Andries Hummel

Computer simulation of ion recombination in irradiated nonpolar liquids

The problem of the diffusion controlled recombination of ions for the case where initially two or more pairs of oppositely charged ions are found in each other’s Coulomb field, as occurs in the track of a high‐energy electron in a nonpolar liquid, is treated by means of computer simulation. Results are presented on the probability of survival as a function of time for ion pairs in clusters containing up to four ion pairs initially. The effect of an external electric field on the probability of escape from recombination is investigated. The multipair results are compared with the calculations based on the single‐pair theory. The consequences of the consideration of multipair effects in addition to the single ion‐pair contribution in the nonhomogeneous kinetics of the track of high‐energy electron in nonpolar liquids are discussed briefly.

The direct observation of a highly mobile positive ion in nanosecond pulse irradiated liquid cyclohexane

Abstract The electrical conductivity induced by pulse irradiation of liquid cyclohexane has been studied by means of microwave absorption. The conductivity in pure cyclohexane, due principally to the excess electron, is reduced to less than 10% of the initial value on addition of 5 × 10 −4 M of the electron scavenger SF 6 . The conductivity remaining after addition of SF 6 is however more than an order of magnitude larger than expected for massive ions in cyclohexane and, since it is almost completely removed by the addition of 4 × 10 −3 M of the positive ion scavenger NH 3 , is attributed mainly to the high mobility of the positive hole in this liquid. The ratio of the electron to hole mobility is determined to be 15. The mean lifetime of the hole under the present conditions is 86 ns. The rate constant for reaction of the hole with NH 3 is determined to be 1.8 × 10 11 M −1 s −1 . From the conductivity remaining after removal of both the electron and the hole the sum of the mobilities of the resulting molecular ions is determined to be 8.4 × 10 −4 cm 2 V −1 s −1 .

Electron migration in hydrated DNA and collagen at low temperatures. Part 1.—Effect of water concentration

The migration of charges formed in hydrated DNA and collagen at low temperatures by nanosecond pulses of 3 MeV electrons has been studied by measuring the microwave conductivity on nanosecond and microsecond timescales. The radiation-induced conductivity is found to be critically dependent on the water concentration of the samples. No radiation induced conductivity could be detected below water concentrations of 0.41 and 0.79 gram water per gram dry collagen and DNA, respectively. Above the critical water concentration the conductivity increases approximately linearly with water concentration. It is suggested that the observed conductivity is due to a highly mobile ‘dry’ electron migrating in the ice-like water layer around the biopolymer, with a mobility similar to that of the excess electron in pure ice (2.5 × 10–3 m2 V–1 s–1) but with a considerably longer lifetime.

Effect of Electron Scavengers on the Positive Ion Reactions in the Radiolysis of Cyclopropane Solutions

Electron scavengers such as CCl4, SF6, N2O and CH3Br are found to increase the yield of the secondary positive ion reactions which occur in the radiolysis of cyclopropane solutions in cyclohexane. The observed increase can be understood in terms of an increase in the lifetime of the ion pairs which results when the electron initially formed is converted to a less mobile negative ion. The quantitative aspects can be treated in terms of the change in the mutual diffusion coefficient of the ion pair if one includes in the model the concentration dependence for ion scavenging found when only one solute is present. In the present study electron capture by CCl4 is found to result in a decrease in the diffusion coefficient of the ions by a factor of 17. From this and available experimental information on the diffusion coefficients of positive and massive negative ions the diffusion coefficient of the negative entity initially produced by the radiation in cyclohexane is estimated as 2.6 × 10−4 cm2sec−1. This diffusion coefficient is considerably higher than those for the massive negative ions so that there seems to be little question but that the electron is involved in a more or less free state. The absolute rate constant for the electron scavenging process is estimated to be of the order of magnitude of 1011M−1·sec−1 and that for the positive ion reactions of cyclopropane a factor of 30 smaller.Electron scavengers such as CCl4, SF6, N2O and CH3Br are found to increase the yield of the secondary positive ion reactions which occur in the radiolysis of cyclopropane solutions in cyclohexane. The observed increase can be understood in terms of an increase in the lifetime of the ion pairs which results when the electron initially formed is converted to a less mobile negative ion. The quantitative aspects can be treated in terms of the change in the mutual diffusion coefficient of the ion pair if one includes in the model the concentration dependence for ion scavenging found when only one solute is present. In the present study electron capture by CCl4 is found to result in a decrease in the diffusion coefficient of the ions by a factor of 17. From this and available experimental information on the diffusion coefficients of positive and massive negative ions the diffusion coefficient of the negative entity initially produced by the radiation in cyclohexane is estimated as 2.6 × 10−4 cm2sec−1. This diff...

Geminate ion decay kinetics in nanosecond pulse irradiated cyclohexane solutions studied by optical and microwave absorption

Abstract The decay kinetics of the ions present, on a nanosecond timescale, in a pulse irradiated solution of biphenyl, φ2, in cyclohexane have been investigated using microwave and optical absorption techniques. The ratio of Gmicro (yield×mobility), determined using microwaves, to Gϵ (yield × extinction coefficient), determined optically at 600 nm, is found to increase with time over a period of several hundred nanoseconds for the same conditions of biphenyl concentration and total dose in the pulse. This effect is thought to be due to electrob transfer from φ2− to cyclohexyl radicals. For times longer than 50 ns, the decrease in Gmicro with time is found to obey quite well a linear dependence on the reciprocal square root of the time for solutions φ2, SF6+NH3 and CO2+C6H6. From the intercepts of Gmicro vs 1 √t plots at infinite time, values of the sum of the mobilities of the ions in these solutions are determined to be 6.3, 8.8 and 9.7×10−4cm2V−1s−1, respectively. The slopes of such plots are in good agreement with values derived from ion scavenging data and transport properties of ions in cyclohexane. The experimental data are also in agreement with theoretical calculations based on solution of the Smoluchowski equation for ion pairs but are limited to a timescale which is not short enough to be able to discriminate between different possible ion pair separation distribution functions.

Rate constants for the reaction of positive ions with solutes in irradiated liquid cyclohexane

Abstract The effect of added solutes on the decay kinetics of primary positive ions in irradiated cyclohexane has been studied using the microwave absorption method. Absolute rate constants, in the range of 1 to 3.5 × 10 11 M −1 s −1 , have been determined for the reaction of positive ioins with ethanol, ammonia, cyclopropane, benzene, dimethylaniline and biphenyl. For all of the above solutes the activation energy for reaction is found to be less than 1 kcal/mole. The rate constants obtained are compared with reactivity parameters available from steady-state radiolysis experiments. Taking for the yield of geminate ions a value of 5.0 (100eV) −1 , the characteristic geminate ion lifetime, λ −1 , in pure cyclohexane is estimated to be 2.2 ± 0.5 ps.

Anomalously high rate constants for the reaction of solvent positive ions with soluts in irradiated cyclohexane and methylcyclohexane

Abstract The formation kinetics of the ions TMPD + and pyrene + have been studied in pulse irradiated dilute solutions of TPmPD and pyrene in cyclopentane, n -hexane, cyclohexane, methylcyclohexane, iso-octane and cyclo-octane. In cyclohexane and methylcyclohexane the rates of formation indicate rate constants, (7 ± 4) × 10 11 M −1 sec −1 in the case of cyclohexane, for the reaction of solvent positive ions with the solute which are more than tenfold greater than expected for the reaction of molecular positive ions. In the other solvents, however, rate constants on the order of 10 10 M −1 sec −1 are found.

Geminate ion recombination in irradiated liquid CCl4: As observed by means of microwave and optical absorption pulse radiolysis and compared with calculations based on a single ion pair model

Abstract The geminate recombination of the ions formed in liquid CCl 4 by high energy radiation has been studied by measuring the microwave absorption at 8.7 GHz (X-band) and 29 GHz (Ka-band) as well as the optical absorption as a result of irradiation with pulses of 3 MeV electrons of (sub)nanosecond duration. The observed kinetics has been analysed using a single ion pair model. The survival probability against recombination as a function of time, W , for pairs of oppositely charged ions has been calculated by numerical integration of the Smoluchowski equation. For quite different distributions of initial separations the survival probability at long times is adequately represented by W ( τ )=(1+0.6 τ -0.6 ) W (∞), where W (∞) is the escape probability and τ = Dt / r c 2 the generalized time with D the sum of the diffusion coefficients of the ions and r c the Onsager radius. This equation describes quite well the geminate ion recombination in pure CCl 4 at different temperatures and also in solutions of cyclohexane. Using this equation the sum of the mobilities of the positive and the negative ions in CCl 4 has been determined to be 4.1, 7.6 and 12.8×10 -4 cm 2 V -1 s -1 at -20, 20 and 60°C, respectively. Values of the free ion yield of 0.08, 0.095 and 0.095 (100eV) -1 have been found at the three temperatures, respectively. A comparison of the results observed at short times using the microwave absorption method with the kinetics calculated for different distributions of initial ion separations shows that good fits are obtained for an exponential distribution with an initial yield of G (0)=5 (100 eV ) -1 as well as a Gaussian distribution with G (0)=2.3 (100 eV ) -1 . With the assignment of the optical absorption with a maximum at 480 nm to a product resulting from the geminately recombining ions, the growth kinetics observed on a subnanosecond time scale (risetime ∼ 200 ps) has been compared with the kinetics calculated for the two above mentioned distributions. It is found that the experimental results are in fair agreement with the kinetics expected for an exponential distribution of separations of ions with an initial yield 5 (100 eV) -1 .

Measurement of ionic mobilities in dielectric liquids by means of concentric cylindrical electrodes

When a liquid, placed in an electric field between coaxial cylindrical electrodes, is ionized uniformly by a short pulse of x rays, the current resulting from the subsequent motion of the inward‐moving ions decays with time differently from that due to the outward‐moving ions. This effect provides a convenient method, here described in detail, for assigning mobilities to the positive and negative ions present. Mobilities were determined as a function of temperature for the ions formed in dilute solutions of the electron‐trapping molecules SF6, CH3I, CH3Br, CH3Cl, or O2 in cyclohexane or 2,2,4‐trimethylpentane. In all these cases the negative ions were more mobile than the positive. Only one major negative ion mobility and one major positive ion mobility was to be seen in the solutions, with the exception of O2 in 2,2,4‐trimethylpentane, which gave two negative ions of different mobilities.

The measurement of a positive charge mobility larger than that of the excess electron in irradiated liquid trans‐decalin

The ion mobility for irradiated trans decalin below 10°C is reported.(AIP)The ion mobility for irradiated trans decalin below 10°C is reported.(AIP)