International Journal for Radiation Physics and Chemistry最新文献

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 effects of γ-radiation on aqueous solutions of myo-inositol hexasulphate in the free acid form have been studied. The results have been explained on the basis of radical initiated chain processes which lead to the observed high G-values for the ultimate inorganic product namely the sulphate ion and to low hydrogen peroxide yields. In addition, steady-state competition procedures and pulse radiolysis experiments show that the reaction between the OH and the substrate is slow.

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 role of excited molecules and excitation transfer in the radiolysis of solid alkanes at 77 K is described. Studies of the effect of electron scavengers in the radiolysis of solid alkanes show that hydrogen formation is mainly due to a nonionic process. Though the yield of hydrogen in the radiolysis of solid isobutane at 77 K is not affected by the addition of N₂O or SF₆, it decreases sharply upon the addition of CCl₄. Similarly, the yield of C₄H₉ radical in the radiolysis of isobutane at 77 K is not changed by the presence of conventional electron scavengers; however, it decreases remarkably upon the addition of CCl₄ or toluene. When 2,3-dimethylbutane (23DMB) containing a small amount of toluene (To) is irradiated with γ-rays or nanosecond pulses of X-rays in the solid phase at 77 K, luminescence from excited toluene is observed. The emission spectrum from 23DMB-To (2 mol%)-N₂O(2 mol%) during γ-irradiation at 77 K is similar to that from 23DMB-To (2 mol%) during U.V.-illumination. The formation of the singlet-excited toluene is not affected by the addition of electron scavengers and hole scavengers. These results are explained by excitation transfer from alkane to additives. The possibility of exciton migration in the radiolysis of solid alkanes is discussed by a simple theoretical treatment.

A study of the radiolysis of monocrystalline, liquid and aqueous solutions of methane sulphonic acid is reported. E.S.R.-measurements on γ-irradiated single crystals show that two radicals ·CH₂SO₂(OH) and CH₃ŻSO₂(OH)⁻ are formed. Sulphonic acid-water mixtures were investigated by pulse radiolysis. A short-lived transient absorption with maximum at 450 nm is assigned to the radical ion CH₃ŻSO₂(OH)⁻ (ϵ = 1740 dm³ mol⁻¹ cm⁻¹). Two longer-lived transient absorptions at 263 and 347 nm in concentrated solution were shifted to 242 and 332 nm respectively with increasing water concentration.

The 242/263 nm absorption is ascribed to the ·CH₂SO₃⁻ radical and its protonated form and the 332/347 nm absorption to the CH₃SO₂· radical formed by the decomposition of the CH₃ŻSO₂(OH)⁻ radical ion. The interpretation of E.S.R. and optical spectra is based on the assumption of a hydrogen bonding structure of the sulphonic acid.

Various picosecond and nanosecond pulse radiolysis studies of excited states in alkane and erene liquids are described. In particular, the origin of the excited states is sought after. In alkanes ion neutralization gives rise to excited triplet states of solutes. Singlet energy transfer from excited alkanes to solutes gives rise to the excited singlet states of the solutes. In aromatic liquids both the excited singlet and triplet state of the solvent are observed, and energy transfer to solutes is readily established. The role of ion neutralization in the formation of excited states is proportionally less pronounced as the observed yield of ions is low. However, indirect experiments suggest that the ion neutralization reaction is too rapid for conventional observation, and even in arenes large yields of excited states are formed by ion neutralization.

The conditions determining the limits of applicability of the diffusion-drift model (DDM) of radiolysis of non-polar organic fluids containing additives of acceptors of charged particles have been obtained. The validity of the DDM for saturated hydrocarbons is associated with the localization of the electrons which move in the medium by thermally activated jumps from one localized state to another. It is shown that the use of the DDM in radiation-chemical kinetics is justified if the lenght of localization l (or the mean length of the free path Λ) of the electrons is small as compared with the characteristic dimension of the problem. For describing the death of ion pairs such a characteristic dimension is the mean initial distance between their components and for calculating the rate constants of the reactions of recombination and capture of the charges by an acceptor the radius of the absorbing sphere R. The corrections arising from calculation of these constants for the systems where l_L (or Λ) is greater than or comparable with R are evaluated.

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.