EFFECT OF ROTATIONAL QUANTUM STATES (J = 0,1) ON THE TUNNELING REACTION H-2+H-] H+H-2 IN PARAHYDROGEN SOLID AT 4.2-K

ESR spectra of H atoms, produced by gamma-irradiation of the p-H-2 solid, and their decay have been studied at 4.2 K and compared with the results in the n-H-2 solid that consists of 75% o-H-2 and 25% p-H-2. Each of two main lines of the ESR spectra of H atoms that are separated by about 500 G accompanies two weak spin flip satellite lines. The observed line width of the main line in the p-H-2 solid is one-third that of the H atoms in the n-H-2 solid. A rotational quantum number (J) of p-H-2 at 4.2 K is 0, and thus the orientation of p-H-2 molecules in the solid is effectively isotropic. The narrowing of the ESR spectrum in the p-H-2 solid is ascribed to the isotropic orientation of p-H-2 molecules. The ratio of the spin flip line intensity to the main line intensity in the p-H-2 solid is about one-tenth that in the n-H-2 solid. The very weak intensity of the spin flip lines in the p-H-2 solid is explained in terms of zero total nuclear spin of p-H-2. Since the decay of H atoms in the p-H-2 solid is associated with the tunneling reaction p-H-2 + H --> H + p-H-2, the rate constant for the reaction is obtained from the decay rate of H atoms in p-H-2. The rate constant for the tunneling reaction p-H-2 (J = 0) + H --> H + p-H-2 (J = 0) was found to be 4.3 X 10 cm3 mol-1 s-1. The rate constant for the tunneling reaction o-H-2 (J = 1) + H --> H + H-2 (J = 0, 1) was estimated as 1.2 X 10 cm3 mol-1 s-1 from the decay rate of H atoms in n-H-2.