Xe adsorption on a C₆₀ monolayer on Ag(111)

Low-energy electron diffraction (LEED) experiments and grand canonical Monte Carlo simulations were carried out to study the adsorption of Xe on a substrate composed of a monolayer of C₆₀ molecules on a Ag(111) surface. LEED adsorption isobars indicated that the adsorption occurs in steps, with the Xe initially adopting a structure having the same unit cell as the C₆₀. Isosteric heats corresponding to the first two steps were measured to be 234±8 and 204±14 meV, respectively. For the simulations, the interaction potential of Xe with the composite substrate was modeled as the sum of two parts: the Xe-Ag part was computed using an ab initio van der Waals potential that varies as an inverse-distance cubed and the Xe-C₆₀ part was computed using a spherically averaged C₆₀ potential [ E. S. Hernandez et al. J. Low Temp. Phys. 134 309 (2004)]. The resulting adsorption potential is highly corrugated, with the most attractive sites located in the threefold hollows between the C₆₀ molecules, forming a honeycomb array. The simulations (at temperatures ranging from 55 to 90 K) show that these attractive sites are filled first, followed by adsorption in two types of secondary sites, where a competition exists due to steric hindrance. The thermodynamic properties of film growth obtained in the simulation are in good agreement with the experiment.