Darkening mechanisms in silver-halide photochromic glasses: Flash photolysis and ESR studies

The darkening and subsequent fading of silver-halide photochromic glass upon irradiation with a Q-switched laser pulse has been investigated. Together with new steady-state optical measurements and ESR observations of photolytic Cu++, the data reveal three major components of the darkening at room temperature: prompt (τ<20 nsec) visible absorption due to Cu++ ions in the silver-halide crystallites, slower-rising (τ?7 μsec) absorption due to silver colloids in the crystallites, and a prompt uv band tail due to ions in the matrix glass. The Cu++ component makes a significant contribution to the photochromic darkening at all excitation intensities, whereas that due to ions in the matrix glass becomes significant only for I≳105 W/cm2. Dependence of fading characteristics on intensity of the excitation pulse is shown to be due in part to variation in the relative magnitudes of the three components. Observations on the growth kinetics and temperature dependence of silver colloid formation are found to be consistent with transport properties of silver-halide thin films, supporting the hypothesis that photochromic action takes place primarily in a noncrystalline surface region surrounding the silver-halide crystallites.