Femtosecond spectroscopy of ultrafast nonadiabatic excited-state dynamics on the basis of ab initio potential-energy surfaces: The S₂ state of pyrazine

A theoretical description of femtosecond time-resolved optical spectroscopy of isolated polyatomic molecules is outlined. Our ultimate goal is the characterization of the system response on a microscopic level, that is, time-dependent quantum dynamics. Employing a simplified model Hamiltonian which includes the most relevant electronic states and vibrational modes, the intramolecular quantum dynamics is treated numerically exactly. The model Hamiltonian is constructed, via a Taylor expansion in terms of ground-state normal coordinates, from ab initio potential-energy surfaces. Additional phase relaxation effects (optical pure dephasing) due to weakly coupled modes have been included in a phenomenological manner using the density-matrix formalism. The radiation-matter interaction, derived from ab initio transition-dipole-moment functions, is treated in perturbation theory (up to third order for the polarization). The implementation of this approach is demonstrated for the S₂(ππ*) state of pyrazine, which is strongly vibronically coupled to the lower-lying S₁(nπ*) state. Basic aspects of ultrafast non-Born-Oppenheimer dynamics on multidimensional conically intersecting potential-energy surfaces and its detection by femtosecond pump-probe spectroscopy are discussed.