Femtosecond time-resolved ionization spectroscopy of ultrafast internal-conversion dynamics in polyatomic molecules: Theory and computational studies

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A framework for the theoretical description of two-pulse time-resolved ionization spectroscopy of ultrafast excited-state dynamics of polyatomic molecules is developed. The radiation–matter interaction as well as intramolecular couplings in the excited-state manifold are treated nonperturbatively by solving the time-dependent Schrödinger equation. The numerical solution is based on a discretization of the ionization continua which becomes particularly efficient for ultrashort laser pulses. With this method converged computations of ionization signals become possible even for complex molecular systems. Computer simulations are performed for a model system representing three-dimensional non-Born–Oppenheimer excited-state dynamics on conically intersecting potential-energy surfaces (the S1 and S2 surfaces of pyrazine). The dependence of the observable time-resolved ionization signals (total ion yield as well as photoelectron spectrum) on the properties of the laser pulses (carrier frequency and pulse duration) is explored. It is demonstrated that ultrafast electronic decay processes as well as coherent vibrational motion in excited states can be monitored by pump–probe ionization with suitable pulses. The dependence of the time-resolved ionization signals on properties of the cation (ionization potentials and potential-energy surfaces) is also discussed.