Decay of H2- anions in solid parahydrogen by quantum tunneling: observations of electron bubbles

Decay mechanism of H2- anions produced by γ-ray or X-ray radiolysis of solid para-H2 (p-H2) has been studied using high-resolution ESR spectroscopy in the temperature range between 2.7-6.6 K. The results can be summarized as follows; First, the decay rate constant of the H2- anion is not proportional to initial yields of reactive species such as H radical and cation but proportional to concentrations of HD and D2 impurities in p-H2. Second, ESR spectra assigned as electron bubbles were observed in solid p-H2 containing large amount of HD or D2 (11 mol %), while they were not observed in pure solid p-H2. Third, the decay rate constant of the H2- anion increases with the decrease in temperature between 2.7-5 K, while it decreases with the decrease between 5-6.6 K. Fourth, the decay of the H2- anion is suppressed by addition of ortho-H2 (o-H2) impurity. The first and second results suggest that the H2- anion reacts with the HD or D2 molecule to produce the electron bubble. The third and fourth results are interpreted in terms of two-phonon scattering quantum diffusion and thermally activated diffusion below and above 5 K, respectively. (author)