Electron impact on helium clusters: Metastable excitation, ionization, and charged minicluster ejection
3He and 4He clusters of some millions of atoms are found to be excited into metastable electronic states of more than 1 ms lifetime as well as to eject charged miniclusters of, on the average, 85 3He atoms, or 68 4He atoms, under electron bombardment. The metastable states are thought to be diatomic excitons resulting from 2 3S atomic excitations. The charged minicluster ejection is taken to be due to the polarization energy release if ionization occurs near the cluster surface. The effectivity of single or multiple cluster ionization is thereby reduced.