Resolution of the apparent disorder of the Rb2 A1Σu1(Ou+) and b3Πu(Ou+) spectra:: A case of fully coupled electronic states

Strong perturbation effects in the infrared fluorescence spectra recorded after the resonant excitation process X(1)Sigma(g)(+) --> A(1)Sigma(u)(+)(0(u)(+)) --> (2)(1)Pi(g) in Rb-2, have been observed by high resolution Fourier transform spectroscopy. They arise from complete coupling of the two states over the whole range of binding energies, yielding unusual molecular spectra with oscillating vibrational and rotational structures. A quantitative interpretation, based on Fourier grid Hamiltonian method calculations, of these newly observed phenomena is given. This opens new opportunities to study the triplet manifold in Rb-2, RbCs, and Cs-2, particularly the lowest state relevant for precise scattering length determination.