CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

concentration quenching lies in very efficient energy transfer among the luminescent centers.
Two mechanisms are generally invoked to explain the concentration quenching of
luminescence.
(i) Due to very efficient energy transfer, the excitation energy can migrate about a large
number of centers before being emitted. However, even for the purest crystals, always a
certain concentration of defects or trace ions can act as acceptors, so that the excitation
energy can finally be transferred to them. These centres can relax to their ground state by
multiphonon emission or by infrared emission. Thus, they act as an energy sink within the
transfer chain and so the luminescence becomes quenched, as illustrated in Fig.9 (a) [42].
These kinds of centers are called killers or quenching traps.
Fig.9. Schemes of possible mechanisms for luminescence concentration quenching: (a)
energy migration among donors (circles) in a chain followed by its migration to a
killer site (black circle) which acts as non-radiative sink; (b) cross relaxation
between pairs of centers. (Sinusoidal arrows indicate nonradiative decay) [42].
(ii) Concentration quenching can also take place without actual migration of the excitation
energy among the luminescent centers. This occurs when the excitation energy is lost from
the emitting state via a cross relaxation mechanism. This kind of relaxation mechanism
occurs by resonant energy transfer between two identical adjacent centers, brought about by
the particular energy-level structure of these centers (Fig.9 (b)).
As the concentration quenching results from energy transfer processes, the decay time
of the emitting ions is reduced along with the luminescence quantum yield. In general, this
20