The coupling of the X¹Σ⁺and a³Σ⁺states of the atom pair Na + Cs and modelling cold collisions

The states X1Σ+ and a3Σ+ correlated to the ground-state asymptote of Na (3s) and Cs (6s) atoms have been experimentally investigated using high resolution Fourier-transform spectroscopy. The hyperfine splitting of the a3Σ+ state levels is partially resolved. Transitions to asymptotic vibrational levels of the a3Σ+ and X1Σ+ states were recorded simultaneously. The joint evaluation of the data of both the a3Σ+ and the X1Σ+ states allows us to determine accurate potential energy curves of both electronic states. Coupled-channels calculations are finally applied for deriving long range dispersion parameters and the exchange contribution of the molecular potentials, yielding a reliable description of cold collisions between Na and Cs atoms in their two different hyperfine ground states. Scattering lengths and Feshbach resonances are predicted for selected quantum states.