Stefan J. Rzad

KINETICS OF ELECTRON SCAVENGING AND ION RECOMBINATION IN THE RADIOLYSIS OF HYDROCARBON SOLUTIONS.

A functional description of the lifetimes of the ion pairs produced in the radiolysis of pure hydrocarbons is derived from the concentration dependence observed for ion scavenging. A number of consequences which follow from this description are explored. In particular it is shown that in the absence of scavenger the decay of the geminate ions must be very closely described by F(t) = eλterfc(λt)1/2, where λ is a constant and F(t) is the fraction of ions present at time t. Appropriate descriptions for the growth and decay of secondary ions are given, and the results are applied to real systems of finite pulses. Comparison is made between the predictions given here for the time dependence of the population of secondary negative ions and the experimental observations of Thomas and co‐workers on the decay of diphenylide negative ion in pulse irradiated cyclohexane solutions of diphenyl. From this comparison it is concluded that the rate constant for electron scavenging by the diphenyl is ∼ 3 × 1011M−1·sec−1 an...

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...

Relevance of the Concentration Dependence of Methyl Radical Formation from Methyl Bromide Solutions to the Distribution of Ion‐Pair Separation Distances in Irradiated Hydrocarbons

A radiochromatographic technique for measuring the yields of methyl radicals formed on electron capture by methyl bromide in irradiated liquid hydrocarbons is described. G(CH3) has been measured in n‐hexane, cyclohexane, and iso‐octane from 3 × 10−6 to 0.3M CH3Br. Below 3 × 10−4M the yields are described by the equation G(CH3) = Gfi + K(CH3Br)1/2 in agreement with theory. The values of Gfi (free‐ion yield) determined are 0.12, 0.13, and 0.36 for n‐hexane, cyclohexane, and isooctane, respectively. Over the entire concentration range studied, G(CH3) can be described by the empirical expression G(CH3) = Gfi + (Ggi(αCH3Br[CH3Br])1 / 2 / {1 + (αCH3Br[CH3Br])1 / 2}). A value of approximately 4 is found for the yield of geminately recombining ion pairs for the three hydrocarbons. It is argued from the high‐concentration data that the distribution of thermalization distances are closely similar for the different hydrocarbons, in contradiction to the predicted longer distances in iso‐octane required to explain the...

Effect of charge scavengers on the scavenging of charges of opposite sign. The consequences on the steady state and pulse radiolysis of liquid hydrocarbons

The scavenging of positive ions and electrons in hydrocarbons is re‐examined in terms of a model where the mobilities of positive and negative charges both decrease upon scavenging. The phenomenological model is applied to steady state and pulse radiolysis and explicit expressions describing such phenomena are obtained. The quantitative results obtained for cyclohexane by treating the available experimental data in this manner are discussed. Specifically the mobility of the hole is estimated to be ∼ 2 × 10−2cm2 V−1sec−1 and λ, the constant characterizing the ion recombination processes in pure cyclohexane is interpreted to be ∼ 7 × 1011sec−1, which indicates that half of the original ions recombine in ∼ 1 psec. Estimates of ∼ 7 × 1011M−1sec−1 and ∼ 1013M−1sec−1 are obtained for the rate constants for scavenging of positive ions and electrons respectively. It is shown that the presence of a hole scavenger should affect drastically the time dependent behavior of the secondary negative ions and optimum condi...

Quenching of aromatic hydrocarbon fluorescence by cesium chloride

Abstract Fluorescence quenching of nine aromatic hydrocarbons by cesium chloride in methanolic solution has been measured using pulse fluorimetry. A log-log relationship between k q , the rate constant for quenching by CsCl, and k S→T , the intersystem crossing rate of the fluor, appears to be applicable to these systems. This behavior is interpreted in terms of heavy atom quenching by CsCl.

Electron Scavenging in γ‐Irradiated Liquid Methanol and Ethanol

Electron scavenging in liquid methanol and ethanol has been examined as a function of 14CH3Br and SF6 concentration in the region from 10−5 to 0.3 M. The empirical scavenging model previously proposed for hydrocarbons can be successfully extended to alcohols if a competitive first‐order reaction of the solvated free electron is included. Solvated free‐electron and geminate‐electron yields have been determined to be 1.05 and 3.1. Analysis of the data from competitive experiments leads to an estimation of the absolute rate constants for reaction of solvated electrons with CH3Br, CH3Cl, SF6, N2O, and acetone in methanol and ethanol and to an estimation of the half‐life of the solvated free electrons in both solvents. The present results in ethanol can be correlated with the observations of Thomas and Bensasson on the nanosecond decay of solvated electrons in terms of an ion‐pair lifetime distribution function similar to that applicable to hydrocarbon radiolysis.

Effect of positive ion scavengers on recombination times: An attempt to use secondary electron transfer reactions as a probe for the existence of a high mobility hole in cyclohexane☆

Abstract In order to probe for the existence of a high mobility hole in cyclohexane, secondary electron transfer reactions between CH 3 Cl − and other electron scavenging solutes have been studied in the absence and presence of positive ion scavengers. There is no evidence in these experiments for a change in the time scale for neutralization of the negative ions which can be attributed to trapping of the positive ions.