PRE-RESONANCE RAMAN INTENSITIES

Abstract

A series expansion for the sum over the vibrational levels of an intermediate electronic state, which occurs in the dispersion theory of the Raman effect, is presented. The convergence properties of the series, as a function of electron-vibration coupling strength and exciting photon energy, have been studied for many model systems which span the range between rigid and diatomic-like molecules. Both Condon type fundamental and overtone transitions and vibronically induced transitions are considered. The single most important result is that the leading term of the above series represents a good approximation to the exact sum, up to ca. 3000 cm$^{-1}$ from resonance with the intermediate electronic state for Raman fundamentals. It should be possible, therefore, to study excited state geometries by pre-resonance Raman spectroscopy. The theory also shows that the Condon and vibronic coupling mechanisms exhibit very different dependences on the exciting photon frequency. The corresponding series for Raman overtones has poorer convergence properties. In particular, it is shown that for the general case frequency changes in excited states are as Important as geometry changes inducing overtone intensities.