Quenchers or Killers of Luminescence

Cations, having unpaired electron spins, function as quenchers of luminescence. Some of these are shown in the following diagram, given as 

While this may seem incongruous to some (because we have already cited sub-group IB as an activator-group), it is the actual valence state that is important. Usually, their electronic configuration involves d- electrons as well. Note that certain of these cations are optically active in the proper valence state. In the wrong valence state, however, they function as quenchers. 

H Cations with Unpaired Spins Which Function as Quenchers of Luminescence He 

For example, in ZnS Cu, if the Cu-cation is not stabilized in the Cu state, it acts as a killer of luminescence. This is particularly true in oxygen-dominated phosphor compositions. This insight has important consequences since the method of preparation thus becomes important. If these cations become stabilized in the wrong valence state, they become energy traps and dissipaters of excitation energy. 

These cations are essentially acceptors, even for resonant energy (dq) transfer. Since they cannot undergo an excitation transition because of ground state coupling to the local phonon modes, they function conversely to the luminescence process. Many of them exhibit strong absorption bands at the frequencies of light commonly used for excitation of phosphors. Therefore, excitation energy is dissipated to the lattice by phonon processes, once it has been captured by this type of site. 

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