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 He-4 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 He-4 cluster exerts an additional electrostatic pressure which further increases the local He-4 density around the ion core. We argue that under such pressure, sufficiently small He-4 nanodroplets may turn solid. The stability of a scolium with respect to electron-ion recombination is investigated.