Intramolecular vibrational redistribution and fragmentation dynamics
of I
2
••• Ne
n
„ n 2–6 … clusters
S. Fernandez Alberti,
a)
N. Halberstadt, and J. A. Beswick
IRSAMC, Universite ´ Paul Sabatier, 31062 Toulouse, France
A. Bastida, J. Zu
´
n
˜
iga, and A. Requena
Departamento de Quı ´mica Fı ´sica, Universidad de Murcia, 30100 Murcia, Spain
Received 23 February 1999; accepted 6 April 1999
Intramolecular vibrational energy redistribution and fragmentation dynamics in I
2
( B , v
22) ••• Ne
n
( n 2 – 6 ) and I
2
( B , v 21) ••• Ne
n
( n 2 – 5) clusters is studied by hybrid quantum/
classical techniques and the results are compared with experiments. A vibrational version of the
molecular dynamics with quantum transitions MDQT treatment is used in which the vibrational
degree of freedom of I
2
is treated quantum mechanically while all the other degrees of freedom are
treated classically. The potential energy surface is represented as a sum of pairwise interactions with
parameters taken from the literature. The calculated product state distributions are in very good
agreement with the experiments. Fragmentation lifetimes were also calculated and agree reasonably
well with those measured in time-dependent experiments. Fragmentation proceeds via sequential
ejection of Ne monomers through three different mechanisms: i sequential intramolecular
vibrational redistribution plus vibrational predissociation in which the I
2
molecule loses more than
one quantum of vibration; ii direct vibrational predissociation in which the I
2
molecule loses
only one quantum of vibration; iii evaporation in which the I
2
molecule remains in the same
vibrational state. © 1999 American Institute of Physics. S0021-96069900425-0
I. INTRODUCTION
Van der Waals clusters consisting of a molecular chro-
mophore and a varying number of rare gas atoms are ideal
model systems for the detailed study of vibrational energy
pathways following photon excitation. The weakness of the
Van der Waals bond allows us to clearly identify energy
transfers, and complexity can be gradually added by increas-
ing the number of rare gas atoms. Lifetimes and final state
distributions for vibrational predissociation, intramolecular
vibrational redistribution, and evaporation, as well as their
dependence on the number of degrees of freedom, can be
measured.
The first experimental studies on lifetimes and product
state distributions for vibrational predissociation in Van der
Waals clusters consisting of a dihalogen molecule and one or
several rare gas atoms have been performed by Levy and
coworkers on I
2
••• X
n
(XHe, Ne, Ar.
1,2
They were fol-
lowed by studies on Br
2
••• Ne
n
,
3
ICl ••• Ne
n
,
4
Cl
2
••• Ne
2
,
5
Cl
2
••• Ar
n
,
6
and Cl
2
••• He
2
.
7
Finally, time-resolved life-
time measurements have been performed on complexes of
I
2
••• Ne
n
( n 2–4).
8
From the theoretical point of view, Van der Waals com-
plexes with one rare gas atom have been extensively
studied.
9,10
Exact full-dimensional quantum calculations
have been conducted on several of these triatomic
systems.
11–31
For larger clusters the only quantum studies available are
of reduced dimensionality.
32–36
Another approach has been
to use classical or quasiclassical trajectories,
37–39
which can
handle larger clusters in full dimensionality. However, in
some cases the latter give very poor results due to the occur-
rence of quantum threshold effects and the possibility for the
cluster to dissociate with only a fraction of a vibrational
quantum of the chromophore.
38
I
2
••• Ne
n
is a clear example
of this situation. Although detailed experimental results
2,8
have been available for more than 20 years now, no satisfac-
tory theoretical modeling has yet been given.
In previous works
40–42
we have adapted the MDQT mo-
lecular dynamics with quantum transitions method of
Tully
43,44
for the description of the vibrational relaxation and
predissociation of I
2
••• Ar and I
2
••• Ne
2
Van der Waals
clusters. We have shown that this method is very well
adapted to study the dynamics of these systems and that it
could allow us to deal with larger clusters.
In the present work, we apply this hybrid quantum/
classical treatment to the study of vibrational energy redis-
tribution and fragmentation dynamics of I
2
( B , v
21– 22) ••• Ne
n
( n 2 – 6) clusters. We have obtained re-
sults in very good agreement with the experimentally mea-
sured product state distributions and reasonably good agree-
ment for the lifetimes. In addition, we have characterized the
following mechanisms for fragmentation:
a
Permanent address: Universidad de Quilmes, Argentina.
JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 1 1 JULY 1999
239 0021-9606/99/111(1)/239/6/$15.00 © 1999 American Institute of Physics