Bond Selective Excitation of Molecules

Abstract

Bond selective photochemical reactions have provided an elusive target for laser photochemists. Early claims of mode selectivity in multiphoton dissociation have largely been discredited [1]. Although the laser pumps a single vibrational mode, energy is transferred to other modes more rapidly than the molecule is excited above dissociation thresholds by sequential absorption of photons. Single photon excitation appears more promising for mode selective vibrational photochemistry [2]. All of the energy is absorbed at once. The strongest high overtone absorptions come from the most anharmonic excitations, local modes (LM), in which a single bond is excited [3–5]. Small differences in C-H stretching potential functions are resolved. In the example presented here the chemically equivalent but spatially inequivalent in-plane and out-of-plane C-H oscillators are resolved in the high overtone spectra for the methyl groups of gaseous propane. The ability to excite a single C-H oscillator in a particular steric position suggests a highly selective form of vibrational photochemistry [2, 6, 7].