Sulphide centres on (111) AgBr surfaces: energy levels and computer simulated sensitometry

Abstract

Sensitometric and spectroscopic techniques are used to characterize sensitizer centres produced by sulphur sensitization of AgBr octahedra. Activation energies for long-wavelength sensitivity were independent of thiosulphate concentration for the single-sulphide centres (550 nm), but showed a concentration dependence for the multiple-sulphide centres (700 nm). The achievement of maximum photographic speed is associated with the production of the multiple-sulphide centres. Using activation energies for long-wavelength sensitivity, with the lowest one for the multiple-sulphide centres being taken as closest to reality, an energy level scheme was constructed for the two sulphide centres. The single-sulphide centres are estimated to have an electron trap depth between 0 and 0.1 eV, whereas the multiple-sulphide centres have trap depths between 0.25 and 0.45 eV. Deconvolution of thiosulphate-induced absorption spectra suggested that only 65 per cent of the converted thiosulphtate is photographically active and that the single-sulphide centres are the predominant sulphide species. Computer simulated sensitometry based on these ideas was consistent with experimental sensitometry. Computer simulation of the sensitometry for the optimum thiosulphate concentration required a small concentration of the multiple-sulphide centres. Attempts to simulate the sensitometry of the oversensitized emulsion were less successful. Computer simulations based on the single-sulphide centres acting as hole traps were inconsistent with the experimental data.