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.