Electron-excited 4d 10 -*4d 9 4/ transitions in clean La metal and tetravelent Ce0 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.PACS numbers: 32.30.Rj, 78.70.Dm Dipole selection rules are well known to apply only in the high-incident-energy, low-momentumtransfer regime for electron-excited transitions, but it is difficult to isolate optically nonallowed transitions for electron scattering in solids. At low and intermediate primary energies in reflection mode it is crucial to be able to distinguish surface features, which are emphasized as the mean free path of incident electrons is reduced, from intrinsic energy dependence in the scattering cross sections. In this Letter we account for spectacular variations in the core electron-energy-loss spectra of elemental La and tetravalent Ce by successive breakdowns of optical selection rules as the energy of the exciting primary electrons is reduced: These give the first confirmation of pseudo selection rules in the intermediate energy regime.There has been intense interest in photoexcitation studies of 4d-~nf,ef transitions in atoms and solids with 54 < Z ^ 72 in the last decade. 1 " 6 Ba, La, and tetravalent Ce all undergo 4d 10 -~ 4d 9 nf,±f transitions characterized by two sharp, but weak, peaks below the ionization threshold, and a broad resonance beyond. The resonance is l P x in character, but the actual wave function of the excited electron involves complex configurational mixing of bound and continuum Hartree-Fock average/ wave functions. 7 ' 8 The two sharp peaks, corresponding to highly localized trapping in an "inner well" are the 3 P X and 2 D X states, which are weakly photon excited through a small admixture of 1 P 1 character due to spin-orbit interaction. The preresonance structure of these systems is less complex than for the other rare-earth systems because of their simpler two-electron multiplet structure.Core electron-energy-loss (CEELS) spectra were measured in approximately specular reflection geometry with a high-resolution concentric hemispherical analyzer under UHV conditions, as in previous studies on rare-earth systems. 9 The full width at half maximum of the elastically reflected primary beam was measured to be ~0.5 eV over the full range of primary energies used. Spectrometer calibration was made against a clean Au reference sample. Clean metal films (with very low contamination levels as monitored by Auger electron spectroscopy) were evaporated from a tungsten coil onto a Mo substrate. Ce was oxidized to the tetravalent state by admitting oxygen into the chamber from a gas dosing system, and then maintaining a stationary oxygen pressure of 10" 8 Torr during the recording of the spectra. For incident ...