Band signatures in the low-energy-electron reflectance spectra of fcc metals

The elastic reflectance spectra for low-energy electrons (0-30 eV) of fcc metals show characteristic structure related to critical points in the energy bands. The (111) surfaces of Au, Ag, Pd, Cu, Ni, and Al have been investigated and all have spectra consisting of a sharp reflectance peak followed by a broad, more intense peak at higher energy. The broad peak correlates with the forbidden gap formed by the (222) Bragg reflection. The sharp peak is insensitive to the angle of incidence and lies very near the ${\Gamma }_2^{\prime }$ (${\Gamma }_{25}^{\prime }$ in Al) energy. From a pseudopotential model we demonstrate the connection between the spectra and the band structure. Good quantitative agreement of the peak positions with previous band calculations and photoemission data is obtained and the behavior with angle is understood qualitatively. Ultrathin metals (a few atomic layers) and thick films deposited on single-crystal or amorphous substrates produce similar reflectance spectra. This technique can be applied to the study of metal surfaces and to the initial growth stages of epitaxial films.