Photoionization and Electron-Transfer Quenching

In this section we will draw attention to another type of radiationless transition, viz. that as a consequence of photoionization. In some cases the luminescence of a center 

Also BaF2 Eu does not show Eu emission, but impurity-bound exciton emission. The. same holds for NaF Cu where the excited state consists of a Jahn-Teller distorted Cu ion which binds an election. The absence of luminescence in La203 Ce has also been ascribed to quenching by photoionizaiion [15]. 

This implies that a combination of a center which has a tendency to become oxidized with a center which has a tendency to become reduced will probably not show efficient luminescence. This is illustrated by some examples  

Note that quenching by electron transfer is a special case of quenching via a charge-transfer state as discussed above (Sect. 4.2.2) for Eu. There is also no principal difference between quenching by electron transfer and quenching via photoionization. However, the former has a localized character, the latter is used in energy band models where delocalization is more important. 

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