Mechanism of condensation. I. Direct determination of coefficients for condensation of molecular beams of sodium chloride on oriented single crystals of NaCl

Abstract

Coefficients for condensation of a composite beam of NaCl(g) and Na₂Cl₂(g) on the 100 face of oriented single crystals of sodium chloride have been measured over a range of crystal temperature from 185–700°K, over a range of 50 in dislocation density at the crystal surface, as a function of molecular beam flux, as a function of deposition time, and as a function of the angle of impingement between the molecular beam and the crystal surface. In one set of experiments a thin layer of silicone oil was spread over the surface of the crystal prior to molecular beam deposition. Crystal surfaces have been studied by chemical etching and microscopic examination. It is concluded that dislocations at the surface of the oriented crystal serve as intrinsic nucleation sites for molecules in a two dimensional, mobile adsorbed surface state. The density of molecules in the mobile surface state depends on the flux of the molecular beam. At sufficiently low crystal surface temperature, centers of nucleation are generated spontaneously on the surface as a result of formation of molecular clusters in the adsorbed surface state. At low surface temperature with an adequate beam flux, the controlling factor in the condensation of sodium chloride appears to be the spontaneous formation of molecular clusters of a critical size from which nucleation and growth can occur. As the surface temperature increases there is a decrease both in the number of intrinsic nucleation sites (surface dislocations) and in the tendency of mobile molecules on the surface to cluster and form spontaneous centers of nucleation. However, as the temperature increases the rate of diffusion along the surface increases. The results of this work suggest that the rate of surface diffusion is a dominant factor in the mechanism of condensation of sodium chloride at high crystal surface temperatures. In the intermediate range of surface temperature all of the previously mentioned factors are in competition as rate determining steps in a complicated mechanism of condensation.