ELECTRON-ATTACHMENT TO HELIUM MICRODROPLETS - CREATION INDUCED MAGIC

The size distribution of He(N)- ions produced by crossing a low energy electron beam with an uncharged helium cluster beam exhibits a thresh old below which no ions are detected. The threshold region is unexpect edly structured, however, and exhibits a sharp ''magic'' mass peak at N = (2.0 +/- 0.1) x 10(5) atoms. We suggest that this behavior derives from the dynamics of the conduction band instability which leads to t he formation of a metastably trapped bubble electron in the droplet.