Luminescence radiative decay

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

Another open question is the relationship between the H-induced radiative recombination centers and the H-induced platelets. Controlled layer removal of the plasma-processed silicon surface reveals that the density of luminescence centers decays nearly exponentially with a decay length that is comparable to the depth over which the platelets form (Northrop and Oehrlein, 1986 Jeng et al., 1988 Johnson et al., 1987a). However, the defect luminescence has also been obtained from reactive-ion etched specimens in which platelets were undetectable (Wu et al., 1988). Finally, substantial changes in the luminescence spectra occur at anneal temperatures as low as 250°C (Singh et al., 1989), while higher temperatures... 

From the practical point of view, the radiative decay rate kr may be assumed to be independent of the external parameters surrounding the excited sensor molecule. Its value is determined by the intrinsic inability of the molecule to remain in the excited state. The radiative decay rate kr is a function of the unperturbed electronic configuration of the molecule. In summary, for a given luminescent molecule, its unperturbed fluorescent or phosphorescent decay rate (or lifetime) may be regarded to be only a function of the nature of the molecule. 

Figure 5.22 Schemes of possible mechanisms for luminescence concentration quenching (a) energy migration of the excitation along a chain of donors (circles) and a killer (black circle), acting as nonradiative sink (b) cross relaxation (including an illustrative energy-level diagram) between pairs of centers. (Sinusoidal arrows indicate nonradiative decay or radiative decay from another excited level.)...

The mechanisms of luminescence decay from an optical center are of critical importance. In particular we have to know if there are any processes internal to the center or external to it, which reduce the luminescence efficiency. It is possible to define two decay times, ir, the true radiative decay time which a transition would have in absence of all non-radiative processes, and r, the actual observed decay time, which maybe temperature dependent, as will usually occur when there are internal non-radiative channels, and which may also be specimen dependent, as when there is energy transfer to other impurities in the mineral. The quantum yield may be close to unity if the radiationless decay rate is much smaller than the radiative decay. 

In the absence of non-radiative decay processes the experimentally observed decay time equals the radiative decay time. When non-radiative processes are present, the experimental value is reduced by a factor equal to the quantiun efficiency of the luminescence. There are many factors, which affect the decay time. One is due to competing non-radiative processes, which shorten the measured decay time. We will consider the latter first. The experimentally observed decay time of the liuninescence is given by... 

Ionic radii of zirconium are of 0.73 A in 4-coordinated form and 0.86 A in 6-coordinated form. The possible substituting luminescence centers are Ti with an ionic radius of 0.75 A in 6-coordinated form, TR +, Cr +, Cr +, Mn ", and Fe. Impurities ofU and Th are also possible, which may radiatively decay with formation of radiation induced luminescence centers. 

Another possible solution that has been under development for three decades is to use a pulsed laser and time-resolved detection to allow the Raman photons to be discriminated from the broad luminescence background. The Raman interaction time is virtually instantaneous (less than 1 picosecond), whereas luminescence emission is statistically relatively slow, with minimum hundreds of picoseconds elapsing between electronic excitation and radiative decay. If we illuminate a sample with a very short (= 1 ps) laser pulse, all of the Raman... 

Due to the competing non-radiative decay routes for the lanthanide excited state, there is an intrinsic limit to the overall quantum yield in luminescent lanthanide complexes. It has been estimated that these values are 0.50 and 0.75 for europium and terbium, respectively (27). Although quantum yields exceeding these have been reported (31,32), care should be taken in analyzing quantum yield results in the literature, as these are often given for the energy transfer process alone, and not the overall quantum yield, and in other cases it is unclear as to which process(es) the quoted quantum yield refers to. 

Merkle et al. (1981) discussed three possible mechanisms of the pressure-enhanced decay rate an increase of the radiative decay rate of each Nd3+ ion, an increase of the nonradiative decay rate of each Nd3+ ion, or an increase in the interaction between Nd3+ ions leading to luminescence quenching. The nonradiative decay rates for the 4F3/2 multiplet were estimated to contribute less than 20% to the total decay rate (Powell et al., 1980) at ambient pressure. 

The increase in the radiative decay rate of 4F3/2 under pressure was estimated from the measured absorption strength of the 4l9/2 4F3/2 transitions and the relative luminescence... 

Direct pumping of poison centers as well as energy transfer from co-activators, and energy transfer from activators all represent an energy loss in the system. In addition, activators and co-activators also have non-radiative decay routes. Because non-radiative decay is usually phonon-assisted, non-radiative decay is exacerbated by increasing temperature and manifests itself by a characteristic temperature at which luminescence is quenched. The crystallographic relations that provide optimum sensitizer -activator energy transfer are outlined by Blasse (9)... 

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