Time- and frequency-gated spontaneous emission as a tool for studying vibrational dynamics in the excited state

The theory of time- and frequency-gated (TFG) spontaneous emission (SE) spectra is elaborated. The present formulation generalizes previous derivations, clarifies the interrelations between different existing expressions, and establishes the validity of certain commonly assumed approximations. We obtain various explicit expressions for TFG SE spectra, which are suitable for performing actual calculations for nontrivial systems and which allow us to establish generic (that is, model-independent) properties of TFG spectra. The doorway-window picture of temporally and spectrally resolved spectra is further developed. It is shown that, to the leading order in the pump and probe pulses, the TFG SE signal is equivalent to the stimulated-emission contribution to the integral pump-probe spectrum in the case of nonoverlapping pulses. The theory is illustrated for the example of an electronic two-level system with a single Condon-active harmonic vibrational mode that is coupled to a thermal bath. The effect of imperfect time and frequency resolution is studied. It is pointed out that the TFG SE spectrum carries information not only on the strength of the system-bath coupling, but also on the relative magnitude of the bath correlation time.