INDEX quantum efficiencies

For vacuum sublimed thin films, Grabuzov et al. [138] reported a photoluminescence quantum efficiency of 32 2%. In the same paper, data on the absorption coefficient at the maximum, a = (4.4 0.1) x 104 cm 1, and the refractive index at 633 nm (n = 1.73 0.05) can be found. Other reported values for the photoluminescence quantum efficiency that can be found in the literature are 30 5% [124] and 25 5% [139]. Naito et al. [109] reported a quantum yield of 5% in the amorphous film compared to 35% in the crystalline state. The fluorescence lifetime is reported to be biexponential with x = 3.4 and 8.4 ns, which is much shorter than in the crystal (17.0 ns). In the amorphous state, the larger free volume allows more vibrations and rotations to take place, which favors nonradiative decay. 

Cr + ions in aluminum oxide (the ruby laser) show a sharp emission (the so-called Ri emission line) at 694.3 nm. To a good approximation, the shape of this emission is Lorentzian, with Av = 330 GHz at room temperature, (a) Provided that the measured peak transition cross section is c = 2.5 x 10 ° cm and the refractive index is = 1.76, use the formula demonstrated in the previous exercise to estimate the radiative lifetime, (b) Since the measured room temperature fluorescence lifetime is 3 ms, determine the quantum efficiency for this laser material. 

Fluorescence quantum efficiencies of several solid materials have been measured by photoacoustic spectroscopy." The photophysics of quantities for some common fluorescence standards have been made with some accuracy the influence of refractive index corrections on yield and lifetimes are discussed, 9,10-diphenylanthracene, quinine bisulphate, and 2-aminopyridine being the selected examples. Correction for inner filter effects in fluorescence spectroscopy have been proposed. ... 

Because of total internal reflection only a fraction (l/2n where n 1.8 is the index of refraction of the polymer) is emitted through the substrate [170]. That fraction of the emitted radiation that is waveguided is partially lost as a result of residual self-absorption in the polymer and, more importantly, as a result of losses in the metal cathode and the semitransparent anode. As a result, the external quantum efficiencies are smaller than the internal quantum efficiencies. 

The excited-state lifetime of the molecule in absence of any radiationless deeay processes is the natural fluorescence lifetime", r . The natural lifetime is a constant for a given molecule and given refraction index of the solvent. Because the absorbed energy can also be dissipated by internal conversion, the effective fluorescence lifetime, is shorter than the natural lifetime, The fluorescence quantum efficiency", i.e. the ratio of the number of emitted photons to absorbed photons, reflects the ratio of the radiative decay rate to the total decay rate. Most dyes of high quantum efficiency, such as laser dyes and fluorescence markers for biological samples, have natural fluorescence decay times of the order of 1 to 10 ns. There are a few exceptions, such as pyrene or coronene, with lifetimes of 400 ns and 200 ns, and rare-earth chelates with lifetimes in the ps range. 

In this description the FRET rate is therefore a fimction of the refractive index of the medium between the two molecules n, the fluorescence lifetime Tp[s] and quantum efficiency Qd of the donor in the absence of FRET, Avogadro s number N, the separation of the two molecules R[cm], the normalized spectral overlap integral / [ cm j and the so-called orientation factor k. Note that different forms of equa-... 

Other benzenoid structures incorporating fused thiophenes are represented by syn (42a) [99] and anti (42b) [100] isomers of a benzene linked trithiophene system. The compounds were obtained via condensation reactions of thiophene carboxaldehydes 41a and 41b in 88 and 24 % yield, respectively. Absorption and emission maxima are given in Chart 3.5, but noteworthy is the reduction in phosphorescence quantum efficiency experienced by the anti conformer (0.04 compared with 0.56 for the syn conformer) [101]. This is not surprising considering the tight crystal packing structure of 42b, which has an effective volume occupation or packing index of 0.75 [100]. 

Here a is a constant (8.8 x 10 cm ), is the refractive index of the medium, is an orientation factor which for random orientations of D and A is 2/3, and 4>q and Xq represent the quantum efficiency and lifetime of the donor D in absence of the acceptor A. At some particular distance Rq, /cet will be equal to the probability of deactivation of D by other processes, represented by the first-order rate constant (/ i -I- k2), which equals 1 /Xq. Hence we obtain for Rq ... 

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