International Journal for Radiation Physics and Chemistry最新文献

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.

In this paper we describe the characteristics of two photodiodes which make them very useful for pulse radiolysis studies. These detectors are the EG & G SHS-100 Si photodiode and the Barnes room temperature A-100 InAs photodiode. The former has a linear output up to at least 6·5 mA, has a 0–98% response time ≌ 15 ns and is distinguished from Si photodiodes previously described by having no detectable slow component to its response. The 0–98% response time of the A-100 photodiode and the circuit developed for it is about 60 ns, and it also shows no detectable slow response component. The output of the A-100 is linear up to at least 2 mA, and we have used it for absorption measurements between 450 and 3200 nm.

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.

A survey is given of recent studies of the primary CH and CC bond dissociations of excited molecules and ions in the gas- and liquid-phase radiolysis of hydrocarbons, which have been mainly carried out by the product analysis method. In the CC bond dissociations evidence has been presented for the fragmentation of the excited parent ion, while in the CH bond dissociations attention has been focused upon an important role of hot hydrogen atoms in the hydrogen formation. A theoretical treatment of highly excited hydrocarbon molecules involving super-excited states is also described.

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⁺.

The decay of O₃⁻ occurs through the thermal dissociation reaction, O₃ ⇌ O₂ + O⁻, for which the absolute rate constant at 22°C was found to be 6·2 ± 1·2 × 10³ s⁻¹.