Spin—Orbit Effects and the Fine Structure in the 3Σg− Ground State of O2

The true microscopic Hamiltonian is used for the spin—orbit interaction in the description of the fine structure of the O2 3Σg− ground state. The inclusion of the true spin—orbit interaction Hamiltonian in place of the commonly used isotropic form AL·S causes a mixing of 1Σg+ into the ground state. The 1Σg+ character causes Hso to contribute to the same order to the effective spin dipole—dipole parameter λe as does the spin dipole—dipole Hamiltonian Hss and also results in slight changes in the experimental values of the spin—rotation coupling constant and the g value of the orbital angular momentum. The amount of 1Σg+ character is estimated to be 1.0×10−4 from the data of Tinkham and Strandberg, and a somewhat larger value 3.6×10−4 from the data of Bowers et al., these are to be compared with 1.3×10−4 obtained by a previous calculation. The amount of singlet character, obtained from a combination of experimental data and estimates of molecular parameters, is compatible with the experimental data obtained by Tinkham and Strandberg or Bowers et al. Our result is supported by agreement with a calculation of 〈Hss〉 and by comparing the values of this parameter for several 3Σ− molecules.