The simple model and its application to interpretation and assignment of 4f-5d transition spectra of rare-earth ions in solids

The simple model for 4f-5d transition spectra of rare-earth ions in crystals is reviewed. We summarize successful applications to the 4f-5d spectra of Nd3+, Sm3+, Eu3+, Eu2+, Tb3+, Ho3+, Er3+ ions in various crystals. Some unsolved issues are also mentioned and discussed briefly.