Relativistic and perturbational calculations of fine structure splittings in F₂ and F₂⁺

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.