Investigation of the B1Π state in NaCs by polarisation labelling spectroscopy

We present experimental observation of the B1Π state in NaCs molecule converging to the Na (32S1/2) + Cs (62P3/2) states of separated atoms. The two-colour polarisation labelling spectroscopy technique was applied to the B1Π ← X1Σ+ band system. The data field for the B state consists of rovibrational levels with v = 0–52 and J = 13–51, the last vibrational level located ca. 7.6 m−1 below the atomic asymptote. The data were incorporated into a fit of a potential energy curve to the level energies using the Inverted Perturbation Approach (IPA) method. The resulting potential, extending between R = 2.4 and 24.8 Å, is employed to partially assign the existing photoassociation spectra of NaCs.