2023MNRAS.522.3183L

To establish the spectroscopy of the solids α- and β-oxygen, we measured the thermal ramping spectra of the solid O₂ in visible and near-infrared (near-IR) wavelength range 450-1020 nm and mid-infrared (mid-IR) range 500-5000 cm^–1 (2-20 μm) at temperature 9-30 K. Besides the solids α- and β-oxygen, the imperfect structure of O₂ ice might co-exist at low temperature. In mid-IR spectra, a triplet was observed at 1545.7, 1548.1, and 1549.6 cm^–1 associated to the imperfect crystallinity of solid O₂; the triplet existed below 27 K, but not at 30 K and on re-cooling to 9 K. The spectra measured at re-cooled 9 and 30 K are evidently responsible only for the solids α- and β-O₂, respectively; overall, the spectra of the solids α- and β-O₂ appear similar in the visible region, but those differ significantly from the spectrum of the solid O₂ contaminated with imperfect structure. For solid α-O₂, IR lines were recorded at 1591 and 1617 cm^–1, which exhibited a reversible behaviour through the α-β phase transition temperature at 24 K. The visible spectrum of the separate solid α-O₂ was obtained for the first time.