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High Resolution Probes of Chemical Dynamics and
Kinetics
Gregory Hall, Gas Phase Molecular Dynamics Group
Chemical dynamics is the field of study that predicts or
explains chemical reactions and their rates, energy levels and energy
flows within in energized or colliding molecules, given the potential
energy surfaces. It forms the microscopic basis of chemistry: the making
and breaking of molecules. The DOE Chemical Physics program supports
research in chemical dynamics as part of its focus on the basic science of
combustion and catalysis. Molecular level understanding of chemical
transformations in the gas phase or on surfaces is an essential component
of predictive models for applications.
The GPMD group at BNL performs experiments in spectroscopy, dynamics and
kinetics with a strong theoretical component, addressing multiple facets
of selected high-priority topics in the chemical physics of combustion and
catalysis. Recent themes and applications of the work I am leading
include:
Collision-induced intersystem crossing via mixed-states in CH2
Spectroscopy, dynamics and kinetics come together to shed
new light on a process that controls the rate of interconversion between
singlet and triplet forms of a reactive hydrocarbon species, methylene.
The two types of methylene – singlet and triplet – differ in their total
electronic spin, and also in their reactivity and product branching
ratios, despite having very similar energies. We are exploring the
mechanism of interconversion between these two forms of methylene.
Spin-orbit coupling in the isolated molecule is responsible for a sparse
set of pairwise perturbations between zero-order singlet and triplet
rovibronic levels, which act as gateways for the spin changing process in
a collisional environment. A combination of state-resolved thermalization
kinetic measurements, high dynamic range observation of double-exponential
kinetics, and double resonance saturation recovery and saturation transfer
experiments provide a highly detailed look at the behavior of mixed states
in methylene collisions.
Coherent and incoherent effects in direct
photodissociation
Direct photodissociation systems serve as a prototype
for chemical branching between different products. In the case of ICN
photodissociation, multiple interacting excited states lead to
distinguishable product states that can be fully characterized by
transient FM Doppler spectroscopy. Angular distributions and polarization
effects allow us to resolve multiple paths to the same products, which
display both coherent and incoherent properties. The observations can be
compared with state-of-the-art nonadiabatic dynamical theory on multiple
potential energy surfaces. Key points are spectroscopic observables to
characterize curve crossing dynamics, and development of instrumental and
analysis techniques for polarized transient FM Doppler spectroscopy.
Rev. GEH/Jan 2007
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