Exchange Forces Between Neutral Molecules and a Metal Surface

Abstract

A Heitler-London treatment of the exchange part of the mutual energy of a neutral atom and a metal is developed. The resulting interactions are evaluated on the basis of a simplified model of the metal and lead to a convenient and simple expression for the total exchange energy, In Section III this expression is applied to the interaction of H? and He with metals where it is found to represent a repulsion. By adding this exchange interaction to the attractive van der Waals interaction between the molecule and the metal, a potential curve of the usual type possessing a minimum is obtained for each system. The depths of these minima are compared with observed heats of van der Waals adsorption. A discussion is given in an appendix of the effects of some of the simplifications and approximations employed.

Theoretical approach to the interaction of a single water molecule with mercury

“…The repulsion energy term, L7rep, is obtained from Pollard's equation. 30 It is admitted that the four electrons of the O-H bonds behave as highly repulsive ones, involving a relatively large radius for the maximum electronic charge probability and consequently a relatively high O-H polarizability. The four electrons of the remaining hybrid orbitals also contribute to repulsion and the latter effect increases as they are located closer to the metal surface.…”

Atomic helium scattering and diffraction from solid surfaces

“…Finally, it should be emphasized that the gas-surface potential well depth for the interaction of helium with the various metal surfaces was assumed equal to the constant value of 300 cal mol-', that is one conduction electron per metal surface atom was used in connection with the theory of Pollard (1941). When experimentally measured initial heats of physical adsorption become available (ideally as a function of crystallographic orientation) those values for D may be used in equation 4 in order to calculate 6.…”

“…The experimental determination is made using the Debye-Waller attenuation of low e n e r g electron diffraction peaks. The value of the gas-metal surface potential well depth, D, is taken as 300 cal mol-' by assuming one conduction electron per atom in the formulation of Margenau and Pollard (1941) and Pollard (1941). This is the same value used by Beeby (1971).…”

“…However, at a surface these normally bonding electrons may occupy surface orbitals and greatly increase the very small number of holes usually associated with the d band in bulk transition metals (eg de Haas-Van Alphen experiments have placed an upper limit of l o F 4 holes in the d band for platinum (Mueller et a1 1970, Andersen 1970). The available surface orbitals are also a function of crystallographic orientation so that the appropriate number of 'conduction electrons' to use in the formulation of Margenau and Pollard (1941) and Pollard (1941) may not even be exactly constant for a given metal (Weinberg and Merrill 1972d). Evidence for this assertion comes from the measured changes in surface potential for xenon physically adsorbed on various crystallographic orientations of Ni (Baker and Johnson 1972) and the experimental binding energies of argon, krypton and xenon on various crystal planes of W (Engel and Gomer 1970).…”

Effect of Spatial Dispersion upon Physisorption Energies: He on Metals

“…4,159 Because of the fundamental nature of chemical reactions at surfaces and their important role in such practical problems as catalysis 1 and oxidation, the understanding of these reactions is recognized as one of the major goals of surface science. Essential to this objective is the complete description of the adsorption process, i.e., an understanding of both chemical and physical adsorption.…”