Abstract

Raman spectra obtained in conditions of resonance with electronic excitations are often dominated by transitions that are very weak in the normal off-resonance spectrum. These enhanced bands can be used to determine the symmetry of the resonant excited electronic state if this state gives rise to a forbidden transition. In this case the most strongly enhanced bands correspond to binary combinations and overtones of the vibronic promoting modes active in the transition. The use of this method to determine the symmetry of excited states of butadiene and benzene is presented.^1,2 The most strongly enhanced vibrations associated with resonance with allowed electronic transitions are those that have large projections along the geometry change associated with excitation. Recent results for a series of substituted ethylene species are presented. Resonance with excitations that have both allowed and induced vibronic character gives rise to characteristic fundamental transitions of the promoting modes. The case of substituted benzene molecules is discussed.

© 1986 Optical Society of America