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

Abstract

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...

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…However, the ejection of He + from the droplet is difficult to explain [200]. A comparison of the ion yield of He + and He*  at subthreshold energies reveals a close similarity [175] (see Figure 20). A possible scheme for forming sub-threshold He + is proposed in the right hand reaction sequence in Figure 21.…”

“…Buchenau et al [77] also observed a threshold energy at 21 eV for He2 + , which they explained via formation and ejection of He2* and subsequent ionization via a collision with a metallic surface (see Figure 21 left). [175] propose a model where the HND is hit by two electrons that form He* and two low-energy electrons. One of these electrons recombines with the neighboring He* and forms He* -, which then polarizes and attracts a second He*.…”

“…All curves exhibit a more or less pronounced contribution below the ionization energy of He (24.59 eV). Renzler et al [175] propose a model where the HND is hit by two electrons that form He* and two low-energy electrons. One of these electrons recombines with the neighboring He* and forms He* -, which then polarizes and attracts a second He*.…”

Helium anion formation inside helium droplets

“…In the meantime several novel processes have been ascribed to the presence of metastable He * − in HND such as the formation and ejection of He + at electron energies below the ionization threshold of the free atom [21], or the self-assembly of salt nanocrystals from chemical reactions [22]. It has been shown that He * − has a high mobility inside the helium droplet.…”

Helium Droplets Doped with Sulfur and C60

“…Small helium cluster anions are unstable although He – and He 2 – have been observed; they are metastable if electronically excited. 20 , 21 …”