Efficient calculation of time- and frequency-resolved spectra: a mixed non-perturbative/perturbative approach

A computational scheme for the calculation of femtosecond pump-probe spectra is presented. The method aims to combine the virtues of a perturbative description of the field-matter interaction with the advantages of a non-perturbative approach. The concept is (i) to evaluate the non-linear electronic polarization Pδ for idealized δ-function pulses through a non-perturbative wavepacket propagation, (ii) to calculate the spectroscopic signals for finite pulses via a convolution of Pδ with the electric fields, and (iii) to use a filter-diagonalization method for the spectral analysis of the polarization. Adopting a two-state three-mode model describing non-adiabatic photoisomerization, the speed-up of the computational scheme compared to a standard non-perturbative calculation is between 1 and 100.