Electron attachment to helium microdroplets: Creation induced magic?

The size distribution of HeN^- ions produced by crossing a low energy beam with an uncharged helium cluster beam exhibits a threshold below which no ions are detected. The threshold region is unexpectedly structured, however, and exhibits a sharp ``magic'' mass peak at N=(2.0±{}0.1)×{}10⁵ atoms. We suggest that this behavior derives from the dynamics of the conduction band instability which leads to the formation of a metastably trapped bubble electron in the droplet.