  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.
