Comparison of Measured and Calculated Appearance-Potential Spectra for Six 3d Metals

Abstract

We report comparisons of measured and calculated L3-shell appearance potential spectra (APS) for Ti, V, Cr, Fe, Co, and Ni. The measurements were made with polycrystalline specimens and carefully calibrated electron energy scale. APS data have usually been interpreted in terms of a simple one-electron model in which both the excited core electron and the incident electron have final states near the Fermi level. According to this model, a self-convolution is made of the empty density of states (DOS) and this is convolved with a Lorentzian function to represent core-hole lifetime broadening. Comparisons of the shapes of measured and calculated APS yield satisfactory agreement for the heavier elements of the 3d series (Co and Ni) but the model breaks down for the lighter elements (Ti and V). As indicated by the final-state rule, we need to include effects of the core hole in the final state. With the use of a Green's function approach, the APS are calculated from the final-state DOS (i.e., modified by the presence of a core hole). For Ti and V, we indeed find satisfactory agreement with the measured data after including effects of the core hole. Core-hole effects are smaller but still noticeable for Cr and Fe; they are nearly negligible for Co and Ni, consistent with the rule.