International Journal for Radiation Physics and Chemistry最新文献

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.

In irradiated methanolic solutions of triphenylphosphine (Ph₃P) deaerated with argon or saturated with N₂O a transient spectrum has been detected. This spectrum is attributed to Ph₃PCH₂OH and a value of k_(CH2OH+Ph3P) = 8.8 x 10⁹mol⁻¹dm³S⁻ has been detemined. No evidence has been obtained for a reaction between e_sol⁻¹ and Ph₃P in methanol.

The gamma-irradiation of oxygen-saturated methanolic solutions of Ph₃P have shown that, under these conditions, Ph₃P is oxidized to Ph₃PO according to a chain reaction mechanism. The experimental results are in good agreement with the equation for G(-Ph₃P) = G(Ph₃PO) obtained by steady-state treatment of the proposed mechanism.

Pulse radiolysis has been used to measure the rates of reactions of O₂⁻ with electron donors and acceptors, either directly or by a competition kinetic method using p-benzoquinone. O₂⁻ readily reduces quinones and various redox dyes and enzymes, and oxidizes ascorbate and catecholamines, but is quite unreactive towards aromatic amino acids, nucleic acids, sugars and alcohols.

Pulse irradiation of PtCl₄²⁻ in aqueous solutions containing Cl⁻ or Br⁻ indicates that oxidation of Pt(II) by OH may be wholly or partially replaced by oxidation by Cl₂⁻ or Br₂⁻, depending on the efficiency of OH scavenging by the particular halide ion. The Pt(III) species produced by the latter reactants absorb respectively at 290 and 310 nm and differ from that produced by OH. Contrary to the similar PdCl₄²⁻ no evidence of Pt(II) co-ordinating more than four Cl⁻ ions has been obtained. The rate constants of the processes and extinction coefficients of the transient species observed are reported.

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.