Antiferromagnetism in the high-pressure phases of solid oxygen: Low-energyelectronic transitions - art. no. 064428

In this work we report a study in the near infrared spectral region, at low temperature and high pressure, of solid delta -O-2 and beta -O-2, concerni ng electronic excitations between the ground state (3)Sigma (-)(g) and the lowest excited states (1)Delta (g) and (1)Sigma (+)(g). These transitions a re essentially due to the simultaneous creation of an exciton, a magnon, an d a vibron, and confinn the antiferromagnetic order of the delta phase and the short-range antiferromagnetic order of beta -O-2. Strong phonon sideban ds are also observed. A simple model let us obtain, from the frequency posi tion of the observed bands, the exchange integral between nearest-neighbor molecules as a function of pressure, i.e., of the intermolecular distance. This result is compared with the available theoretical calculations at high pressure and other experimental data at ambient pressure. The comparison m akes it possible to estimate the spin-correlation function in the beta p ha se. Finally, we measure a dramatic change of the spectrum at the delta-epsi lon phase transition, which is consistently interpreted on the basis of the formation of the O-4 molecule, confirming previous vibrational data. The a ntiferromagnetic coupling in solid oxygen appears to be the driving force l eading to the formation of the diamagnetic O-4 molecule.