International Journal for Radiation Physics and Chemistry最新文献

The decay of $\text{C}\text{}$HO radicals in U.V.- and X-irradiated methanol-water matrices has been studied. The decay rate is not dose dependent in the dose range investigated (45–300 krad) and a certain fraction of the radicals are practically stable at 113 K. The decay is significantly different in X-irradiated and U.V.-irradiated samples. This is probably due to the existence of spurs in the X-irradiated samples.

Information on the effect of γ-irradiation on the emission behaviour of dopants benz(f)indan and anthracene in a single crystal of fluorene is presented. Quenching is of the Stern-Volmer type at low doses (up to 10.6 Mrad) but the effect tends to saturate at higher doses. The same quenching parameter for the irradiation products is found for benz(f)indan and anthracene.

Gamma irradiation of a 3-methylpentane (3MP) glass containing triphenylamine (TPA) at 72–74K produces the trapped electron (e_3MP⁻) and another negative species (e_TPA⁻). Like e_TPA⁻ absorbs in the I.R. with a peak at 1.6 μm. In contrast with e_3MP⁻, e_TPA⁻ is not bleached by 1.9 μm light, has a narrower absorption band, and exhibits a broader E.S.R. singlet with ΔH_{ms = 6.6 G}. At 72K, the 100 eV yield, G(e_TPA⁻), increases with increase in TPA concentration and has the value 1.3 at 10⁻² mol dm⁻³ TPA while G(e_3MP⁻ decreases from 0.4 in neat 3MP to 0.1 at 10⁻² mol dm⁻³ TPA. Gamma irradiation of an ethanol glass containing TPA at 77K does not produce an infrared absorption but produces peaks at 440 and 410nm in addition to the trapped-electron peak at 540nm. The peaks at 440 and 410nm are attributed to TPA⁻ because absorbance at these peaks increases with increase in TPA concentration and that at 540nm decreases. The totality of the results for alcohol and 3-MP glasses suggests that e_TPA⁻ is an electron in a trap similar to that of e_3MP⁻ but modified by the presence of a TPA molecule whose positive nitrogen end is oriented toward the electron.

The rate constants for the reactions of electrons with biphenyl, pyrene, perylene and carbon tetrachloride in n-hexane can all be expressed by the relation $\text{k = 3·6(±1) × 10}{}^{14}\text{exp}\text{[}\text{−14,600(±1000)}\text{RT}\text{]}\text{mol}{}^{\mathrm{-1}}\text{dm}{}^3\text{s}{}^{\mathrm{-1}}$ over the temperature range 300-185 K. These are considered normal diffusion-controlled reactions, the activation energy (in J mol⁻¹) being that for electron diffusion. For reaction with oxygen, $\text{k = 5×10}{}^{12}\text{exp (}\text{−9600}\text{RT}\text{]}$ and the relatively slower rate for oxygen thus derives from the pre-exponential factor not from a higher activation energy, and is explained in terms of a reversible electron addition.

In iso-octane there is more variation with solute than in n-hexane. Forcarbon tetrachloride, biphenyl and oxygen the activation energies are 2000, 2800 and 3200 J mol⁻¹, respectively, and the pre-exponential factors 1·5×10¹³, 4·0×10¹³ and 1·0×10¹², respectively. The rate constant with pyrene does not follow the Arrhenius expression and is about twice those for carbon tetrachloride and biphenyl at room temperature.

Although some uncertainty concerning the yield of triplet excited states exists, the yields of ions and atoms predicted from electron impact cross-sections and other information are in relatively good agreement with experimental data for the radiolysis of H₂. A substantial contribution to the atom yield comes from excited states lying just below and above the ionization potential. Less information is available for hydrogen halides. However, while rapid auto-ionization occurs for some superexcited states of HCl, there is again evidence for disscoiations- in this case to form H(1s) and excited Cl. The occurence of this type of process is supported by energy balance calculations for both HCl and HBr. The behaviour of the low-lying excited states of these molecules is relatively well understood.

Electron impact dissociation following molecular electronic excitation is shown to be one of the main dissociation channels for most molecules and the main one for a number of them. The calculated cross-sections and the rate constants for the dissociation process are given for a number of molecules. The influence of vibrational and rotational excitation of the target molecules on the cross-sections and the rate constants are analysed.

The radioluminescence of pure, frozen methylcyclohexane (MCH) has been investigated. λ-Irradiation of MCH at 77 K produces at least three negative species—trapped electrons, e^-_t, and two types of anions, which are formed in the reactions of electrons with radicals, R^-, and with stable radiolysis products, P^-. It is shown that the radiothermoluminescence (RTL) emission is due to the recombination of e_t-(RTL peak at 90 K) and R^-, P^- (RTL peak at 95 K) with cations. The intensity-dose dependence of the RTL peaks has been interpreted using a simple kinetic model.

This paper summarizes results obtained earlier from E.S.R. studies of γ-irradiated n-alkane single crystals. It also contains some new experimental results that serve to give a more complete picture of the deposition of radiation energy in solid alkanes. The experiments performed with solid n-alkanes have thus far provided structural data that permit the nature and even the conformation of alkyl radicals to be clearly understood. Two types of radical exist, namely, one where the unpaired electron is located next to the end methyl group and one with the unpaired electron in the interior of the chain. The first type has a conformation which differs from that of the undamaged molecule. Microwave saturation data show that there is a difference in relaxation properties of these radicals which can be understood in terms of a difference in mobility. Relative yield measurements give the distribution of isomeric alkyl, the result differing from that obtained using product analysis in liquids. For protiated n-alkanes n-alkyl is lacking and the 2-alkyl concentration is higher than expected. For deuterated n-alkanes the E.S.R. spectrum is mainly that of radicals with the unpaired electron located in the interior of the carbon chain. This isotope effect is again contrary to observations in liquid n-alkanes.

The broad lines observed in protiated alkanes irradiated at 77 K and deuterated alkanes irradiated at 4.2 K are not believed to arise from strong spin-spin interactions. They are thought instead to arise from distorted crystal and radical structures relating to the damage regions of the crystals.

Radical pairs exist with different stability, yield and structure. Our estimate, in deuterated alkanes, that as much as 40% of the radicals are formed in pairs or clusters at 4.2 K shows that radical pair formation is an important radiolytic process in solid n-alkanes. Energy transfer from deuterated to protiated molecules has also been experimentally verified. This energy transfer is temperature dependent and occurs efficiently at 77 K, but less efficiently at 4.2 and 273 K. A relationship also exists for deuterated alkanes between the amount of radical pairs formed at 4 K and the long-range energy transfer at 77 K. This can be readily accounted for by an exciton transfer mechanism analogous to that in aromatic crystals. It might also be qualitatively described by a hydrogen abstraction process. In this case, however, the properties of the deuterium atoms are not understood.