Radiative decay time

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... 

Fig. 4. Experimental radiative decay times and fluorescence quantum yields for aromatic molecules in inert solvents. Data from (a) I. B. Berlman 202 (b) Strickler and Berg.28...

Both the E and Tstates can radiate to the ground A2 level, so the rate of radiative emission per unit volume will be AeIIe + AjTif. Since the total number density of excited atoms is nj + iie, the radiative decay time is... 

The radiative decay time of the red PL is of the order of several tens of microseconds in all cases. The lack of the blue shift in the compact films has been explained in terms of the surface state model [19]. The theory predicts an increase of the transition probability for absorption and PL with decreasing crystallite size [9, 10, 13]. Fig. 5 shows the measured dependence of the PL intensity on the average crystallite size in the compact ncSi/SiO films [19,25]. 

The fast decrease of the decay time of the luminescence of the (UO Vp) centre above about 60 K (Fig. 17) indicates that at higher temperatures a higher energy level becomes populated which has a considerably shorter life time than the A2 level. When the temperature dependence of the decay time is described in terms of a three level scheme, the data reveal that the higher energy level has a radiative decay time of about 5 ts. The temperature dependence of the relative intensities of the patterns associated with the two electronic origins in the emission spectrum and the... 

Figure 11. Dependence of the exciton radiative decay time on the effective radius, a, at 10 K (open circles) and 77 K (closed circles). A solid line represents theoretical calculation from Eq. (10). (Taken from reference 58 with permission.)...

The expression (2.1) is of course also valid for the emission transition. As for absorption transitions, the nuclear part determines the emission band shape. The electronic part determines the value of the radiative decay time. 

Schuurmans MFH, van Dijk JMF (1984) On radiative and non-radiative decay times in the weak coupling limit. Physica 123B 131-155... 

So the first example of real superradiance was in fact the free induction decay and die decay of the photon echo observed in ruby by Kumit, Abella and Hartmann (1964). When a pulse from a ruby laser was sent onto a ruby crystal, the free induction decay and the echo decay observed were about 50 ns, when compared to the usual Cr " radiative decay time of 4 ms, showing clearly the radiation emission from the macro-dipole. 

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