Recombination region

Exciton decay When an exciton decays radiatively a photon is emitted. When the excitons form in fluorescent materials radiative decay is limited to singlet excitons and emission occurs close to the recombination region [7] of the OLED due to the relatively short lifetime of the excited state (of the order of 10 ns). For phosphorescent materials, emission can occur from triplet excitons. Due to the longer excited state lifetime (of the order of hundreds of nanoseconds), triplet excitons can diffuse further before decaying. 

Summation on the r.h.s. runs over all site on the surface of the recombination region. Taking into account the normalisation condition of the defect production, the recombination probability reads... 

Calculation done by Schroder and Eberlein [3, 83], has shown than even for small recombination regions, e,g., that containing only nearest to a vacancy sites in the b.c.c. lattice, Rett 0.81ao which very close to the linear size of the recombination region, Rmax = ao /3/4 0.866ao. 

These results yield an impressive justification for the use of the continuous diffusion approximation even for short distances and small recombination regions. 

It is convenient to consider a model of an anisotropic recombination region the reflecting recombination sphere (white sphere) with black reaction spots on its surface [77, 78], The measure of the reaction anisotropy here is the geometrical steric factor Q which is a ratio of a black spot square to a total surface square. Such a model could be actual for reactions of complex biologically active molecules and tunnelling recombination when the donor electron has an asymmetric (e.g., p-like) wavefunction. Note the non-trivial result that at small Q, due to the partial averaging of the reaction anisotropy by rotational motion arising due to numerous repeated contacts of reactants before the reaction, the reaction rate is K() oc J 1/2 rather than the intuitive estimate Kq oc Q. 

To establish the fundamental laws of the process of the kinetics of particle accumulation in solids, initially the quasicontinuum model of a crystal was studied [107]. A one-dimensional crystal was represented in the form of a segment containing L cells with periodic boundary conditions (the ends of the segment are closed). The simulation was conducted for different dimensions L of the crystals and magnitudes l of the recombination region. The results of the simulation are given in Table 7.2. 

In the two-dimensional case the value of Uo is smaller than for the onedimensional case at the same magnitudes of the recombination region (thus, when vp = 400, Uo = 3.2 in the one-dimensional case). However, the aggregation effect is expressed rather strongly. 

The quantity Uo characterizes the degree of aggregation only on the average. Therefore it is important in each particular case to analyze also the spatial distribution of the defects. Thus, the low-temperature accumulation of the Frenkel defects in the two- and three-dimensional cases was simulated in [36, 114] and the obtained values of Uo for d = 2 considerably exceed the same in [113]. In contrast to the latter, the authors of [36, 114] used a circle as the recombination region. In its turn, the values of u0 obtained in [115] are considerably larger than those found in [114]. We note that it was assumed in [115] that, when an interstitial atom occurs at a site where the recombination spheres of several vacancies overlap, it recombines with the closest vacancy. This demonstrates very well how any details, insignificant at first glance, can affect considerably the accumulation kinetics. 

A two-dimensional simulation has also been performed of the accumulation of defects with an asymmetric recombination region [113] chosen in the form of a rectangle with vp = a x b. The anisotropic case in which the... 

Figure 9. Comparison of OH and temperatures in both primary reaction zone and recombination regions. The fuel-air equivalence ratio was = 0.93. The probed region was 1.5 mm from the curved wall. The uncertainty in the hydroxyl temperature is 100 K. CH,-air flame (9), Ng ( ), OH ( 3).

Fig. 1. General lay-out of ATHENA experiment. Shown are the positron accumulator and the main magnet system holding the antiproton catching trap, the recombination trap, and the annihilation detector surrounding the recombination region...

MAIN REACTION ZONE AND RECOMBINATION REGION IN HYDROGEN-OXYGEN IGNITION... 

In the burnt gas recombination region of fast, fuel-rich hydrogen-nitrogen—oxygen flames the observed intensities of chemiluminescence for sodium and other metal additives were found [134] to obey the relation... 

In fast flames and shock tube flows such as are considered here, the concentration gradients in the recombination region are such that diffusion effects can be neglected. The recombination can also be considered as taking place in the presence of effectively constant concentrations of the bulk species Hj, HjO and N2 or Ar. As was first pointed out by Sugden and co-workers [133] the radical concentrations do not behave independently during the approach to full equilibrium. The observed relationships... 

Since the ratio 2fe2/ 4 is reliably known from second explosion limit work, the three kinetic unknowns in the system are now 3/ 20 smd 17. Again initially, 2 was assigned the fixed value 2.05 x 10 exp (—8,250/T). It was found that the best fit of the burning velocity, the relative H atom concentration decay profile in the recombination region (measured by intensity of sodium chemiluminescence), and the temperature and composition profiles were obtained with feg /fe2 0 = 5 1 and fej 7 = (4.5 1.5) X 10, assuming equal efficiencies of all the molecules in the... 

Figures 25—27 show the temperature and composition profiles calculated for the standard flame by the refined treatment using set 2 of the rate coefficients of Table 30. Figure 25 also includes for comparison a number of points representing the observed temperature profile. Agreement is excellent. The composition profiles for the stable species in the flame were measured by means of a mass spectrometric probe, using the unbumt gas ratios of each species concentration to that of nitrogen as calibration standards. Realistic comparison is then in terms of these ratios, and is shown in Fig. 28. The relative intensities of sodium chemiluminescence in the recombination region of the low temperature flames are proportional to the square of the H atom concentrations. A comparison between theory and experiment on this basis, with intensities normalized with respect to the maximum H atom concentration and the...

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