Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He*– and He2*–

Mauracher, Andreas; Daxner, Matthias; Postler, Johannes; Huber, S.; Denifl, Stephan; Scheier, P.; Toennies, J. P.

doi:10.1021/jz500917z

Cited by 57 publications

(148 citation statements)

References 42 publications

(83 reference statements)

Supporting

Mentioning

128

Contrasting

Order By: Relevance

“…[17] Recent work has shown that He À is a highly mobile electron donor within a helium droplet. [16,18] The ion yield curve for (NaF) 4 F À matches that of He À (which is also shown in Figure 3) very closely and we conclude that He À is the dominant source of anions in the current study. The anion peaks occur at approximately 22 rather than 20 eV because additional energy is required for an electron to enter a helium droplet.…”

Section: Methodssupporting

confidence: 81%

See 1 more Smart Citation

Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

Daxner¹,

Denifl²,

Scheier³

et al. 2014

Angewandte Chemie

Self Cite

View full text Add to dashboard Cite

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 SF 6 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 À . In the case of anions the self-assembly is not directly initiated by electrons: the dominant process involves resonant electron-induced production of metastable electronically excited He À anions, which then initiate anionic chemistry by electron transfer.Neutral and ionic alkali-metal halide clusters have been widely studied, both experimentally and theoretically. Part of the motivation to study these species is to see if the threedimensional structures of the crystalline salts are retained in relatively small clusters. Such information can be derived from mass spectrometry through the observation of anomalously intense peaks (so-called magic number features). Several methods have been used to produce alkali-metal halide cluster ions in the gas phase, including sputtering, [1] laser ablation, [2] electrospray, [3] and ion-molecule reactions in a flowing afterglow. [4] The general finding, whether detecting cations or anions, is that enhanced signal intensity is seen for cluster ions (magic-number ions) of composition consistent with one or more complete unit cells. On these grounds it is reasonable to suppose that the cluster ions adopt structures based on the normal crystalline structure of the extended solid.Herein we show that it is possible to form alkali-metal halide clusters by reactions between clusters of sodium and SF 6 in liquid helium nanodroplets. The low intrinsic temperature (ca. 0.4 K) and the rapid cooling of dopants in these droplets, [5] which is assisted by the high thermal conductivity of superfluid helium, should inhibit chemical reactions. However, reaction between sodium and SF 6 can be triggered by electron impact on the droplet, leading to a rich range of cationic and anionic salt clusters. Particularly surprising is that self-assembly into structures based on the unit cell of NaF occurs even when the chemistry is initiated inside a liquid helium nanodroplet.The two reagents were added separately to the helium droplets, with sodium vapor coming from an oven containing solid sodium while the SF 6 was supplied from a gas cylinder. SF 6 was added to the first pick-up cell and sodium vapor to the second pick-up cell. The partial pressures of the dopants were set so that the most probable process was pick-up of a relatively small number of dopant atoms/molecules. However, the statistical nature of the pick-up process means that, in practice, clusters spanning a relatively wide range of sizes were generated. Reaction products derive...

show abstract

Section: Methodssupporting

confidence: 81%

“…He À has only recently been identified as a product from electron impact of helium droplets. [16] Although atomic helium has a negative , and He À are expanded vertically relative to that of (NaF) 4 Na + .…”

mentioning

confidence: 99%

Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

Daxner¹,

Denifl²,

Scheier³

et al. 2014

Angewandte Chemie

Self Cite

View full text Add to dashboard Cite

show abstract

“…He2 -(not shown) were also observed at a much smaller (factor 100) yield; the signal reached a maximum at 22.9 eV. 36 Another dramatic change occurs when the helium droplet size is reduced by two orders of magnitude (Fig. 1c) 4 , respectively (T0 = 8.5 and 10.5 K, respectively); the electron energy was 47 eV.…”

Section: 42mentioning

confidence: 95%

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

Renzler

Daxner

Weinberger

et al. 2014

Phys. Chem. Chem. Phys.

Self Cite

View full text Add to dashboard Cite

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...

show abstract

“…Therefore the investigation of the physics and chemistry behind it prove to be challenging for this anion in the gas phase. Mauracher et al [20] recently discovered that He * − is efficiently formed upon electron…”

Section: Introductionmentioning

confidence: 99%

Helium anion formation inside helium droplets

et al. 2016

Self Cite

View full text Add to dashboard Cite

Abstract. The formation of He* − is examined with improved electron energy resolution of about 100 meV utilizing a hemispherical electron monochromator. The work presented provides a precise determination of the three previously determined resonance peak positions that significantly contribute to the formation of He * − inside helium nanodroplets in the energy range from 20 eV to 29.5 eV. In addition, a new feature is identified located at 27.69 ± 0.18 eV that we assign to the presence of O 2 as a dopant inside the droplet. With increasing droplet size a small blue shift of the resonance positions is observed. Also for the relatively low electron currents used in the present study (i.e., 15-70 nA) a quadratic dependence of the He * − ion yield on the electron current is observed.

show abstract

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He^– and He₂^–

Cited by 57 publications

References 42 publications

Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

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

Helium anion formation inside helium droplets

Contact Info

Product

Resources

About

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He*– and He2*–

Cited by 57 publications

References 42 publications

Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

Electron‐Driven Self‐Assembly of Salt Nanocrystals in Liquid Helium

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

Helium anion formation inside helium droplets

Contact Info

Product

Resources

About

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He^– and He₂^–

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