Mobility of Electrons and Positive Holes in Spectral Sensitization and in Desensitization of the Photographic Process by Dyes

Abstract

The method of displacement, by pulsed electric fields synchronized with the exciting radiation, of the photographic images caused by electrons and positive holes, respectively, has been applied to the study of spectral sensitization of latent-image formation by dyes, and to the desensitizing effect of certain dyes, in thin, macroscopic, sheet crystals of silver bromide. In the spectrally sensitized process, free electrons are produced, with the same mobility as those excited by self-absorption by the silver bromide, but no mobile holes have been observed in the interior of the crystal in this process. The inability of the positive holes associated with the spectrally sensitized process to migrate into the interior is consistent with theoretical concepts regarding this process, but hole trapping by the dye may contribute to the observed nonmobility. Electron trapping by readily reducible dyes is demonstrated, consistent with a mechanism of photographic desensitization in which these desensitizing dyes act as alternative electron traps to the sensitivity centers in the silver halide crystal.