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. Although the molecular anion is heliophobic and migrates toward the surface of the helium droplet, the excited metastable atomic helium anion is bound within the helium droplet and exhibits high mobility. The atomic anion is shown to be responsible for the formation of molecular dopant anions upon charge transfer and thus, we clarify the nature of the previously unidentified fast exotic negative charge carrier found in bulk liquid helium.

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. In some cases, depending on the neutral target, the process also leads to a subsequent Coulomb explosion of the dopant.

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. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C is added. The sole exception is atomic Cs, which remains at the surface.

The effects of interactions between He − and clusters of fullerenes in helium nanodroplets are described. Electron transfer from He − to (C 60 ) n and (C 70 ) 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 − . We suggest a mechanism which involves long range electron transfer followed by the conversion of He + into He 2 + , where formation of the He-He bond in He 2 + releases sufficient kinetic energy for the cation and the dianion to escape their Coulombic attraction. By analogy with the corresponding dications, the observation of a threshold size of n ≥ 5 for formation of both (C 60 ) n 2− and (C 70 ) n 2− is attributed to Coulomb explosion rather than an energetic constraint. We also find that smaller dianions can be observed if water is added as a co-dopant. Other aspects of He − chemistry that are explored include its role in the formation of multiply charged fullerene cluster cations and the sensitivity of cluster dianion formation on the incident electron energy. C

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. Research into helium nanodroplets, originally a scientific niche driven by curiosity about the minimum droplet size that supports superfluidity, 1 has matured to a point where 4 He droplets provide a novel method to synthesize and characterize unusual molecules, large aggregates in unusual morphologies, metallic foam, or nanowires from a wide range of materials.2-7 Still, not only do the droplets provide new ways for synthesis but the products also provide new insight into properties of helium droplets. For example, the shape of silver aggregates grown in very large droplets reflects the presence of quantized vortices in superfluid droplets. 7,8 A topic that has been of interest ever since large helium droplets were efficiently produced in supersonic jets 9 is the mechanism by which droplets become charged by ionizing radiation. How do small Hen + ions containing as few as two atoms emerge from a very large neutral, undoped droplet? 10,11 How do monomer or dimer ions form when the energy of the ionizing radiation is below their thermodynamic threshold? [12][13][14] How do large Hen -cluster anions form upon electron impact? [15][16][17][18] What role do metastable electronically excited species play? 12,19 What is the local structure near a positive or negative charge in undoped helium droplets, and how does it compare to that in bulk helium or helium films? [20][21][22][23] Our present work addresses the formation and subsequent ejection of bare He + from undoped droplets. For ionizing radiation exceeding the ionization threshold of atomic He (24.59 eV) small Hen + cluster ions (n > 1) are thought to result from a two-step mechanism. 12,22,[24][25][26][27][28] The process commences with the formation of He + in the droplet. Direct formation of Hen + cluster ions (n > 1) is disfavored by very small Franck-Condon factors. The hole will hop, on the time scale of femtoseconds, by resonant charge exchange with adjacent helium atoms. After about 10 hops the charge will localize by forming a vibrationally excited He2 + . Its excess energy will be large given the large (about 2.4 eV) dissociation energy of He2 + . 29 This energy would be sufficient to boil off thousands of helium atoms (the bulk cohesive energy of helium is 0.62 meV) but a thermal process appears unlikely; evaporation of even thousands of helium atoms from a primary droplet containing »10 4 helium atoms would still result in a helium cluster ion whose size lies outside the range of...