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Intramolecular dynamics of substituted acetylene compounds

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Abstract

Intramolecular vibrational energy redistribution (IVR) continues to be a subject of wide interest to chemical physics. Most previous work has found that IVR is rapid (on the time scale of 10 psec or less) whenever the density of rovibrational states is of the order of 100 states per wavenumber or greater. We have been studying IVR in a series of substituted acetylene compounds using optothermal detection of the acetylenic C-H stretch or its first overtone absorption spectrum in a cold (few degrees K), collimated molecular beam. This gives a resolution of 10 MHz, which allows us to study IVR on time scales up to 30 nsec. Our results show that these molecules, when their density of states is high enough, undergo statistical IVR (energy spreads over all vibrational modes) but with lifetimes of 200 psec or greater. This is clearly long enough to allow bimolecular collisions before reactions, and thus possible mode specific chemistry, even in the gas phase at modest pressure. The observed stability of the lifetimes suggests that the relaxation rate is determined by the acetylene chromophore and not by the modes that ultimately receive the vibrational energy.

© 1991 Optical Society of America

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