Soft-x-ray appearance-potential spectra of rare-earth elements are interpreted in terms of strong resonance interactions between the incident electrons and atomic 4/ states. These interactions account for both the large intensity and the threshold shifts of the measured signal. This resonance persists in La, Ce, Pr, arid Nd when present as only a few atomic percent in a NiCr alloy reflecting the local character of the interaction.

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We have measured the M5, M4 Soft X-Ray Appearance Potential Spectra (SXAPS) of the lanthanide series rare earths. Lutetium and ytterbium do not show the huge signal response characteristic of the other rare earths. This is to be expected since both metals have a closed 4f shell containing 14 electrons. More striking is the lack of significant SXAPS response for ytterbium oxide (Yb2O3) in which a single 4f Yb vacancy is known to exist. The contribution to this spectrum from the local density of states is more than an order of magnitude smaller than the intense peaks seen in the other rare earths. The M5 and M4 spectra of the other rare earths are split into a series of lines. All of the intense SXAPS lineshapes are explained in terms of a resonance in the bremsstrahlung emission. The excitations involve a resonant scattering interaction in which both the incident electron and a d core electron scatter into the atomic 4f level.

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