5s-electron excitation energies in the rare-earth metals

Abstract

We report calculations of $5s$ electron excitation energies for the rare-earth metals. Aside from the lack of a direct estimate of correlation effects, the method is quite similar to that we have used in calculating $4f$ level positions which are in excellent correspondence with experiment. Our aim here is to assess the applicability of the technique to closed-shell core states, and we have chosen the $5s$ levels in view of the simplicity of their x-ray photoelectron spectra (XPS). Within the approximation of a completely screened final state, in which the atomic cell having the $5s$ hole is electrically neutral, the results are in good qualitative and semiquantitative agreement with XPS measurements. The disparity between theory and experiment is about 4 eV across the lanthanide series, and its sign is consistent with correlation effects. Available free-atom information only permits us to estimate the correlation energy associated with $5s$ excitation in ionized Cs; the estimate is of the proper sign and magnitude to bring our results into accord with experiment. We also assess the effect of the complete screening assumption by estimating excitation energies corresponding to ionized final-state cells; we find that the presence of a screening electron lowers the $5s$ binding energy by 4-5 eV.