Growth and evaporation of crystals from the vapour phase

The rates of growth and evaporation of single crystal faces of benzophenone [110], benzophenone [111], polycrystalline benzophenone and rhombic sulphur [111] have been measured by means of a torsion balance technique. The method aims at providing correct information concerning the temperature of the crystal face and the pressure of vapour in contact with the face; special attention has also been directed to reducing the partial air pressure to a point at which diffusional resistance becomes negligible. Growth and evaporation curves have in each case been obtained over a range of supersaturations, and for several temperatures. At high supersaturations the curves of growth and evaporation as a function of supersaturation are straight lines: at lower supersaturations both growth and evaporation lines become slightly curved in the sense tending to reduce the slope as the curves pass through the origin. At high supersaturations values of α, the condensation coefficient, are obtained which in all cases lie between 0.1 and unity. The question whether the shape of the curves can be explained in terms of Frank’s theory of crystal growth is discussed. A short discussion is also given of the theory of momentum transfer to the surface for the case where the probability of adsorption is a function of the velocity of approach of the molecule.