Communication: Dopant-induced solvation of alkalis in liquid helium nanodroplets.

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C.

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.

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.

The interaction of He(-) with fullerenes.

The effects of interactions between He(-) and clusters of fullerenes in helium nanodroplets are described. Electron transfer from He(-) to (C60)n and (C70)n clusters results in the formation of the corresponding fullerene cluster dianions. This unusual double electron transfer appears to be concerted and is most likely guided by electron correlation between the two very weakly bound outer electrons in He(-).

Electron-driven self-assembly of salt nanocrystals in liquid helium.

The self-assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF6 molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)n Na(+) and (NaF)n F(-) .

Formation of dianions in helium nanodroplets.

The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n(2-) and (C70)n(2-) were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He(-) ion.

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.

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He and He.

Helium droplets provide the possibility to study phenomena at the very low temperatures at which quantum mechanical effects are more pronounced and fewer quantum states have significant occupation probabilities. Understanding the migration of either positive or negative charges in liquid helium is essential to comprehend charge-induced processes in molecular systems embedded in helium droplets. Here, we report the resonant formation of excited metastable atomic and molecular helium anions in superfluid helium droplets upon electron impact.

Electron attachment to CO2 embedded in superfluid He droplets.

Electron attachment to CO2 embedded in superfluid He droplets leads to ionic complexes of the form (CO2)n(-) and (CO2)nO(-) and, at much lower intensities, He containing ions of the form Hem(CO2)nO(-). At low energies (<5 eV), predominantly the non-decomposed complexes (CO2)n(-) are formed via two resonance contributions, similar to electron attachment to pristine CO2 clusters. The significantly different shapes and relative resonance positions, however, indicate particular quenching and mediation processes in CO2@He.

Sequential penning ionization: harvesting energy with ions.

We report the observation of the ejection of electrons caused by collisions of excited atoms with ions, rather than neutrals, leading to the production of doubly charged ions. Doping superfluid He droplets with methyl iodide and exposing them to electrons enhances the formation of doubly charged iodine atoms at the threshold for the production of two metastable He atoms. These observations point toward a novel ionization process where doubly charged ions are produced by sequential Penning ionization.