A study of copper ions in silver-halide photochromic glasses

Measurements of optical absorption, kinetics, electron-spin resonance, and flash photolysis, as well as x-ray fluorescence and wet-chemical analysis, have been applied to a series of silver-halide photochromic glasses having precisely controlled variations in silver and copper concentrations as well as in the size of the precipitated silver-halide microphases. These combined experimental data have enabled us to clarify the manner in which copper enters the microphases and the mechanisms whereby it affects their photochromic properties. Most notably, we have succeeded in determining the site and valence-state distribution of copper ions, produced by the separation of the microphases. We have demonstrated, for example, that copper enters the silver-halide phase only as cuprous ions, which then act as the dominant hole traps in the photochromic darkening process. Optical absorption of these cuprous ions in the silver-halide phase was also found to make a major contribution to the absorption edge observed in the undarkened photochromic glasses. The increase of thermal fade rate with increasing copper content is discussed in light of these new experimental results.