Apparent recombination coefficient

One resolution of the problem that gained acceptance for a while was proposed by Adams et al.18 In their flowing afterglow measurements, the apparent recombination coefficient of fell off to a small value in the late afterglow . This finding... 

In this work, the apparent recombination coefficient, 7a, i.e., the probability that atoms colliding on the surface will recombine, was measured in a well-defined system built. Our results were compared with the recombination probability Ta estimated from the surface temperatures measured during the re-entry of the SSO (5). 

Other reactions, such as C-H bond scissions and Hj eliminations, although possible, have been shown to be unimportant under the conditions studied. In Table XIV, the parameters needed for the QRRK analysis of the recombination of CH2CI radicals are presented. The methods and sources used to obtain these data are the same as those noted in the discussion of the unimolecular QRRK method. In Fig. 22, the apparent rate coefficients for... 

Selim M, Fink JN, Kumar S, Caplan LR, Horkan C, Chen Y, Linfante I, Schlaug G (2002) Predictors of hemorrhagic transformation after intravenous recombinant tissue plasminogen activator prognostic value of the initial apparent diffusion coefficient and diffusion-weighted lesion volume. Stroke 33 2047-2052... 

Fig. 4.14. Maps of the apparent diffusion coefficient (ADC) measured before and after embolic occlusion of the right middle cerebral artery in an animal without therapy (upper row) and in two animals with thrombolytic treatment initiated 1.5 h (middle row) and 4.5 h (lower row) after onset of ischemia. In the untreated animal, a decline of ADC was observed immediately after MCA occlusion that increased in size over time. Thrombolysis with recombinant tissue-type plasminogen activator (tPA) lead to the partial reversal of the ADC lesion over the first 5 h of therapy if started early. Late-onset thrombolysis at 4.5 h post occlusion did not reverse lesion growth, but was followed by a further lesion enlargement of the ischemic lesion. [Reproduced with permission from Hoehn et al. (2001)]...

This has some characteristics of diffusion, but is not well described by a simple diffusion equation. Both the apparent diffusion coefficient and the apparent recombination time are electron density dependent. 

As mentioned earlier, ascorbate and ubihydroquinone regenerate a-tocopherol contained in a LDL particle and by this may enhance its antioxidant activity. Stocker and his coworkers [123] suggest that this role of ubihydroquinone is especially important. However, it is questionable because ubihydroquinone content in LDL is very small and only 50% to 60% of LDL particles contain a molecule of ubihydroquinone. Moreover, there is another apparently much more effective co-antioxidant of a-tocopherol in LDL particles, namely, nitric oxide [125], It has been already mentioned that nitric oxide exhibits both antioxidant and prooxidant effects depending on the 02 /NO ratio [42]. It is important that NO concentrates up to 25-fold in lipid membranes and LDL compartments due to the high lipid partition coefficient, charge neutrality, and small molecular radius [126,127]. Because of this, the value of 02 /N0 ratio should be very small, and the antioxidant effect of NO must exceed the prooxidant effect of peroxynitrite. As the rate constants for the recombination reaction of NO with peroxyl radicals are close to diffusion limit (about 109 1 mol 1 s 1 [125]), NO will inhibit both Reactions (7) and (8) and by that spare a-tocopherol in LDL oxidation. 

As a result of light trapping and intercalation, thin film solar cells can be made of a thickness /, which apparently violate the condition Le,h l 1 /a. Even with light trapping, the thickness l must be of the order of the penetration depth /a of the light. The diffusion length, on the other hand, can be arbitrarily small, if caused by a small diffusion coefficient. The recombination lifetime should always be as large as possible and should approach the radiative lifetime. 

Porter et al flashed I2-NO mixtures and calculated a third-order rate coefficient for recombination of iodine atoms by nitric oxide of A = 3 x 10 P.mole. sec At very high NO pressures, the recombination rate falls off concurrently, a new transient intermediate appears in the absorption spectrum which is considered to be NOI. Porter et al found nitric oxide to be 20 times more effective than I2 in the recombination of iodine atoms. This is about a factor of ten less than observed by Engleman and Davidson but no account of possible NOI absorption was included in Engleman and Davidson s work. This could account for part of the discrepancy but not all of it. Because of the fall-off of the apparent third-order rate coefficient at high NO pressures, it is apparent that this mechanism is valid only at very low pressures. For high pressures bimolecular reactions of NOI must become rate controlling. 

There are some similarities between these two sites the lifetime of 0-, the rate of proton escape (k23), and even the apparent rate of proton recombination (A2i-[H+]). The implication of these values will be discussed below. What markedly differentiates the two sites is the rate of proton dissociation (ki2). In both sites, the rate of proton dissociation is significantly slower than in water, implying that in these sites the water molecules are at a state that is not suitable for rapid (sub-picosecond) hydration of the discharged proton. The equivalent water activity coefficients, as estimated from the kinetic method described in Section III. are... 

R, (iii) the possible participation of excited electronic states and (iv) the density dependence of After these have been dealt with adequately, it can be shown that for many solvent bath gases, the phenomenon of the turnover from a molecular reaction into a diffusion-controlled recombination follows equation (A3.6.26) without any apparent discontinuity in the rate coefficient k at the gas-liquid phase transition, as illustrated for iodine atom recombination in argon [36, 37]. For this particular case, is based on and extrapolated from experimental data, R is taken to be one-half the sum of the Lennard-Jones radii of iodine atom and solvent molecule, and the density-dependent contribution of excited electronic states is implicitly considered by making the transition from the measured vin dilute ethane gas to in dense liquid ethane. 

Dissociation is already strongly apparent when evaporating metal sulphides. However, during condensation a strong recombination is also evident. For example ZnS decomposes completely into Zn and S2 according to mass spectrometrical analyses [299,294]. Nevertheless, stoichiometric films are usually obtained [300]. The dissociation of ZnS into its components supplies an explanation for the clear temperature dependency of the condensation coefficient of ZnS, which is observed even at low temperatures of 25°C to 200°C [301]. 

Northrup and Hynes [103] have remarked that the effects of the potential of mean force as well as hydrodynamic repulsion are very much more apparent in their effect on the survival (and escape) probability of a reactant pair of radicals than their effect on the rate coefficient. For instance, considering the escape probability of Fig. 20, suppose that an escape probability of 0.75 had been determined experimentally. Initial distances of separation Tq = 4i or 312 would have been deduced from the diffusion equation analysis alone or from the diffusion equation with the potential of mean force and hydrodynamic repulsion included. Again, the effect of a moderately slow rate of reaction of encounter pairs further reduces the recombination probability. Consequently, as the inherent uncertainty in the magnitudes of U r), D(r) and feact may be as much as a factor of 2, the estimation of an initial separation distance, Tq, of a radical pair from experimental measurements of escape probabilities may be in doubt by a factor of 30% or more. Careful and detailed analysis of the recombination of radical pairs has been made by Northrup and Hynes... 

The rate coefficients of many of the important elementary steps at high temperatures are now well established, particularly and the functions / /[M] and A a2)2)[M] which describe the recombination kinetics for those gas compositions which have been studied directly. Improved experimental accuracy is apparently needed in the determination of the rate of recombination before more definitive values of the coeflScients for individual collision partners, kf and kf, can be anticipated. Also, more quantitative information is desirable concerning the high-temperature rate coefficients of the other important bimolecular steps, k, kc and k. Some of this can be provided, without major advances in experimental technique, by further study of the nonequilibrium excursions of intermediate species concentrations toward the end of the ignition process under selected conditions in nonstoichiometric mixtures, and from further resolution of the exponential branching behaviour of lean mixtures, as discussed in section 2.3.2. 

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