Optical study on electronic structure of rare-earth sesquioxides

Reflectivity spectra of nine rare-earth sesquioxide (R2O3; R = La, Pr; Nd; Sm, Eu, Er, Tm, Yb and Lu) single crystals have been measured in the photon energy range from 0.5 to 50 eV at 300 K, The fundamental optical absorption has been clarified in connection with the electronic structure of R2O3. The absorption due to an intra-atomic optical transition of R4f --> R5d6s was observed in the same energy region as the charge transfer excitation of O2p --> R5d6s in the energy range of 6-15 eV. The absorption structure of the R4f --> R5d6s transition and the R5p --> R5d6s transition are explained by using the crystal field splitting of the 5d band and the multiplet structure of the 4f(n-1) final state in photoemission process. Fine structures due to the absorption of the R4f --> O3s transition in the energy region of the O2p --> O3s absorption were observed and also can be explained by the 4f(n-1) final state.