Geminate recombination time scale

Consider particles i, j,k which can all react with each other at geminate times tij, tik and tjk- If particles i and j react to produce I (which replaces i), then the diffusive trajectory for Ik will be the same as ik had reaction not occurred but it will be followed with the new relative diffusion coefficient. Hence, the new species I follows the same trajectory path as species i would have, with a new relative diffusion coefficient up to the encounter distance point The new scaled reaction time for the geminate recombination time Ik is then given by... 

Experiments on 1 -CO using benzene in place of CTAB were also done to examine the effects solvent and environment on the photodissociation. None were found. The photointermediates arrived at the same time, had the same peak wavelengths, extinction coefficients and band shape. In so far as the dynamics observed in these experiments are independent of CO pressure and since there is no detectable geminate CO recombination, it is reasonable to expect effects on the photodissociation due to solvation to be minimal as diffusion has not yet occurred on the time scale studied. 

Fast pulse radiolysis studies have shown that geminate recombination occurs on the picosecond time scale [12,13]. Bartczak and Hummel [14] predicted that for -dodecane, 82% of the geminate ions still remain at 5 psec for 1 MeV irradiation. Future accelerators, with pulses of a few picoseconds length, may soon provide experimental measurements of Gtot directly. 

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... 

It is to be noted that, after geminate recombination, when diffusion takes place on the nanosecond and longer time scale, reactions between the radiolytic species still occur and, in the absence of any other solutes, those reactions are responsible of the disappearance of the solvated electron. 

Figure 5 depicts the decay of the solvated electron due to spur reactions in two different solvents, water and tetrahydrofuran. In both liquids, the solvent relaxation is very fast (less than 1 ps), therefore, the absorption signals on the picosecond time scale are due to the fully solvated electron. As the dielectric constant of tetrahydrofuran is low (e = 7.6 compared to 80 for water), the electrostatic attraction is not screened by the solvent and geminate recombination between the solvated electron and the cation can occur over long separation distances in contrast to water. Moreover, the mobility of ej in THF is roughly three times higher than that in water. That explains why the decay ofthe solvated electron is more important in tetrahydrofuran compared to water [19]. 

It is to be noted that, after geminate recombination, when diffusion takes place on the nanosecond and longer time scale, reactions between the radiolytic species still occur and, in the absence of any other solutes, those reactions are responsible for the disappearance of the solvated electron. Besides, the metastability of the "blue" solutions of alkali metals in liquid ammonia is due to the fact that the solvated electron does not react with another solvated electron and that it reacts extremely slowly with the protonated form (NH/) which is at an extremely low concentration. 

After dissociation, a part of the separated atoms combines by a geminate recombination within a picosecond time scale and the diffusive geminated recombination is negligible after more than 1 ns. Since the thermal energies... 

With the intensive development of ultrafast spectroscopic methods, reaction dynamics can be investigated at the subpicosecond time scale. Femtosecond spectroscopy of liquids and solutions allows the study of sol-vent-cage effects on elementary charge-transfer processes. Recent work on ultrafast electron-transfer channels in aqueous ionic solutions is presented (electron-atom or electron-ion radical pairs, early geminate recombination, and concerted electron-proton transfer) and discussed in the framework of quantum theories on nonequilibrium electronic states. These advances permit us to understand how the statistical density fluctuations of a molecular solvent can assist or impede elementary electron-transfer processes in liquids and solutions. 

Formation of excited states of hydrocarbons via geminate recombination has been studied by detection of emission on the picosecond time scale using a streak camera [18]. Single-shot streak cameras with resolution on the order of 200 fs are now available. 

Complex mechanism of class (ii) and (iii) reactions may account for the puzzling result in the studies on radiation-induced fluorescence in cis- and trans-decalins containing 3-100 mM of benzene [54], where it was concluded that on the time scale of geminate recombination of primary pairs in trans-decalin (< 1 ns), the hole is scavenged by benzene with rate constant of 7.7x1010 g-l (vs. (5-5.5)xl09 M l s-1 observed in the transient conductivity experiments [7,8,12,14]). This was taken as evidence for the involvement of short-lived, reactive excited solvent holes. 

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