M-Series Soft-X-Ray Appearance-Potential Spectroscopy ofYb2O3

The $M_{5,4}$ soft-x-ray appearance-potential spectrum of ${\mathrm{Yb}}_2$ ${\mathrm{O}}_3$ does not show the intense structure characteristic of the lighter rare-earth elements even though a single $4f$ vacancy exists in ${\mathrm{Yb}}_2$ ${\mathrm{O}}_3$. This indicates that the strong $M_{5,4}$ spectral response of each rare-earth element previously measured using the soft-x-ray appearance potential technique is determined primarily by the availability of vacant $4f$ atomic states to accept both the excited $d$ electron and the incident electron.