Calculations of some weakly bound diatomic molecular negative ions

We examine the existence of stable bound states of some diatomic molecular ions where the neutral molecule itself is either unbound or barely bound. Two bound states have been found for the ${\mathrm{HeH}}^{-}$ ion even though HeH is known to be unbound. We have found no bound states for ${\mathrm{He}}_2^{-}$ even though ${}^4{\mathrm{He}}_2$ is known to have one bound state. The predicted binding energies for ${\mathrm{HeH}}^{-}$ are of the order of 1 K and depend sensitively on the isotope combinations. Other weakly bound negative ions such as ${\mathrm{NeH}}^{-}$ and ${\mathrm{ArH}}^{-}$ have also been investigated. These weakly bound molecular ions are good candidates for cold collision experiments.