Experimental Observation of ^–₂ Anion and its para → ortho Conversion in Solid para hydrogen Using High–Resolution ESR Spectroscopy

Abstract

We present experimental observations and study of ${\text{H}}_{\text{2}}^ -$ in solid parahydrogen. Since the parahydrogen molecule does not produce local magnetic fields, high–resolution ESR spectra of trapped radicals can be observed in the solid parahydrogen matrices. Using this high–resolution ESR spectroscopy, new quartet ESR signals were observed in γ–rays irradiated solid parahydrogen and assigned as ${\text{H}}_{\text{2}}^ -$ In addition, para– ${\text{H}}_{\text{2}}^ -$ was observed to convert into ortho– ${\text{H}}_{\text{2}}^ -$ on the storage at 4.2 K. On the other hand, ortho–H ₂ molecule converts into para– ${\text{H}}_{\text{2}}^ -$ at cryogenic temperatures. The difference in the conversion between the H ₂ molecule and the ${\text{H}}_{\text{2}}^ -$ anion is explained by the parity conservation law of wavefunctions on exchanging the protons in homonuclear diatomic molecules such as the ${\text{H}}_{\text{2}}^ -$ anion and H ₂ molecule.