Cu/Ag(111) interface studied by surface electron energy-loss fine-structure spectroscopy

The structure of Cu overlayers evaporated on a Ag(111) substrate has been studied with the Cu $M_2$,3 surface extended energy-loss fine structure (SEELFS), recorded with an angle-resolved energy-loss spectrometer using both normal- and glancing-incidence electron beams. For coverages less than 2 monolayers (ML) an expansion of the Cu-Cu nearest-neighbor distance is observed, consistent with epitaxial growth in registry with the larger lattice size of the Ag(111) substrate. Above 2 ML the Cu-Cu distance decreases with increasing coverage until at coverages above 5 ML the Cu-Cu distance is essentially identical to that of bulk Cu. The variation of the low-energy electron diffraction pattern with Cu coverage is consistent with the SEELFS results. The Debye-Waller factors derived from the SEELFS amplitudes indicate increased thermal motion of the Cu surface atoms. The variation of the Debye-Waller factor with incidence angle for a 60-Å-thick Cu(111) film (i.e., bulk Cu) indicates anisotropic thermal motion at the surface, with motion perpendicular to the surface having a larger amplitude than that parallel to the surface.