Residence Time Analysis of Rare-Gas Atoms Trapped At A Solid Surface

Abstract

Recently a model based on an attractive square well and impulsive repulsive potential between a gas and a solid surface has been employed to estimate the fraction of gas atoms trapped in the potential well at the solid surface. A gas atom was considered trapped if it transferred enough energy to the solid (as judged by the thermal accommodation coefficient) to fall into the potential well, i.e., to have negative total energy where the zero of energy is assumed to be the infinitely separated gas atom and solid with the gas atom at rest. The criterion for gas trapping when considering experimental scattering data has been the assumption that the trapped gas is emitted from the surface with a random (cosine) distribution. To assess the possibility that rare-gas atoms may fall into the interatomic well with the metal surface (i.e., be trapped according to the definition of Ref. 1) and still be re-emitted from the surface in a nonrandom distribution, some estimates of surface residence times for the gas should be made. If such a phenomenon as suggested above were to occur, then the calculated trapping probability of the square well model would be expected to be larger than those found experimentally, as has been observed to be the case for krypton and xenon interactions with a tungsten (110) surface. The idea that a gas particle may execute several vibrations at a surface and still not reach thermodynamic equilibrium is not a new one. In this paper the pertinent theory describing this effect is presented, and empirical calculations estimating the magnitude of the effect are presented.