Sulphide centres on (111) AgBr surfaces: the effect of thiocyanate on electronic properties

Abstract

Sensitometric and spectroscopic techniques are used to study the effect of sodium thiocyanate addition to sulphur-sensitized AgBr octahedra. Thiocyanate increased the speed of an oversensitized emulsion, causing it to have a speed comparable to the optimally suphur-sensitized emulsion at high irradiance and greater speed at low irradiance. Thiocyanate decreased the absorption by sulphide centres at shorter wavelengths (500–650 nm), but increased the absorption at longer wavelengths. The long-wavelength sensitivity (> 500 nm) was also increased by thiocyanate, whereas the activation energy for this response was reduced. These results are interpreted with a model that assumes that thiocyanate interacts with multiple-sulphide centres to lower their ground state energy level. This interaction reduces the degree of free-hole/trapped-electron recombination which occurs at these centres and which is responsible for the over-sensitization effect. A computer simulation based on this model gave simulated reciprocity failure data which were qualitatively consistent with the experimental trends. The effects of thiocyanate on absorption are interpreted as a change in extinction coefficient.