The origin of lambda doubling effect for the B1Pi and D1Pi states of NaK

The origin of Lambda-doubling effect (q factors) for the regularly perturbed NaK B 1[product] and D 1[product] states has been investigated by means of ab initio many-body multipartitioning perturbation theory calculation of the electronic L-uncoupling matrix elements between the examined 1[product] and five lowest 1[summation]+ states for both 23Na39K and 23Na41K isotopomers. The hypothesis of pure precession was found to be valid for the B 1[product]-A 1[summation]+ and D 1[product]-C 1[summation]+ pairs of the interacted states, while the unique perturber approximation works properly for the D 1[product] state and completely breaks down for the B 1[product] state. The theoretical rovibronic q-factor values estimated for both states by means of the approximate sum rule agree well with their experimental counterparts and demonstrate high sensitivity to vibrational and rotational quantum numbers.