Recombination spin-dependent

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. 

Two aminyl radicals combine to form the N—N-dimer with the rate constant proportional to the spin density on the nitrogen atom [96]. This recombination also depends on the steric factor. Thus, diethylaminyl radicals recombine with a rate constant of 109 L mol 1 s 1, while diisopropylaminyl radicals are only able to disproportionate with a rate constant of... 

A spin-dependent recombination rate is another consequence of the electron-hole correlation. The conservation of spin selection rule is preserved in amorphous materials. The final state of the recombination process has zero spin and both radiative and phonon-assisted non-radiative transitions occur without change in spin, so that recombination can only proceed from an initial state of zero spin. A weakly interacting electron-hole pair forms four possible spin states, one singlet and one triplet. Of the four states, only the singlet and one... 

The theory underlying this effect depends critically on two selection principles the nuclear spin-dependence of intersystem crossing in a radical pair, and the electron spin-dependence of the rates of radical pair reactions. The combination of these selection principles causes a sorting of nuclear spin states into different products, formed by geminate recombination (allowed for singlet pairs but spin-forbidden for triplet pairs) or by free-radical ( escape ) products (whose formation is electron spin-independent). As a result, geminate reaction products are formed with characteristic non-equilibrium populations of nuclear spin levels, whereas escape products show complementary non-equilibrium spin level populations. 

In the conventional ODMR techniques, one monitors the luminescence intensity to detect ESR signals in terms of changes in luminescence at resonance. These changes are a consequence of spin-dependent recombination processes. For trapped electron - hole pair recombination, illustrated in Fig. 1, the spin-dependent nature of the radiative recombination can be easily understood by considering that radiative recombination is allowed when spins of an electron and a hole are antiparallel and is forbidden, in principle, when they are parallel. 

Photoinduced absorption (PA) measurements provide useful information about relaxation processes of photocarriers and tail and gap states in a-Si H [Tauc, (1982) see also Chapter 9 by Tauc in Volume 21B]. Photoinduced absorption also depends on the recombination processes of those trapped electrons and holes that are created under optical excitation. Thus it is expected that PA will be spin-dependent, as is luminescence. Hirabayashi and Morigaki (1983a,b) have observed, for the first time, spin-dependent PA at 2°K in a-Si H, monitoring the PA intensity while irradiating the sample with microwaves at 9.6 GHz. This is called photoinduced absorption-detected ESR (PADESR), which provides complementary information to the conventional ODMR (luminescence-detected ESR). However, since the PADESR technique can be applied to nonluminescent materials, it may provide a more general means for highly sensitive detection of ESR than conventional ODMR. 

The measurements of spin-dependent photoconductivity in a-Si H provide information about recombination processes and tail and gap states that is complementary to ODMR measurements (Dersch et al, 1983). 

Here, "recombination" stands for all types of radical pair collapse reactions, such as coupling, disproportionation, back electron transfer etc. This nuclear spin dependent reactivity is a consequence of two basic premises of the radical pair theory ... 

The kinetics of multiplet CIDEP generation for radical pair recombination (F-pairs) in low-dimensional media have been studied theoretically by Shushin et al The results obtained show that the polarization of the multiplet CIDEP is sensitive to dimensionality only in the limit of weak spin-dependent interaction and large diffusion coefficient. The theoretical expressions have been discussed in relation to some experimental CIDEP results obtained for radical reactions in porous solids and on surfaces. ... 

The spin-dependent decay of P+H" is complex. It comprises two decay pathways as shown in the kinetics scheme of Fig.3. We will briefly explain how spin-dependent recombination dynamics are responsible for inherent deviations from monoexponentiality in Fig.2 and can fully account for them, as shown by rigorous simulations. On the basis of a brief summary of radical pair dynamics, the effect will be outlined in the following section. 

DEVIATIONS FROM MONOEXPONENTIAL DECAY OF P+H" DUE TO SPIN-DEPENDENT RECOMBINATION... 

Equation (44) shows that (1) the sign of the ODMR is opposite that of the ADMR and (2) the H-ADMR spectrum for a particular spin species is directly reflected in the PL-ODMR spectrum, since in Eq. (4 5N, is multiplied by a constant factor independent of H. This is very important for triplet resonances. If triplets indirectly influence exciton photoluminescence, then we expect the PL-ODMR and the H-ADMR powder patterns to be almost identical but of opposite sign. This important finding can explain the triplet ODMR spectrum in P30T (Fig. 22.7a). From the similarity of ODMR and ADMR (Figs. 22.7a and 22.7b, respectively) we conclude that the ODMR in P30T is due to changes induced in spin-dependent recombination of nonradiative channels with spin 1/2 and 1, respectively. In this case the information obtained in ADMR essentially covers that in PL-ODMR also. In other words, the PL-ODMR resonances do not directly detect excitons but are sensitive to recombination centers in the sample, which can be more directly studied by ADMR. 

In principle, the spin 1/2 ADMR signal of the HE band could arise from three possible sources (1) spin-dependent generation of spinless excitation (2) spin-dependent recombination of spinless excitation or (3) spin-... 

We found a positive ADMR signal at 0.45 eV that is associated with the photogenerated 5". As mentioned in Section III.B.5, however, a situation where a spinless particle shows an ADMR signal is not uncommon in ODMR spectroscopies. This comes about via spin-dependent recombination processes of companion excitations having a cross-recombination channel with the spinless particle. The experimental correlation found between 6N(HE) (<0) and SN(LE) (>0) may show that an apparent conversion process from into 5 -5 ... 

Another important factor in such discrimination is determined by electron spin (Franzen 2002 Jensen and Ryde 2004 Minaev 2010 Shikama 2006 Sigfridsson and Ryde 2002 Stryer 1995). The O2 binding to heme is higlily non-exponential at ambient temperature with a rapid phase of 3 ps, a longer phase of similar geminate recombination (20-200 ps), and a slow bimolecular process of 1 ms all phases are spin-dependent processes (Franzen 2002 Jensen and Ryde 2004 Petrich et al. 1988 Shikama 2006). At this point we need to consider it in more detail. 

Several other methods have been employed to access the conditions of bubble collapse. Misik et al. studied H20—D20 mixtures and through measurements with the use of spin traps, were able to determine the temperature from the relative rates of O—H and O—D cleavage [21]. They reported temperatures ranging from 2,000 to 4,000 K. Hart et al. developed a method based on the gas phase recombination of methyl radicals (MRR method), formed from the decomposition of methane [22]. They calculated temperatures of 2,000-2,800 K depending on the methane concentration. 

Since the two-spin state forms can lead to different products, the products obtained will be a mixture that reflects the initial fractionation of the reaction between the two-spin states. The fractionation in turn is a reflection of the interplay and the probability of cross-over between the two-spin states (8). Thus, the two-state reactivity paradigm resolves the dilemma of whether a radical recombination or a direct insertion mechanism governs cytochrome P450-catalyzed hydroxylation actually they are both involved and the degree to which either is expressed depends upon the specific substrate hydroxylated and the specific enzyme. 

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