The spectrum of the molecular eigenstates (ME) belonging to the various rotational
members of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi /><mml:mn>1</mml:mn></mml:msup><mml:msub><mml:mi>B</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mi>u</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> (0&#8211;0) transition of pyrazine was measured with a very narrow
band laser in a molecular beam with a Doppler width of 30 MHz.It is shown that, when the ME&#39;s belonging to a single rotational state are Fourier-transformed,
the beating decays of these states are obtained. A problem is constituted
by the lack of a fast component in these reconstructions, while it is observed in
experiments on higher J-states.

Barend J. van der Meer

Harry Th. Jonkman

Jan  Kommandeur

The Spectrum of the Molecular Eigenstates of Pyrazine and the Reconstruction of Decays of 
 Rotational States of the <svg style="vertical-align:-3.3907pt" height="20.0625" version="1.1" viewBox="0 0 29.6875 20.0625" width="29.6875" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.012,-0,0,-.012,.062,7.613)"><path id="x31" d="M384 0h-275v27q67 5 81.5 18.5t14.5 68.5v385q0 38 -7.5 47.5t-40.5 10.5l-48 2v24q85 15 178 52v-521q0 -55 14.5 -68.5t82.5 -18.5v-27z" /></g><g transform="matrix(.017,-0,0,-.017,6.4,15.775)"><path id="x1D435" d="M594 511q0 -122 -171 -157l1 -2q158 -33 158 -159q0 -52 -34.5 -95t-90.5 -65q-76 -33 -217 -33h-223l8 28q63 5 79.5 19t26.5 72l83 426q9 48 -2.5 60t-77.5 17l6 28h259q195 0 195 -139zM499 509q0 59 -37 83t-91 24q-36 0 -51 -9q-17 -9 -22 -44l-35 -195h62&#xA;q82 0 128 37t46 104zM481 199q0 71 -48 102.5t-121 31.5h-56l-37 -201q-11 -58 7.5 -77t80.5 -19q76 0 125 44.5t49 118.5z" /></g><g transform="matrix(.012,-0,0,-.012,16.887,19.863)"><path id="x33" d="M285 378v-2q65 -13 102 -54.5t37 -97.5q0 -57 -30.5 -104.5t-74 -75t-85.5 -42t-72 -14.5q-31 0 -59.5 11t-40.5 23q-19 18 -16 36q1 16 23 33q13 10 24 0q58 -51 124 -51q55 0 88 40t33 112q0 64 -39 96.5t-88 32.5q-29 0 -64 -11l-6 29q77 25 118 57.5t41 84.5&#xA;q0 45 -26.5 69.5t-68.5 24.5q-67 0 -120 -79l-20 20l43 63q51 56 127 56h1q66 0 107 -37t41 -95q0 -42 -31 -71q-22 -23 -68 -54z" /></g><g transform="matrix(.012,-0,0,-.012,22.599,19.863)"><path id="x1D462" d="M515 96q-37 -44 -84.5 -76t-71.5 -32q-12 0 -18 7t-6 34t10 75l19 89h-2q-87 -113 -181 -176q-43 -29 -83 -29q-46 0 -46 63q0 31 9 67l52 222q7 36 -1 36q-10 0 -76 -50l-13 24q47 45 92 71.5t67 26.5q18 0 21 -16.5t-8 -65.5l-56 -242q-16 -67 13 -67q41 0 114 74&#xA;t114 146l32 145l74 26h11q-52 -199 -80 -347q-8 -39 6 -39q7 0 32 17.5t47 39.5z" /></g></svg> (0&#x2013;0) Transition of Pyrazine

A.  Kaldor

E.  Rohlfing

D. M. Cox

Molecular Surfaces&#8212;Photoionization Probes of Metal Clusters

Recent developments in laser and molecular beam technology have now made it
possible to produce supersonic beams of virtually any element in the periodic table.
Using laser vaporization of the appropriate target, the beam source conditions may
be adjusted to produce either the cold free atoms alone, or clusters of these atoms
with each other&#8212;or with another element. Since the vaporization laser heats only a
small spot on the target, extremely high local temperatures can be obtained without
heating any other part of the apparatus; and beams of even the highest boiling element
(tungsten) are readily obtained both in atomic and cluster form. The physics and
chemistry of these exotic cluster species is almost completely unknown on the fundamental
level. Even for clusters containing 100 atoms, most of these atoms lie on the
surface, and the chemical and physical properties will be predominately surface
phenomena. Initial studies of these clusters have entailed the use of one- and two- photon
laser ionization with time-of-flight mass selective detection. Using a variety of
fixed frequency lasers, the work function of copper clusters has been examined as a
function of cluster size in the range from 2 to 29 atoms per cluster. Considerable
detailed information has also been obtained for the electronic structure and bond
lengths of a number of transition metal dimers and trimers (including Cu2, Cr2, V2,
Mo2, and Cu3) through the use of high resolution laser spectroscopy with mass-selective
photoionization detection.

R. E. Smalley

Laser Studies of Metal Cluster Beams

In this review we describe the development and the applications of the picosecond-jet
technique, which utilizes a picosecond laser and a supersonically-cold jet beam of large
molecules. The applications include studies of coherence (quantum beats), photodissociation,
isomerization, and partial solvation. The results emphasize the role of
intramolecular energy redistribution, and provide evidence for intramolecular threshold effect
for rates vs. excess molecular energy. Simple theory for this redistribution of
energy among certain modes in molecules is given, and comparison with RRKM
calculation is also made to assess the nature of the statistical behavior for the redistribution.

Ahmed H. Zewail

Picosecond Laser Chemistry in Supersonic Jet Beams

Hydrogen bonding in isolated, cooled dimers of benzoic acid is explored following
preparation in a free jet expansion. Analysis of the laser excitation spectrum in the
low energy region allows for the identification of the modes involved in hydrogen
bonding in the excited state of the dimer, and cluster formation is noted between the
ground state dimers. Van der Waals complexes of the dimer with rare gas atoms are
also identified. Assignments of the observed dimer modes have been made and the
results are compared with previous studies in solution and matrices and with normal
mode analyses.

David E. Poeltl

Jeffrey K. McVey

Low Energy Modes of the Hydrogen-Bonded Dimer of Benzoic Acid

Recent experimental and theoretical studies of very low energy collision induced
vibrational relaxation in diatomic and polyatomic molecules are surveyed. Emphasis
is placed on the novel features of the very low energy process; these require a full
quantum mechanical treatment of the collision to account for the observations.

Stuart A. Rice

Charles  Cerjan

Very Low Energy Collision Induced Vibrational Relaxation: An Overview

Pyrazine (1,4-diazabenzene) has played a central role in the development of the theory
of radiationless transitions. In this paper, we will describe studies of the decay behavior
and fluorescence quantum yield of the isolated molecule and several of its isotopically
labelled derivatives following pulsed dye laser excitation of &#8220;single&#8221; rotational levels
of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi /><mml:mn>1</mml:mn></mml:msup><mml:msub><mml:mi>B</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mi>u</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> (n&#960;&#42;) state in a seeded supersonic jet, both in the presence and absence
of an external magnetic field. We shall demonstrate that pyrazine can be transformed
from a &#8220;small&#8221; molecule to a &#8220;large&#8221; molecule by very small perturbations; e.g., by
climbing up the rotational ladder of the first excited singlet state, by isotopic labeling,
and/or by applying dc magnetic fields of the order of 100 G. Thus, the experiments
provide important tests of the predictions of the theory for the first time. Specifically,
we will show that, under &#8220;intermediate-case&#8221; conditions, (1) the fast component of
the decay is indeed coherent in nature, (2) the coupled state is triplet in character
(and, thus, that the process we are monitoring is intersystem crossing (ISC)), (3) nuclear
spin is conserved in ISC, (4) rotations and small magnetic fields play an important role
because of the angular momentum selection rules for ISC, and (5) the levels mixed
by the magnetic field are most likely the fine-structure components of the different
angular momentum levels of the mixed, singlet&#8211;triplet state. The relationship of these
results to those of other workers in the field will also be discussed.

Y.  Matsumoto

L. H. Spangler

D. W. Pratt

<svg style="vertical-align:-3.3907pt" height="20.0625" version="1.1" viewBox="0 0 29.6875 20.0625" width="29.6875" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.012,-0,0,-.012,.062,7.613)"><path id="x31" d="M384 0h-275v27q67 5 81.5 18.5t14.5 68.5v385q0 38 -7.5 47.5t-40.5 10.5l-48 2v24q85 15 178 52v-521q0 -55 14.5 -68.5t82.5 -18.5v-27z" /></g><g transform="matrix(.017,-0,0,-.017,6.4,15.775)"><path id="x1D435" d="M594 511q0 -122 -171 -157l1 -2q158 -33 158 -159q0 -52 -34.5 -95t-90.5 -65q-76 -33 -217 -33h-223l8 28q63 5 79.5 19t26.5 72l83 426q9 48 -2.5 60t-77.5 17l6 28h259q195 0 195 -139zM499 509q0 59 -37 83t-91 24q-36 0 -51 -9q-17 -9 -22 -44l-35 -195h62&#xA;q82 0 128 37t46 104zM481 199q0 71 -48 102.5t-121 31.5h-56l-37 -201q-11 -58 7.5 -77t80.5 -19q76 0 125 44.5t49 118.5z" /></g><g transform="matrix(.012,-0,0,-.012,16.887,19.863)"><path id="x33" d="M285 378v-2q65 -13 102 -54.5t37 -97.5q0 -57 -30.5 -104.5t-74 -75t-85.5 -42t-72 -14.5q-31 0 -59.5 11t-40.5 23q-19 18 -16 36q1 16 23 33q13 10 24 0q58 -51 124 -51q55 0 88 40t33 112q0 64 -39 96.5t-88 32.5q-29 0 -64 -11l-6 29q77 25 118 57.5t41 84.5&#xA;q0 45 -26.5 69.5t-68.5 24.5q-67 0 -120 -79l-20 20l43 63q51 56 127 56h1q66 0 107 -37t41 -95q0 -42 -31 -71q-22 -23 -68 -54z" /></g><g transform="matrix(.012,-0,0,-.012,22.599,19.863)"><path id="x1D462" d="M515 96q-37 -44 -84.5 -76t-71.5 -32q-12 0 -18 7t-6 34t10 75l19 89h-2q-87 -113 -181 -176q-43 -29 -83 -29q-46 0 -46 63q0 31 9 67l52 222q7 36 -1 36q-10 0 -76 -50l-13 24q47 45 92 71.5t67 26.5q18 0 21 -16.5t-8 -65.5l-56 -242q-16 -67 13 -67q41 0 114 74&#xA;t114 146l32 145l74 26h11q-52 -199 -80 -347q-8 -39 6 -39q7 0 32 17.5t47 39.5z" /></g></svg> Pyrazine. Experimental Tests of the Theory of Radiationless Transitions

s-Tetrazine argon complexes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mtext>T</mml:mtext><mml:mo>&#8722;</mml:mo><mml:msub><mml:mrow><mml:mtext>Ar</mml:mtext></mml:mrow><mml:mi>n</mml:mi></mml:msub></mml:mrow></mml:math> (n &#61; 1, 2) are formed in a supersonic expansion
of argon seeded with s-tetrazine. The expansion was conducted through a nozzle of
50 or 100 &#956;m with an argon stagnation pressure between 1 and 1.5 bar. From spectrally
resolved measurements it is clear that vibrational redistribution processes as well as
vibrational predissociation processes take place after SVL excitation within the
complex.From rise and decay time experiments it can be concluded, that after excitation of
the 6a1 complex level, the above mentioned processes are consecutive and not parallel.
It appears that the out of plane mode 16a couples with the Van der Waals stretching
mode. The predissociation rate of the 16a2 complex is observed to be 2.3 &#215; 109 s&#8722;1.

J. J. F. Ramaekers

L. B. Krijnen

H. J. Lips

J.  Langelaar

R. P. H. Rettschnick

Consecutive Vibrational Redistribution and Predissociation Processes in s-Tetrazine&#8211;Argon Van Der Waals Complexes

Vibrational predissociation (VP) of glyoxal complexes formed in a supersonic
expansion, has been studied in several vibrational levels of the first excited singlet
electronic state S1(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi /><mml:mn>1</mml:mn></mml:msup><mml:msub><mml:mi>A</mml:mi><mml:mi>u</mml:mi></mml:msub></mml:mrow></mml:math>) of glyoxal. Light complexes as H2-glyoxal display a nice state
to state dissociation. Whereas the analysis of the fluorescence spectrum, together with
lifetime measurements, have shown that VP is in competition with other intramolecular
vibrational energy redistribution processes in these systems, as well as inter-system
crossing (ISC). Glyoxal-Ar and glyoxal-Kr behave very similarly. In particular, two
isomers exist with different dissociation schemes (but similar for the same isomer of
Ar or Kr-complex), and Kr does not seem to enhance ISC. Thus selectivity in the
dissociation routes has to be understood as a competition between these deactivation
channels.

N.  Halberstadt

B.  Soep

From Selective to Non-Selective Vibrational Predissociation in Glyoxal <small.letters>van der</small.letters> Waals Complexes

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