COMPUTER-SIMULATION STUDIES OF SILVER CLUSTER FORMATION ON AGBR MICROCRYSTALS

Photoinduced silver cluster formation on AgBr microcrystals is modeled by a nucleation-and-growth process in competition with electron-hole recombination. Nucleation involves the trapping of an electron at a si lver atom, followed by the migration of an interstitial silver ion to yield a stable two-atom cluster. The growth stage is the enlargement o f the two-atom cluster by additional trapping of electrons and capture of interstitial silver ions to yield the photographically developable latent image. The simulation of silver cluster formation is accomplis hed with a Monte Carlo procedure which randomizes the sequence of even ts but weights each event according to a probability determined by the simulation parameters. Events followed are photon absorption, trappin g and detrapping of electrons and holes, silver atom formation and dec ay, nucleation, growth, and recombination. Prior to the simulations, t he microcrystal size and shape must be specified, along with the densi ty, depth, and trapping radii of electron traps. Other parameters incl ude the diffusion coefficient of the electron and hole, the recombinat ion radius, the time for capture of interstitial silver ions by trappe d electrons, and the lifetime of the silver atom. Simulation over an e nsemble of independent microcrystals is done with a transputer-based p arallel processor. Exemplary results showing the dependence of silver cluster formation efficiency on trap depth and trap density as produce d by chemical pretreatment of the microcrystals are included. For the high-irradiance condition studied, the trends show that as the cluster size increases, the required trap depth and trap density for maximum formation efficiency decrease.