Are surface-atom vibrational amplitudes along the normal always larger than in the plane?

Abstract

Temperature- and polarization-dependent surface extended x-ray absorption fine-structure measurements from higher-neighbor shells are used to study the anisotropy of surface vibrational amplitudes. In Cu(100)c(2\ifmmode\times\else\texttimes\fi{}2)-Cl, mean square displacements of both Cl and surface Cu atoms are found to be approximately twice as large within the surface as along the normal. This anisotropy is opposite to that reported in recent experimental and theoretical studies of other systems.