James R. Klein

Substrate-mediated dispersion interaction between adsorbed atoms and molecules☆

Abstract The approximation of G. Vidali and M.W. Cole, Surface Sci. 110 (1981) 10, for the polarization potential coefficients is extended to the coefficients in the McLachlan theory of the substratemediated dispersion interaction between physisorbed atoms and molecules. Values for the parameters entering in the approximation are given for several adsorbates and substrates. The values for the strength coefficients obtained with the approximate formulae are in good agreement with values obtained by detailed numerical integrations.

The interaction between noble gases and the basal plane surface of graphite

Abstract Calculations are presented of the laterally averaged interaction between a noble gas atom and the basal plane surface of graphite. The proposed model is seiempirical, based in part on a previous successful treatment of the He/graphite case. The calculated potentials have minima which are 5–10% closer to the surface than earlier values. The results are consistent with LEED and SEXAFS data.

Many-body interactions in physisorption: Nonadditivity of film and substrate contributions to the holding potential

Abstract Calculations are presented of non-pair-additive contributions to the interaction energy of a probe atom with a physisorbed monolayer film and substrate. The Axilrod-Teller-Muto triple dipole energy is summed for triplets composed of probe atom, monolayer atom, and substrate atom. The lateral average, over probe atom positions, of the sum vanishes. A similar result and a nonvanishing term from a higher many-body interaction are derived also from the McLachlan theory of the dispersion interaction of two atoms in the presence of a continuum substrate.

Is there a "universal" potential of physical adsorption?

Abstract Evidence is presented concerning the hypothesis that all physisorption pontetials have a common form. Input comes from atomic and molecular beam scattering. LEED and thermodynamic data and theoretical calculations. We conclude that a smaller force constant and softer repulsion occur for light gases interacting with insulators than with graphite or metals.

Tunneling from electron bubbles beneath the surface of liquid helium

Schoepe and Rayfield have measured the lifetime Τ for escape of electrons from bubbles beneath a liquid He surface. We compute Τ using the tunneling Hamiltonian method of Bardeen. If accepted bubble parameters are assumed, Τ is found to be approximately two orders of magnitude smaller than the measured value. Agreement with experiment can be obtained if the radius is taken to be 50% larger than currently believed. We consider the effects of diffuseness of the bubble boundary, diffuseness of the liquid-vapor interface, and polarizability of the bubble. A discrepancy remains which may be explicable in terms of surface deformation when the bubble is very close to the surface.

Adsorption interactions and the two-dimensional critical temperature

Thermodynamic properties of films depend on both the interactions within the adsorbate and those between the adsorbate and the substrate. We investigate these in relation to the problem of the two-dimensional (2D) liquid–vapour critical point. Our focus is the value of the critical temperature Tc for physically adsorbed atoms and methane molecules. The calculations employ perturbation theory, using the equation of state of a 2D Lennard-Jones (LJ) fluid as a reference. Explicitly included effects are deviations of the two-body potential from LJ shape, substrate modification of the interaction (leading to different 2D and 3D potentials) and three-body interactions within the adsorbate. Each of these contributes a measurable shift of Tc from the value computed with LJ theory and the 3D potential. The final predictions are in semiquantitative agreement with experimental data for gases on graphite.

Liquid-vapor critical point of physisorbed films☆

Abstract A tutorial overview is presented concerning the problem of the liquid-vapor critical point of physically adsorbed films. Emphasis is placed on the behavior of atoms and spherical molecules adsorbed on graphite. The critical temperatures are interpreted in terms of a theory based on “realistic” interactions. These deviate from Lennard-Jones form and include the substrate-mediated contribution. The role of the periodic potential in suppressing a critical point is discussed briefly.