Intensities of spin‐forbidden transitions in molecular oxygen and selective heavy‐atom effects

Intensities of a1Δg←X 3∑g− and b 1∑g+ ← X 3∑g− transitions in molecular oxygen have been calculated on the basis of the INDO method taking into account spin‐orbit coupling by perturbation theory. The transitions are magnetic dipole in nature. The first of them (a − X) steals its intensity from 3Πg‐3∑g− and 1Πg‐1Δg transitions, which are determined by the orbital angular‐momentum operator. This source is not the principal one for the intensity of the second (b‐X) transition. Its intensity is stolen principally from microwave transitions between spin sublevels of the ground 3∑g− state. The last source explains the large difference in intensities of the a‐X and b‐X transitions. Calculated oscillator strengths are in a good agreement with experiment. The same integrals that determine the intensity also determine the parameters of the spin Hamiltonian for the ground 3∑g− state. These parameters are in a good agreement with experiment also, showing the validity of the whole calculation. In a condensed phase the investigated transitions are enhanced by intermolecular exchange interaction. It is known that an external heavy atom (EHA) enhances the b‐X transition of oxygen in solution, but does not influence the a‐X transition. In the collision complex O2‐EHA, which has a geometry without inversion symmetry, the microwave transitions between spin sublevels of the “3∑g−” state obtain electric‐dipole moments, which are stolen from the charge‐transfer transition. This mechanism explains the selective effect of EHA.