Pure and alkali–ion-doped droplets of He4

Abstract

We present the results of variational Monte Carlo calculations of clusters of He4 systems: We study the pure He4 case and the case of a cluster doped with a single alkali–ion impurity. The results are compared with similar calculations in bulk He4. Our trial wave function is a glue-shadow wave function that can describe successfully self-binding and localization in space. The local Bose–Einstein condensate in the pure clusters is calculated. We give the results on the microscopic structure of the doped cluster in terms of the radial density profile around the ion and of angular correlations. We have studied the case of Na+ and K+. In both cases the local order of the atoms in the first shell around the ion is quite distinct compared to that present in the bulk liquid.

Bosonic helium droplets with cationic impurities: Onset of electrostriction and snowball effects from quantum calculations

“…Another important feature which characterizes the ionic dopants in He droplets is the presence of a more structured solvent environment around the ion [14,15]: positive ions are thus surrounded by many He atoms that are strongly compressed as a result of electrostriction. The resulting ionic core is thought to be as a solid, with a diameter of severalÅ containing many He atoms, and is referred to as a "snowball".…”

“…The resulting ionic core is thought to be as a solid, with a diameter of severalÅ containing many He atoms, and is referred to as a "snowball". To investigate the possible existence and the geometric structure of such snowballs we have mapped the position of the He atoms surrounding the central ion in terms of polar angles defined via a convenient spherical coordinates system [14]. The system we use here defines a z axis joining the impurity and one He atom and the xz plane as the plane that contains this z axis and a second He atom: all the remaining (n−2) He atoms are mapped in terms of their polar angles θ and φ with respect to that reference frame.…”

Alkali-helium snowball complexes formed on helium nanodroplets

“…Mainly alkali and alkaline earth ions have been addressed so far since reliable Me + -He potentials are available for these species. Using variational Monte Carlo simulations, Reatto and coworkers find that all alkali and alkaline-earth cations form snowball structures featuring shells of He atoms with high average density [8,9,11]. In addition to a modulated radial density profile around the impurity ions, snowballs are characterized by angular correlations in the first He shell as well as by the degree of radial localization of He atoms and the ability of He atoms to move from the first to the second shell and vice versa.…”

“…A number of theoretical techniques have been applied to studying the solvation of positive ions in He droplets [8,9,10]. Mainly alkali and alkaline earth ions have been addressed so far since reliable Me + -He potentials are available for these species.…”

Theoretical modeling of ion mobility in superfluid4He

“…Thus the difference in mobilities between the positive and negative ions should be related to the difference in their solvation cavity size and the accompanying solvent shell structure (i.e., the overall effective ion size in the liquid). Previous theoretical investigations of the ion solvation structures in superfluid helium have relied on the well-known semiempirical bubble model [11][12][13] or more accurate quantum Monte Carlo (QMC) based methods, [14][15][16][17][18] whereas the low-temperature ion mobility has been discussed mostly in terms of the ion-roton collision model 3,19,20 or in terms of Stokes' law. 21,22 However, since the ion-roton collision cross section depends on the square of the ion radius in superfluid helium, the use of Stokes' law is not justified in this case.…”