Multiplet effects and breakdown of dipole selection rules in the3d→4fcore-electron-energy-loss spectra of La, Ce, and Gd

Electron-energy-loss spectra of La, Ce, and Gd in the region of the $3d^{10}4f^n\to 3d^{9}4f^{n+1\mathrm{}}$ excitations show doublet structures associated with each spin-orbit component. The relative intensities of the structures vary systematically with excitation energy, the lower-energy component being emphasized at lower primary energies. The effects are interpreted in terms of the multiplet structure of the final state coupled to progressive breakdown of optical selection rules with decreasing energy of the exciting beam.