Comparison of potential calculations with helium diffraction and thermal desorption data ofCF4andCF3Cladsorbed on Cu(110)

Abstract

Helium atom scattering and thermal desorption spectroscopy are used to determine the adsorption properties of the tetrahedral and pyramidal molecules ${\mathrm{CF}}_{4}$ and ${\mathrm{CF}}_{3}\mathrm{Cl},$ respectively, on the Cu(110) surface. High-order commensurate structures with large unit cells are obtained for ${\mathrm{CF}}_{4}$ and ${\mathrm{CF}}_{3}\mathrm{Cl}.$ The thermal desorption spectra reveal a zero-order desorption process for both species. A ``leading edge'' analysis yields monolayer adsorption energies of $161\ifmmode\pm\else\textpm\fi{}6\mathrm{meV}$ for ${\mathrm{CF}}_{4}$ and $260\ifmmode\pm\else\textpm\fi{}6\mathrm{meV}$ for ${\mathrm{CF}}_{3}\mathrm{Cl}.$ Semiempirical potential calculations corroborate these values and show that the shape of the potential energy maps for the two molecules is not very different, leading to tripod-down equilibrium configurations. The calculations also indicate some trends for an orientationally ordered structure for ${\mathrm{CF}}_{3}\mathrm{Cl}$ along the Cu troughs which could be at the origin of a ferroelectric phase.