An ENDOR spectrum of H atoms in solid H2

An ENDOR spectrum of H atoms produced in the γ-ray irradiated solid H2 was measured at 4.2 K in order to elucidate the structure of the local environment of the H atoms in solid H2. We found that the H atoms were not trapped in interstitial sites but in substitutional sites of the solid, and almost all the ortho-H2 molecules at the first nearest sites from the H atoms converted into para-H2 molecules. This result shows that the ortho–para conversion is induced by electron spins of the H atoms.