Selection Rules in Electron-Excited4d→4fTransitions at Intermediate Incident Energies

Electron-excited $4d^{10}\to 4d^{9}4f$ transitions in clean La metal and tetravelent Ce${\mathrm{O}}_2$ show systematic deviations from those observed in x-ray absorption. At intermediate primary energies simple selection rules allow a limited range of new singlet excitations, while triplet excitation becomes very prominent as the primary energy approaches threshold. With this technique a complete multiplet analysis of atomiclike core excitations has become possible for the first time.