Squeezing a helium nanodroplet with a Rydberg electron

We have investigated, by means of density functional theory, the structure of a "scolium", that is, an electron circulating around a positively charged (4)He nanodroplet, temporarily prevented from neutralization by the helium-electron repulsion. The positive ion core resides in the center of the nanodroplet where, as a consequence of electrostriction, a strong increase in the helium density with respect to its bulk value occurs. The electron enveloping the (4)He cluster exerts an additional electrostatic pressure which further increases the local (4)He density around the ion core. We argue that under such pressure, sufficiently small (4)He nanodroplets may turn solid. The stability of a scolium with respect to electron-ion recombination is investigated.