15min:

J. WEI, B. KARPICHEV, H. REISLER, Department of Chemistry, University of Southern California, Los Angeles, CA 90089.

Infrared spectra in the fundamental, the first, and the second OH-stretch overtone transitions ( ₁, 2 ₁, 3 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 ₁ energy region around 10 490 cm^-1, which is 960 cm^-1 above the thermochemical threshold for dissociation to H + CH₂O.

The spectrum in the third overtone region (4 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 CH₂OH (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 CD₂OH excitation shows that tunneling through the barrier followed by direct O-H bond fission is the dominant pathway following 4 ₁ excitation. Theoretical calculations predicted that dissociation via isomerization to the methoxy radical, CH₃O, 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 ₁ region.

Ongoing work involves accessing higher energy regions that may exceed the reaction barriers.

Research funded by DOE.