LOCALIZED EXCITON-STATES IN SILVER-HALIDES

Recent advances in exploring the properties and nature of various localized exciton states in the silver halides AgBr and AgCl are surveyed. Knowledge about these states comes primarily from optical studies which, besides conventional high resolution absorption and luminescence, include selective excitation and resonant Raman spectroscopy combined with time-discrimination of the spectra and hydrostatic pressure effects. Three distinctly different types of exciton states are considered and results for these are presented and analyzed. Doping with divalent metal and chalcogen impurities, such as Cd2+ and S2-, gives rise to weakly localized states with localization energies of several millielectron volts (E1 < 9 meV and < 17 meV in AgBr and AgCl, respectively). They are interpreted in terms of excitons bound to neutral and positively charged donors of type (D0, X) and (D+, X). Excitons bound to intrinsic donors as well as non-linear effects suggested to be due to donor-bound biexcitons are also reported. Isoelectronic impurities (I- in AgBr, Br- and I- in AgCl) produce localized states with E1 up to several tens to hundreds of millielectron volts. These excitons are described in a simplified configurational coordinate model suggesting lattice relaxation to be essential in stabilizing the exciton states. High power excitation in these systems causes additional luminescence also associated with bound biexciton recombination. Finally, new results concerning exciton states in mixed silver halides AgBr1-xCx (0 ≤ x ≤ 1) are presented. Related to the disorder, besides free excitons, different types of localized exciton states are observed and discussed within a consistent model describing exciton localization in these alloys. © 1990.