Fourier-transform spectroscopy, direct-potential-fit, and electronic structure calculations on the entirely perturbed (4)1Π state of RbCs

Abstract

We performed a high-resolution Fourier-transform spectroscopic study of the (4)1Π state of the RbCs molecule by applying two-step (4)1Π ← A1Σ+ ∼ b3Π ← X1Σ+ optical excitation followed by observation of the (4)1Π → X1Σ+ laser induced fluorescence (LIF) spectra. In many LIF progressions the collision-induced satellite rotational lines were observed thus increasing the amount of term values and allowing to estimate the Λ-doubling effect in the (4)1Π state. The Direct-Potential-Fit (DPF) of experimental term values of 777 rovibronic levels of both 85RbCs and 87RbCs isotopologues has been performed by means of the robust weighted non-linear least squares method. The PFanalysis based on adiabatic approximation and analytical Expanded Morse Oscillator potential revealed numerous regular shifts in the measured level positions. The spectroscopic studies of the (4)1Π state were supported by the electronic structure calculations including the potential energy curves of the singlet and triplet states manifold and spin-allowed transition dipole moments. The subsequent estimates of radiative lifetimes and corresponding vibronic branching ratios elucidated a dominant contribution of the (4)1Π → A ∼ b channel into the total radiative decay of the (4)1Π state. The relative intensity distributions simulated for (4)1Π → X1Σ+ LIF progressions agree well with their observed counterparts even for the profoundly shifted levels of the entirely perturbed (4)1Π state. To get an insight into the origin of the intramolecular perturbations the relevant spin-orbit and L-uncoupling electronic matrix elements were evaluated