Low-temperature phases of Xe on Pd(111)

Abstract

Low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD) were used to determine the adsorption properties and the structural phase diagram of Xe on the clean Pd(111) surface. The binding energy of a Xe atom to the substrate is found to be quite strong $(320\ifmmode\pm\else\textpm\fi{}10\mathrm{meV}),$ whereas the effective Xe-Xe lateral interaction is extremely weak ${(V}_{\mathrm{lat}}\ensuremath{\ll}10\mathrm{meV})$ and most likely attractive on the clean Pd(111) surface. Within the monolayer regime, a phase transition sequence from a commensurate $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase to a hexagonal incommensurate phase (HI) and, finally, to a hexagonal incommensurate rotated phase (HIR) is observed. The resulting phase diagram is compared to theoretical predictions and other rare-gas adsorption systems. The Xe adlayer structure turns out to be extremely sensitive to the presence of impurities, such as H and CO. This sensitivity can be rationalized by the weak Xe-Xe lateral interaction.