Size limits and fission channels of doubly charged noble gas clusters.

Small, highly charged liquid droplets are unstable with respect to spontaneous charge separation when their size drops below the Rayleigh limit or, in other words, their fissility parameter exceeds the value 1. The absence of small doubly charged atomic cluster ions in mass spectra below an element-specific appearance size has sometimes been attributed to the onset of barrierless fission at = 1. However, more realistic models suggest that marks the size below which the rate of fission surpasses that of competing dissociative channels, and the Rayleigh limit of doubly charged van der Waals clusters has remained unchartered.

Doubly charged dimers and trimers of heavy noble gases.

Many doubly charged heteronuclear dimers are metastable or even thermodynamically stable with respect to charge separation. Homonuclear dicationic dimers, however, are more difficult to form. He was the first noble gas dimer predicted to be metastable and, decades later, observed.

Correction: Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations.

Correction for 'Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations' by Siegfried Kollotzek , , 2023, , 462-470, https://doi.org/10.1039/D2CP03841B.

Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations.

We report a novel method to reversibly attach and detach hydrogen molecules to positively charged sodium clusters formed inside a helium nanodroplet host matrix. It is based on the controlled production of multiply charged helium droplets which, after picking up sodium atoms and exposure to H vapor, lead to the formation of Na(H) clusters, whose population was accurately measured using a time-of-flight mass spectrometer. The mass spectra reveal particularly favorable Na(H) and Na(H) clusters for specific "magic" numbers of attached hydrogen molecules.

Solvation of Large Polycyclic Aromatic Hydrocarbons in Helium: Cationic and Anionic Hexabenzocoronene.

The adsorption of helium on charged hexabenzocoronene (Hbc, CH), a planar polycyclic aromatic hydrocarbon (PAH) molecule of symmetry, was investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of HeHbc complexes versus size features prominent local anomalies at = 14, 38, 68, 82, and a weak one at 26, indicating that for these "magic" sizes, the helium evaporation energies are relatively large. Surprisingly, the mass spectra of anionic HeHbc complexes feature a different set of anomalies, namely at = 14, 26, 60, and 62, suggesting that the preferred arrangement of the adsorbate atoms depends on the charge of the substrate.

Adsorption of Helium and Hydrogen on Triphenylene and 1,3,5-Triphenylbenzene.

The adsorption of helium or hydrogen on cationic triphenylene (TPL, CH), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, CH), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HeTPL complexes are particularly stable if = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for = 2 and on either side of the three outer rings for = 6. Theory suggests that HeTPL is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings.

Stabilization of phenanthrene anions in helium nanodroplets.

It has been debated for years if the polycyclic aromatic hydrocarbon phenanthrene exists in its anionic form, or, in other words, if its electron affinity (EA) is positive or negative. In this contribution we confirm that the bare phenanthrene anion Ph created in a binary collision with an electron at room temperature has a lifetime shorter than microseconds. However, the embedding of neutral phenanthrene molecules in negatively charged helium nanodroplets enables the formation of phenanthrene anions by charge transfer processes and the stabilization of the latter in the ultracold environment.

Phenanthrene: establishing lower and upper bounds to the binding energy of a very weakly bound anion.

Quite a few molecules do not form stable anions that survive the time needed for their detection; their electron affinities (EA) are either very small or negative. How does one measure the EA if the anion cannot be observed? Or, at least, can one establish lower and upper bounds to their EA? We propose two approaches that provide lower and upper bounds. We choose the phenanthrene (Ph) molecule whose EA is controversial.

Splashing of Large Helium Nanodroplets upon Surface Collisions.

In the present work we observe that helium nanodroplets colliding with surfaces can exhibit splashing in a way that is analogous to classical liquids. We use transmission electron microscopy and mass spectrometry to demonstrate that neutral and ionic dopants embedded in the droplets are efficiently backscattered in such events. High abundances of weakly bound He-tagged ions of both polarities indicate a gentle extraction mechanism of these ions from the droplets upon collision with a solid surface.

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation.

Complexes of atomic gold with a variety of ligands have been formed by passing helium nanodroplets (HNDs) through two pickup cells containing gold vapor and the vapor of another dopant, namely a rare gas, a diatomic molecule (H, N, O, I, P), or various polyatomic molecules (HO, CO, SF, CH, adamantane, imidazole, dicyclopentadiene, and fullerene). The doped HNDs were irradiated by electrons; ensuing cations were identified in a high-resolution mass spectrometer. Anions were detected for benzene, dicyclopentadiene, and fullerene.

SF: Stabilizing Transient Ions in Helium Nanodroplets.

There are myriad ions that are deemed too short-lived to be experimentally accessible. One of them is SF. It has never been observed, although not for lack of trying.

Proton transfer at subkelvin temperatures.

We demonstrate a novel method to ionize molecules or molecular clusters by proton transfer at temperatures below 1 K. The method yields nascent ions and largely eliminates secondary reactions, even for notoriously 'delicate' molecules. Protonation is achieved inside liquid helium nanodroplets (HNDs) and begins with the formation of (H2)mH+ ions as the proton donors.

Isotope enrichment in neon clusters grown in helium nanodroplets.

Neon cluster ions Ne grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope Ne that depends on cluster size s and the experimental conditions. For small sizes, the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs.

Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe.

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and just a few (or no) silver atoms.

Atomic Gold Ions Clustered with Noble Gases: Helium, Neon, Argon, Krypton, and Xenon.

High-resolution mass spectra of helium droplets doped with gold and ionized by electrons reveal HeAu cluster ions. Additional doping with heavy noble gases results in NeAu, ArAu, KrAu, and XeAu cluster ions. The high stability predicted for covalently bonded ArAu, KrAu, and XeAu is reflected in their relatively high abundance.

Charged Clusters of C and Au or Cu: Evidence for Stable Sizes and Specific Dissociation Channels.

We have doped helium nanodroplets with C and either gold or copper. Positively or negatively charged (C) M ions (M = Au or Cu) containing up to ≈10 fullerenes and ≈20 metal atoms are formed by electron ionization. The abundance distributions extracted from high-resolution mass spectra reveal several local anomalies.

Electron Attachment and Electron Ionization of Formic Acid Clusters Embedded in Helium Nanodroplets.

We report the results of an experimental study of electron ionization of large helium nanodroplets doped with formic acid (FA). Several homologous series of cluster anions are observed, including [FA-H], undissociated FA, and these ions complexed with one or more HO. Some major features resemble those observed upon sputtering of frozen FA films but they differ significantly from results obtained by electron attachment to bare FA clusters in the gas phase.

Doubly charged coronene clusters-Much smaller than previously observed.

The smallest doubly charged coronene cluster ions reported so far, Cor, were produced by charge exchange between bare coronene clusters and He [H. A. B.

The structure of coronene cluster ions inferred from H uptake in the gas phase.

Mass spectra of helium nanodroplets doped with H and coronene feature anomalies in the ion abundance that reveal anomalies in the energetics of adsorption sites. The coronene monomer ion strongly adsorbs up to n = 38 H molecules indicating a commensurate solvation shell that preserves the D symmetry of the substrate. No such feature is seen in the abundance of the coronene dimer through tetramer complexed with H; this observation rules out a vertical columnar structure.

Positively and Negatively Charged Cesium and (C) Cs Cluster Ions.

We report on the formation and ionization of cesium and CCs clusters in superfluid helium nanodroplets. Size distributions of positively and negatively charged (C) Cs ions have been measured for ≤ 7, ≤ 12. Reproducible intensity anomalies are observed in high-resolution mass spectra.

Fission of multiply charged alkali clusters in helium droplets - approaching the Rayleigh limit.

Electron ionization of helium droplets doped with sodium, potassium or cesium results in doubly and, for cesium, triply charged cluster ions. The smallest observable doubly charged clusters are Na9(2+), K11(2+), and Cs9(2+); they are a factor two to three smaller than reported previously. The size of sodium and potassium dications approaches the Rayleigh limit nRay for which the fission barrier is calculated to vanish, i.

Helium Droplets Doped with Sulfur and C.

Clusters of sulfur are grown by passing superfluid helium nanodroplets through a pickup cell filled with sulfur vapor. In some experiments the droplets are codoped with C. The doped droplets are collided with energetic electrons and the abundance distributions of positively and negatively charged cluster ions are recorded.

Doubly charged CO clusters formed by ionization of doped helium nanodroplets.

Helium nanodroplets are doped with carbon dioxide and ionized by electrons. Doubly charged cluster ions are, for the first time, identified based on their characteristic patterns of isotopologues. Thanks to the high mass resolution, large dynamic range, and a novel method to eliminate contributions from singly charged ions from the mass spectra, we are able to observe doubly charged cluster ions that are smaller than the ones reported in the past.

Ordered phases of ethylene adsorbed on charged fullerenes and their aggregates.

In spite of extensive investigations of ethylene adsorbed on graphite, bundles of nanotubes, and crystals of fullerenes, little is known about the existence of commensurate phases; they have escaped detection in almost all previous work. Here we present a combined experimental and theoretical study of ethylene adsorbed on free C and its aggregates. The ion yield of [Formula: see text] measured by mass spectrometry reveals a propensity to form a structurally ordered phase on monomers, dimers and trimers of C in which all sterically accessible hollow sites over carbon rings are occupied.

On subthreshold ionization of helium droplets, ejection of He(+), and the role of anions.

The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are He(+) ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of He(+) and He2(+) as a function of electron energy, electron emission current, and droplet size reveals that metastable He*(-) anions play a crucial role in the formation of free He(+) at subthreshold energies. The proposed model is testable.