Pair recombination

Fig. 2. Surface temperature dependence of the vibrational excitation of NO(v = 0 — 1) in collisions with a clean Ag(lll) surface. The observed thermal activation was attributed to hot electron-hole-pair recombination transferring energy to NO vibration. This work provided some of the first strong evidence that metal electrons can interact with an adsorbate molecule strongly enough to change its vibrational quantum numbers. (See Ref. 24.)...

FIGURE 7.10 Comparison of long-time decay kinetics for ion-pair recombination. The authors find r 0 ) decay describes the kinetics over a wider range of time than r05. Here T is normalized time. See text for details. Reproduced from Bratczak et al. (1997), with permission of Elsevier . 

In conclusion we may state that there is evidence for multiple ion-pair recombination in spurs yet a theoretical analysis of free-ion yield and scavenging at low-LET based on the geminate ion-pair picture is meaningful in view of the similarity of the recombination process in the geminate and multiple ion-pair cases. However, if this analogy holds, the geminate ionization yield has to be somewhat less than the true ionization yield. 

For very long time (> 1 year), electron-hole pair recombination occurs and spectral features of neutral occluded DPB molecule are found to reappear (figure 5). Then, electron trapping in zeolite framework shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

Mg2+ ion. 49 has been used to deposit MgO by atomic layer epitaxy,222 and is commonly employed as a />-type dopant for semiconductors, particularly GaAs,223 GaN,224,225 and AlGaN.226 In GaN, Mg doping induces a blue 2.8 eV photoluminescence band arising from donor-acceptor (D-A) pair recombination.227 It is likely that isolated Mg... 

The study of the decomposition of optically active 1-methyl-2,2-diphenylcyclopropanoyl peroxide proved the retention (37%) of the product of the geminate radical pair recombination [90]. The radical center in the formed cyclopropyl radical is so strained that the racemization rate is unusually slow. 

The higher the viscosity of the solvent, the higher the amount of the parent molecules formed due to the geminate recombination of radicals. The observed rate constant of decomposition of the initiator decreases with an increase in viscosity [3,90], This was observed in the decomposition of peresters and diacetyl peroxide in various solutions. Subsequently, the fraction fT of the radical pairs recombining to the parent molecule increases with an increase in the viscosity ... 

The yield (y) of the product of geminate pair recombination (ROCMe3) as a function of the subsequent rate constants [104,105] is given by... 

The oriented elongation of the polymer increases the packing of macromolecules and decreases the molecular mobility in the polymer. This was observed by the EPR spectra of the nitroxyl radical in these films. Therefore, one can expect an increase in radical pair recombination in the cage with an increase in y. However, experiment showed an opposite pattern the more the y, the higher the e value. These results found explanation within the scope of the... 

This is the result of cage effect. The cage of a solid polymer matrix is rigid (see earlier) and the most part of the forming radical pairs recombine in the cage. Hence, the probability of... 

Leite, R. C. C. DiGiovanni, A. E. 1967. Frequency shift with temperature as evidence for donor-acceptor pair recombination in relatively pure n-type gallium arsenide. Phys. Rev. 153 841-843. 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

W Wohlgenannt, XM Jiang, C Yang, OJ Korovyanko, and ZV Vardeny, Spin-dependent polaron pair recombination in TT-conjugated polymers enhanced singlet exciton densities, Synth. Met., 139 921-924, 2003. 

All these quantities are shown in figure 3.2, where Ais the the activation enthalpy for the cage radical pair recombination and Ais the activation enthalpy for diffusive cage escape. 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

Figure 5. Plot of log for clgree separation,, and log for radical ion pair recombination, j [, vs edge-to-edge distance between the 7T systems of the ZnTPP donor and anthraquinone acceptor in compounds 1-3 in butyronitrile at 21 .

A formal synthesis of ( )-Eseroline (69) via a 3-aza-4-oxa-Cope rearrangement was reported. An iV-aryl A-hydroxycarbamate was reacted with 2-phenylsulfanylpropanoic acid to yield the O-acylhydroxamic acid derivative 65, R = H that rearranged in the presence of potassium bis(trimethylsilyl)amide. The [3,3] and [3,5] rearrangement products, respectively 66 and 67, were formed (equation 22). If the para-substituted hydroxamic acid 65, R = OCOBu-t is used, no [3,5] rearrangement product is observed and the [3,3] rearrangement product 68 is the only product formed (equation 23). The authors proposed two parallel mechanisms, a concerted pathway and an ionic mechanism by an ion-pair recombination. 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

As shown in reactions (6.5.13) and (6.5.14), metal ion dopants influence the photoreactivity of metal oxides by acting as electron (or hole) traps thereby altering the e /h -pair recombination rate. The energy level of should be less negative than that of the... 

Ramsey theory, 22 201-204 Random-fragmentation model, Szilard-Chalmers reaction and, 1 270 Random-walk process, correlated pair recombination, post-recoil annealing effects and, 1 288-290 Rare-earth carbides, neutron diffraction studies on, 8 234-236 Rare-earth ions energy transfer, 35 383 hydration shell, 34 212-213 Rare gases... 

Giebink NC, Lassiter BE, Wiederrecht GP, Wasielewski MR, Forrest SR (2010) Ideal diode equation for organic heterojunctions, n. The role of polaron pair recombination. Phys Rev B... 

Let us assume that fci is equal to k9, the rate constant for the gas phase decomposition (15), where no cage effect is expected. This assumption does not always hold (15, 18). For example, it is known (18) that di-f erf-butyl peroxide (DPB) decomposes about 30% slower in the gas phase than in solution. We can calculate from our value of k8 and the known value of kg, from the work of Szwarc (7, 21), a value for the fraction of acetoxy radical pairs recombining, fR, where... 

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