Squeezing a Helium Nanodroplet with a Rydberg Electron † F. Ancilotto,* ,‡ M. Pi, § R. Mayol, § M. Barranco, § and K. K. Lehmann | INFM-CNR DEMOCRITOS National Simulation Center and Dipartimento di Fisica “G. Galilei”, UniVersita ` di PadoVa, Via Marzolo 8, I-35131 PadoVa, Italy, Departament ECM, Facultat de Fı ´sica, and IN 2 UB, UniVersitat de Barcelona. Diagonal 647, 08028 Barcelona, Spain, and Departament of Chemistry and Physics, UniVersity of Virginia, 22904-4319 CharlottesVille, Virginia ReceiVed: July 31, 2007; In Final Form: October 10, 2007 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. 1. Introduction It has been proposed in the past the possibility of creating long-lived surface Rydberg atoms 1,2 consisting of a positive ion adsorbed below the liquid helium surface, while the external Rydberg electron is located outside the helium and cannot penetrate inside the liquid because of the positive energy barrier for electron penetration, V 0 ∼ 1 eV. A similar system, that is, a metastable Rydberg atom where the positive ion is located under a solid hydrogen surface, has been also proposed. 3 Similarly, a system made by an electron circulating around a positively charged 4 He nanodroplet, prevented from neutraliza- tion by the helium-electron repulsion, was studied by Golov and Sekatskii, 4 and named Scolium after Giacinto Scoles. 5,6 Recent experiments 7 on the Rydberg nature of high-lying electronically excited states of small 4 He clusters seem to point to the presence of Rydberg states of the same nature as those originally proposed by Golov, 4 that is, an excited electron largely localized outside the He cluster, under the attraction of a positive He ion in the interior of it. The temporary trapping of photoelectrons into Rydberg states largely located outside the 4 He cluster has also been invoked in the interpretation of photoelectron imaging of helium nanodroplets. 8 Other works on highly electronically excited states of helium clusters and nanodroplets have recently been reported. 9,10 Different routes than that used in ref 7 could be followed for a practical realization of a scolium. One possibility consists of a two-step process where first a 4 He cluster doped with a positive ion is created, and later, an electron is picked up. The first step has been proved possible in recent experiments where metal ions have been attached to helium droplets by means of laser evaporation techniques, 11 allowing isolation of charged particles inside 4 He clusters at temperatures below 1 K. The electron attachment to 4 He droplets can be realized similarly as in the experiments reported in refs 12-14 where a low-energy electron beam is crossed with a helium cluster beam. A second way for producing a scolium could be through a gentle collision between an atom, previously excited into a high-n Rydberg state and a pure 4 He nanodroplet. Yet, another way of realizing in practice a scolium could follow the excitation in a Rydberg state of an neutral atom impurity deeply bound to the surface of a 4 He nanodroplet like, for instance, an alkaline-earth atom. 15 Following the excitation process, the electron wavefunction could be forced outside the interface region, while the ion core will sink toward the interior of the 4 He nanodroplet because of electrostriction, leading eventually to a state where the electron wavefunction completely surrounds the nanodroplet, remaining largely confined outside its surface. The excitation into Rydberg states of silver atoms initially embedded in the center of 4 He nanodroplets has been experi- mentally studied, but the impurity atom was found to move, after the excitation, to the surface of the droplet and eventually desorb. 16 An experiment is planned to look for these Rydberg atoms using Pb + cations. 17 Interestingly, the results of a more recent experiment, where the excited-state dynamics of silver atoms in 4 He nanodroplets have been investigated, 18 can be interpreted as providing some evidence for the existence of a scolium. 19 The positive ion core in a scolium resides in the bulk of the nanodroplet where, as a consequence of electrostriction, it strongly perturbs the 4 He density around it. In particular, a local increase in the helium density with respect to its bulk value appears near the ion site, whose magnitude depends on the strength of the ion-He interaction potential. 20-22 Strongly attractive ions tend to form a solid-like structure, the so-called “snowball”, characterized by a very inhomoge- neous, solid-like helium density distribution in the surroundings of the ion. Alkali ions are believed to belong to this category. 21,22 In contrast, singly charged alkaline-earth cations, due to their larger radii and lower strength interaction with the He atoms, are expected to produce a cavity surrounded by compressed, less structured, and likely not solidified helium. 23,24 † Part of the “Giacinto Scoles Festschrift”. * Corresponding author. ‡ Universita ` di Padova. § Universitat de Barcelona. | University of Virginia. 12695 J. Phys. Chem. A 2007, 111, 12695-12701 10.1021/jp076069b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/06/2007