Electronic processes in the silver halides, silver atoms and clusters : an ESR study
During bandgap exposure, the initial steps of latent image formation in the silver halides are dominated by the shallow trapping of free carriers (electrons and holes). Electron Spin Resonance studies of irradiated silver halides in powder, single crystal and de-gelled emulsion formats revealed signals assigned to electrons shallowly trapped at intrinsic sites. These signals were only observed at temperatures below ca. 50K. Experiments were carried out to probe the nature of these sites which appeared to be concentrated at structural dislocations in the lattice. The shallowly trapped electron signal was detected in silver bromide, silver chloride and a range of binary halide silver halide powders. The observation of this signal was accompanied in many cases by the simultaneous observation of a Cyclotron Resonance transition of free conduction band electrons. In addition doping with divalent lead ions capable of acting as "extrinsic" shallow traps resulted in an enhancement of the lifetime of the trapped electron ESR signal. The radiolytic formation of silver atoms and charged clusters (dimers, trimers and tetramers) in frozen solutions of silver perchlorate in methanol, were studied by ESR, this work includes the probable identification of the novel pentameric unit, Ag54+. The formation and detection of clusters of this type in the silver halide matrix is also discussed, together with the detection of latent image centres at liquid helium temperatures, where ionic motion does not occur.