Kinetics of Silver Cluster Formation in Silver Halide

Abstract

In silver halide photographic grains, silver cluster formation is a major means of relaxation of the induced photocharge. In these systems, the maximum photocharge is reached in less than 1 microsecond. The decay process is in general not adequately described by simple first or second order decay kinetics. A combination of first plus second kinetics conforms well with the experimental data. From measurements of photocharge decay of this type, it is possible to evaluate the first order, k 1 , and the second order, k 2 , decay constants for emulsion coatings which had undergone systematic variation in degree of reduction sensitization and chemical sensitization. Based on these determinations and on photographic evaluations of the same series of emulsion coatings, a mechanism is formulated involving k 1 and k 2 , which explains the variation in photographic efficiency as a function of reduction or chemical sensitization. In this mechanism, k1 is the rate constant for the reaction of a trapped photoelectron with an interstitial silver ion. The product of this reaction is in turn a trap for another photoelectron. A continuation of these processes leads to the formation of small, stable silver clusters (latent image centers), which serve as catalytic sites for further reduction of the silver halide grain during the development stage of photography.