Experimental determination of the mechanism of the tunneling diffusion of H atoms in solid hydrogen: Physical exchange versus chemical reaction

The pressure effect on the recombination rate constant of H atoms in solid ${\mathrm{H}}_2$ has been studied by electron spin resonance spectroscopy. It has been found that the rate constant was independent of pressure up to 13 MPa in the temperature range below 4 K, while it significantly decreases with an increase in pressure in the range above 5 K. The low temperature behavior observed indicates that H atoms diffuse in solid ${\mathrm{H}}_2$ “chemically” through the $\mathrm{H}+{\mathrm{H}}_2\to {\mathrm{H}}_2+\mathrm{H}$ reaction not by the physical exchange mechanism.