Decay Dynamics of H Atoms in Solid Hydrogen at 4.2 K. Controlling Factor of Tunneling Reaction H + para-H2 → para-H2 + H

H atoms produced by the γ-ray irradiation of solid para -H 2 ( p -H 2 ) and solid normal -H 2 ( n -H 2 ) decay gradually when the irradiated sample is stored at 4.2 K. ortho -H 2 ( o -H 2 ) molecules at the sites nearest an H atom in solid n -H 2 convert to p -H 2 molecules by interacting with the H atom. H atoms in solid H 2 decay by the process of tunneling diffusion H + p -H 2 → p -H 2 + H and finally recombine with other H atoms. The rate constants for the tunneling reaction were obtained from the decay rates of H atoms in solid H 2 at various concentrations of o -H 2 . When solid n -H 2 is stored for 10 h at 4.2 K after γ-ray irradiation, a number of o -H 2 molecules change into p -H 2 molecules by a catalytic chain reaction induced by mobile H atoms. The G value of the o -H 2 yields from H atoms amounts to 1−2.6 × 10 4 . The rate constant for the tunneling reaction H + p -H 2 → p -H 2 + H decreases drastically with increasing numbers of o -H 2 molecules, which exist as orientational defects in solid p -H 2 . It was concluded that a very small amount of energy (ca. 2 cal mol -1 ) affects the tunneling reaction at very low temperatures.