Gas-Phase Molecular Dynamics
The Gas-Phase Molecular Dynamics Group is dedicated to developing and
applying spectroscopic and theoretical tools to challenging problems in
chemical physics related to reactivity, structure, dynamics and kinetics of
transient species.
Recent theoretical work has included advances in exact variational solution
of vibrational quantum dynamics, suitable for up to five atoms in systems
where large amplitude motion or multiple strongly coupled modes make simpler
approximations inadequate. Other theoretical work, illustrated below,
applied direct dynamics, quantum force trajectory calculations to
investigate a series of reactions of the HOCO radical.
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The potential energy surface for the OH + CO/ H + CO2
reaction, showing two barriers (TS1 and TS2) and the deep HOCO
well along the minimum energy pathway. The inset figure shows
the experimental and calculated reactivity of HOCO with selected
collision partners. See J.S. Francisco, J.T. Muckerman and H.-G.
Yu, “HOCO radical chemistry,”
Acc. Chem. Res. 43, 1519 (2010). |
A recent experimental highlight in sub-Doppler saturation spectroscopy,
illustrated below left, demonstrates very high resolution optical spectroscopy to
measure excited-state nuclear hyperfine splittings in the CN radical. Using
the nuclear spin as a probe, experiments like this were used to measure the
electric dipole moment of the excited state and details of the electron spin
localization in the excited state, properties that had not been previously
measured for this radical, or even for many stable molecules. A further
application of the sub-Doppler hyperfine spectroscopy is in the
characterization of the mixed singlet-triplet doorway states important in
collision-induced intersystem crossing in CH2. The hyperfine splitting in
singlet methylene is typically negligible unless mixing with accidentally
degenerate triplet states introduces a non-zero electron spin that couples
with the hydrogen nuclear spins to cause a measurable triplet splitting. The
unresolved, sharp saturation spectrum of an unmixed singlet CH2 transition
is contrasted with the triplet splitting observed in a pair of mixed states
in the figure below right.
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Sub-Doppler saturation spectroscopy of CN radicals.
Counter-propagating laser beams bleach and probe velocity groups of CN
radicals to resolve the hyperfine splitting patterns of single
rotational states caused by the nuclear spin of the nitrogen. The inset
energy level diagram shows the nuclear spin components of the rotational
levels that participate in the R1(1/2) rotational line of the
CN A(v=1) ← X (v=0) transition near 900 nm. The arrows in the energy
level inset are sorted by increasing energy and aligned below the
corresponding spectral lines above. The strong transitions at the
positions labeled X in the level diagram are crossover resonances,
observed midway between pairs of transitions sharing a common upper or
lower level. The entire patterns is repeated with an inverted sign 191
MHz to the blue, since the probe spectroscopy uses frequency modulation
(FM) and each sideband contributes independently to the observed
saturation signals.
M. L. Hause, G. E. Hall and T. J. Sears, J. Mol. Spectrosc. 253
122-128 (2009)
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Hyperfine-resolved sub-Doppler spectra of selected CH2 transitions,
measured with FM saturation spectroscopy. Top spectrum shows single
saturation feature for the pure singlet rotational state 212,
detected with opposite signs by the two first-order sidebands. Below are
scans for the two mixed eigenstates with parentage in the singlet 818
rotational level. The hyperfine splitting into three lines is dominated
by the electron-spin/nuclear-spin coupling. The relative magnitude of
the splitting in these spectra directly gives the relative triplet
character of the two mixed eigenstates.
See Chang et al. J. Chem. Phys. 133, 144310 (2010). |
The
Gas-Phase Molecular Dynamics Program,
is supported by the
Chemical Physics Research Program of the
Division
of Chemical Sciences, Biosciences, and Geosciences of the
Office of Basic Energy
Sciences of the
Office of
Science under contract No. DE-AC02-98CH10886 with the
U.S.
Department of Energy.
Learn more about the Gas-Phase Molecular Dynamics Group
Last Modified: June 28, 2012
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