2022MNRAS.514.2815L

We report spectra in the visible and near-infrared wavelength range 350-1155 nm and in the mid-infrared range 500-5000 cm^–1 (2-20 µm) of molecular oxygen at temperatures 6-33 K. In mid-infrared spectra, a sharp line at 1552.4 cm^–1 was observed at a deposition temperature of 6 K. The intensity, profile and position of this feature depended sensitively upon the deposition temperature as a result of a connection with lattice imperfections; instead, a triplet feature at 1545.7, 1548.1 and 1549.6 cm^–1 appeared at temperatures 9 and 12 K, but disappeared at 18, 24 and 30 K. Additional infrared lines of solid α-O₂ were observed at 1591 and 1617 cm^–1, which exhibited a reversible behaviour through the α-β phase transition temperature at 24 K. The transmissions of solid O₂ showed nearly no change in wavenumber range 500-2000 cm^–1 (5-20 µm) at temperatures 6-33 K; above 2000 cm^–1 (<5 µm), the transmission of solid O₂ depended strongly on temperature. Roughly, the transmission of solid O₂ at temperatures 6-21 K fluctuated slightly in the visible and near-infrared region, whereas it decreased greatly at 24 K and continuously diminished further at higher temperatures. Notably, a broad absorption of solid β-O₂ was observed in the wavelength region 350-700 nm at temperatures 27, 30 and 33 K.