Coupled system a 3Σ+ and X 1Σ+ of KLi: Feshbach resonances and corrections to the Born-Oppenheimer approximation

High-resolution Fourier transform spectroscopy was used to record fluorescence progressions of KLi formed in a heat pipe. Lithium isotope effects were used to derive the adiabatic correction to the Born-Oppenheimer potential for the electronic ground state over a wide range of internuclear separations. With a short extrapolation from the experimentally determined potential functions of the coupled states X (1)Sigma(+) and a (3)Sigma(+) toward their common atomic asymptote, we have improved the modeling of recently observed Feshbach resonances for the fermionic atom pair Li-6 and K-40. Predictions for cold collisions between different isotopologues are presented using mass scaling with or without Born-Oppenheimer corrections. Predictions for Feshbach resonance positions differ by up to 0.5 G within reach of current experimental verification.