Absence of recombination of neighboring H atoms in highly purified solid parahydrogen: Electron spin resonance, electron-nuclear double resonance, and electron spin echo studies

Diffusion and recombination of H atoms were studied in solid hydrogen containing ortho-${\mathrm{H}}_2$ molecules at relative concentration $X_o\text{=0.001→0.75}$ using electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and electron spin echo (ESE) methods at around 4 K. When the rate-determining step for recombination is assumed to be the diffusion of H atoms, the rate constant for recombination at $X_o\text{⩾0.1}$ is consistent with the diffusion coefficient estimated from the analysis of ENDOR spectra and longitudinal spin relaxation behaviors. The recombination rate constant at $X_o\text{<0.1,}$ however, is too slow to be explained using the diffusion coefficient estimated from longitudinal spin relaxation and forbidden spin-flip satellite transition studies. This result suggests that, even if one H atom finds another H in its immediate neighborhood, these H atoms do not react to form a ${\mathrm{H}}_2$ molecule at $X_o\text{≪0.1.}$ The absence of recombination of H atoms is due to lack of the energy dispersion path required for the recombination of diatomic molecules. Since the absence of recombination becomes less significant at higher $X_o,$ ortho-${\mathrm{H}}_2$ molecules are found to play an important role in the energy dispersion which accompanies the recombination reaction.