COMPUTER-SIMULATION OF PHYSICAL ADSORPTION ON STEPPED SURFACES

A stepped surface has been modeled by taking stacks of graphite basal planes truncated to form an effectively infinite set of steps of constant width and height. The gas-solid interaction potential used exhibits a deep trough at the bottom of each step and a large barrier to free translation at the top. Four widths were studied, ranging from 25 to 8 angstrom (step heights were held fixed at 3.4 angstrom). The adsorption of krypton on these surfaces was simulated at temperatures of 110, 130, and 150 K and coverages ranging from 1/2 to 2 nominal monolayers. It was found that the presence of the steps produces strong ordering in these layers. In addition to structural information, as measured by the local densities of these systems, the coverage dependence of the energy of adsorption is also reported. The pronounced heterogeneity in the gas-solid interaction does not give the expected decay in the adsorption energy curves. Reasons for this behavior (and other results) are suggested. In addition to the thermodynamic data, the molecular dynamics data have been used to extract diffusive fluxes of molecules jumping from one step to another. The coverage and temperature dependence of these fluxes are presented and discussed.