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The oscillation continuum in laser-induced fluorescence spectra of bound-free c3Σ+ → a3Σ+ and (4)1Σ+ → X1Σ+ transitions of the KCs molecule was recorded by a Fourier-transform spectrometer and modeled under the adiabatic approximation. The required interatomic potentials for ground a3Σ+ and X1Σ+ states were reconstructed in an analytical Chebyshev-polynomial-expansion form in the framework of the regularization direct-potential-fit procedure based on the simultaneous consideration of experimental line positions from Ferber et al. [Phys. Rev. A 80, 062501 (2009)] and the present ab initio calculation of short-range repulsive potential data. It was proved that the repulsive part over the dissociation limit of the derived a3Σ+ potential reproduces the experiment better than the potentials reported in the literature. It is also shown that all empirical and semi-empirical potentials available for the X1Σ+ state reproduce the bound-free (4)1Σ+ → X1Σ+ spectrum with equal quality in the range of observations. [ABSTRACT FROM AUTHOR]

• 673 experimental term values belonging to both e / f -components of the c 3 Σ Ω = 1 + state of KCs were determined with about 0.01 cm − 1 accuracy applying high resolution Fouriertransform spectroscopy. • High fidelity relativistic coupled cluster calculation on the electronic structure of the KCs low-lying states was performed. • Semi-empirical point-wise interatomic potential was reconstructed for the c 3 Σ 1 + state. • Ω -doubling effect in the c 3 Σ 1 + state was investigated. • Radiative properties of the c 3 Σ + state were simulated for the lowest vibrational v c ≤ 22 levels. The Ti:Saphire laser operated within 13800 - 11800 cm − 1 range was used to excite the c 3 Σ + state of KCs molecule directly from the ground X 1 Σ + state. The laser-induced fluorescence (LIF) spectra of the c 3 Σ + → a 3 Σ + transition were recorded with Fourier-transform spectrometer within 8000 to 10000 cm − 1 range. Overall 673 rovibronic term values belonging to both e / f -components of the c 3 Σ + (Ω = 1 ±) state of 39 KCs, covering vibrational levels from v = 0 to about 45, and rotational levels J ∈ [ 11 , 149 ] were determined with the accuracy of about 0.01 cm − 1 ; among them 7 values for 41 KCs. The experimental term values with v ∈ [ 0 , 22 ] were involved in a direct point-wise potential reconstruction for the c 3 Σ + (Ω = 1 ±) state, which takes into account the Ω -doubling effect caused by the spin-rotational interaction with the nearby c 3 Σ + (Ω = 0 −) state. The analysis and interpretation were facilitated by the fully-relativistic coupled cluster calculation of the potential energy curves for the B 1 Π , c 3 Σ + , and b 3 Π states, as well as of spin-forbidden c − X and spin-allowed c − a transition dipole moments; radiative lifetimes and vibronic branching ratios were calculated. A comparison of relative intensity distributions measured in vibrational c → a LIF progressions with their theoretical counterparts unambiguously confirms the vibrational assignment suggested in [ J. Szczepkowski, et al., JQSRT, 204 , 133–137 (2018)]. [ABSTRACT FROM AUTHOR]

• The detection of the exremely weak A 1 S i g m a + b ˜ 3 P i → a 3 S i g m a transitions in laser-induced fluorescence of RbCs. • The scalar- and full-relativistic ab initio calculations of the transition dipole moments. • The comparison of the experimental and theoretical branching ratios of transitions to the triplet and singlet ground states. • The explanation of pecularities in the doublet progressions to the triplet ground state. We observed the A 1 Σ + ∼ b 3 Π → a 3 Σ + / X 1 Σ + laser-induced fluorescence (LIF) of the RbCs molecule excited from the ground X 1 Σ + state by the Ti:Sapphire laser. The LIF spectra from the common perturbed levels of the singlet-triplet A ∼ b complex was recorded by the Fourier-transform (FT) spectrometer with the instrumental resolution of 0.03 cm − 1. The relative intensity distribution in the rotationally resolved A ∼ b → a 3 Σ + (v a) / X 1 Σ + (v X) progressions was measured, and their branching ratio was found to be about of 1 ÷ 5 × 10 − 4 in the bound region of the a 3 Σ + and X 1 Σ + states. The experiment was complemented with the scalar- and full-relativistic calculations of the A / b − X / a transition dipole moments (TDMs) as functions of internuclear distance. The relative systematic error in the resulting ab initio TDM functions evaluated for the strong A − X transition was estimated as few percent in the energy region, where the experimental LIF intensities are relevant. The relative spectral sensitivity of the FT registration system, operated with the InGaAs diode detector and CaF beam-splitter, was calibrated in the range [6 500, 12 000] cm − 1 by a comparison of experimental intensities in the long A ∼ b → X (v X) LIF progressions of the K 2 and KCs molecules with their theoretical counterparts evaluated using the ab initio A − X TDMs. Both experimental and theoretical transition probabilities can be employed to improve the stimulated Raman adiabatic passage process, a → A ∼ b → X , which is exploited for a laser assembling of ultracold RbCs molecules. [ABSTRACT FROM AUTHOR]