Photolytic Effects Relevant to Theories of Solarization

Abstract

By means of a heavy high-intensity latent-image exposure and partial development. tabular silver bromide grains were coated with a layer of very small particles of developed silver. On further exposure in the absence of a halogen acceptor this silver was bleached off the sur(ace of the majority of grains and replaced by internal silver. This constitutes a direct demonstration of the feasibility of the rehalogenation theory of solarization.

On the remaining grains rehalogenation of surface silver also occurs, since the initial uniform distribution of small surface specks is converted into a few large surface particles. usually near the centre of the grain. This process continues up to very heavy exposures, but ultimately conversion to an internal silver distribution occurs. Apparently in such grains the electrons released at the same time as the holes responsible for the bleaching are preferentially trapped at certain of the surface sites, and. together with silver ions. form silver there. As a result there is no net loss of surface silver over a very wide exposure range. This emphasizes that extensive rehalogenation processes can occur without necessarily leading to solarization. The essential requirement for solarization is the existence of internal trapping sites at which the silver formed is less susceptible to halogen attack than that on the grain surface.

The large particles of silver confer developability in a surface-image type developer. From this it is concluded that whatever truth there may be in the coagulotion theory of solarization, it must be something other than mere size of the latent-image specks which is responsible for the loss of developobility.