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...

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Bibliographic Details
Published in:Physical chemistry chemical physics : PCCP 2014-11, Vol.16 (41), p.22466-2247
Main Authors: Renzler, Michael, Daxner, Matthias, Weinberger, Nikolaus, Denifl, Stephan, Scheier, Paul, Echt, Olof
Format: Article
Language:English
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Summary: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 He 2 + 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. How are atomic helium cations formed from helium nanodroplets at sub-threshold electron energies? Surprisingly, metastable helium anions play a crucial role.
ISSN:1463-9076
1463-9084
DOI:10.1039/c4cp03236e