Resolution of the apparent disorder of the Rb-2 A(1)Sigma(1)(u)(O-u(+)) and b(3)Pi(u)(O-u(+)) spectra: A case of fully coupled electronic states

Strong perturbation effects in the infrared fluorescence spectra recorded a fter 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 Fouri er transform spectroscopy. They arise from complete coupling of the two sta tes over the whole range of binding energies, yielding unusual molecular sp ectra with oscillating vibrational and rotational structures. A quantitativ e interpretation, based on Fourier grid Hamiltonian method calculations, of these newly observed phenomena is given. This opens new opportunities to s tudy the triplet manifold in Rb-2, RbCs, and Cs-2, particularly the lowest state relevant for precise scattering length determination.