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44025-B6
Atom-Molecule Inelastic Collision Dynamics in Open Shell Systems: O + NO and Cl + NO
Thomas A. Stephenson, Swarthmore College
Our
work on collision-induced energy transfer continues along two parallel tracks: investigating
the electronic energy transfer dynamics that occur when Br2 collides
with inert partners and studies of the dynamics of O + NO collisions.
We
have made substantial progress in our studies of electronic energy transfer in
Br2 + He, Ar. In these
experiments, we prepare Br2 in the E ion-pair state using two color double resonance laser excitation. By monitoring the emission spectrum as
a function of collision partner pressure, we map out the electronic energy
transfer pathways and the distribution of vibrational energy within each
electronic state populated. This
work is carried out in collaboration with Professor Alexei Buchachenko (Moscow
State University) and his students, who have examined the same collision events
from the standpoint of fully quantum scattering calculations. Experimentally, we find that collisions
with He and Ar induce electronic energy transfer to the D, D' and β ion-pair states. He collisions
favor population of the D state, while Ar collisions direct population predominately to the
β
state, results that are both consistent with our previous studies of I2(E) collisions. The vibrational distributions
within each electronic state also vary with the identity of the collision
partner, with He collisions populating vibrational levels with significant
Franck-Condon overlap with the initial E state vibrational level. On the other hand, collisions with Ar
favor population in vibrational levels that are closer in energy to the
initially prepared level. These
trends are in accord with the commonly invoked vibrational "sudden"
approximation, which suggests that Franck-Condon dominated vibrational
distributions will be favored when the relative collision velocities are high, i.e., when He
and not Ar is the collision partner.
Collision-induced population of the D state occurs with a rate constant in Br2
than is significantly larger than the corresponding process in I2, a
result of the existence of energy transfer pathways that simultaneously
maximize the Franck-Condon overlap and minimize the vibronic energy gap. All of these features are reproduced in
the scattering calculations, and the agreement of experiment and theory is a
hopeful sign that the methodology developed to date is adequate to address a
wide range of energy transfer phenomena. More
recently, we have extended these investigations to include Br2 + Xe
and Br2 + CF4 collisions. Our goals in these studies were to explore further the
effect of collision partner mass on the vibrational distributions and to
explore the role, if any, that internal degrees of
freedom play in the energy transfer process. Consistent with our expectations, Br2 + Xe
collisions have much higher cross sections for electronic energy transfer, by
factors of between 4 and 12 (relative to He collisions), depending on the final
electronic state. In addition, Br2
+ Xe collisions favor population of near resonant vibrational levels in the D state, also
consistent with the trend identified with the lighter rare gases. In Br2 + CF4
collisions, we observe the effect of the CF4 internal degrees of
freedom only indirectly. For these
collision events, we observe greatly enhanced population in the D' and
β
electronic states, far in excess of what we would expect based on the effects
of mass and polarizability. We
assign this enhanced propensity to the ability of the CF4 rotational
and vibrational degrees of freedom to absorb large amounts of vibronic energy
that would have otherwise been released to translation. Our
experiments on O + NO are now underway in the lab. We have successfully characterized our NO free jet expansion
and our photolytic source of O atoms (from the 193 nm photodissociation of SO2)
is now fully functional. We hope
to have preliminary results from this study by the end of 2008.
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