Fast decay of the visible band electron in e-irradiated crystalline ice at low temperature: The isotope effects and the role of a mobile proton in the decay

The trapped electron which absorbs in the visible region in crystalline ice e−vis has been studied by pulse radiolysis in the low temperature range 6 to 77 K using 2 μs pulses, and above 77 K using pulses of 40 ps to 6 ns width. The half-life of e−vis in ice around 77 K is unusually short, 8 ns in H2O ice and 120 ns in D2O ice. The decay of e−vis in ice is found to fit Hummel’s empirical equation for the decay by geminate ion recombination in a spur. Several other pieces of evidence indicate that the decay occurs in a spur. Electron tunneling from e−vis to the OD radical does not occur in D2O ice. It is concluded that a proton (H3O+) or a deuteron (D3O+) produced in a spur by the irradiation migrates through ice to react with immobile e−vis in the same spur, and that the half-life of e−vis is determined by the mobility of the proton or deuteron. The Arrhenius plot of the half-life in the range 100 to 6 K is nonlinear and shows an activation energy of 20 meV at higher temperatures and much smaller values at temperatures below 50 K (1 meV below 15 K). The causes of the unusual Arrhenius plot and the isotope effect on the proton mobility are discussed.