In general, rare earth elements exist as oxides. The luminescence mechanism of rare earth elements is determined by their own energy levels and crystal sites. When the rare earth element is in the free state, the transition of each energy level in the configuration is called parity forbidden transition, and this state cannot undergo radiation transition and produce spectrum. However, when the rare earth element is in the condensed state, the parity forbidden transition of each energy level in the rare earth element configuration is lifted by the action of the matrix crystal field, so in this state, the radiation transition will occur between the energy levels in the rare earth element configuration and the spectrum will be generated. Generally, the spectra produced by electronic transitions in rare earth elements can be divided into two categories: one is sharp linear spectrum, mainly the electronic transitions between energy levels in 4/ “structure, that is, 4f-4f energy levels, and the other is wide band spectrum, mainly the electronic transitions between energy levels in 4f” structure and other energy levels, such as 4f-5d energy levels! . In addition to these two types of electronic transitions, there is also a charge transfer transition, mainly between rare earth elements and adjacent ligands (oxygen or halogen, etc.)
The sharp linear emission spectrum is caused by the electronic transition of trivalent rare earth ions. According to the transition selection rule, it is forbidden by parity, but these transitions are still often seen. This is mainly because the rare earth ions are influenced by the external crystal field and break the parity prohibition, so that the 4f-4f transition spectrum is detected. Because the spectrum of this transition is very narrow, the light color is very pure, which is widely used in display and optical fiber communication. The broad band spectrum is caused by the transition of electrons from the inner 4f layer to the outer 5d layer. According to the transition selection rule, this transition can occur. Generally speaking, band-shaped spectra have high spectral intensity, such as Ce, Eu and Tb, which are widely used in lighting and display fields. According to the rule of transition selection, electron migration transition can also occur, which shows a wide excitation band in the excitation spectrum and generally appears on the high energy side (ultraviolet band). The characteristics of this transition are very similar to those of 4f-5d transition. According to the research, almost all rare earth elements have electron migration transition bands, and most of them exist in high-energy ultraviolet or near-ultraviolet bands.