Formation of He via electron impact of helium droplets.

Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of He ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 10 or more atoms), however, the He signal intensity is anomalously large. We have studied the mechanism for the formation of He ions in large helium droplets by varying the duration of the electron impact excitation pulse.