Vibrational coherence in ultrafast electron-transfer dynamics of oxazine 1 in N,N-dimethylaniline: Simulation of a femtosecond pump-probe experiment

Ultrafast intermolecular electron transfer in the oxazine/dimethylaniline system has been investigated with a time resolution of better than 20 fs. The time- and frequency-resolved differential transmission spectrum as well as the frequency-integrated pump-probe signal are reported. A detailed theoretical simulation of the spectroscopic data is performed, adopting a microscopic quantum mechanical model for the material system. The model includes electron-transfer coupling, intra-state electronic-vibrational coupling to two vibrational modes, as well as vibrational dissipation. The laser-matter interaction is included in a nonperturbative manner, thus fully including strong-field and pulse-overlap effects. The relationship between the electronic and vibrational relaxation dynamics of the molecular system and the structures observed in the transient spectra is discussed in detail. It is found that the population probability of the optically excited electronic state exhibits a biexponential decay characteristics. The rapid (≈50 fs) initial decay is caused by the few vibrational modes strongly coupled to the ET reaction. The subsequent slower decay on a time scale of a few hundred femtoseconds reflects the vibrational cooling of the hot photoproducts. In particular, the simulations reproduce the coherent beating with a period of 55 fs observed in the experiment, which corresponds to an intramolecular vibrational mode that drives the ET process.