Ag_2 molecules in silver halide crystals have three functions in the mechanisms of photographic sensitivity. (1) They introduce donor centers which are dissociated into Ag_o~+ ions and electrons by the annihilation of photons or of excitons of polarization waves and exited dye molecules. (2) They provide latent image growth nucleus precursors at which Ag_o~+ ions and conduction electrons compine to give Ag_3 clusters. These adsorb a silver ion from an adjacent site to give Ag_4~+ laten image growth nuclei. Positive holes are created by the annihilation of photons and excitons at these and larger Ag_n~+ acceptor centers. (3) The Ag_2 molecules then provide effective trpas for positive holes giving Ag_2~+ centers which dissociate. The silver atoms released in both (1) and (3) have a lifetime of about 1 sec at room temperature before dissociation.
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