The Structure and Properties of Adsorbed Layers by X-Ray and Neutron Scattering.

Available from UMI in association with The British Library. Requires signed TDF. X-ray and neutron diffraction, together with low angle neutron scattering and incoherent elastic neutron scattering, have been used to study the structure and properties of halomethane monolayers and methane multilayers adsorbed on graphite. Structures of all the adsorbed halomethane monolayers, and the phase transitions that have been found to occur in these layers, have been described. These experimentally determined structures have been used to assess the influence of dipolar ordering relative to other factors such as close -packing requirements and halogen-halogen interactions. In fitting trial structures to the experimental diffraction patterns, the problem of uniqueness has been greatly reduced by the combination of both X-ray and neutron data. The method of assignment of the experimental patterns has been developed by a modification of the technique of Hull and Davey for two variable problems. Although limited to rectangular unit cells, this modified Hull-Davey technique is applicable to the great majority of 2D structures, such as triangular commensurate and incommensurate lattices. In the study of multilayer methane on graphite MAT, there is clear evidence for the onset of melting of the methane below the bulk triple point and the existence of a single solid monolayer above. More detailed analysis of the scattering from the solid methane has indicated incomplete wetting below the triple point. If the thickness of the wetting film is taken to include the fluid methane then the system appears to undergo triple point wetting. The conclusions in this study are subject to the nature of the powdered substrate employed and evidence of capillary condensation has been presented.