Excitation of symmetric and anti-symmetric stretching motion in the continuum resonance Raman scattering from ABA-type molecules

We theoretically investigate the continuum resonance Raman spectrum from symmetric ABA-type molecules using a model of the pseudo triatomic molecule X(=CH2)I-2. The experimental Raman spectrum of this molecule shows pronounced excitation of the asymmetric stretching mode which is in contrast to what is found for other molecules like H2O. We show within time-dependent quantum calculations that this behaviour, in principle, gives important information about the excited state potential energy surface and in particular about the location of the reaction barrier relative to the ground-state equilibrium configuration. (C) 1998 Elsevier Science B.V.