Geminate pair, lifetime

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. 

A more tractable theory based on the probability that a reactant pair will react at a time t (pass from reactants to products) is that due to Szabo et al. [282]. If the survival probability of a geminate pair of reactants initially formed with separation r0 is p (r0, t) at time t, the average lifetime of the pair is /dr0 p(r0, t)t and this is longer for larger initial separation distances. It provides a convenient and approximate description of the rate at which a reactant pair can disappear, but it does so without the need of a full time-dependent solution of the appropriate equations. Nevertheless, as a means of comparing time-dependent theory and experiment in order to measure the value of unknown parameters, it cannot be regarded as satisfactory. 

The reconciliation of these two apparently conflicting results is quite interesting and is connected to the broad distribution of recombination lifetimes. Those electron-hole pairs which are created with small separations are more likely to result in geminate recombination than the more distant pairs. The close pairs are also more likely to contribute to the luminescence and the distant pairs to LESR. Thus the two experiments are selectively measuring different parts of the distribution. The density of geminate pairs of separation, R, is given by. 

The diffusive kinetics of geminate pairs have been predicted to show a time-dependent decaying behavior [117-122]. Early experiments showed, in contrast, a decay, with a being dependent on the proton concentration [123]. Experiments on longer time ranges with improved sensitivity are prerequisites for an accurate determination of the asymptotic behavior [124]. In fact, recent measurements on HPTS have demonstrated the validity of the theoretically predicted decay law (see Fig. 14.4) [125]. For 5-(methanesulfonyl)-l-naphthol a kinetic transition from power law to exponential has been reported due to a short photobase lifetime [126]. 

Two papers have been presented on the photochemistry of 5-methylphena-zinium salts in aqueous solution. Fluorescence, optical flash photolysis, and electron paramagnetic resonance (e.p.r.) techniques have been used to elucidate various aspects of product formation and quantum yield. Two products have been identified, namely the 5-methyl-10-hydrophenazinium cation radical (MPH ) and the pyocyanine (l-hydroxy-5-methyl-phenozinium) cation (PyH ) in a stoicheiometric ratio of 2 1. The quantum yield of formation of (MPH ) was found to be 0.29 0.03 at pH 7.0 and 1.1 0.1 at pH 3.0. The triplet state of MP (Ti) has also been detected by triplet-triplet absorption and is found to have a lifetime of 0.5 ns. Flash photolysis and e.p.r. have also been used to study a geminate triplet radical pair obtained from hydrogen abstraction by excited triplet acetone from propan-2-ol. The authors demonstrate that the geminate pairs contribute most of the polarization in photochemically-induced dynamic electron polarization (CIDEP) as compared with free random-phase pairs. 

In order to answer this question, Mihailetchi et al. [39] measured photogeneration in a blend of poly(2-methoxy-5-(3, 7 -dimethyloctyloxy)-phenylene vinylene (OCICIO-PPV) and PCBM as function of applied voltage and temperature. Presuming that the essential intermediate is an optically generated coulombically bound e-h pair located at an internal donor-acceptor hetero-junction that can either dissociate completely or recombine geminately, they analyzed their data in terms of Braun s model [22]. Based upon a broad distribution of e-h pair distances centered at 1.3 nm and invoking a value of 1 [is for the e-h pair lifetime they were able to rationalize their experimental results that include a measured 60% carrier yield at a built-in field of 7.5 X 10 Vcm at room temperature. However, an open question appears to be their choice of a pair lifetime of 1 [is. Recent experiments on a blend of a polyfiuo-renecopolymer and PCBM reveal a broad distribution of lifetimes albeit centered at a value as short as 1 ns [40]. 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

In the first case, pairs are isolated from each other and the recombination is monomolecular, with a rate which is independent of illumination intensity. The non-geminate electrons and holes act as independent particles and the recombination rate is proportional to the product of the two densities (here assumed equal). The observable difference is a recombination lifetime which is independent of the excitation intensity for geminate recombination, but which decreases with increasing illumination intensity for non-geminate recombination. The simple rate equations also predict a different form of the time dependence, but a more realistic model must also include a distribution of recombination rates due to the tunneling recombination. 

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