Antiferromagnetism in the high-pressure phases of solid oxygen: Low-energy electronic transitions

In this work we report a study in the near infrared spectral region, at low temperature and high pressure, of solid delta and beta oxygen, concerning electronic excitations between the ground state and the lowest excited states. These transitions are essentially due to the simultaneous creation of an exciton, a magnon, and a vibron, and confirm the antiferromagnetic order of the deltaphase and the short-range antifer- romagnetic order of beta oxygen. Strong phonon sidebands are also observed. A simple model let us obtain, from the frequency position 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 makes it possible to estimate the spin-correlation function in the beta phase. Finally, we measure a dramatic change of the spectrum at the delta-epsilon phase transition, which is consistently interpreted on the basis of the formation of the O4 molecule, confirming previous vibrational data. The antiferromagnetic coupling in solid oxygen appears to be the driving force leading to the formation of the diamagnetic O4 molecule.