Nucleation and growth of rare-gas crystals

Abstract

An in situ electron microscope study has been made of the nucleation and growth of crystals of Ar, Kr and Xe grown by vapour deposition on to cooled graphite and amorphous carbon substrates. In particular, the saturation density of nuclei has been measured as a function of temperature and gas arrival rate. An important feature of this experiment is that the relevant parameters for these systems, such as surface diffusion and cluster energies, are known theoretically. Consequently, it has been possible to attempt an absolute check on nucleation theory. The theory is found to hold within a factor less than 3 for Kr and Xe, a t all temperatures and arrival rates studied. Despite good agreement with the theory using the parameters for nucleation taking place on the bare graphite surface it is almost certain that growth of three-dimensional crystals does not occur until a strongly bound monolayer of adsorbed gas atoms approximately covers the substrate. The possible ways in which this feature may be understood are explored. In the case of argon the measured saturation densities (though less accurate) seem to be significantly below the theoretical values, and possible reasons for this are given.