Theory of femtosecond pump–probe spectroscopy of ultrafast internal conversion processes in polyatomic molecules

A theoretical description of stimulated-emission pumping with femtosecond laser pulses on complex absorption bands of polyatomic molecules is developed. The strong non-Born–Oppenheimer coupling associated with conical intersections of multidimensional excited-state potential-energy surfaces is included in a fully microscopic manner for appropriate model systems. Additional relaxation effects in the excited-state manifold (lifetime broadening and pure dephasing) are included in the usual phenomenological manner by using the density-matrix formalism. The calculation of the stimulated-emission pump–probe signal by means of density-matrix perturbation theory is briefly discussed and numerically realized using a time-dependent eigenstate-free treatment of the intramolecular dynamics. Some results obtained for a simple model of the S1–S2 conical intersection in pyrazine are presented.