UNIMOLECULAR PROCESSES IN CH$_2$OH BELOW THE DISSOCIATION BARRIER STUDIED BY O-H STRETCH OVERTONE EXCITATION
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Date
2005
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Ohio State University
Abstract
Infrared spectra in the fundamental, the first, and the second OH-stretch overtone transitions ($\nu_1$, 2$\nu_1$, 3$\nu_1$) were obtained by using double resonant ionization via the 3p$_z$ Rydberg state. The rotationally well-resolved spectra show that intramolecular vibrational redistribution (IVR) is restricted even in the 3$\nu_1$ energy region around 10 490 cm$^{-1}$, which is 960 cm$^{-1}$ above the thermochemical threshold for dissociation to H + CH$_2$O. The spectrum in the third overtone region (4$\nu_1$) at 13 600 cm$^{-1}$, which is 3 950 cm$^{-1}$ higher than the dissociation threshold, was obtained by monitoring atomic hydrogen photofragments. The overtone spectrum is partially rotationally resolved with a line width of 1.1 cm$^{-1}$. The long dissociation lifetime ($>$ 6 ps) and the similar O-H stretch anharmonicties (obtained from the Birge-Sponer plot) for CH$_2$OH (91 cm$^{-1}$) and other molecules with much higher O-H dissociation energies indicate that the examined energy region is below the barrier to direct O-H bond fission. The lack of signal from deuterium fragments in CD$_2$OH excitation shows that tunneling through the barrier followed by direct O-H bond fission is the dominant pathway following 4$\nu_1$ excitation. Theoretical calculations predicted that dissociation via isomerization to the methoxy radical, CH$_3$O, has a barrier lower by 1 000 cm$^{-1}$ - 2 000 cm$^{-1}$ than that for direct O-H bond fission, but this channel appears to be unimportant in the 4$\nu_1$ region. Ongoing work involves accessing higher energy regions that may exceed the reaction barriers. Research funded by DOE.
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Author Institution: Department of Chemistry, University of Southern California, Los Angeles, CA 90089