Negatively charged helium microdroplets

The size distribution of He - N ions produced by crossing a low-energy electron beam with an uncharged helium droplet beam exhibits a threshold size below which no ions are detected. The threshold region is unexpectedly structured, however, and contains a sharp ‘magic’ mass peak at N=(2.0±0.1)×10 5 atoms. We will discuss possible models for electron attachment to pure He microdroplets, with particular attention paid to the possibility of metastably trapped electron bubbles. We suggest that the magic peak may arise from the dynamics of the conduction band instability which leads to the creation of the bubble state in the droplet and not, as is normally the case, from some special stability of the resulting ion.