Review of luminescence studies of mechanisms of spectral sensitization and supersensitization: Chemically sensitized emulsions

The sulfur sensitization of photographic emulsions leads to the formation of silver sulfide clusters on the surface of emulsion microcrystals (MC). The size of the clusters is between 3 and 10 nm. The clusters of small size (Ag2S)(n) are hole traps, the mixed clusters (Ag2S)(p)Ag-k(+) (k < m = 4) are the sensitivity centers, but larger clusters (Ag2S)(p)Ag-m(+) (m 4) are the fog or latent image centers. In addition, emulsion dye can be adsorbed not only by AgBr MC but also by the silver sulfide clusters. We have shown that light absorption by dye can give rise to the cluster luminescence and, vice versa, the light absorption by the clusters can excite the luminescence of adsorbed dye. We also have shown that when a TAI layer is formed between clusters and adsorbed dye, the dye luminescence disappears if light is absorbed by the cluster. These results prove that in the presence of the TAI layer the relocalization of charge carriers from the excited levels of clusters to dye is impossible. Thus, at room temperature, the TAI layer can inhibit the process of desensitization of Type I, and therefore, TAI works as supersensitizer. We have also established, using luminescence studies, the important role of the surface anions of the emulsion MC in the mechanism of spectral sensitization.