International Journal for Radiation Physics and Chemistry最新文献

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.

The mechanism of cyclohexane radiolysis in the condensed phase is discussed with particular attention paid to the cleavage of CC bonds. The contribution of ions and various neutral excitation states to the formation of radiolysis end-products is analysed. As follows from the experimental data shown, short-lived (≈10⁻¹²) superexcited states (S.E.S.) take part in the cyclohexane radiolysis reactions in the liquid phase. The basic process accompanying the decay of S.E.S. molecules is the formation of hydrogen atoms which may acquire excess kinetic energy (“hot atoms”). It is assumed that inspection of hot H atoms along the CC bond constitutes the main source of saturated products formed by cleavage of the cyclohexane ring (C.R.C.P.=cyclohexane ring cleavage products): CH₄, C₂H₆, C₆H₁₄ and so on. In condensed medium, the cleavage of two or more CC bonds in the same molecule, giving rise to unsaturated C.R.C.P. (C₂H₄, C₃H₆, etc.), originates mainly from S.E.S. A smaller amount of unsaturated C.R.C.P. is probably formed via the decay of excited molecular ions. Cleavage of a single CC bond with formation of hexene proceeds effectively from lower excited states which derive particularly from ion-recombination.

Experimental data concerning the incidence of polymorphous transformation in cyclohexane on the C.R.C.P. yields are given. A general scheme for cyclohexane radiolysis, based on the cleavage of CH bonds, is postulated.

The experimental arrangements used by the authors for the study of optical vacuum ultra-violet and electron energy loss spectra of organic compounds are described and some theoretical aspects of studies of higher excited states are considered. Results for alkanes, benzene, naphthalene, anthracene and some more complex hydrocarbons are reviewed. Recent results obtained by reflection and electron energy loss spectroscopy for single crystals of anthracene are included and their relevance for gas phase work as well as for the understanding of exciton effects in organic solids is described.

The reaction of OH radicals with aqueous tris(1,10-phenanthroline)iron(II) leads to the formation of an adduct, which exhibits a broad absorption band at rmpH = 6, λ_{max = 460 nm}, and ϵ₄₆₀ = 6700 (molar, decadic, 1 mol⁻¹ cm⁻¹). The rate of formation of the adduct is first order in complex concentration with a bimolecular rate constant $\text{k = (1 >}\text{s}\text{̇}\text{;05±0 >}\text{s}\text{̇}\text{;03)× 10}{}^{10}\text{1 mol}{}^{\mathrm{-1}}\text{s}{}^{\mathrm{-1}}$ independent of pH in the range pH 3–11. The adduct decays by mixed-order kinetics, but at 310 nm a second-order formation of a decay product can be directly observed.

The reaction of OH radicals with aqueous 1,10-phenanthroline leads also to the formation of an adduct which absorbs in the whole visible region with a maximum at 425 nm and ε₄₂₅ = 2612 (molar, decadic, 1 mol⁻¹ cm⁻¹) in neutral solution. The adduct exhibits a red shift in acidic and alkaline media. The formation is first order in 1,10-phenanthroline with a bimolecular rate constant $\text{k = (8 >}\text{s}\text{̇}\text{;6±0 >}\text{s}\text{̇}\text{;7)×10}{}^9\text{1 mol}{}^{\mathrm{-1}}\text{s}{}^{\mathrm{-}}$ at pH = 6; the rate of formation is pH dependent. The adduct decays according to second-order kinetics leading to the formation of a product that exhibits the presence of an additional functional group. A similar product, determined to have pK⋟4 > ṡ;2 and pK₂⋟8 > ṡ;1, has been isolated after γ-radiolysis of 1,10-phenanthroline.

The influence of electron acceptors on the yield of trapped electrons in frozen aqueous solutions is examined, and the results of picosecond radiolysis is discussed. It has been shown that the correlations observed cannot be explained if thermal electrons only take part in reactions occurring in such matrices, whereas they can be correctly interpreted assuming that epithermal electrons are also capable of reacting with acceptors in a condensed medium.

Experimental data obtained are discussed on the basis of these concepts. The times needed for thermalization, solvation and capture have been evaluated, and the rate constants of the reactions of epithermal electrons computed.

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 e_aq⁻ or H.

Due to their diffusive motion at energies ⩽10²eV and to their comparatively long lifetimes in the subvibrational energy range at the end of their tracks, positrons may form positronium by a reaction with (epi) thermal electrons. The order of the reactivity of solutes towards the precursors of positronium is the same as that towards precursors of the molecular hydrogen yield and is in agreement with the order of their relative rate constants towards the non-solvated electron.

It seems that the non-solvated electron takes part in the formation of positronium and of radiolytic H₂.

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 reaction of e_aq⁻ with halogenated aromatic compounds (fluoro-, chloro-, bromobenzene, and 2-, 3-, 4-chloro-aniline) was studied by conductometric pulse radiolysis and under steady-state conditions. It was established that in the pH range from 4 to 10 all e_aq⁻ are reacting with the compound in question by quantitative splitting of the halide anion (X⁻). Considering certain experimental factors G(X⁻)=2.7 was obtained by both methods for all investigated substances. This fact is in disagreement with previous observations on various halogenated compounds. However, it can be explained taking all possible reactions for each system into account.

Exposure of aqueous alkaline glasses to ⁶⁰Co γ-rays followed by photobleaching to remove e_t⁻ resulted in E.S.R. spectra dominated by O_t^- and H·_t^-, but at ca. 110 K H·_t was lost and e_t⁻ reappeared.

Electron spin resonance studies of phosphorus compounds containing PH bonds, after exposure to ⁶⁰Co γ-rays, show that electron addition is followed by movement of hydrogen into the axial position of the resulting phosphoranyl radical. Subsequent thermally induced, irreversible, pseudo-rotations placed the hydrogen ligands in the equatorial position.

Electron loss was always accompanied by loss of the proton bonded to phosphorus, giving phosphoryl radicals. A range of other species were also detected, and their identities and reactivities are discussed.