Alkali-helium snowball complexes formed on helium nanodroplets

Abstract

We systematically investigate the formation and stability of snowballs formed by femtosecond photoionization of small alkali clusters bound to helium nanodroplets. For all studied alkali species Ak=(Na, K, Rb, Cs) we observe the formation of snowballs Ak + HeN when multiply doping the droplets. Fragmentation of clusters AkN upon ionization appears to enhance snowball formation. In the case of Na and Cs we also detect snowballs Ak + 2 HeN formed around Ak dimer ions. While the snowball progression for Na and K is limited to less than 11 helium atoms, the heavier atoms Rb and Cs feature wide distributions at least up to Ak + He41. Characteristic steps in the mass spectra of Cs-doped helium droplets are found at positions consistent with predictions on the closure of the 1 st shell of helium atoms around the Ak + ion based on variational Monte Carlo simulations.

Picosecond solvation dynamics of alkali cations in superfluidHe4nanodroplets

“…Using this procedure the first solvation shell is found to host 19.2 (21.4) atoms for Rb + (Cs + ). These values are somewhat larger than those found experimentally [32,34] and by QMC calculations [3]. Figure 3 shows the equilibrium configuration of Ba + @ 4 He 1000 .…”

Cs atoms on helium nanodroplets and the immersion of Cs+ into the nanodroplet

“…135, 074306 (2011) in Ref. 15. The observation of large Rb + -He N aggregates (N > 1000) with Rb + in a He snowball configuration inside a He nanodroplet was recently reported.…”

Modelling the mobility of positive ion clusters in normal liquid helium over large pressure ranges

“…Mixed quantum/classical dynamics calculations and, more recently, density functional theory studies identified the linear triatomic He + 3 ion as the centre of an isolated snowball cluster 14,15 , which should have an effect on the mobility 16 . The picture of a solid snowball forming around a positive core was also found valid for foreign ions and has received both experimental and theoretical support: on the experimental side the long life-time of nuclear spin polarised 12 B + and 8 Li + ions injected into liquid helium 17 has been attributed to helium snowballs forming around the positively charged foreign ion; the observation of magic numbers in the time-of-flight mass spectrum of He n Cs + and He n Ar + clusters ejected from large doped helium droplets 18,19 showed the prevalence of specific -hence solid -structures; on the theoretical side, Diffusion Monte Carlo (DMC) calculations for small 4 He clusters containing alkali ions revealed solid-like snowballs surrounded by a liquid-like environment of additional solvent atoms [20][21][22] . Also radial density profiles from DMC calculations for Pb 2+ He n for several relevant sizes n show peak helium density values much higher than the density of solid helium 23 .…”