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.

Results of studies carried out in this laboratory on radiolytic formation and deactivation of solute excited triplet states in rigid organic solutions are reviewed. Energy transport from matrix to solute molecules was shown to occur in these systems. In matrices formed by aromatic molecules (toluene, polystyrene) this energy transport is realized via processes of singlet-singlet and triplet-triplet transfer of excitation energy. Three processes of triplet quenching have been found and discussed: (i) transfer of electronic excitation energy to solvent radicals, (ii) annihilation of solvent molecular excitations upon interaction with solute triplet molecules and (iii) specific deactivation of solute triplet molecules by solvent radiation products.

A comparison has been made of the experimental and calculated relative values of the efficiency of capture by the acceptor molecule of an electron generated by ionizing radiations (the α coefficient in Hummel's theory) in liquid hydrocarbons at room temperature; pentadecane, n-hexane, iso-octane and neopentane. For the experimental determination of the values of α the authors used the measurements of luminescence induced by a pulse of fast electrons in solutions of pyrene (10¹⁵−10¹⁶ cm⁻³) in hydrocarbons. Calculation of the values of α for the diffusion description of the process of recombination of an electron is based on the relation: α≈K_Sr_m^3/u, where K_S is the rate constant of capture of an electron by the molecule of pyrene; u is the drift mobility of the electron; r_m is the most probable distance of thermolysation of an electron. The values of K_S and u for pentadecane and neopentane were determined by investigation of the electrical conductivity induced by a pulse of fast electrons in solutions of pyrene; for K_S and u in the case of n-hexane and iso-octane and also for r_m the published data were used. It is shown that while for hydrocarbons with relatively low mobility of the electron the calculated and experimental values of α agree, for the hydrocarbon with the highest mobility of the electron (neopentane) use of diffusion concepts for the calculation gives an overstated value of α.

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.