Europium: electron excitation and decay of 4d core holes

Abstract

Electron emission and electron energy loss spectra of atomic europium excited by electron impact with primary energies (140 - 400 eV) in the region of the 4d giant resonance are presented. The observed emission peaks are identified and the strong energy dependence of their relative intensities near the 4d giant resonance is investigated. The emission spectra show seven intense structures in the energy range 50 - 150 eV which arise from different decay channels after 4d subshell excitation. Auger and direct recombination peaks can be identified by comparing our electron-excited spectra with photon-excited spectra. For primary energies above 300 eV the emission spectra are very similar to spectra excited with high-energy electrons . Near the 4d ionization threshold the relative intensities of the peaks change drastically with decreasing primary energy. Within the region = 180 - 140 eV the Auger peaks decrease and direct recombination channels become dominant. Under these conditions the giant resonance in the excitation step is no longer visible. These near-threshold spectra give insight into the decay mechanism of the 4d giant resonance of Eu in electron scattering. The dominance of direct recombination channels indicates the enhancement of discrete 4d - 4f excitations with impact energies near the resonance.