Theoretical description of secondary emission reflecting ultrafast nonadiabatic isomerization

The fluorescence and resonance Raman emission of molecular systems undergoi ng ultrafast nonadiabatic isomerization is considered. Adopting a multidime nsional model of vibronic coupling, explicit expressions for the continuous -wave spectra as well as for the time-resolved fluorescence spectrum are de rived. To facilitate a simple eigenstate-free evaluation of the spectra of interest, various approximations are introduced. Employing a two-mode, two- state model of the photoisomerization of retinal, the validity of the appro ximations is studied numerically. The time-dependent eigenstate-free scheme is shown to provide an accurate approximation to the exact fluorescence sp ectrum. On the other hand, the well-known assumption that the emission stem s from a vibrationally relaxed excited electronic state is not applicable i n the case of ultrafast nonadiabatic isomerization processes under consider ation.