Fragmentation of ionized liquid helium droplets: A new interpretation

Pure liquid helium droplets of mean size 〈N〉=100–15 000 atoms ionized by electron impact show surprisingly similar ion fragment distributions. For all cluster sizes He2+ is the most probable cluster ion fragment, accounting for 30%–70% of the total ion yield. The high relative intensity of He2+ for the larger clusters shows that the droplets dissipate the ionization energy through an impulsive process, which ejects He2+ from the cluster, rather than by thermal evaporation. The other helium ion fragments that have been the focus of previous studies are most likely formed by a similar mechanism.