Model studies on femtosecond spectroscopy of multidimensional photoisomerization and internal-conversion dynamics. A nonperturbative approach

Femtosecond real-time pump-probe signals are evaluated for a model which describes ultrafast internal-conversion dynamics triggered by the twisting of a double bond. Apart from the torsional mode, which is assumed to lead to electronic surface crossing, a vibronically active coupling mode and an additional accepting mode are considered. Using an expansion of the time-dependent state vector in a direct-product basis of diabatic electronic states, free-rotor states and harmonic oscillator states, the time-dependent Schrödinger equation including the light-matter interaction is solved (numerically) exactly. The calculation of nonlinear transient signals from the time-dependent state vector is briefly discussed. As an example, the time- and frequency-resolved stimulated-emission spectrum is evaluated for a simple model of a photochemical funnel. This signal monitors the ultrafast internal-conversion process in real time.