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The mechanism of ionization of helium droplets has been investigated innumerous reports but one observation has not found a satisfactory explanation:How are $He^+$ ions formed and ejected from undoped droplets at electronenergies below the ionization threshold of the free atom? Does this path existat all? A measurement of the ion yields of $He^+$ and $He_2^+$ as a function ofelectron energy, electron emission current, and droplet size reveals thatmetastable $He^{*-}$ anions play a crucial role in the formation of free $He^+$at subthreshold energies. The proposed model is testable.

On Subthreshold Ionization of Helium Droplets, Ejection of He+, and the Role of Anions