The Concentration Quenching of Luminescence

In principle, an increase in the concentration of a luminescent center in a given material should be accompanied by an increase in the emitted light intensity, this being due to the corresponding increase in the absorption efficiency (see expression (1.15)). However, such behavior only occurs up to a certain critical concentration of the luminescent centers. Above this concentration, the luminescence intensity starts to decrease. This process is known as concentration quenching of luminescence. 

As the concentration quenching results from energy transfer processes, the decay time of the emitting ions is reduced when one concentration quenching mechanism occurs. In general, this decay-time reduction is much easier to measure than the reduction in the quantum efficiency. In fact, the easiest way to detect luminescence concentration quenching is to analyze the lifetime of the excited centers as a function of the concentration. The critical concentration is that for which the lifetime starts to be reduced. 

Finally, it is important to mention that besides the possibility of energy transfer, a high concentration of centers can lead to new kinds of centers, such as clusters formed by aggregation or coagulation of individual centers. Thus, these new centers can have a different level scheme to that of the isolated centers, giving rise to new absorption and emission bands. This is, of course, another indirect mechanism of concentration quenching for the luminescence of the isolated centers, as happens in the next example. 

EXAMPLE 5.6 The low-temperature emission lifetime of a very low concentration ofCr ions in ruby (AfOs Cr) is about 4 ms. For a 1% 

Cf2 O j concentrated sample, the measured lifetime becomes L7 ms. Estimate the loss of efficiency due to this concentration quenching of the luminescence. 

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