Solvation of ions in helium

Abstract

In recent years several reviews of 4 He nanodroplets (HNDs) have been published in this Journal. The present one focuses on the solvation of atomic, molecular or cluster ions X ± in HNDs. After briefly reviewing the properties of so-called snowballs in bulk helium we discuss experimental conditions for the efficient synthesis of charged, doped HNDs. We show that the cluster ions observed in conventional mass spectrometers originate from fission of highly charged HNDs. Mass spectra of alkali clusters recorded near the ionization threshold of HNDs reveal the size limit beyond which the neutral cluster will be fully embedded in the HND. The abundance distributions of He N X ± ions solvated in helium frequently reveal local anomalies or magic numbers. We demonstrate that the ion abundance is approximately proportional to the dissociation energy D N = E N − E N −1 . A compilation of observed magic numbers will be presented together with theoretical data, including data for ions solvated in molecular hydrogen. Alternative methods to forming He N X + that do not rely on nozzle expansions will be summarized. Electronic excitation spectra of C + 60 and polycyclic aromatic hydrocarbon ions with up to 100 adsorbed helium atoms reveal the properties of the helium adsorption layer in quantitative detail. Next we discuss theoretical efforts to describe the interaction between ions and helium. We close with summarizing the size dependence of physical quantities computed for atomic alkali and alkaline earth cations in helium, such as binding energy, superfluid fraction, structural order, radial density profiles, and the existence of first and higher solvation shells.

Multiply Charged Helium Droplet Anions

“…The possibility of higher charge states in helium droplets wasn ot obvious given the very low binding energies (about0 .06 meV per atom) and other unique characteristics of the liquid. 4 He droplets formed in vacuum have an equilibrium temperature of 0.37 Ka nd are in as uperfluid state, which resultsi na ne nvironment within the droplets with no internal friction and an extremelyh igh thermal conductivity.H elium droplets are capable of capturing ands olvating aw ide range of atomica nd molecular species, which, together with their aforementioned properties, makest hem au seful toolf or experimental studies in, for instance, chemistry and chemical physics. [2,3,4] Examples of processes that have been studied by using doped He droplets are reaction productso fm etal nanoparticles and organic molecules, [5] reactionsb etween atomic radicals and complex molecules, [6] and sub-Kelvin protont ransfer reactions.…”

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation

“…The ligand most weakly bound to gold is helium [9] and thus ligand-exchange with every other ligand can be expected inside a helium nanodroplet (HND). In the case of neutral helium droplets, this was successfully demonstrated by the growth of pristine and mixed gold clusters containing a few gold atoms [10][11][12][13][14][15][16][17][18], as well as nanoparticles and nanowires that contain many thousand atoms [19][20][21][22][23][24][25].…”

Roadmap on dynamics of molecules and clusters in the gas phase

“…The theoretical studies of ions solvated in He present a number of technical challenges due to the low binding energies involved as well as the low mass of the He atoms. Because of this, quantum descriptions of nuclear motion are generally required to properly sample the dynamics of He solvation layers [170,179].…”