Xe adsorption on aC60monolayer on Ag(111)

Abstract

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 ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$. Isosteric heats corresponding to the first two steps were measured to be $234\ifmmode\pm\else\textpm\fi{}8$ and $204\ifmmode\pm\else\textpm\fi{}14\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, 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 $\mathrm{Xe}\text{\ensuremath{-}}{\mathrm{C}}_{60}$ part was computed using a spherically averaged ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ molecules, forming a honeycomb array. The simulations (at temperatures ranging from $55\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) 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.