title: 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.
authors.affiliation: Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, A-6020 Innsbruck, Austria. paul.scheier@uibk.ac.at.
authors.forename: Michael
authors.initials: M
authors.lastname: Renzler
authors.forename: Matthias
authors.initials: M
authors.lastname: Daxner
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weinberger
authors.forename: Stephan
authors.initials: S
authors.lastname: Denifl
authors.forename: Paul
authors.initials: P
authors.lastname: Scheier
authors.forename: Olof
authors.initials: O
authors.lastname: Echt