ИСТИНА

We present results of systematic high resolution Fourier transform study and direct deperturbation fit performed for the lowest excited $A^1\Sigma^+$ and $b^3\Pi$ states of KCs and RbCs molecules converging to K(4$^2$S)+Cs(6$^2$P) and Rb(5$^2$S)+Cs(6$^2$P) dissociation limits, respectively. Such strongly spin-orbit coupled singlet and triplet states are of continual interest for selecting optimal optical paths for producing and monitoring cold polar diatomic molecules. The collisionally enhanced laser induced fluorescence (LIF) spectra corresponding to the $A^1\Sigma^+\sim b^3\Pi \to X^1\Sigma^+$ and $E^1\Sigma^+\to A^1\Sigma^+\sim b^3\Pi$ transitions of both molecules were recorded by Fourier Transform Spectrometer (Bruker IFS 125HR) with the instrumental resolution of 0.03-0.05 cm$^{-1}$. More than 4600 rovibronic term values assigned to the $A\sim b$ complex of $^{85}$RbCs isotopologue were obtained in the energy range [10066,~12857] cm$^{-1}$ above the minimum of the ground state. The experimental data set of the KCs $A\sim b$ complex currently consists of 5900 term values which covers even wider energy interval [9190,~13560] cm$^{-1}$. Data of the both molecules were treated in the framework of the $4\times 4$ coupled-channel deperturbation Hamiltonian constructed in Hund's coupling case (a) basis functions. The elaborated model takes into account explicitly: (1) direct spin-orbit coupling between $A$-state and $b^3\Pi_{\Omega=0}$ components of the $b$-state; (2) indirect spin-orbit-electronic-rotational interaction between the $A$-state and $b^3\Pi_{\Omega=1}$ component; (3) spin-rotational interaction between the $\Omega=0,1,2$ components of the triplet state. The required initial potential energy curves (PECs) and spin-orbit coupling (SOC) functions were extracted from a large scale quasi-relativistic \emph{ab initio} calculations. The resulting analytically defined empirical PECs and SOC functions reproduce about 96\% of the experimental term values with a standard deviation of 0.005 cm$^{-1}$ which is consistent with 0.003-0.01 cm$^{-1}$ of the uncertainty of the experiment upper limited by the Doppler effect. The Moscow team is grateful to the Russian Foundation for Basic Researches for support by the Grant 10-03-00195-a. The support from the Latvian Science Council grant 09.1567 is greatly acknowledged by Riga team.