Low-energy electron diffraction study of the surface of thin crystals and monolayers of normal paraffins and cyclohexane on the Ag(111) crystal surface

Abstract

The surfaces of the normal paraffins (C4–C8) and cyclohexane in the form of thin single crystal films (102 Å) condensed upon an Ag(111) single crystal surface have been studied using low-energy electron diffraction (LEED). The physical adsorption of monolayers of these molecules was also studied on the Ag(111) surface. The data were compared with similar data obtained from experiments on the Pt(111) surface (Ref. 1) to yield the following results : n-Octane condenses on either substrate with its unreconstructed (101̄) plane parallel to the substrate. Cyclohexane similarly condenses exposing its (001). n-Heptane multilayers on either substrate give the same LEED pattern, but the surface unit mesh is different from any of the expected planes of the n-heptane crystal structure. n-Hexane and n-pentane multilayers form pseudomorphic crystal structures, that are dependent upon the structure of the substrate. Physically adsorbed monolayers of n-octane, n-heptane, and n-hexane adsorb in ordered structures with their molecular axes parallel to each other and the Ag<11̄0≳. Cyclohexane adsorbs in a (9×9) structure in a hexagonal array with its molecular plane parallel to the Ag(111) surface. The generally lower temperatures necessary to order monolayers on Ag(111) indicate that unsaturated hydrocarbons are less strongly adsorbed on this surface than on the Pt(111).