X-ray photoemission and Auger energy shifts in some gold-palladium alloys

Abstract

We report the results of x-ray photoemission (XPS) and Auger electron spectroscopies in the alloy ${\mathrm{Au}}_{\mathit{x}}$${\mathrm{Pd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ for various concentrations. By comparing the predictions of the excited-atom version of the quasiatomic model with the dilute Au results, we show that charge-transfer corrections to the Au Auger kinetic-energy shift, \ensuremath{\Delta}${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{i}\mathit{j}\mathit{k}}^{\mathrm{Au}}$, are small. This result supports the validity of the relation \ensuremath{\Delta}${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{i}\mathit{j}\mathit{k}}^{\mathrm{Au}}$\ensuremath{\simeq}-\ensuremath{\Delta}${\mathit{E}}_{\mathit{F}}^{\mathrm{Au}}$==${\mathit{E}}_{\mathit{F}}^{\mathrm{Au}}$-${\mathit{E}}_{\mathit{F}}$(x) [where ${\mathit{E}}_{\mathit{F}}^{\mathrm{Au}}$ and ${\mathit{E}}_{\mathit{F}}$(x) are the Fermi energies in pure Au and the alloy, respectively] and permits determination of the order of magnitude of the charge transfer from Auger measurements alone (i.e., hundredths of electrons/atom). Such small charge transfer seems to hold also for the Ag and Cu atoms in Pd-Ag, Au-Ag, Pd-Cu, and Au-Cu. Furthermore, these results are consistent with interpretation of the Au Auger parameter shift, \ensuremath{\Delta}${\ensuremath{\xi}}^{\mathrm{Au}}$, as well as with analysis of \ensuremath{\Delta}${\mathrm{\ensuremath{\omega}}}^{\mathrm{Au}}$, the valence-electron contribution to the Au XPS core binding energy; we predict that \ensuremath{\Delta}\ensuremath{\xi}\ensuremath{\simeq}\ensuremath{\Delta}\ensuremath{\omega} for noble metals in general. This analysis yields the result that the dilute Au d-valence-electron occupations change \ensuremath{\simeq}-0.2 electrons/atom, approximately the negative of the sp change from isomer shifts. The Pd results are consistent with small charge transfer and a small gain of d electrons.