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

We report on the formation and ionization of cesium and C 60 Cs clusters in superfluid helium nanodroplets. Size distributions of positively and negatively charged (C 60 ) m Cs n ± ions have been measured for m ≤ 7, n ≤ 12. Reproducible intensity anomalies are observed in high-resolution mass spectra. For both charge states, (C 60 ) m Cs 3 ± and (C 60 ) m Cs 5 ± are particularly abundant, with little dependence on the value of m . Distributions of bare cesium cluster ions also indicate enhanced stability of Cs 3 ± and Cs 5 ± , in agreement with theoretical predictions. These findings contrast with earlier reports on highly Cs-doped cationic fullerene aggregates which showed enhanced stability of C 60 Cs 6 building blocks attributed to charge transfer. The dependence of the (C 60 ) m Cs 3 - anion yield on electron energy shows a resonance that, surprisingly, oscillates in strength as m increases from 1 to 6.