Experimental investigation of the 61Σ+ ‘shelf’ state of KCs

We report the first experimental observation of the 61Σ+ state in the KCs molecule. Polarization labelling spectroscopy technique was applied to determine energies of about 1000 rovibrational levels in this state, most of them strongly perturbed by the neighbouring 43Π state levels. The inverted perturbation approach was used to construct the irregular potential energy curve of the 61Σ+ state. It is compared with the recent theoretical calculations to show that Hundʹs coupling case (a) is proper for description of this state.