Electron-excitation energy-dependent features in theM-series x-ray spectrum of lanthanum

$M$-series (14 to 15 Å) x-ray spectra from lanthanum are presented for near-threshold electron excitation. Several structures in the spectra which are not the ordinary characteristic-x-ray lines exhibit very strong intensity and wavelength (energy-position) dependence on the energy of the exciting electrons. Consideration of a sequence of spectra recorded for closely spaced ($\Delta E=1\mathrm{}$ eV) incident electron energies and of the shape of the continuum-limit spectrum from lanthanum for other energies show that the several structures are actually a single feature of the continuous spectrum. This linelike feature exactly follows the energy of the incident electrons and has large intensity resonances in the neighborhoods of the threshold excitation energies of the lanthanum $M_5$ and $M_4$ inner levels. The interpretation offered suggests that the observed spectral variations are due to resonances in the cross section for scattering incident electrons into vacant $4f$ levels and that these resonances exist because of the excitation of sets of excited negative-ion bound states involving $3d$ inner-shell electrons. Consequences of the observed spectral variations for valence-band x-ray spectroscopy, appearance-potential spectroscopy, and continuum-limit spectroscopy are discussed.