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

The states X-1 Sigma(+) and a(3)Sigma(+) 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 a(3)Sigma(+) state levels is partially resolved. Transitions to asymptotic vibrational levels of the a(3)Sigma(+) and X-1 Sigma(+) states were recorded simultaneously. The joint evaluation of the data of both the a(3)Sigma(+) and the X-1 Sigma(+) 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.