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Abstract
A matched pair of 0.73 μm AgBr core-shell octahedral emulsions, sulphur-plus-gold sensitized either internally or on the surface, was used to study dye desensitization by J-aggregating dyes. Six dyes having a systematically varying electrochemical oxidation potential, Eox, were studied at 20, 40, 60, 80 and 100 per cent monolayer coverage. Except for one of the dyes, there was no desensitization of the internal image. However, several dyes showed desensitization of the surface image, which increased with increasing dye coverage, and which in some cases prevented the measurement of dye desensitization at monolayer coverage (>2 log E speed loss). Dye desensitization occurred at a threshold dye Eox of about 1.1 V (versus Ag/AgCl in acetonitrile) and increased dramatically at lower Eox. Desensitization was about the same or greater at low irradiance as compared with high-irradiance results, and the development time had little, if any, impact on desensitization. It is argued that these results support a desensitization mechanism for these dyes that involves photohole processes and not dye-induced dispersity of the latent image. The photohole processes involve oxidized dye either as a recombination centre for electrons or in the direct oxidation of the latent image or latent subimage. The absence of internal image desensitization suggests that the latter process is the dominant desensitization mechanism.