Abstract
The mechanism of IR multiple-photon excitation through the dense manifold of vibrational states, usually called the quasi-continuum, of a vibrationally excited molecule is an unresolved issue in laser chemistry. Due to the clearly resolved spectral identity of the GH vibrational modes in propylene (C3H6), it is a particularly good molecule in which to study the influence of discrete molecular structure on the IR multiple-photon excitation process. 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 higher-frequency CH mode on the adjacent carbon. This result implies that (1) 3-μm excitation involves nonlocalized energy deposition into several spatially discrete molecular features and (2) certain combinations of vibrational modes may be useful to promote efficient multiple-photon absorption.
© 1983 Optical Society of America
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