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 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. 
Michael Renzler, Matthias Daxner, Nikolaus Weinberger, Stephan Denifl,   Paul Scheier, Olof Echt
physics.atm-clus