The a3Σ+ state of KCs revisited: Hyperfine structure analysis and potential refinement

Laser-induced fluorescence spectra of the c³Σ⁺(v_c ,J_c =N_c) →a³Σ⁺(v_a ,N_a =J_c ± 1) transitions excited from the ground X¹Σ⁺ state of ³⁹K>¹³³Cs 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 a³Σ⁺ 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 X¹Σ⁺ and a³Σ⁺ 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 a³Σ⁺ 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 X¹Σ⁺ and a³Σ⁺ 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.