Structure and dynamics of the Cu(001) surface investigated by medium-energy ion scattering

The structural and vibrational properties of the Cu(001) surface at temperatures between 107 and 685 K have been determined by medium-energy ion scattering (MEIS). At ≤305 K, a top-layer contraction (Δ${\mathit{d}}_{12}$) of -2.0% is found. Above 520 K the contraction may be slightly larger at -2.3%. Small second-layer expansions (≃+1%) exist. Monte Carlo modeling of the data shows that the vibrations of the Cu(001) surface atoms are strongly enhanced over those of the bulk, with ratios of surface to bulk vibrational amplitudes increasing from ∼1.6 at 107 K to more than 2.0 at 685 K. Analysis of multiple-scattering geometries which preferentially probe either the in-plane or the near-perpendicular vibrational amplitudes shows anisotropic surface vibrations at all temperatures examined. Lattice vibration correlation effects have been included in the analysis. The in-plane vibrational amplitude changes from 11% to 37% larger than the perpendicular vibrational amplitude over the temperature range studied. The effect on the results of isolated, diffusing Cu adatoms on the surface at high temperature is considered. Due to the influence of the methods of data reduction upon the results, an analysis of the ion energy loss in the high-resolution MEIS energy spectra is presented.