Geminate pair

Fluctuating Gap Model Fowler-Nordheim fluorine-doped tin dioxide full width at half maximum geminate pair... 

Immediately after formation of the geminate pair (time, [Pg.64]

In the case of radical formation by the main-chain scission of the polymer molecule, a high concentration of the radicals initiates the reactions involving the geminate pair [30]. 

So in summary, the presence of a chiral centre in a molecule can render nearby geminal pairs of protons non-equivalent. Nearby is not an exact term and varies according to circumstance. Let s consider our molecule again, but this time, replace the -CH2- with an alkyl chain (Structure 6.2). 

As before, the chiral centre renders Ha and Hb non-equivalent and for the reasons already covered, Hx will couple to both with all three couplings (Ha-Hb, Ha-Hx and Hb-Hx) likely to be different. So the classical presentation of an ABX system is that of three multiplets, each of four lines. (Note that in Spectrum 6.2, the size of the Ha-X and the Hb-X couplings are almost identical so the X proton appears as an approximate triplet. This is quite common.). The AB part indicates that the geminal pair are likely to be relatively close in terms of chemical shift, whilst the X proton is someway distant from both. Obviously, the scope for variation in the appearance of ABX systems is enormous. The difference in chemical shift between Ha and Hb is a major factor in this but we have also come across ABX systems... 

Mozumder and Magee, 1967), the success of the geminate pair model in kinetics (Warman et al., 1969 Rzad et al, 1970) and in free-ion yield determination (Hummel and Allen, 1966 Freeman, 1963a, b) has been rationalized on the basis that in multi-ion pair spurs all but the final e-ion pair would quickly neutralize due to intense internal coulombic interaction. Therefore, emphasis is laid in this section on the geminate pair with two caveats ... 

The slope-to-intercept ratio of the variation of free-ion yield with the external field is about the same for multiple ion-pairs as for the isolated geminate pair, to within -12%. 

At this stage, it should be recalled that although the S/I ratio has been derived in the Onsager model for a geminate pair, there is extensive Monte Carlo simulation work to extend the same value for multiple-ion-pair cases and, therefore, to the entire track as well (see Sect. 9.3). Hence, the comparison of the experimental value of S/l with the theoretical value of Onsager is meaningful. For example, Hummel and Allen (1967) measure S/I = 6.0 x 10-5 cm/V in n-hexane at 298 K, whereas the theoretical value is 5.81 x 10-5 cm/V. 

The temperature dependence of luminescence from the sample irradiated at 1 x 1013 cm-2 with 28Si+ indicates, above —110 K, an activation energy of 90 meV for the competing nonradiative recombination process— this competing process may be the thermal dissociation of geminate pairs or bound excitons at donorlike or acceptorlike centers. The 0.09-eV value of activation energy is consistent with the results of Troxell and Watkins (1979). 

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

In the photolysis of. -dibromo-diphenyl-diazomethane in toluene the geminate pair effect is observed 87). However, it is accompanied by the enhanced emission by the escape product 7. This means that the carriers of the original polarization were the free radicals, whose lifetime is obviously shorter than the nuclear relaxation time. 

Comparing experimental cross-relaxation rates from table 3 with crossrelaxation rates from the model (table 1), one can see that discrepancy beyond the estimated error limits exists only for proton 7 and 6 geminal pairs. Because the differences are not far from SAcr", it is hard to interpret them in more detail. A relatively large discrepancy between the diagonal elements can be explained by the fact that in the model calculations uniform external relaxation rate was assumed for all spins (i ex = 1-426 s ). 

At low temperature, mixing times of about 1 s are excessively long. This can be seen from the buildup curve for geminal pair GlyH /H , which reached saturation around 300 ms. 

Albrecht U, Bassler H (1995) Yield of geminate pair dissociation in an energetically random hopping system. Chem Phys Lett 235 389... 

The probability density w(r,t) contains the complete information about the behavior of the geminate pair. However, it is difficult to solve Eq. (2) in most cases of interest. Moreover, the probability density w(r,t) itself is usually experimentally unobservable. The quantities of greater practical importance are the pair survival probability defined as... 

The latter quantity is called the escape probability, and describes the probability that the two oppositely charged particles of the geminate pair will never recombine with each other and become free ions. 

In the equations written so far, the reaction between the particles of the geminate pair is not accounted for. This can be performed by introducing a boundary condition at a distance R, which is called the reaction distance. If the reaction on encounter is very fast, one can use the totally absorbing boundary condition... 

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