ИСТИНА

We present results of high-resolution experimental study, direct potential fit and radiative properties calculation on the RbCs $E(4)^1\Sigma^+$ state converging to Rb(5$^2$S) + Cs(5$^2$D) atomic limit; such shelf-like states [1,2] are of particular interest for searching optical paths suitable for STIRAP producing of stable ultracold polar molecules. The collisionally enhanced laser induced fluorescence (LIF) spectra corresponding to both spin-allowed $E(4)^1\Sigma^+ \to X(1)^1\Sigma^+$ and spin-forbidden $E(4)^1\Sigma^+\to a(1)^3\Sigma^+$ transitions of RbCs were recorded in visible region by Fourier Transform Spectrometer (Bruker IFS 125HR) with the instrumental resolution of 0.03 cm$^{-1}$. Overall about 2300 rovibronic term values of the $E(4)^1\Sigma^+$ state of $^{85,87}$RbCs isotopologues were determined in the energy range $[16750,18170]$ cm$^{-1}$ above the minimum of the ground $X$-state. Experimental data field is limited by vibrational levels $v^{\prime}\in [3,88]$ with rotational quantum numbers $J^{\prime}\in [2,276]$. The closed analytical form for potential energy curve (PEC) based on Chebyshev polynomial expansion was implemented to a direct potential fit of the experimental term values of the most abundant $^{85}$RbCs isotopologue. The mass-invariant properties of the empirical PEC were tested by the prediction of rovibronic term values of the $^{87}$RbCs isotopologue. The absorbtion and emission Einstein coefficients were predicted in a wide range of vibrational $v$ and rotational $J$ quantum numbers for both singlet-singlet $E-X$ and singlet-triplet $E-a$ rovibronic transitions along with radiative lifetimes of the upper $E$-state and branching ratios of spontaneous emission into the low lying electronic states, including both bound-bound and bound-continuum parts of the spectra. The required spin-allowed $E^1\Sigma^+-X;A;C^1\Sigma^+;B^1\Pi$ transition dipole moments were obtained in the framework of quasi-relativistic electronic structure calculations. The regular spin-orbit coupling effect with the nearest $^3\Pi$ states is found to be sufficient to induce the spin-forbidden $E^1\Sigma^+ -a;c^3\Sigma^+;b^3\Pi$ transitions by borrowing probabilities of the relevant $^3\Pi-^3\Lambda$ transitions. Reliability of the derived empirical $E$-state PEC and \emph{ab initio} $E-X;a$ transition dipole moments is additionally confirmed by good agreement between the calculated and experimental relative intensity distributions in the long $E(v^{\prime})\to X(v^{\prime\prime})$ and $E(v^{\prime})\to a(v^{\prime\prime})$ LIF progressions.