(a). We have computed an approximate absorption spectrum for unstable intermediates
 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mtext>H</mml:mtext><mml:mn>3</mml:mn><mml:mo>&#x2260;</mml:mo></mml:msubsup></mml:mrow></mml:math> which constitute the transition states in the fundamental exchange reaction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>H</mml:mtext><mml:mo>+</mml:mo><mml:msub><mml:mtext>H</mml:mtext><mml:mn>2</mml:mn></mml:msub><mml:mo>&#x2192;</mml:mo><mml:msubsup><mml:mtext>H</mml:mtext><mml:mn>3</mml:mn><mml:mo>&#x2260;</mml:mo></mml:msubsup><mml:mo>&#x2192;</mml:mo><mml:msub><mml:mtext>H</mml:mtext><mml:mn>2</mml:mn></mml:msub><mml:mo>+</mml:mo><mml:mtext>H</mml:mtext></mml:mrow></mml:math>. Reaction takes place across the Siegbahn, Liu, Truhlar, Horowitz
(SLTH) ground state energy surface, and electronic excitation is to the bound state
that correlates with H&#42;(22P)+H2. Features in the spectrum are indicative of the
favored turning points for the mean of many classical trajectories across the SLTH
surface. (b). Preliminary experimental data are reported which give evidence of emission
arising from the transition states of NaI&#8800;&#42; in the (cw or pulsed) photolysis of
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>NaI</mml:mtext><mml:munder><mml:mo>&#x2192;</mml:mo><mml:mrow><mml:mn>222</mml:mn><mml:mtext>nm</mml:mtext></mml:mrow></mml:munder><mml:mtext>NaI</mml:mtext></mml:mrow></mml:math>&#8800;&#42;<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>&#x2192;</mml:mo></mml:mrow></mml:math>Na&#42;(32P)+I.
The observed transition state emission is compared with a simple calculation.

H.-J.  Foth

H. R. Mayne

R. A. Poirier

J. C. Polanyi

H.  H. Telle

The Spectroscopy of Transition States in H+H2 Reaction, and NaI Photodissociation

Evidence is presented concerning the onset of intramolecular vibrational redistribution
(IVR) as one climbs the vibrational manifold of room temperature S1 aromatics. The
onsets of IVR occur at S1 vibrational energies between 670 and 2000 cm&#8722;1 for various
single and double ring aromatic molecules. The vibrational level densities in the onset
region seem too small to support the extensive IVR that is observed in every molecule.
Comparisons of room temperature fluorescence spectra with cold supersonic beam
spectra from p-difluorobenzene support the proposition that Coriolis vibration&#8211;rotation
coupling makes important contributions to the effective density of states in room
temperature IVR.

D. A. Dolson

K. W. Holtzclaw

S. H. Lee

S.  Munchak

C.  S. Parmenter

B. M. Stone

A. E. W. Knight

Intramolecular Vibrational Redistribution at Low State Densities in S1 Aromatics. Evidence for Rotational Contributions

A density matrix approach for describing intramolecular dynamics, with special application
to vibrational relaxation in excited electronic states, is presented. We derive the
master equations governing intramolecular transfer of excitation energy between the
states in a zeroth-order basis defined by considering the excitation and detection
conditions in the time-resolved experiments. It is shown that, in this formalism, the
memory function plays a central role. We note that the form of intramolecular memory
is determined by the importance of quantum mechanical mixing of the zeroth-order
states. A distinction is made between the subsystems of states with strong and weak
mixing properties; while the former account for quasiperiodic character of coherent
motion, the latter display a Markovian behavior. The physical conditions fixing the
relative importance of quasiperiodic and statistical dynamics in individual systems are
discussed. In the application to time resolved intramolecular vibrational relaxation,
special consideration is given to the nature of the initially excited doorway states and
the intermode couplings. The symmetry restrictions and the possible role of rotational
motion in vibrational relaxation are also discussed, before considering the recent results
obtained by Zewail et al. with anthracene in the first excited singlet state.

R.  Voltz

A.  Boeglin

A. A. Villaeys

S. H. Lin

Intramolecular Vibrational Relaxation in Electronically Excited States

Unimolecular decomposition following state-specific laser excitation is considered. If
the transition state for the unimolecular reaction possesses any geometrical symmetry,
then it is shown that this can lead to mode-specificity in the decay rates (i.e., different
states with essentially the same total energy and angular momentum react at significantly
different rates). This is illustrated for a model problem of two coupled oscillators (the
Henon&#8211;Heiles potential) and also for the unimolecular dissociation of formaldehyde,
H2CO&#8594;H2+CO, which has a planar transition state. Dynamical calculations indicate
that there may also be a significant degree of mode-specificity for the formaldehyde
reaction beyond that due to symmetry.

William H. Miller

On the Question of Mode-Specificity in Unimolecular Reaction Dynamics

K.  Kleinermanns

J.  Wolfrum

Laser Stimulation and Observation of Elementary Chemical Reactions in the Gas Phase

The possible photoreactions leading to the formation of &#8220;laser snow&#8221; in cell experiment
by laser irradiation of Cesium atoms to the (7p) level will be analyzed through crossed
beam experiment. A single collision process would be excluded if an activation barrier
was present in the inverse ground state reaction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>CsH</mml:mtext><mml:mo>+</mml:mo><mml:mtext>H</mml:mtext><mml:mo>&#8594;</mml:mo><mml:mtext>Cs</mml:mtext><mml:mo>+</mml:mo><mml:msub><mml:mtext>H</mml:mtext><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>; accurate ab initio
calculations exclude the existence of such a barrier. The ionic state Cs+ + <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mtext>H</mml:mtext><mml:mn>2</mml:mn><mml:mo>&#x2212;</mml:mo></mml:msubsup></mml:mrow></mml:math> is suggested to make possible the electronic into vibrational energy transfer in the
perpendicular collision.

X. F. Gadea

G. H. Jeung

M.  Pelissier

J. P. Malrieu

J. L. Picque

G.  Rahmat

J.  Verges

R.  Vetter

Preliminary Investigations About the Cs&#42;+H2 Collision

Application of the method of molecular beam photofragmentation translational spectroscopy
to the investigation of primary photochemical processes of polyatomic
molecules is described.Examples will be given to illustrate how information concerning the energetics,
dynamics and mechanism of dissociation processes can be obtained from the precise
measurements of angular and velocity distributions of products in an experiment in
which a well defined beam of molecules is crossed with a laser.

Yuan T. Lee

Molecular Beam Studies of Primary Photochemical Processes

The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>CN</mml:mtext><mml:mrow><mml:mo>&#x2009;</mml:mo><mml:mo>(</mml:mo><mml:mrow><mml:mi>A</mml:mi><mml:mo>&#x2009;</mml:mo><mml:msup><mml:mi /><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mrow><mml:mtext>&#928;</mml:mtext></mml:mrow><mml:mi>i</mml:mi></mml:msub></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> and (X <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>&#931;</mml:mi><mml:mprescripts /><mml:none /><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math>+) vibrational distributions from the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>C</mml:mtext><mml:mo>+</mml:mo><mml:msub><mml:mtext>N</mml:mtext><mml:mn>2</mml:mn></mml:msub><mml:mtext>O</mml:mtext><mml:mo>&#x2192;</mml:mo><mml:mtext>CN</mml:mtext><mml:mo>+</mml:mo><mml:mtext>NO</mml:mtext></mml:mrow></mml:math> reaction at 300 K were determined in a low pressure flow reactor. From the chemiluminescence
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>A</mml:mi><mml:mo>&#x2009;</mml:mo><mml:msup><mml:mi /><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mrow><mml:mtext>&#928;</mml:mtext></mml:mrow><mml:mi>i</mml:mi></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>&#x2192;</mml:mo></mml:mrow></mml:math>X<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&#x2009;</mml:mo><mml:mmultiscripts><mml:mi>&#931;</mml:mi><mml:mprescripts /><mml:none /><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math>+ the vibrational distribution of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>A</mml:mi><mml:mo>&#x2009;</mml:mo><mml:msup><mml:mi /><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mrow><mml:mtext>&#928;</mml:mtext></mml:mrow><mml:mi>i</mml:mi></mml:msub></mml:mrow></mml:math> was found to peak at &#965;&#8242; &#61; 1. From the laser induced fluorescence
 B<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&#x2009;</mml:mo><mml:mmultiscripts><mml:mi>&#931;</mml:mi><mml:mprescripts /><mml:none /><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math>+<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>&#x2192;</mml:mo></mml:mrow></mml:math>X<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&#x2009;</mml:mo><mml:mmultiscripts><mml:mi>&#931;</mml:mi><mml:mprescripts /><mml:none /><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math>+, the vibrational distribution of X<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&#x2009;</mml:mo><mml:mmultiscripts><mml:mi>&#931;</mml:mi><mml:mprescripts /><mml:none /><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math>+ was found to peak at &#965;&#8243; &#61; 4.A C2H2&#8212;N2O combustion flame, stabilized at 2.8 torr with a temperature of 2500 K,
 gave a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>CN</mml:mtext><mml:mrow><mml:mo>&#x2009;</mml:mo><mml:mo>(</mml:mo><mml:mrow><mml:mi>A</mml:mi><mml:mo>&#x2009;</mml:mo><mml:msup><mml:mi /><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mrow><mml:mtext>&#928;</mml:mtext></mml:mrow><mml:mi>i</mml:mi></mml:msub></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> vibrational distribution peaking at &#965;&#8242; &#61; 2 which can be attributed to the C+N2O 
reaction at 2500 K. However the CN ground state vibrational distribution was found to be relaxed.

G.  Dorthe

J.  Joussot&#8211;Dubien

G.  Nouchi

Ph.  Kottis

Determination of Vibrational Energy Distributions of <svg style="vertical-align:-3.3907pt" height="20.0625" version="1.1" viewBox="0 0 78.724998 20.0625" width="78.724998" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,15.775)"><path id="x43" d="M614 175l29 -10q-33 -109 -57 -154q-121 -26 -184 -26q-90 0 -160.5 29t-112.5 77t-63.5 105.5t-21.5 119.5q0 157 108 253t277 96q36 0 71.5 -5t69 -13.5t36.5 -8.5q15 -102 20 -150l-29 -8q-20 79 -66.5 114t-128.5 35q-119 0 -187.5 -86t-68.5 -207&#xA;q0 -140 73.5 -227.5t188.5 -87.5q73 0 119.5 37.5t86.5 116.5z" /></g><g transform="matrix(.017,-0,0,-.017,11.367,15.775)"><path id="x4E" d="M719 650v-28q-43 -2 -62 -15t-22 -44q-6 -47 -6 -169v-403h-31l-426 524h-2v-251q0 -111 6 -169q4 -37 24 -50.5t72 -16.5v-28h-237v28q45 2 64.5 16t23.5 49q6 62 6 171v220q0 54 -3 68.5t-17 32.5q-16 19 -34.5 26.5t-54.5 10.5v28h147l418 -502h3v246q0 117 -7 166&#xA;q-4 34 -24.5 47t-73.5 15v28h236z" /></g><g transform="matrix(.017,-0,0,-.017,26.819,15.775)"><path id="x28" d="M300 -147l-18 -23q-106 71 -159 185.5t-53 254.5v1q0 139 53 252.5t159 186.5l18 -24q-74 -62 -115.5 -173.5t-41.5 -242.5q0 -130 41.5 -242.5t115.5 -174.5z" /></g><g transform="matrix(.017,-0,0,-.017,32.7,15.775)"><path id="x1D434" d="M716 28l-8 -28h-254l8 28q55 5 66 17.5t5 47.5l-30 145h-232l-74 -138q-21 -43 -11 -54.5t72 -17.5l-6 -28h-235l8 28q53 7 73.5 21t53.5 70l319 540l33 8q6 -43 33 -176l80 -381q10 -49 26.5 -63t72.5 -19zM495 281l-49 264h-2l-149 -264h200z" /></g><g transform="matrix(.012,-0,0,-.012,48,7.613)"><path id="x32" d="M412 140l28 -9q0 -2 -35 -131h-373v23q112 112 161 170q59 70 92 127t33 115q0 63 -31 98t-86 35q-75 0 -137 -93l-22 20l57 81q55 59 135 59q69 0 118.5 -46.5t49.5 -122.5q0 -62 -29.5 -114t-102.5 -130l-141 -149h186q42 0 58.5 10.5t38.5 56.5z" /></g><g transform="matrix(.017,-0,0,-.017,54.338,15.775)"><path id="x1D6F1" d="M840 650l-8 -28q-60 -5 -77 -20.5t-28 -75.5l-76 -402q-11 -60 3 -76t81 -20l-5 -28h-280l6 28q63 5 81.5 20.5t29.5 75.5l88 461q3 17 -3 22t-27 5h-288q-20 0 -26 -5t-10 -25l-87 -458q-11 -60 3 -76t85 -20l-6 -28h-279l5 28q61 4 79 20t29 76l76 402q11 60 -2 75.5&#xA;t-83 20.5l6 28h713z" /></g><g transform="matrix(.012,-0,0,-.012,68.912,19.863)"><path id="x1D456" d="M244 607q0 -25 -15.5 -43t-37.5 -18q-19 0 -32 13t-13 35q0 21 15 41t39 20q20 0 32 -14t12 -34zM222 91q-29 -33 -79 -68t-75 -35q-13 0 -19 7.5t-6 31t10 65.5l62 253q5 26 -1 26q-21 0 -72 -43l-13 24q43 40 91 68t71 28q30 0 10 -78l-71 -274q-8 -30 3 -30&#xA;q16 0 76 48z" /></g><g transform="matrix(.017,-0,0,-.017,72.775,15.775)"><path id="x29" d="M275 270q0 -296 -211 -440l-19 23q75 62 116.5 174t41.5 243t-42 243t-116 173l19 24q211 -144 211 -440z" /></g></svg> and (X
<svg style="vertical-align:-0.0pt" height="15.8375" version="1.1" viewBox="0 0 19.4625 15.8375" width="19.4625" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.012,-0,0,-.012,2.9,7.613)"><use xlink:href="#x32"/></g><g transform="matrix(.017,-0,0,-.017,9.225,15.775)"><path id="x3A3" d="M570 168l-39 -168h-498v23l262 301l-248 304v22h471q4 -71 11 -146l-29 -4q-17 65 -37 88q-12 15 -36 20.5t-79 5.5h-181l211 -256l-243 -287h280q51 0 77 20.5t49 83.5z" /></g></svg> +) from the C+N2O Reaction by Chemiluminescence and Laser Induced Fluorescence

Advantages of molecular ions for the study of electronic radiationless transitions are
discussed. Several coincidence techniques for measuring key relaxation parameters are
briefly described. The different cases of radiationless transition studies of molecular
ions are outlined and examples mentioned. Coincidence measurements between threshold
photoelectrons and ion fluorescence photons emitted by VUV-excited
hexafluorobenzene are presented. The radiative, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mi>r</mml:mi></mml:msub></mml:mrow></mml:math>, and nonradiative, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi>n</mml:mi><mml:mi>r</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>, relaxation rates for specific vibrational levels in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mover accent="true"><mml:mi>B</mml:mi><mml:mo>&#8764;</mml:mo></mml:mover><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mi>A</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mi>u</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mover accent="true"><mml:mi>C</mml:mi><mml:mo>&#8764;</mml:mo></mml:mover><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mi>B</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mi>u</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> states of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>C</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:msubsup><mml:mi>F</mml:mi><mml:mn>6</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math> are derived from the measured lifetimes and quantum yields of ion fluorescence. The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mi>r</mml:mi></mml:msub></mml:mrow></mml:math> rate for excitation of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mi>C</mml:mi><mml:mo>&#8764;</mml:mo></mml:mover></mml:math>, 0&#176; level is found to be about 25&#37; of that for the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mover accent="true"><mml:mi>B</mml:mi><mml:mo>&#8764;</mml:mo></mml:mover><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mi>A</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mi>u</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> state where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mi>r</mml:mi></mml:msub></mml:mrow></mml:math> &#61; (18&#177;3) &#215; 106 s&#8722;1. The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi>n</mml:mi><mml:mi>r</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> rates for the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mn>1</mml:mn><mml:mi>n</mml:mi></mml:msup></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mn>1</mml:mn><mml:mi>n</mml:mi></mml:msup><mml:msup><mml:mn>2</mml:mn><mml:mn>1</mml:mn></mml:msup></mml:mrow></mml:math> progressions in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mi>B</mml:mi><mml:mo>&#8764;</mml:mo></mml:mover></mml:math> state increase with n but exhibit a mode selective behavior. Model calculations of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mrow><mml:mi>n</mml:mi><mml:mi>r</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> confirm the mode selectivity. 
These calculations were carried out on a noncommunicating, harmonic oscillator basis. 
Progressive deviations between experimental and calculated results are interpreted as indicating the 
gradual onset of vibrational redistribution with increasing vibrational energy. 
The results illustrate a new method for demonstrating the growing in of intramolecular vibrational relaxation.

Sydney  Leach

G&#233;rald   Dujardin

Ionic State Relaxation Processes in VUV-Excited Polyatomic Molecules

The theoretical basis for the local mode model is reviewed. The model is applied to
gas phase overtone spectra of aromatic molecules to investigate both substituent
induced CH bond length changes and conformationally inequivalent hydrogens. The
dynamic implications of the local mode model are discussed.

Bryan R. Henry

Kathleen M. Gough

Applications of the Local Mode Model to CH Bond Length Changes, Molecular Conformations and Vibrational Dynamics

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