DIPOLE FORBIDDEN F-F EXCITATION IN YTTERBIUM OXIDE

The dipole forbidden f-f excitation in Yb2O3 has been investigated with EELS. The oxide with its large band gap provides in EEL-spectra a reliable separation of the low lying dipole forbidden 4f excitations from other, optically allowed transitions. For primary energies ranging from 1245 eV down to 10 eV we have performed EELS measurements on thin oxide films. In the loss spectra the intensity of the 1.33 eV loss increases drastically with decreasing primary electron energy. Additional EELS measurements on differently prepared films of ytterbium oxide and on Lu2O3 throw light on the nature of the 1.33 eV loss. The origin of a previously reported unexplained 2.5 eV loss is discussed.