Relative Fermi energies in copper-palladium alloys

We report the results of measurements, by x-ray excited Auger spectroscopy, of the shifts of the Cu ${\mathit{L}}_{2,3}$ ${\mathit{M}}_{4,5}$ ${\mathit{M}}_{4,5}$ energy spectrum in the alloy ${\mathrm{Cu}}_{\mathit{x}}$ ${\mathrm{Pd}}_{1\mathrm{-}\mathit{x}}$ for various concentrations. Comparison with self-consistent coherent-potential-approximation calculations of ${\mathit{E}}_{\mathit{F}}^{\mathrm{Cu}}$-${\mathit{E}}_{\mathit{F}}$(x), the negative of the alloy Fermi energy relative to that of Cu, manifests a remarkable agreement in magnitude and concentration dependence. By resorting to a simple version of the quasiatomic model of excited-state electron screening, we demonstrate the physical origin of the various contributions to the Auger shift and show under what conditions the noble-metal Auger shift can be identified with the negative of the relative Fermi energy. Experimental evidence indicates that the charge transfer is very small (of the order of hundredths of electrons per atom) in a number of alloys containing noble metals. The nonlinear systematics exhibited by the data emphasizes the importance of considering a large number of alloys.