Determination of Energy Levels in AgBr Emulsion Crystals from Optical Measurements

Abstract

The width of the forbidden energy gap and the position of the top of the valence band cannot be determined as precisely for AgBr at room temperature as for many ionic crystals. Some choices for these parameters, which are necessarily arbitrary, are given here to provide good agreement with the variety of optical and photographic measurements made with small AgBr crystals. Changes of the energy levels with temperature, iodide incorporation, and the presence of surfaces were measured. Surface states located at lattice ions are too shallow to act as electron traps or to be detected in our optical absorption experiments. Luminescence of pure AgBr was investigated at 77°K. The luminescence centre is at an electron trap which is 0.75 eV below the bottom of the conduction band. This centre, which is stable when formed by exposures at 77°K, is the same as the unstable latent image centre at room temperature investigated by Webb and is thought to be a single electron trapped at two silver ions. The stable latent image centre at 300°K has an electron trap depth of 1.20 eV. which is precisely the Fermi level in bulk silver.