NEUTRON SCATTERING AND PHYSISORPTION.

Abstract

Neutron scattering experiments on methane and ammonia adsorbed on a graphitized carbon black are described. Diffraction from adsorbed deuterated methane shows that, at a coverage of 0. 7, it forms an epitaxial layer with a 3** one-half multiplied by 3** one-half structure. Between 50 and 60 K it undergoes a phase transition from two-dimensional solid to liquid. The carbon atom of the methane is estimated to be 3. 3 plus or minus 0. 2 A from the surface. Ammonia-d//3 follows a type III isotherm and at low temperatures desorbs from the surface to form bulk ammonia. This has anomalous melting properties which are shown to be related to adsorption isobars for the system. The detailed interpretation of the results emphasizes the close link between adsorption and heterogeneous nucleation. Quasielastic experiments on the ammonia-graphon system show that the adsorbed ammonia is undergoing translational diffusion on the surface which is much faster than in the bulk. This is attributed to the breaking up of the hydrogen bonded network normally present in the liquid. Show more