The 0 2-0 2 Dimer: Magnetic Coupling and Spectrum

A theoretical analysis has been made of the van der Waals vibration-rotation-electron spin states of the 0 2-0 2 dimer in its 32 g "-32 g-electronic ground state. This analysis is based on a Hamiltonian that includes a spin-dependent 0 2-0 2 interaction potential and it involves also the permutation-inversion symmetry of the system. We have constructed some hindered internal rotor models for the vibrational states of the 0 2-0 2 dimer which correspond with different equilibrium geometries, and for each of these models we have numerically calculated the spin-rotation fine structure. This fine structure appears to be determined not only by the Heisenberg exchange interaction between the 0 2 monomer triplet states, but also by intramolecular spin-orbit and spin-spin coupling and, to a smaller extent, by the intermolecular spin-spin (magnetic dipole) interaction and by the Coriolis terms in the kinetic energy. The resulting fine-structure spectrum is very complex, and very sensitive to the geometry of the 0 2-0 2 dimer, to the nature of its internal motions and to the various magnetic couplings. This implies that detailed measurements of this spectrum, which can be interpreted with the help of the theory presented here, will yield interesting information on the properties of the 0 2-0 2 dimer and, at the same time, verify our assumptions on the magnetic interactions between 0 2 molecules which have important consequences for the properties of solid oxygen.