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.

 


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.

 



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)
 

   


 

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.

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Last Modified: June 28, 2012