The growth, structures and thermodynamics of Xe adsorbed on Pb(111) were studied with low-energy electron diffraction (LEED). Measurements of equilibrium adsorption isobars indicate layer-by-layer growth for at least two layers in the temperature range 60-90 K, and an isosteric heat of adsorption of 191 ± 10 meV for the first Xe layer and 158 ± 20 meV for the second Xe layer. The monolayer has an incommensurate hexagonal structure with a lattice parameter similar to that found in bulk Xe, and its thermal expansion coefficient was measured to be 9 ± 1 × 10 −4 K −1 . The average overlayersubstrate spacing was determined from a constant-momentum-transfer analysis of the LEED intensities to be 3.95 ± 0.10 Å. Vibrational information was obtained from measuring the decay of the diffraction intensities as a function of temperature. The perpendicular root mean square displacement of the Xe atoms at 50 K is 0.12± 0.01 Å, giving an effective Debye temperature for adsorbed Xe of 37 ± 2 K. The corresponding vibrational energy for the perpendicular mode of Xe is 3.2 ± 0.3 meV.

The physisorption of N2 molecules has long been a model system of molecular adsorption. We present a low-energy electron diffraction (LEED) study of the adsorption structures and thermodynamics of monolayer N2 on Pb(1 1 1). The results indicate that the monolayer structure has a triangular incommensurate center-of-mass lattice, and that the N2-substrate interaction is weaker than that observed on other metal surfaces. The N2 monolayer undergoes a phase transition between an orientationally ordered phase (low-temperature) and an orientationally disordered phase at a temperature of 20 K. Potential energy and quasiharmonic calculations indicate that the weak N2-Pb(1 1 1) interaction is the main contributing factor for the difference in orientational order of incommensurate N2 monolayers on Pb(1 1 1) and other similar metal surfaces.