Fission of multiply charged alkali clusters in helium droplets – approaching the Rayleigh limit

Abstract

Electron ionization of helium droplets doped with sodium, potassium or cesium results in doubly and, for cesium, triply charged cluster ions. The smallest observable doubly charged clusters are Na₉²⁺, K₁₁²⁺, and Cs₉²⁺; they are a factor two to three smaller than reported previously. The size of sodium and potassium dications approaches the Rayleigh limit n_Ray for which the fission barrier is calculated to vanish, i.e. their fissilities are close to 1. Cesium dications are even smaller than n_Ray, implying that their fissilities have been significantly overestimated. Triply charged cesium clusters as small as Cs₁₉³⁺ are observed; they are a factor 2.6 smaller than previously reported. Mechanisms that may be responsible for enhanced formation of clusters with high fissilities are discussed.