Photophysics of Sulfur-Containing Centers on AgBr Surfaces

Abstract

Silver halide microcrystals, the basic element in photographic products, are commonly reacted with labile sulfur-containing compounds to increase their photographic efficiency (speed). Such AgBr microcrystals have been examined with computational and photophysical techniques. Density functional and lattice-relaxation computations have given estimates of the optical transition energies and the electron- and hole-trapping abilities of a number of small sulfur-containing clusters that may be deposited on, or buried in, an AgBr surface. These data indicate that single sulfides, whatever their structure, are not deep traps for electrons or holes. Disulfides, in general, can trap holes and certain structures; for example, a disulfide at a double-kink site can trap electrons. Computations indicate that single sulfides substituted for a halogen in the (001) surface and charge compensated with an interstitial silver ion in the surface or in the bulk do not trap electrons; neither do disulfides in the bulk. A single sulfide substituted for a bromide at a positive kink and compensated with an interstitial silver ion appears to be capable of initiating the formation of small, stable silver clusters (Ag n , n 4) that become latent image, the centers that eventually lead to a visible photographic image. Photographic data from sulfur-sensitized AgBr microcrystals indicate that the optimum formal sulfur level is about 10 000 S atoms/μm 2 . Radio-frequency photoconductivity measurements show that deposited sulfur centers trap photoelectrons. The optimum and higher levels of sulfur deposited on the surface of AgBr microcrystals produce an absorption band (∼525 nm) and associated low-temperature luminescence (∼725 nm). The luminescence lifetime behavior and ODMR data clearly indicate that this sulfur-related absorption and emission is due to a donor-acceptor processes. ODMR spectra obtained by monitoring the intrinsic AgBr luminescence (590 nm) have a resonance that is assigned to a Ag 2 + center. This Ag 2 + species is thought to arise from a silver dimer associated with a sulfur species at the surface.