The Role of Holes in the Photographic Process

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Abstract
Investigations on the photoresponse of silver halides carried out on large crystals have provided new information on the properties of photoexcited holes and electrons. This information may now be used to describe the mechanism of the photographic process, paying due attention to the fate of holes which until now has been more or less obscure. The experimental evidence now available favours the assumption that in less than a microsecond both holes and electrons, released on the absorbtion of light quanta in a silver bromide grain, are independently and separately trapped. According to the transport mechanism of Gurney and Mott, interstitial silver ions neutralize the trapped electrons, forming in this way the silver atoms of the latent image specks. The temperature dependence of the lifetime and mobility of photoexcited holes indicate that, in the volume of the crystal, trapped boles are also neutralized by mobile point defects of the lattice. Silver ion vacancies are likely to participate in this neutralization process forming complexes with the trapped holes. Diffusion studies of photoexcited holes have indicated an apparent diffusion coefficient of these complexes comparable with the mobility of other ionic defects of the crystal lattice. The hole complexes were found to dissociate on the surface of the crystals, thus establishing a concentration gradient relative to the interior. In this way the bromide equivalent to the photolytic silver formed during exposure is transferred on the surface of the grain. The proposed scheme provides also a natural explanation of the solarization process observed with heavier exposures.

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