Effect of dose-rate, and L.E.T., on the radiation-induced cis-trans isomerization of butene-2 in liquid benzene solution

Abstract

Previous work with low dose-rate ⁶⁰Co γ-rays has shown that the irradiation of cis-butene-2 dissolved in liquid benzene at room temperature causes isomerization to trans-butene-2, the G value rising with the concentration of cis-butene-2 to a plateau of G(trans)= 2.4 at ≈ 0.3 mole/1. of cis-butene-2. Experiments in which known triplet state scavengers (e.g., O₂, anthracene) were added suggested that the triplet state of benzene was the main agent of the isomerization, and the concentration dependence was attributed to the competition between the normal decay of the triplet state, and reaction with cis-butene-2. [graphic omitted]

The same measurements were done with a beam of 0.5 MeV protons as the radiation source, and G(trans) was depressed at all concentrations of cis-butene-2, e.g., at 0.3 mole/1. G(trans) was 1.2, These results could be explained on the basis of diffusion and reaction of the species involved if the self-annihilation of triplet states occurred B^T+ B^T→B^s+ B to form a singlet excited state and a ground-state molecule. Diffusion calculations show that for a normal spur size of ≈30 Å diameter the rate-constant for this reaction would be high, ≈10¹⁰1. mole^–1 sec^–1. The effect of the decay of singlet states to form triplets during diffusion was considered.

A dose-rate effect was also found with fast (2–4 MeV) electrons in the sense of lower values of G(trans) at high dose-rate and substantial effects were noted for effectively steady irradiation at 3.9 × 10²⁰ eV cm^–3 sec^–1. This dose-rate is much lower than that expected on the mechanism above, using the rate constants needed to explain the L.E.T. effect with protons. It is suggested that radicals formed in the bulk of the solution at high dose-rates are efficient quenching agents of excited states, and that the effective collision diameter for the quenching of excited molecules by radicals is ∼100 Å.