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 $\alpha $, the condensation coefficient, are obtained which in all cases lie between 0$\cdot $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.