Negative impurity ions in liquid He-4

We present theoretical results, based on zero-temperature density-functional theory, for the formation and properties of a negative impurity ion in bulk liquid (4)He. We first consider Ca which, due to its very low electron affinity, does not easily form a negative ion in vacuum. We show that a neutral Ca atom in bulk liquid (4)He can easily capture a nearby electron bubble leading to the exothermic formation of a Ca(-) ion trapped inside a spherical cavity of similar to 15 A degrees radius. The Ca negative ion in bulk (4)He turns out to be a metastable state, the lowest-energy configuration being represented instead by a weakly bound Ca(-) ion floating over the nearly unperturbed free surface of liquid (4)He. We have computed the threshold negative pressure at which the trapped Ca(-) ion bubble explodes and we discuss our results in light of recent experimental measurements. We have also considered the possible ion formation in the case of a Ne atom, i.e., an atomic impurity that does not form a negative ion in vacuum. Despite the long-range attraction between the electron bubble and the Ne atom due to polarization forces, in the minimum-energy configuration the electron bubble and the Ne atom rather than merge together remain spatially separated in bulk liquid (4)He, forming a weakly bound state that has no analog in vacuum.