The Moderation and Tunneling Reaction of Recoil T Atoms in Solid Xenon–Hydrogen Mixtures at 77 K

Abstract

Recoil T atom reactions were studied in Xe–H2–D2 mixtures at 77 K. The isotope effect on the T-atom reaction with H2 and D2 (k(T+H2→HT+H)/k(T+D2→DT+D)) was measured on the basis of the HT and DT yields. The isotope effect (k(T+H2)/k(T+D2)) is 1.1 in the range of hydrogen concentrations of 0.1–1.0 mol%. Since the small isotope effect (1.1)is similar to the isotope effect (1.0) for a hot T-atom reaction in the gas phase, recoil T atoms are not thermalized at these concentrations in solid xenon and form HT and DT by means of hot-atom reactions. At concentrations of hydrogen of less than 0.1 mol%, the recoil T atoms are thermalized before they encounter the solute hydrogen. The isotope effect increases with a decrease in the hydrogen concentration and amounts to 3.1 at a hydrogen concentration of 0.01 mol%. Since a large isotope effect is expected for the tunneling reaction of T atoms, the large isotope effect (3.1) at 0.01 mol% hydrogen was interpreted in terms of the tunneling reaction of thermalized T atoms. The present results in the solid xenon–hydrogen mixtures were compared with those in the gaseous xenon–hydrogen mixtures reported previously. It was concluded that hot T atoms in the solid xenon at 77 K are deactivated much less effectively than in the gas phase and that they migrate long distances with excess kinetic energies.