Structure and dynamics of rare-gas layers on pt(111)

Abstract

The structural behavior of rare-gas monolayers on the (111)-surfaces of Ag and of Pt is radically different. On Ag the adlayers show a uniform behavior, they are all “floating” incommensurate. This corresponds to a laterally perfectly flat holding potential; the substrate structure is evidenced only in the alignment of the hexagonal adlayers with the substrate. In contrast, on Pt the adlayers exhibit a great variety of phases and phase transitions. In particular for Xe-adlayers, the phases range from “locked” commensurate and high order commensurate to various “floating” incommensurate ones. Temperature and coverage changes induce first and second order as well as symmetry breaking and conserving transitions between these phases. This fascinating plurality of phases and phase transitions is due to a delicate balance between the adatomadatom interaction and the corrugation of the Pt holding potential. This corrugation is thus, in the case of Pt, by no means negligible; its average value - as deduced from direct measurements - is larger than 30 meV, i.e. 10 % of the holding potential. The lattice dynamics of these rare-gas layers on Pt(111) accessible via the perpendicular modes as observed so far in He inelastic scattering appears to be much less sensitive to the corrugation of the holding potential. All the features predicted by assuming a completely uncorrugated holding potential, i.e. almost dispersionless monolayer mode, leakage into the bulk around \\ ̄ gG and hybridization with the substrate Rayleigh wave have been confirmed in detail by high resolution He Spectroscopy of Ar, Kr and Xe adlayers on Pt(111). No frequency difference between the Einstein oscilations of the commensurate and incommensurate Xe-layers could be detected so far showing that, in spite of the corrugation, the perpendicular curvature of the holding potential does not vary significantly along the surface. Substantial differences are expected for the in-plane adlayer modes.