The $a^3\Sigma^+$ state of KCs revisited: hyperfine structure analysis and potential refinement

Laser-induced fluorescence spectra of the $c^3\Sigma^+(v_{c},J_{c}=N_{c})\rightarrow a^3\Sigma^+(v_{a},N_{a} = J_{c} \pm 1)$ transitions excited from the ground $X^1\Sigma^+$ 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 $a^3\Sigma^+$ state for levels with $v_{a} \in [0, 27]$ and $N_{a} \in [24, 90]$ shows that the splitting monotonically increases with $v_{a}$. The spectroscopic study was supported by ab initio calculations of the magnetic hyperfine interaction in $X^1\Sigma^+$ and $a^3\Sigma^+$ 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 $a^3\Sigma^+$ 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 $X^1\Sigma^+$ and $a^3\Sigma^+$ 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.