Analysis of transient-absorption pump-probe signals of nonadiabatic dissipative systems: 'Ideal' and 'real' spectra

We introduce and analyze the concept of the "ideal" time and frequency resolved transient-absorption pump-probe (PP) signal. The ideal signal provides the most direct link between the "real" (measurable) PP signal and the material system dynamics. The simulation of PP signals involves two steps. (i) The ideal signal, which exhibits perfect time and frequency resolution, is calculated. For this purpose, the probe pulse is replaced by an auxiliary continuous-wave pulse. (ii) The real signal is obtained by the convolution of the ideal signal with the appropriate time- and frequency-gate function, which depends on the envelope of the actual probe pulse. This concept has been used to simulate integral and dispersed PP signals for a model system exhibiting nonadiabatic and dissipative dynamics. The ideal signal is computed with the two-pulse equation-of-motion phase-matching approach which has been extended to take excited-state absorption into account. We demonstrate how the ideal signal, an object exhibiting the features of moving wave packets as well as stationary spectra, is related to real signals detected with short (good temporal resolution) or long (good frequency resolution) probe pulses.