Core-electron excitation edges in metallic Ni, Cu, Pt, and Au

Abstract

The shapes of core excitation edges with ${E}_{B}<150$ eV are reported for Ni, Cu, Pt, and Au. The spectra are interpreted as arising from the interference of excitations of core electrons into conduction-band states of $d$ symmetry with excitations of $d$ electrons into continuum states of $f$ symmetry well above the vacuum level. In all cases excitation of core electrons into $d$ conduction-band states hybridized into the $\mathrm{sp}$ conduction bands is important, but in the metals Ni and Pt, which have partially filled $d$ bands, the excitation of core electrons into unhybridized $d$ states outweighs the contributions of the hybridized $d$ states at the Fermi edge. The line shapes due to the spectral densities resulting from excitations into the unhybridized part are accurately described by Fano interferences due to configuration interaction between a spin-orbit component of a core-hole state and the $d$-electron continua of partial waves indexed by the same total angular momentum. Off-diagonal interactions between the core-hole spin-orbit components appear to be unimportant, as are contributions from many-body screening effects. No theory is available for the interference involving the hybridized $d$-band states. The energies of the edges in electron-energy-loss or x-ray-absorption measurements are shown to be the same within experimental error (about 0.2 eV) as the corresponding values of fully screened core-electron binding energies referenced to the Fermi level. These latter values were determined by Wertheim and coworkers, who fitted the threshold peaks in x-ray-photoemission spectra with a many-body screening theory. It is shown that such agreement between the core-excitation thresholds measured by electron-energy loss or by x-ray absorption and the fully screened binding energies implies that the screening of core holes in Ni and Pt is due almost entirely to the $d$ conduction electrons.