The [formula omitted] state of KCs revisited: Hyperfine structure analysis and potential refinement.

• High-resolution spectroscopy. • Coupled channels deperturbation. • Hyperfine structure. • Relativistic ab initio calculations. Laser-induced fluorescence spectra of the c 3 Σ + (v c , J c = N c) → a 3 Σ + (v a , N a = J c ± 1) transitions excited from the ground X 1 Σ + state of 39 K 133 Cs molecule were recorded with Fourier-transform spectrometer IFS125-HR (Bruker) at the highest achievable spectral resolution of 0.0063 cm − 1. Systematic study of the hyperfine structure (HFS) of the a 3 Σ + state for levels with v a ∈ [ 0 , 27 ] and N a ∈ [ 24 , 90 ] shows that the splitting monotonically increases with v a. The spectroscopic study was supported by ab initio calculations of the magnetic hyperfine interaction in X 1 Σ + and a 3 Σ + states. The discovered variation of the electronic matrix elements with the internuclear distance R is in a good agreement with the observed v a -dependencies of the HFS. Overall set of available experimental data on the a 3 Σ + state was used to improve the potential energy curve particularly near a bottom, providing the refined dissociation energy D e = 267.21 (1) cm − 1. The ab initio HFS matrix elements, combined with the empirical X 1 Σ + and a 3 Σ + PECs in the framework of the invented coupled-channel deperturbation model, reproduce the experimental term values of both ground states within 0.003 cm − 1 accuracy up to their common dissociation limit. [ABSTRACT FROM AUTHOR]