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Summary of Research Interests
Our group studies the gas phase kinetics of reactions of atomic and free radical
species with small atmospheric molecules and free radicals. Techniques employed
include discharge flow with detection of free radical and molecular species
using collision-free sampling mass spectrometry and flash photolysis, coupled
with atomic resonance fluorescence detection. These studies provide
quantitative rate data as a function of temperature and pressure for models of
the atomspheres of planets and satellites such as Jupiter, Saturn, Titan. Venus,
etc., and for models of the upper atmosphere of Earth, with particular emphasis
on the ozone layer and its possible perturbation by anthropogenic and natural
phenomena. Atomic species studied have included H, O, N, Cl and Br, while free
radical reactions have included those of OH, NH2 SO, ClO, BrO, CH2OH and CH3.
Recent research has provided the first measurement of the absoloute rate
constant for the reaction of N(4S) with CH3 (precursor of HCN observed on
Titan), production of the CH2OH radical and its reaction with O2 (hydrocarbon
oxidation in the stratosphere of Earth) and reaction of O(3P) with PH3
(pertinent to the chemistry of phosphine on Jupiter and Saturn). Current
research includes the first studies of the effect of temperature on the rate
constants for
CH2OH + O2 -> HO2 + H2CO and for N(4S) + CH3 -> products
(both have a complex, non-Arrhenius temperature dependence), and for
O(3P) + PH3 -> H2PO + H (no temperature dependence). We are also making
quantitative measurements of the fractional yield for primary reaction channels
in the reaction N(4S) + CH3 -> HCN + H2 (a), -> HCN + 2H (b), and -> H2CN + H
(c). Channel (a) is favored energetically but forbidden by spin and orbital
symmetry correlation rules; channel (b) is nearly thermoneutral. We find the
major channel to be (c) yielding H2CN + H, and this can be rationalized in terms
of both symmetry and energetics. Since we find the overall yield for the
conversion of CH3 to HCN in excess is unity, formation of HCN in this system, as
well as in the atmosphere of Titan, occurs largely via the sequence N + CH3 ->
H2CN + H, N + H2CN -> HCN + NH rather than the previously expected direct
formation of HCN in channels (a) or (b).
- Selected Publications:
"Temperature Dependence of the Reaction Between
Atomic Oxygen (3P) and Chlorine Oxide (OClO) at Low
Pressure." Gleason, J. F.; Nesbitt, F. L.; Stief, L.
J. J. Phys. Chem. 1994, 98, 126.
"Hypobromous Acid Kinetics: Reactions of Halogen
Atoms, Oxygen Atoms, Nitrogen Atoms, and Nitric Oxide with
HOBr." Monks, P. S.; Nesbitt, F. L.; Scanlon, M.;
Stief, L. J. J. Phys. Chem. 1993, 97, 11699.
"A Discharge-Flow Photoionization
Mass-Spectrometric Study of the Bromine Oxide (BrO)(X2.PI.)
Radical. Photoionization Spectrum and Ionization
Energy." Monks, P. S.; Stief, L. J.; Krauss, M.; Kuo,
S. C.; Klemm, R. B. Chem. Phys. Lett. 1993, 211, 416.
"A Discharge-Flow Photoionization
Mass-Spectrometric Study of Hydroxymethyl Radicals (H2COH
and H2COD): Photoionization Spectrum and Ionization
Energy." Tao, W.; Klemm, R. B.; Nesbitt, F. L.; Stief,
L. J. J. Phys. Chem. 1992, 96, 104.
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