Low-energy helium scattering from ordered physisorbed layers of polar molecules

Abstract

Atomic beam diffraction is discussed as a useful method for the structural characterization of mono- and multilayers of gases physisorbed on single-crystal surfaces with some emphasis on experimental techniques. The method is applied to the study of physisorbed phases of polar molecules on the (0001) face of single crystal graphite. On bare graphite CH{sub 3}F forms a commensurate {radical}3 {times} {radical}3R30{degree} structure with all the molecular dipoles parallel to each other while CH{sub 3}Cl and CH{sub 3}Br form uniaxially incommensurate phases with two molecules per unit cell that have the dipoles antiparallel to each other. If Xe-covered graphite is used as a substrate, the unit cell of CH{sub 3}F doubles with the two dipoles becoming also antiparallel, while the study of CH{sub 3}Cl and CH{sub 3}Br is complicated by layer mixing phenomena. HCl on graphite forms an incommensurate closed packed structure, rotated by 30{degree} with respect to the substrate lattice, with a lattice parameter of 3.80 {angstrom}. On the same substrate ammonia shows instead ringlike diffraction patterns indicative of a two-dimensional polycrystalline phase in which the crystalline domains have a large degree of azimuthal disorder.