High-pressure infrared spectroscopy of solid oxygen

The infrared IR absorption spectra of molecular solid oxygen ${\mathrm{O}}_2$ have been studied to 92 GPa at room temperature. The IR-active vibron fundamental around $1500{\mathrm{cm}}^{-1}$ consisted of at least four absorption bands and their frequencies showed a turnover at 25 GPa with increasing pressure and then monotonically increased to 87 GPa after an initial decrease. The significant increase in the frequencies of the IR- and Raman-active vibrons has suggested a strengthening of the O-O intramolecular bond with pressure. An increase in the frequency difference between the IR and Raman vibrons with a pressure dependence similar to those of librons has also indicated an increase in the ${\mathrm{O}}_2-{\mathrm{O}}_2$ intermolecular bond. The absorbance saturation over all frequencies in the present observable region above 92 GPa was attributed to the insulator-metal transition due to a discontinuous band-gap closure. The structure of the ɛ phase has been examined based on the spectroscopic data.