Crystal-field effects on the apparent spin-orbit splitting of core and valence levels observed by x-ray photoemission

Several anomalous relativistic effects in x-ray-photoemission spectra of metals and binary compounds are reviewed and explained in terms of combined spin-orbit and crystal-field interactions. The apparent spin-orbit splitting does not appear to be enhanced by renormalization effects, which would affect the expectation value of ξ→l·→s itself. The variation of ξ→l·→s with charge state is not large enough to be important in solids. Rather for both outer p and d shells, the splitting appears to be affected by "crystal-field" terms that carry the lattice symmetry. In III-V and II-VI compounds only the tellurium $4d$ shell may have a spin-orbit splitting different from that expected from free-atom data. Howerer the enhancement is small (3%) and consistent with a tetrahedral crystal field. The enhancement of d-shell spin-orbit splitting in Zn and Cd arises from the Y₂ terms in the crystal field because of the large c/a ratio in these lattices. There is no enhancement for Cd in a cubic lattice, while the enhancement in several lattices follows the quadropole coupling constant of ¹¹¹Cd, which presumably also arises from Y₂-symmetry terms. The d-band density of states in fcc Au and Ag is consistent with expectations based on a ξ→l·→s and Y₄ interaction, but band-structure effects so complicate these cases as to preclude such a simple interpretation. The absence of enhanced splitting in valence-shell p shells in Pb and Bi is explained in terms of the higher symmetry of the p wave functions as compared to that of the d electrons and the partial filling of p-derived valence bands in these metals.