Aspects of Bonding in Silver Clusters

Abstract

Publisher Summary The latent image in silver halide photography is usually thought to be a cluster of 4–10 silver atoms and it is one of the most dramatically effective catalysts known to chemistry. As it is small and not amenable to direct characterization by usual experimental methods, it has been the subject of several theoretical investigations. This chapter extends the previous methodologies to (a) refine the results; (b) provide some insight into the mechanism of latent image formation (cluster growth) in the silver halide crystal environment; and (c) establish a kinetic basis for the selectivity of the development reaction—that is, the catalytic action of the cluster. Both in vacuo and in the crystal, three dimensional, defect octahedral cluster geometries are found to be optimum, based on criteria of magnitude of overlap integral, and bond length to bond order ratio. Mechanisms proposed for latent image growth include, in addition to schemes involving atomic condensation, ones based on alternate accumulation of electrons and interstitial silver ions. Selectivity of development of exposed crystals is based on kinetic factors. The probability of electron transfer to clusters of varying size can be estimated by using a Marcus–Hush model in which only the cluster size dependent terms are evaluated. Variation of this probability with cluster size provides the basis for selectivity of development, provided the oxidation of developing agent is rate determining.