Electron attachment to helium microdroplets : creation induced magic ?

The size distribution of He[sub [ital N]][sup [minus]] 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 [ital N]=(2.0[plus minus]0.1)[times]10[sup 5] 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.