This paper reviews the work of the author and his co-workers on the radiation-induced formation of excited states of aromatic compounds in solution. The experimental methods used are surveyed and in particular the method of measuring the yields of triplet and singlet excited states of the solute are described. The problems discussed are: (1) the effect of solvent on the yields of excited states, (2) formation of excited states in cyclohexane and other alicyclic hydrocarbons, (3) the formation of excited states in benzene and (4) the identification of T-T absorption spectra.
The main kinetic patterns of the radiophotoluminescence (RPL) of organic substances at 77 K have been studied. It was established that the kinetics of the long-lived RPL component is determined by the sum of two processes: monomolecular, corresponding to the spontaneous deactivation of the charges of the triplet molecules formed on recombination of charges, and quasi-monomolecular the kinetics of which is determined by the recombination of electroncation pairs. For a number of aliphatic substances at radiation doses of more than 1 Mrad it was possible to find quenching of RPL manifest in an appreciable change in the luminescent characteristics. From the experimental findings it may be concluded that the quenching observed is due to the free radicals formed in the course of radiolysis and which are effective electron acceptors. A theoretical model of RPL is proposed based on the concepts of the tunnelling mechanism of charge migration in the irradiated frozen organic substances. In examining the processes of paired recombination of spatially separated charges consideration was given to the role of the processes competing with them for capture of one of the components of the pair (electron) by free radicals. The model satisfactorily explains the experimentally observed hyperbolic character of the decay of the long-lived RPL component, the shortening of the duration of luminescence with a rise in the radiation dose and an absence of the effect of temperature on the kinetics of RPL.
As difficulties are encountered with model theories in describing the nature of solvated electrons, it is suggested that possible non-model relations in the theory of solvated electrons be used. In the present work the results that follow from the sum rules, virial theorem and threshold formulae of the quantum theory of scattering are examined. In this way it is possible to establish relationships that can be used for determining some of the physical characteristics of solvated electrons directly from optical data.
Almost all theoretical papers on the mechanism of radical reactions in the tracks of ionizing particles implicitly assume the independence of the radical distributions. In this paper the effect of correlations arising between the radical distributions in the course of the reaction process on its change in time and the product yields is studied. The problem has been formulated in terms of the joint probabilities of radical pairs according to a one-spur-one-radical model. It is found that the combination and recombination processes in real spurs must be more effective than prescribed by one-radical models assuming the independence of the distributions of the reacting particles. The results of those theories correspond with the results from our model only for local concentrations which are too low to be realistic. On the other hand, for the relative lowering of the molecular yield with increasing scavenger concentration this model gives the same deviations from the experiment for higher scavenger concentrations as other one-radical models.
The kinetics of positive ion and electron scavengin gin certain n-alkanes, cycloalkanes, branched alkanes, aromatic hydrocarbons and in some polar compounds (ethers, dimethyl-formamide) have been investigated.
Pulse irradiation of PtCl42− in aqueous solutions containing Cl− or Br− indicates that oxidation of Pt(II) by OH may be wholly or partially replaced by oxidation by Cl2− or Br2−, depending on the efficiency of OH scavenging by the particular halide ion. The Pt(III) species produced by the latter reactants absorb respectively at 290 and 310 nm and differ from that produced by OH. Contrary to the similar PdCl42− no evidence of Pt(II) co-ordinating more than four Cl− ions has been obtained. The rate constants of the processes and extinction coefficients of the transient species observed are reported.
Electron scavenging efficiencies have been measured at 77 and 4 K in ethylene glycol-water glass for the following scavengers which span a 250-fold range of scavenger efficiencies at 77 K: HCl, NaNO3 and K2CrO4. The range of scavenging efficiencies decreases to 62 at 4 K with the largest relative change occurring for the less efficient scavengers. These results are suggested to be most consistent with a model in which scavenging occurs by tunneling from shallowly and deeply trapped electrons at 4 and 77 K, respectively.
The authors have investigated the current of electrical conductivity I induced by a pulse of 2 × 10−7 s of high energy electrons in 3-methylpentane (3MP) over the temperature range 94–296 K. The determination of the drift mobility of the excess electron u is based on measurement of the amplitude of I in conditions in which the lifetime of the excess electron in 3MP τ ⪢ 2 × 10−7 s and the total number of free charges q generated in the course of one pulse. In liquid 3MP q was measured by integrating I and in solid 3MP by integrating the current of thermostimulated conductivity on heating the sample to melting point. At T = 296 K the value u = 0·22 cm2 V−1 s−1 and agrees with the value obtained from measurements of the time of transit by the electron of the distance between electrodes d≈0·1 cm.
Several fundamental reactions have been investigated using nanosecond pulse radiolysis. The following reaction rate constants were measured: k(N2O−→N2+O−)>108s−1, k(H++O2−)= (5 ± 1) × 1010 dm3 mol−1 s−1. k(HO2→H++O2−) = (7 ± 2) × 105 s−1, k(CO2−+O2) = (4.2 ± 0.4) × 109 dm3 mol−1 s−1.
Gas phase radiation chemistry yield data and electron impact cross-section data are used to derive excitation mechanisms and to discuss the role of excited states in the radiation chemistry of O2, N2, N2O, CO, CO2, H2S, H2O and NH3. For each of these systems available cross-sections for ionization and neutral excitation are listed, together with relevant reaction rate data and a summary of the radiation chemistry studies at both high and low dose rates. In general, fairly complete mechanisms are derived and further tested by energy balance calculations. In order to present as complete a picture as possible, a summary of rates and products of ion-neutralization reactions is given at the end of the paper.
Scanning electron microscopy has been used to examine γ-irradiated purine and pyrimidine bases in dry powder form. Channelling is observed in some of the samples. Surface changes appear to be correlated with radiosensitivity as measured by other techniques.
Tryptophan and naphthalene in boric acid glass at 77 K give rise to a delayed fluorescence under U.V. excitation and to a photostimulated delayed fluorescence under visible light excitation of an irradiated sample. It is shown that these emissions are due to electron-cation recombination and that, beside the chemical trapping of electrons by protons which is the main process, a physical electron trapping occurs in this strongly acidic medium. This unexpected property is related to the inhomogeneous structure of boric acid glasses which plays an important role in the unique properties of this medium for photoionization studies of aromatic compounds.
A pulse generator is described which produces the trigger pulses required by a linear accelarator and also the pre-pulses required for pulse radiolysis experiments. The latter are adjustable in 1 μs increments from 0 to 9999 μs and in wider steps to 2 s.
Exposure of aqueous alkaline glasses to 60Co γ-rays followed by photobleaching to remove et− resulted in E.S.R. spectra dominated by Ot- and H·t-, but at ca. 110 K H·t was lost and et− reappeared.
Radiolysis of frozen aqueous or methanolic glasses containing sodium borohydride gave E.S.R. features characteristic of ·BH3− as the major radical product. These radicals were strongly associated with the solvent lattice in contrast with their behaviour in the parent salt. Additional E.S.R. features separated by ca. 200 G are assigned to BH4, but since this spectrum could never be obtained in the absence of the intense BH3− features, this assignment is tentative.
Pulse radiolysis of adrenaline in acid aqueous solutions (pH 1–3) was carried out. The rate constants for the reactions of adrenaline with H and OH were determined: k(H + adr.) = (0·9±0·1) × 109 dm3 mol−1 s−1; k(OH + adr.) = (1·65±0·15) × 1010 dm3 mol−1 s−1. The H-adduct of adrenaline has two λmax, at 280 and 355 nm, with ϵ280 = 420 m2 mol−1 and ϵ355 = 390 m2 mol−1, which disappears according to a first order reaction, k1 = 1·4 × 103 s−1. The spectra formed by OH attack was assigned to the corresponding benzoxy radical with absorption maxima at 285 and 365 nm and ϵ285 = 620 m2 mol−1 and ϵ365 = 105 m2 mol−1. Due to the overlapping of the intermediates, no decay kinetics could be obtained.
Short-lived free radicals formed in γ-irradiated methanol, ethanol and 2-propanol have been detected by E.S.R. combined with spin trapping method using 2,4,6-tri-t-buthylnitrosobenzene (BNB) as a spin trap. The observed spectra could be identified clearly by the help of deuterated alcohols. The hydrogen spin adduct, which had not been clearly identified in earlier works, was observed together with alkoxy radicals and other radicals. It was revealed that a part of the hydrogen adduct could come from proton transfer to the anion radical of BNB, which was produced by electron capture during irradiation. From the change of ∗CH2OH/CH3O∗ ratio with that of added spin trap concentration one may conclude that the methoxy radical and hydrogen atom are primary radicals in radiolysis of liquid phase methanol. This conclusion is also supported by both the change of the ratio with irradiation temperature and the isotope effect on the spin adduct yields.
The dependence of H2 yields on the concentration of certain solutes indicates that molecular hydrogen is formed via recombination of its precursors in spurs. The order of efficiency of solutes in decreasing the formation of H2 does not correlate with the rate constants of their reactions with eaq− or H.
The reactivity of the oxide radical ion, O−, and the decay kinetics of the ozonide ion, O3−, have been investigated in aqueous solutions at pH 12·8, in the presence of tetramethyl-, tetraethyl-, tetrapropyl- and tetrabutylammonium ions, O2 and N2O. Using pulse radiolysis to follow the O3− kinetics gives information on the competition of O2 and R4N+ for O−. Absolute rate constants for these reactions are: 3·0 × 108 dm3 mol−1 s−1 for O− + (CH3)4N+, 1·0 × 109 dm3 mol−1 s−1 for O− + (C2H5)4N+, 1·3 × 109 dm3 mol−1 s−1 for O− + (C3H7)4N+ and 2·2 × 109 dm3 mol−1 s−1 for O− + (C4H9)4N+.
