The laser induced fluorescence (LIF) spectra A 1 Σ + ∼ b 3 Π(E J ) → X 1 Σ + of KCs dimer were recorded in near infrared region by Fourier Transform Spectrometer with a resolution of 0.03 cm −1 . Overall more than 200 collisionally enhanced LIF spectra were rotationally assigned to 39 K 133 Cs and 41 K 133 Cs isotopomers yielding with the uncertainty of 0.003-0.01 cm −1 more than 3400 rovibronic term values of the strongly mixed singlet A 1 Σ + and triplet b 3 Π states. Experimental data massive starts from the lowest vibrational level vA = 0 of the singlet and nonuniformly cover the energy range E J ∈ [10040, 13250] cm −1 with rotational quantum numbers J ∈ [7, 225]. Besides of the dominating regular A 1 Σ + ∼ b 3 ΠΩ=0 interactions the weak and local heterogenous A 1 Σ + ∼ b 3 ΠΩ=1 perturbations have been discovered and analyzed. Coupled-channel deperturbation analysis of the experimental 39 K 133 Cs e-parity termvalues of the A 1 Σ + ∼ b 3 ΠΩ=0,1,2 complex was accomplished in the framework of the phenomenological 4 × 4 Hamiltonian accounting implicitly for regular interactions with the remote 1 Π and 3 Σ + states manifold. The resulting diabatic potential energy curves of the interacting states and relevant spin-orbit coupling matrix elements defined analytically by Expanded Morse Oscillators model reproduce 95% of experimental data field of the 39 K 133 Cs isotopomer with a standard deviation of 0.004 cm −1 which is consistent with the uncertainty of the experiment. Reliability of the derived parameters was additionally confirmed by a good agreement between the predicted and experimental termvalues of 41 K 133 Cs isotopomer. Calculated relative intensity distributions in the A ∼ b → X LIF progressions are also consistent with their experimental counterparts. Finally, the deperturbation model was applied for a simulation of pump-dump optical cycle a 3 Σ + → A 1 Σ + ∼ b 3 Π → X 1 Σ + proposed for transformation of ultracold colliding K+Cs pairs to their ground molecular state vX = 0; JX = 0.

High resolution Fourier-transform spectroscopy data of term values in the spin-orbit (SO) coupled first excited A(1)Σ(+) and b(3)Π states in KCs were obtained from (4)(1)Σ(+) → A(1)Σ(+) - b(3)Π, A(1)Σ(+) - b(3)Π → X(1)Σ(+), and (1)(3)Δ1→b(3)Π(0(±)) spectra of laser-induced fluorescence (LIF). About 3000 new rovibronic term values of the A(1)Σ(+) and b(3)Π(Ω) states were obtained with an uncertainty about 0.01 cm(-1) and added to the previously obtained 3439 term values in Kruzins et al. [Phys. Rev. A 81, 042509 (2010)] and 30 term values of the b(3)Π(0(+)) state levels below the A(1)Σ(+) state in Tamanis et al. [Phys. Rev. A 82, 032506 (2010)]. The data field was extended considerably, going down to vibrational level v(b) = 0 and up in energy to 13,814 cm(-1), as compared to previously achieved v(b) = 14 and E = 13,250 cm(-1). Overall 6431 e-symmetry term values of (39)K(133)Cs were included in 4 × 4 coupled-channel deperturbation analysis. The analytical Morse-Long-Range (MLR) function yielded empirical diabatic potentials for the A(1)Σ(+) and b(3)Π(0(+)) states while the morphing of the SO ab initio points [J. T. Kim et al., J. Mol. Spectrosc. 256, 57 (2009)] provided the empirical diagonal and off-diagonal SO functions. Overall 98.5% of the fitted term values were reproduced with a rms (root mean square) uncertainty of 0.004 cm(-1). The reliability of the model is proved by a good agreement of predicted and measured term values of the (41)K(133)Cs isotopologue, as well as of measured and calculated intensities of (4)(1)Σ(+) → A(1)Σ(+) - b(3)Π LIF progressions. Direct-potential-fit of low-lying v(b) levels of the b(3)Π(0(-)) component yielded the MLR potential which represents the 204 f-symmetry experimental term values with a rms uncertainty of 0.002 cm(-1). The Ω-doubling of the b(3)Π0 sub-state demonstrates a pronounced vb-dependent increase.

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 diode-laser-induced fluorescence spectra b 3 + =0 → X 1 + originated from the rovibronic levels of the "dark" triplet b 3 + =0 state of KCs dimers were recorded with a Fourier transform spectrometer with a resolution of 0.03 cm −1 . Term values of 30 rovibronic levels (v * b0 ∈ [14,18]; J ∈ [47,134]) below the minimum of perturbing A 1 + state were determined with 0.003-0.01 cm −1 uncertainty. The optimal excitation and detection lines for very weak spin-forbidden b 3 + =0 -X 1 + transitions have been properly predicted in the framework of the coupled-channels deperturbation analysis recently performed for the singlet-triplet A 1 + -b 3 + =0,1,2 complex [A. Kruzins et al., Phys. Rev. A 81, 042509 (2010)]. The lowest observed triplet level has only 8.6% of singlet admixture.

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.