DETECTION OF ULTRAFAST MOLECULAR-EXCITED-STATE DYNAMICS WITH TIME-RESOLVED AND FREQUENCY-RESOLVED PUMP-PROBE SPECTROSCOPY

A unified theoretical description of various forms of time- and frequency-resolved stimulated-emission pump-probe (PP) spectroscopy of molecules is outlined. The expressions for the integrated and the dispersed PP signal are derived in the limit of an optically thin medium. In the limit of ultrashort pulses, the connection between the PP signals and the appropriate time-dependent observables of the molecular system are established. The PP signals obtained with probe pulses of finite duration are expressed in terms of the simpler quantities pertaining to the ultrashort limit. The apparent paradox that it seems possible to achieve simulataneously high time and frequency resolution is discussed. It is shown that the dispersed PP spectrum carries information on electronic population dynamics, coherent excited-state vibrational motion, as well as electronic dephasing processes. It may thus be difficult to interpret in general. The integrated PP signal, on the other hand, possesses a particularly simple interpretation in the limit of ultrashort pulses: It directly reflects the electronic population dynamics of dipole-allowed excited states. The general conclusions are illustrated by computational results for a model system representing multidimensional vibrational motion and ultrafast internal conversion in excited singlet states of pyrazine.