OVERTONE SPECTROSCOPY AND DYNAMICS OF HCFC COMPOUNDS
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Date
2000
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
Photoacoustic spectra of the second, third and fourth overtones of methyl C-H stretches of $CH_{3}CF_{2}Cl$ and $CH_{3}CFCl_{2}$ and of the $N = 3, N = 7/2$ and $N = 4 C-H$ stretch-bend polyads of $CHFCl_{2}$ were measured at room temperature. For the last two compounds also the action spectra were obtained via photodissociation of the jet-cooled vibrationally excited molecules combined with mass spectroscopic detection of the photofragments. In the action spectra fragments of $H, Cl(^{2}P_{3/2})[Cl]$ and $Cl(^{2}P_{1/2}) [Cl^{\ast}]$ were detected, due to enhanced C-Cl and C-H bond breaking indicating energy flow out of the initially prepared states. A simplified local mode model for C-H stretching, including the stretch-deformation Fermi resonances, was used to interpret the multi-peak structure of the spectra of $CH_{3}CF_{2}Cl$ and $CH_{3}CFCl_{2}$. The action spectra are significantly narrower than the room temperature photoacoustic spectra due to reduction of the rotational inhomogeneous structure. In $CHFCl_{2}$ the action spectra enabled to resolve the components arising from the different isotopomers of the precursor and the resonance splitting attributed to a local resonance of the $7/2_{1}$ polyad component with a combination of the $7/2_{3}$ component and the ClCCl bending. This splitting reflects an oscillation period of $^{-}3$ ps for the vibrational redistribution and indicates that the coupling of the stretch-bend mixed state to the rest of the molecule is weaker than the Stretch-bend coupling itself. The yield of $Cl^{\ast}$ photofragments was found to be about half that of Cl for $^{-}235$ nm photolysis of vibrationally excited $CH_{3}CFCl_{2}$ and $CHFCl_{2}$. The initial vibrational state preparation increases the $Cl^{\ast}/Cl$ branching ratio, as compared to the nearly isoenergetic one-photon 193 nm photolysis of vibrationless ground state $CH_{3}CFCl_{2}$, implying that it alters the photodissociation dynamics. Possible reasons for this enhancement are discussed.
Description
Author Institution: Department of Chemistry, UC Berkeley; Department of Physics, Ben-Gurion University of the Negev; The Institutes for Applied Research, Ben-Gurion University of the Negev; Department of Physics, Ben-Gurion University of the Negev