On the vibronic theory of resonance Raman scattering

Resonance Raman excitation profiles of totally symmetric vibrational modes are investigated using a model that analytically includes the complete subspace of Franck-Condon active vibrations associated with each intermediate electronic state. This model is used to fit data obtained in resonance with the Soret band of cytochrome c. The excitation profiles are asymmetric and peak distinctively to the blue of the Soret absorption maximum. Large damping factors and/or inhomogeneous site distributions by themselves cannot account for the observed data. The theoretical results imply that the excited state lifetime associated with the Soret band of ferrocytochrome c has a lower limit on the order of 50 fs and that, compared to the ferrous form, the ferric cytochrome has a larger x-y splitting and a shorter lifetime.In constrast to the large multidimensional heme system, we also present the results of a simple model calculation applicable to smaller molecules. A two dimensional subspace is explored, where Raman Franck-Condon (RFC) and Franck-Condon (FC) factors are calculated for different potential energy surface parameters. Under certain conditions the RFC based scattering profiles of one vibration are strongly coupled to the FC based behaviour of the other vibration. Rather complex profiles are then predicted even for the simple two- dimensional case.