Positively and Negatively Charged Cesium and (C₆₀) Cs Cluster Ions

We report on the formation and ionization of cesium and C₆₀Cs clusters in superfluid helium nanodroplets. Size distributions of positively and negatively charged (C₆₀) Cs ^± ions have been measured for m ≤ 7, n ≤ 12. Reproducible intensity anomalies are observed in high-resolution mass spectra. For both charge states, (C₆₀) Cs₃^± and (C₆₀) Cs₅^± are particularly abundant, with little dependence on the value of m. Distributions of bare cesium cluster ions also indicate enhanced stability of Cs₃^± and Cs₅^±, in agreement with theoretical predictions. These findings contrast with earlier reports on highly Cs-doped cationic fullerene aggregates which showed enhanced stability of C₆₀Cs₆ building blocks attributed to charge transfer. The dependence of the (C₆₀) Cs₃^- anion yield on electron energy shows a resonance that, surprisingly, oscillates in strength as m increases from 1 to 6.