Relativistic and perturbational calculations of fine structure splittings in F 2 and F 2+

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Abstract
Dirac-Fock calculations near the SCF limit using the recently developed basis set expansion technique of Mark and Schwarz have been performed on the F2 ground state with particular consideration of the relativistic splitting of the π g and π u orbitals. The magnetic contribution to the Breit interaction has been included by first-order perturbation theory. Fine structure splittings of the ionic states F2 +(X 2Π g ) and F2 +(A 2Π u ) have been calculated by first-order perturbation theory within the Breit-Pauli framework at three levels of approximation for the zero-order wavefunction. The results of the Dirac-Breit and the Breit-Pauli approaches are compared with experimental data. The calculated splittings are analysed by partitioning them into physically meaningful contributions. Aided by results of numerical Dirac-Fock calculations on atoms the general conclusion is drawn that ionic reorganization of the wavefunction increases the fine structure splitting at the Breit-Pauli level, whereas it decreases the splitting at the Dirac-Breit level. Using a model of Ishiguro and Kobori the ratios of Π g and Π u spin-orbit splittings of halogen molecular ions X 2 + are discussed.