Observation of Intermode Coupling in the 3μm IR Multiple Photon Excitation of Propylene

The mechanism of IR multiple photon excitation through the dense manifold of vibrational states, usually called the quasicontinuum, of a vibrationally excited molecule is one of the unresolved issues in the field of laser chemistry. The effects of deuterium substitution on propylene IR multiple photon excitation are used to identify the vibrational modes leading to efficient excitation. Optoacoustic energy deposition data show that for propylene, 3 μm multiple photon excitation occurs most efficiently at the methyl group, and furthermore that efficient methyl group excitation requires the CH group on the adjacent carbon. Thus 3 μm multiple photon excitation of propylene, while involving energy deposition directly into several spatially discrete intramolecular groups, is found to be enhanced by specific intramolecular couplings. Implications of this result for mechanisms of IR multiple photon excitation are discussed.