Evidence of the formation of the O4 molecule in solid oxygen under pressure

We report a detailed experimental characterization of the infrared absorption properties of the high pressure, red phase of oxygen. Spectra are measured in the far infrared, down to 100 cm^-1, in the fundamental vibration and in the overtone regions up to 63 GPa. A new, strong peak is observed at about 300 cm^-1. Samples less than 0.5 mm thick were produced to measure the strong vibron mode avoiding saturation. We find that the diatomic oxygen molecule is not a good basis to describe the vibrational spectrum of the e phase. The spectra can be interpreted in a straightforward way on the basis of an association among the oxygen molecules leading to the formation of O4 units.