources provided with filters. We focus at the thermal and IR-photochemical
methods in obtaining information of conformational energetics of matrix isolated
molecules, at the usefulness of ab initio calculations in assigning the spectra of
conformers involved and in finding the reaction path on the ground state surface. The
data obtained from a number of molecules is used to discuss possible mechanisms in both
intramolecularly hydrogen-bonded and non-bonded molecules. Also possible indications
of mode-selective phenomena in these processes are dealt with. In addition,
common trends found for photorotamerization in different hosts are discussed.

Markku  R&#228;s&#228;nen

Henrik  Kunttu

Juhani  Murto

Infrared Induced Conformer Interconversion Processes in Low-Temperature Matrices

Laser induced charge transfer reactions of halogens in rare gas solids and liquids provide
a powerful means for the study of condensed phase dynamics. Many-body effects with
respect to both electronic and nuclear coordinates, and cooperative interactions with
radiation fields, are some of the studied phenomena that are highlighted in this article.The pertinence of these ionic reactions to chemistry in solids is demonstrated in
photodissociation studies of molecular halogens in rare gas matrices. The coexistence of
both delocalized and localized charge transfer states in solid xenon doped with atomic
halogens is presented and dynamical consequences&#8212;charge separation, self-trapping
and energy storage&#8212;are discussed. Static and dynamic solvent effects in liquid phase
harpoon reactions are considered. The characterization of cooperative excitations&#8212;
two-photon, two-electron transitions&#8212;in liquid solutions is presented.

Mario E. Fajardo

R.  Withnall

J.  Feld

F.  Okada

W.  Lawrence

L.  Wiedeman

V. A. Apkarian

Condensed Phase Laser Induced
Harpoon Reactions

In this article, we review recent work in the authors&#39; laboratory on the subject of free
radical chain reactions of chlorine with small hydrocarbons in amorphous solids. The
solids are formed as thin films by spray deposition of the gaseous reagents onto a
cryogenic window. Reactions are initiated by excimer laser photolysis at 308 nm, which
dissociates a small fraction of the chlorine molecules to atoms. Product yields and
branching ratios are determined by infrared absorption spectroscopy. Reactions of
chlorine with cyclopropane or cyclobutane proceed by true chain reactions, as evidenced
by high product quantum yields (number of product molecules formed per laser photon
absorbed by the sample). Measurements of the dependence of the product yield on the
relative concentrations of chlorine and hydrocarbon provide clues to the reaction
mechanism in the solid state. The cyclobutane reaction appears to involve H atom
transfer from cyclobutane to cyclobutyl radical as an intermediate step in the overall
reaction. Reaction of chlorine with propane, n-butane, or isobutane does not appear to
involve chain propagation and is dominated by radical recombination processes which
result in low quantum yields. All of these results are discussed in terms of reactions
which occur in a solid state environment where molecular motion is severely restricted.

Arthur J. Sedlacek

Charles A. Wight

Photochemical Chain Reactions
in Amorphous Solids

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