Transient hole-burning and time-resolved fluorescence spectra of dye molecules in solution: Evidence for ground-state relaxation and hole-filling effect

A picosecond transient hole-burning (THB) spectroscopy has been performed for organic dyes in solution. The THB spectra of rhodamine 640 have been found to show a time-dependent spectral change. This phenomenon corresponds to the solvent relaxation effect observed in the time-resolved fluorescence (TRF) spectrum. Although TRF spectrum is related only to the excited-state relaxation, THB spectrum is affected by both ground- and excited-state relaxations. Comparing with the TRF spectrum measured under the same exciting energy, we have clarified the presence of the ground-state relaxation. Further, the THB spectrum of styryl-8 gives an antihole between well-separated two holes corresponding to ground- and excited-state contributions. This may originate from the hole-filling effect due to the nonadiabatic relaxation from the nonequilibrium excited state to the ground state. The analysis based on a configuration coordinate model gives an overall understanding for these phenomena.