Multiplet Splitting of Metal-Atom Electron Binding Energies

Abstract

X-ray photoelectron spectroscopy (XPS) is used to measure splittings of metal-atom electron binding energies in both inorganic solids and gases. These splittings are due to the various possible multiplet states formed by coupling a hole in a metal-atom subshell to an unfilled valence subshell. Splittings are observed in various solids containing $3d$-series atoms. In particular, the $3s$ binding energy is split into a doublet with as much as 7.0-eV separation between the two components. The instrumental resolution is ∼ 1.0 eV. $3s$ splittings are exhibited by inorganic compounds containing Mn and Fe, as well as by Fe metal, Co metal, and Ni metal. Theoretical predictions are in good agreement with experiment, provided that the effects of covalency in chemical bonding are taken into account. For Fe metal, the $3s$ splitting is identical both above and below the Curie point. The $3p$ binding energies of these solids also appear to show multiplet effects, but the interpretation of these results is less straight-forward. The $2p$ binding energies in Mn${\mathrm{F}}_2$ are broadened by at least 1.3 eV, and this is shown to be consistent with multiplet splitting. XPS results for gaseous monatomic Eu also indicate the presence of multiplet splittings. The two components in the $4d$ photoelectron spectrum are found to have an intensity ratio in disagreement with observed ratios for neighboring atoms with filled valence subshells. Also, the width of the $4f$ photoelectron peak above the instrumental contribution can be explained in terms of multiplet effects.