We review the solvation of atomic, molecular or cluster ions in HNDs. After briefly discussing the properties of snowballs in bulk helium we consider 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. The ionization threshold of HNDs doped with alkalis reveals the minimum cluster size required for full immersion. The abundance distributions of He ${}_N$X ${}^{\pm }$ ions frequently reveal local anomalies or magic numbers. We demonstrate that the abundance is approximately proportional to the evaporation energy. Observed and calculated magic numbers will be compiled, including data for ions solvated in molecular hydrogen. Alternative methods to forming He ${}_N$X ${}^{+}$ that do not employ HNDs will be summarized. Electronic excitation spectra of C ${}_{60}^{+}$ and polycyclic aromatic hydrocarbon ions 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.

Helium atoms bind strongly to alkali cations which, when embedded in liquid helium, form so‐called snowballs. Calculations suggest that helium atoms in the first solvation layer of these snowballs form rigid structures and that their number (n) is well defined, especially for the lighter alkalis. However, experiments have so far failed to accurately determine values of n. We present high‐resolution mass spectra of Na⁺He_n, K⁺He_n, Na₂⁺He_n and K₂⁺He_n, formed by electron ionization of doped helium droplets; the data allow for a critical comparison with several theoretical studies. For sodium and potassium monomers the spectra indicate that the value of n is slightly smaller than calculated. Na₂⁺He_n displays two distinct anomalies at n=2 and n=6, in agreement with theory; dissociation energies derived from experiment closely track theoretical values. K₂⁺He_n distributions are fairly featureless, which also agrees with predictions.

Helium nanodroplets doped with argon, krypton, or xenon are ionized by electrons and analyzed in a mass spectrometer. He_nNg_x⁺ ions containing up to seven noble gas (Ng) atoms and dozens of helium atoms are identified; the high resolution of the mass spectrometer combined with advanced data analysis make it possible to unscramble contributions from isotopologues that have the same nominal mass but different numbers of helium or Ng atoms, such as the magic He₂₀⁸⁴Kr₂⁺ and the isobaric, nonmagic He₄₁⁸⁴Kr⁺. Anomalies in these ion abundances reveal particularly stable ions; several intriguing patterns emerge. Perhaps most astounding are the results for He_nAr⁺, which show evidence for three distinct, solid-like solvation shells containing 12, 20, and 12 helium atoms. This observation runs counter to the common notion that only the first solvation shell is solid-like but agrees with calculations by Galli et al. for He_nNa⁺ [ J. Phys. Chem. A 2011 , 115 , 7300] that reveal three shells of icosahedral symmetry. He_nAr_x⁺ (2 ≤ x ≤ 7) ions appear to be especially stable if they contain a total of n + x = 19 atoms. A sequence of anomalies in the abundance distribution of He_nKr_x⁺ suggests that rings of six helium atoms are inserted into the solvation shell each time a krypton atom is added to the ionic core, from Kr⁺ to Kr₃⁺. Previously reported strong anomalies at He₁₂Kr₂⁺ and He₁₂Kr₃⁺ [ Kim , J. H. ; et al. J. Chem. Phys. 2006 , 124 , 214301] are attributed to a contamination. Only minor local anomalies appear in the distributions of He_nXe_x⁺ (x ≤ 3). The distributions of He_nKr⁺ and He_nXe⁺ show strikingly similar, broad features that are absent from the distribution of He_nAr⁺; differences are tentatively ascribed to the very different fragmentation dynamics of these ions.

High‐resolution mass spectra of helium nanodroplets doped with hydrogen or deuterium reveal that copious amounts of helium can be bound to H⁺, H₂⁺, H₃⁺, and larger hydrogen‐cluster ions. All conceivable He_nH_x⁺ stoichiometries are identified if their mass is below the limit of ≈120 u set by the resolution of the spectrometer. Anomalies in the ion yields of He_nH_x⁺ for x=1, 2, or 3, and n≤30 reveal particularly stable cluster ions. Our results for He_nH₁⁺ are consistent with conclusions drawn from previous experimental and theoretical studies which were limited to smaller cluster ions. The He_nH₃⁺ series exhibits a pronounced anomaly at n=12 which was outside the reliable range of earlier experiments. Contrary to findings reported for other diatomic dopant molecules, the monomer ion (i.e. H₂⁺) retains helium with much greater efficiency than hydrogen‐cluster ions.