Appearance-potential study of the4fstates of holmium

The soft-x-ray appearance-potential spectra (SXAPS) of $M_{4,5}$ and $N_{4,5}$ levels of rare-earth metal holmium (${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$) have been obtained in the 130-1400-eV range with a simple, nondispersive spectrometer which measures the derivatives of the total x-ray fluorescence of the anode surface as a function of bombarding electron energy. The spectrum represents the differential excitation probability of $3d$ and $4d$ core levels and mirrors the self-convolution of the density of empty $4f$ sates in Ho. The $M_5$ peak shows two distinct components, 1.8 eV apart, followed by two satellites at 5.7 and 8.7 eV and a secondary peak at 33.5 eV above the $M_5$ threshold. The $N_{4,5}$ spectrum is more complex, consisting of two prominent peaks. The more intense peak appears at 10.2 eV above the $N_{4,5}$ threshold in agreement with theoretical calculations. The spectral features have been discussed in terms of the existing excitation state and satellite line theories. The binding energies of the $M_4$, $M_5$, and $N_{4,5}$ levels of Ho were, however, found to be consistently a few eV lower than those found by most other techniques.