Fourier-transform A(1)Σ(+) - b(3)Π → X(1)Σ(+) laser-induced fluorescence spectra were recorded for the natural mixture of (39,41)K(85,87)Rb isotopologues produced in a heatpipe oven. Overall 4200 rovibronic term values of the spin-orbit coupled A(1)Σ(+) and b(3)Π states were determined with an uncertainty of about 0.01 cm(-1) in the energy range [10 850, 14 200] cm(-1) covering rotational quantum numbers J' ∈ [3, 280]. Direct deperturbation analysis of the A ∼ b complex performed within the framework of the A(1)Σ(+) ∼ b(3)ΠΩ=0,1,2 coupled-channel approach reproduced experimental data with a standard deviation of 0.004 cm(-1). Initial parameters of the internuclear potentials and spin-orbit coupling functions along with the relevant transition dipole moments were obtained by performing the quasi-relativistic electronic structure calculations. The mass-invariant molecular parameters obtained from the fit were used to predict energy and radiative properties of the A ∼ b complex for low J levels of (39)K(85)Rb as well as for (41)K(87)Rb isotopologues, allowing us to identify the most reasonable candidates for the stimulated Raman transitions between the initial uppermost vibrational levels of the a(3)Σ(+) and X(1)Σ(+) states, the intermediate levels of the A ∼ b complex, and the lowest absolute ground X(1)Σ(+)(v = 0, J = 0) state.

The article presents a study of the strongly spin-orbit coupled singlet A(1)Σ(+) and triplet b(3)Π states of the RbCs molecule, which provide an efficient optical path to transfer ultracold molecules to their rovibrational ground state. Fourier-transform A(1)Σ(+) - b(3)Π → X(1)Σ(+) and (4)(1)Σ(+) → A(1)Σ(+) - b(3)Π laser-induced fluorescence (LIF) spectra were recorded for the natural mixture of the (85)Rb(133)Cs and (87)Rb(133)Cs isotopologues produced in a heat pipe oven. Overall 8730 rovibronic term values of A(1)Σ(+) and b(3)Π states were determined with an uncertainty of 0.01 cm(-1) in the energy range [9012, 14087] cm(-1), covering rotational quantum numbers J ∈ [6, 324]. An energy-based deperturbation analysis performed in the framework of the four A(1)Σ(+) - b(3)Π(Ω = 0, 1, 2) coupled-channels approach reproduces 97% of the experimental term values of both isotopologues with a standard deviation of 0.0036 cm(-1). The reliability of the deperturbed mass-invariant potentials and spin-orbit coupling functions of the interacting A(1)Σ(+) and b(3)Π states is additionally proved by a good reproduction of the A - b → X and (4)(1)Σ(+) → A - b relative intensity distributions. The achieved accuracy of the A - b complex description allowed us to use the latter to assign the observed (5)(1)Σ(+) → A - b and (3)(1)Π → A - b transitions. As is demonstrated, LIF to the A - b complex becomes as informative as to the ground X(1)Σ(+) state, which is confirmed by comparing the results of (4)(1)Σ(+) state analysis based on (4)(1)Σ(+) → A - b LIF with the data from V. Zuters et al. [Phys. Rev. A 87, 022504 (2013)] based on (4)(1)Σ(+) → X LIF.

We combined high resolution Fourier-transform spectroscopy and large scale electronic structure calculation to study energy and radiative properties of the high-lying (3) 1 Π and (5) 1 Σ + states of the RbCs molecule. The laser induced (5) 1 Σ + ,(4) 1 Σ + ,(3) 1 Π →A(2) 1 Σ + ∼ b(1) 3 Π fluorescence (LIF) spectra were recorded by the Bruker IFS-125(HR) spectrometer in the frequency range ν ∈ [5500, 10000] cm −1 with the instrumental resolution of 0.03 cm −1 . The rotational assignment of the observed LIF progressions, which exhibit irregular vibrational -rotational spacing due to strong spin-orbit interaction between A 1 Σ + and b 3 Π states was based on the coincidences between observed and calculated energy differences. The required rovibronic term values of the strongly perturbed A∼b complex have been calculated by coupled -channel approach for both 85 Rb 133 Cs and 87 Rb 133 Cs isotopolgues with accuracy of about 0.01 cm −1 , as demonstrated in [A. Kruzins et. al., J. Chem. Phys. 141,184309 (2014)]. The experimental energies of the upper (3) 1 Π and (5) 1 Σ + states were involved in a direct-potential-fit analysis performed in the framework of inverted perturbation approach. Quasi-relativistic ab initio calculations of the spin-allowed (3) 1 Π,(5) 1 Σ + → (1-4) 1 Σ + ,(1-3) 1 Π transition dipole moments were performed. Radiative lifetimes and vibronic branching ratios of radiative transitions from (3) 1 Π and (5) 1 Σ + states were evaluated. To elucidate the origin of the Λ-doubling effect in the (3) 1 Π state the angular coupling (3) 1 Π-(1-5) 1 Σ + electronic matrix elements were calculated and applied for the relevant q−factors estimate. The intensity distributions simulated for the particular (5) 1 Σ + ;(3) 1 Π →A∼b LIF progressions have been found to be remarkably close to their experimental counterparts.