On the interaction of NH(X³Sigma⁻) molecules with rubidium atoms: implications for sympathetic cooling and the formation of extremely polar molecules

The Rb-NH interaction is investigated as a prototype for interactions between alkali metal atoms and stable molecules. For spin-aligned Rb and NH that interact on a quartet surface ($^4A''$), the interaction is relatively weak, with a well depth of 0.078 eV. However, there are also doublet surfaces of ion-pair character that are very much deeper (well depth 1.372 eV) and may be important for atom - molecule collision rates. Similar deeply bound ion-pair states are likely to exist for other alkali atom -- molecule pairs. It is shown that a transition to a dipolar superfluid phase will be easier to achieve for low-mass than for high-mass species.