Interaction of Rare Gases with Metal Surfaces. I. A, Kr, and Xe on Tungsten

Abstract

The interactions of argon, krypton, and xenon with a tungsten surface have been examined in the field emission microscope. Bombardment by rare gas ions results in a penetration of the lattice and an apparent diminution of the work function; bombardment damage is partly annealed between 79° and 300°K. Adsorption lowers the work function by 1.38 ev for Xe, on saturation at 79°K, 1.18 ev for Kr at 20°K, and 0.87 ev for A at the same temperature. The work function diminishes monotonically with Xe coverage even after more than a single layer is formed. Binding to the surface is structure sensitive—it is strongest around the 100 pole, in the vicinity of the 116 and 130 planes, and weakest at the 111. This sequence of binding energies corresponds to that calculated for different lattice sites on the assumption of dispersion forces. The lowering of the work function caused by adsorption is interpreted as a polarization of the gas atoms by the dipole layer of the metal surface; formally, the interactions at a metal resemble those of the rare gases adsorbed on ionic crystals. Interaction of polarized adatoms and the anti‐parallel field applied for electron emission increases the free energy of the adlayer, bringing about rearrangements particularly important for adsorbed argon. Observations of changes in emission with temperature yield a heat of desorption of 8 kcal/mole for Xe, as well as the following values for the activation energy for surface diffusion: Xe over the (310), toward the (100), E_D=3.8 kcal/mole, over the (111), 1.5*lt;E_DE_D>1.1 kcal/mole and E_D<0.9 kcal/mole, respectively.