Optical absorption studies of the volume photolysis of large silver chloride crystals

Abstract

The exposure-induced volume darkening of large AgCl crystals has been studied at very low exposure levels, in both pure crystals and in crystals containing known concentrations of cationic impurities. It has been found that the volume photoproduct that can be produced may be controlled by such impurities, of which cuprous ion is a striking example. The cuprous ion apparently provides deep hole traps at room temperature, thereby allowing the photoprocess to proceed with unit quantum efficiency until these traps have been filled. In the purest crystals grown, the volume darkening saturates at optical densities, measured at the peak of the absorption band, of less than 0·001 per cm thickness. The addition of 5×10 16 cuprous ions per cm 3 allows the volume darkening to proceed to optical densities 1000 times higher. The shape of the exposure-induced absorption band is approximately constant over this range of densities, suggesting that there is no marked change in the size and distribution of the photoproduct centers as their number increases. In addition to photochemical effects, certain cationic impurities (particularly the elements commonly classed as Group VIIIA) produce impurity absorption bands in the silver halides whose intensity and shape may depend on the valence state of the impurity.