Femtosecond transient absorption and luminescence decay studies of spectrally sensitized photographic emulsions

Abstract

Fast electronic processes taking place immediately after excitation of sensitized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). The fluorescence decays of the J aggregate obtained by SPC measurements were also analyzed as a sum of three exponentials. For the J aggregates of a benchmark dye, a dimethyl-μ-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was attributed to J aggregates of the sensitizer adsorbed on the AgBr microcrystals. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopic measurements. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrystals and monomers of the dye on AgBr surface. The fluorescence decay was found to be independent of the crystal coverage. Prolonged illumination bleached the fluorescence of the fast component, which is consistent with depletion of the J aggregates by electron injection. The decay time of this fast component shows strong dependence on both the structure of the dye and the composition of the silver halide. Excitation in the narrow absorption band of the J aggregates with a femtosecond pump pulse resulted in ground state bleaching followed by a recovery that was analyzed as a sum of three exponentials. The fast (170 fs and 2 ps) components of the recovery are related to the relaxation of bi- or multiexciton states and to singlet-singlet annihilation processes. The remaining component (1 ns) can be attributed to the radiative relaxation of dye molecules not adsorbed on the silver halide crystals. The observed bleaching recovery demonstrated a complex dependence on both the excitation flux and the total illumination dose.