Ab initio strong-pump strong-probe doorway-window simulation protocol

We have developed an on-the-fly methodology for the simulation of strong-field pump-probe (PP) spectra using semiclassical trajectory methods. This methodology generalizes the weak-field semiclassical doorway-window (DW) simulation protocol to pump and probe pulses of any intensity. The strong-field DW functions are defined by explicit analytical formulas which are fully determined by the molecular transition dipole moments and electronic energies evaluated along the semiclassical trajectories, so that the weak-field and strong-field PP signals can be simulated at roughly the same computational cost. The developed methodology has been applied for the simulation of strong-field PP spectra of pyrazine. The obtained results vividly demonstrate the advantage of using strong pump and (notably) probe pulses for increasing the information content of PP measurements. In particular, the strong probe pulse significantly enhances the weak transitions originating from the lowest excited electronic states of pyrazine, thereby rendering the nonadiabatic wavepacket dynamics in these weakly allowed states visible.