Deperturbation treatment of the A Σ+1 -b Π3 complex of NaRb and prospects for ultracold molecule formation in X Σ+1 (v=0;J=0)

High resolution Fourier transform spectra (FTS) of laser induced fluorescence (LIF) of C Σ+1; D Π1 →A Σ+1 -b Π3 and A Σ+1 -b Π3 →X Σ+1 transitions in Na85 Rb and Na87 Rb were obtained. An analysis of the direct LIF spectra together with the rotational relaxation satellites provided highly accurate rovibronic term values for (4≤J≤163) of the A Σ+1 -b Π3 complex, covering about 1950 mostly singlet levels 0≤ vA ≤49 and a considerable number (>360) of the predominantly triplet b Π 0,1 3 sublevels. The direct deperturbation analysis of the singlet-triplet A-b complex was performed by means of the inverted channel-coupling approach with Hund's coupling case a basis functions. The electronic matrix elements of the model 4×4 Hamiltonian were defined as piecewise analytical functions of the internuclear distance. Besides the Born-Oppenheimer potential energy curves of the mutually perturbed states and the off-diagonal spin-orbit (SO) A-b coupling function, the SO splitting of the b Π3 state was determined due to the pronounced electronic-rotation interaction between the b Π03 and b Π13 components observed for high J levels. Overall, 24 mass-invariant fitting parameters have been required to reproduce about 2300 experimental term values of both isotopomers with a standard deviation of 0.012 cm-1, which is consistent with the uncertainty of the FTS experiment. An analytical mapping procedure based on a reduced variable representation of the radial coordinate was used to diminish the computational effort for the uniform finite-difference grid solution of the coupled-channel equations. The derived nonadiabatic A-b wave functions were used to evaluate the A Σ+1 -b Π3 -D Π1, a Σ+3, X Σ+1 rovibronic transition probabilities. The relative intensity distributions predicted for the D→A-b and A-b→X LIF progressions agree with their experimental counterparts within the accuracy of the measurements. The calculated A-b-a, X transition probabilities were applied for simulation of the stimulated Raman a→A-b→X processes, which can lead to efficient formation of ultracold NaRb molecules in the ground level vX =0; JX =0.