Interaction between cesium and graphite for use in the study of surface phenomena

Surface diffusion has been hypothesized as the fast mode of an unusual fast-slow, two-mode transport process that has been observed in recent diffusion experiments with cesium in graphite. An interaction potential between a cesium atom and a graphite surface is obtained in order to study this surface diffusion by computer simulation (molecular dynamics method). At low surface coverage, the interaction between cesium atoms can be ignored so that the motion of only one cesium atom need be followed, albeit in a very complicated potential energy surface. Cesium is spontaneously ionized by graphite, so that the interaction of cesium with the graphite surface contains pair-wise Cs$sup +$ - C terms (valence, induction, and dispersion forces) as well as an image-charge model of the bulk electrostatic interaction. All parameters but the strength of the repulsive Cs$sup +$ $sup -$ C force are obtained by theoretical estimates, while this last parameter is determined by requiring that the adsorption Cs$sup +$ - C bond length be the same as observed in cesium-graphite lamellar compounds. Results indicate that the adsorption energy for a pit in the graphite surface of one to five missing carbon atoms is not greatly increased over that for the perfect surface (the one-atom hole is slightly repulsive compared to the perfect surface). For the hexagonal six-atom pit, the adsorption energy increases dramatically from about 120 kcal/mole for the perfect surface to about 200 kcal/mole and remains essentially constant for larger holes. Results for potential energy barriers to migration indicate that a cesium ion on a perfect graphite surface moves along the surface like a free particle at temperatures above 1000$sup 0$K. Thus, truly diffusive or random-walk behavior at such temperatures requires the presence of defects in the graphite surface. (auth