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

Fast electronic processes taking place immediately after excitation of sens itized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). Th e 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 ben chmark dye, a dimethyl-mu-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was at tributed to J aggregates of the sensitizer adsorbed on the AgBr microcrysta ls. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopic measurement s. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrysta ls and monomers of the dye on AgBr surface. The fluorescence decay was foun d to be independent of the crystal coverage. Prolonged illumination bleache d the fluorescence of the fast component, which is consistent with depletio n of the J aggregates by electron injection. The decay time of this fast co mponent shows strong dependence on both the structure of the dye and the co mposition of the silver halide. Excitation in the narrow absorption band of the J aggregates with a femtosecond pump pulse resulted in ground state bl eaching followed by a recovery that was analyzed as a sum of three exponent ials. 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 annihil ation processes. The remaining component (1 ns) can be attributed to the ra diative relaxation of dye molecules not adsorbed on the silver halide cryst als. The observed bleaching recovery demonstrated a complex dependence on b oth the excitation flux and the total illumination dose. (C) 1999 American Institute of Physics. [s0021-8979(99)04303-0].