Origin and structural sensitivity of surface Auger extended fine structure

Extended fine Auger structure (EXFAS) spectra of Ag(111) and of thin (4-60-Å) Cu films deposited on Ag(111) have been recorded with a retarding grid electron spectrometer. The temperature dependence of the $\mathrm{Ag} N_{23}\epsilon V$ EXFAS of the Ag(111) surface provides strong evidence of a local rather than a long-range origin of the phenomenon. The utility of EXFAS for surface structural studies is demonstrated by its sensitivity to structural changes as a function of the thickness of Cu overlayers evaporated on the Ag(111) substrate. For Cu coverages of 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 substrate. At coverages above 5 ML the Cu-Cu distance is essentially identical to that of bulk Cu. These EXFAS results are consistent with low-energy electron-diffraction and surface-extended energy-loss fine-structure results on the same system.