Geminate ion-pair recombination

The coulombic interaction between ions [eqn. (39)] in any solvent is sufficiently strong to swamp the potential of mean force. Even in very polar solvents, the distance at which the coulomb potential is is 

The distance to which the ion-pair separates before the ions have vibrationally relaxed (and can then be considered as discrete well-characterised species) has been the subject of much theoretical research [315—317] (see Sect. 4). It has proved a very difficult subject to develop [318], though it does appear that the majority of the distance travelled by ejected electrons is travelled once their energy is little greater than thermal (epithermal). Typical separation distances may be 5—lOnm in 

The diffusion coefficients of the ion are usually estimated from their mobilities (or conductances), which can be measured independently. Diffusion coefficients (or mobilities) vary over very large ranges for the solvated electron in different solvents (neopentane, D 2 x m and 7 X 10 m s hexane, D 2 x 10 m s and p 8x 10 m V s ) [320], There is considerable evidence that the mobility of electrons is not constant, but on the contrary, the mobility depends on the applied electric field, increasing approximately proportionately with electric field at high fields in solvents where p is small and decreasing with electric field in solvents where p is large. If the solvated electron mobility depends on the electric field, then the diffusion coefficient may also depend on the electric field. The implications of these complications are discussed in Sect. 2.2 and in Chap. 8, Sect. 2.7. 

Other probes of the initial distribution have been tried. A few studies of the time-dependent ion-pair recombination probability have been made recently with picosecond pulse radiolysis equipment. A magnetic field alters the rate of interconversion between a triplet and singlet ion-pair. If this rate is fast enough to compete with the recombination rate of ions, the yield of recombined ion-pairs is markedly affected by the magnetic field. 

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The recombination of ions formed from the same solvent or solute molecule by ionisation (geminate ion-pair recombination) is considered in Chap. 7. In the following, only reactions of one ion with homogeneous distributions of the other reactant ion are discussed. Chapter 7 discusses the relationship between these two types of reaction. 

Experimental studies of geminate ion-pair recombination 3.1 INTRODUCTION... 

K. They noted a decay over timescales 95 and < 35 ns, respectively, which was attributed to geminate ion-pair recombination (see Fig. 33). The decay of the optical absorption is independent of the dose of radiation received and continues for about lps. Rather than displaying a dependence on time as eqn. (153), i.e. at f 3/2, the experimental results are more nearly represented by either at f 1 decay to an optical density about one tenth of the maximum or by a decay as t 1/2 to zero absorption. These effects may be the recombination of ions within a spur (or cluster of ion-pairs), which is more nearly like a homogeneous reaction. The range of electrons in propane at 100 K is 10 nm [334] and the extrapolated diffusion coefficient is 10 11 m2 s 1 [320]. The timescale of recombination is 10 ps. The locally greater concentration of ions within a spur probably leads to a faster rate of reaction and is consistent with the time-scale of the reaction observed. Baxendale et al. [395] observed the decay of the infrared optical absorption of the solvated electron in methylcyclo-hexane at 160 K. They noted that the faster decay occurring over < 50 ns was independent of dose and depended on time as t 1/2, i.e. the reaction rate decays as t 3/2, see eqn. (153). It was attributed to recombination of... 

Bakale et al. [397] pulse irradiated the hydrocarbons cyclopentane, cyclohexane and n-hexane with 0.9 MeV electrons of duration 10 or 100 ns. The transient conductivity decreased approximately exponentially with time for low doses of radiation. The first-order decay of the conductance is probably due to electrons reacting with impurities. With higher doses, the conductance decays approximately as inverse time, characteristic of a second-order recombination of free ions. No evidence for time-dependent geminate ion-pair recombination effects was observed. 

THE EFFECT OF A MAGNETIC FIELD ON GEMINATE ION-PAIR RECOMBINATION... 

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