International Journal for Radiation Physics and Chemistry最新文献

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.

When alkane containing a small amount of toluene is irradiated by γ-ray or X-ray in the solid state at 77 K, luminescence from excited toluene is observed. The formation of the excited toluene is not due to direct excitation of toluene by γ-ray or Čerenkov light, but to the rapid energy transfer from the irradiated alkane to toluene. The intensity of luminescence from the excited toluene in the cyclopentane matrix does not correspond at all to the amounts of toluene ions, which are formed by pre-irradiation. The energy transfer is not caused by a free hole and an electron, but by an exciton. Furthermore, the effect of additives on the fluorescence suggests that the Wannier exciton or mobile ion-pair is responsible for the energy transfer in cyclopentane containing toluene in the solid phase at 77 K.

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.

Dissociative electron attachment reaction to CH₃I, CH₃Cl and CH₃F in a 3-methylhexane glassy matrix was studied by determining the yield of trapped electrons and that of methyl radicals immediately after γ-irradiation at 77 K as a function of the scavenger concentration. The efficiency of conversion from the trapped electrons to the methyl radicals was also studied by photobleaching the trapped electrons. The results obtained are (1) the dissociative electron attachment occurs to CH₃F, for which the gas phase data indicate that the reaction is endothermic by 1·2 eV, during either the γ-irradiation or the photobleaching, and (2) CH₃F is relatively less efficient in scavenging photo-liberated electrons than in scavenging the electrons during the γ-irradiation, whereas CH₃I and CH₃Cl are efficient scavengers for both the electrons. The dependence of the yields of the trapped electrons and the methyl radicals is discussed in terms of the electron-tunneling mechanism and the epithermal electron-scavenging mechanism.

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.

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.

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.

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.