Photodissociation Dynamics of
I2¯
in CO2 Clusters
Claudio J. Margulis and David F. Coker
Department of Chemistry
Boston University
We investigate the photodissociation, geminate recombination and relaxation
dynamics in size-selected
I2¯ ·
(CO2)n
cluster ions using an electronically nonadiabatic molecular dynamics method and
a model Hamiltonian gained from diatomics-in-ionic systems (DIIS).
These theoretical studies make contact with recent time resolved pump-probe,
and photoelectron detachment experiments. Our studies reveal a rich excited
state dynamics in which various competing electronic relaxation channels as
well as vibrational relaxation influence the recovery of signal in these
experiments.
The first two video segments for the smaller
I2¯ ·
(CO2)8
show how nonadiabatic electronic state changes can lead to switching of the
excess charge from an initially externalized charge state of the cluster, to a
state in which the excess charge is solvated by the
CO2 molecules and the
I2¯
dissociates. Recombination can also take place if this charge switching
is initially ineffective. The third video sequence shows relaxation in a larger
cluster
I2¯ ·
(CO2)16
with a completed first solvation shell. The electronic states of the
completely solvated molecule are very different than in the smaller cluster
leading to very different relaxation dynamics. These findings are supported by
earlier laboratory results.
Video Segments
An iodine molecular ion (I2¯) embedded
in a cluster of carbon dioxide molecules
(CO2)
is excited electronically. We study the transfer of this excitation
energy to the CO2 molecule motions in
different sized clusters.
Iodine atoms are purple spheres. CO2
molecules are formed from grey carbon atoms and red oxygen atoms.
Video Sequence
Segment 1: I2¯(CO2)8
photo-excitation leading to I2¯
dissociation.
Video Sequence
Segment 2: I2¯(CO2)8
photodissociation followed by recombination and complete evaporation of
cluster.
Video Sequence
Segment 2: I2¯(CO2)16
photodissociation and recombination, followed by heating of large cluster.
Evaporation is slow due to strong attractions between
CO2 molecules.
Hardware: SGI Power Challenge Array and SGI Origin 2000.
Software: Fortran 90. Visualization done using IDL, C, IRIS Performer.
Graphics programming and video production:
Erik Brisson and Robert Putnam,
Scientific Computing and Visualization Group, Boston University.
Acknowledgments: We gratefully acknowledge financial support for this
work from the National Science Foundation (Grant No. CHE-9521793), and a
generous allocation of supercomputer time from Boston University's center for
Scientific Computing and Visualization.
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