Charged Clusters of C₆₀ and Au or Cu: Evidence for Stable Sizes and Specific Dissociation Channels

We have doped helium nanodroplets with C₆₀ and either gold or copper. Positively or negatively charged (C₆₀) _mM _n^± ions (M = Au or Cu) containing up to ≈10 fullerenes and ≈20 metal atoms are formed by electron ionization. The abundance distributions extracted from high-resolution mass spectra reveal several local anomalies. The sizes of the four most stable (C₆₀) _mAu _n^± ions identified in previous calculations for small values of m and n ( m ≤ 2 and n ≤ 2, or m = 1 and n = 3) agree with local maxima in the abundance distributions. Our data suggest the existence of several other relatively stable ions including (C₆₀)₂Au₃^± and (C₆₀)₃Au₄^-. Another feature, namely the absence of bare (C₆₀)₂^±, confirms the prediction that (C₆₀)₂M^± dissociates by loss of C₆₀^± rather than loss of M. The experimental data also reveal the preference for loss of (charged or neutral) C₆₀ over loss of a metal atom from some larger species such as (C₆₀)₃M₃⁺. In contrast to these similarities between Au and Cu, the abundance distributions of (C₆₀)₃Au _n^- and (C₆₀)₃Cu _n^- are markedly different. In this discussion, we emphasize the similarities and differences between anions and cations, and between gold and copper. Also noteworthy is the observation of dianions (C₆₀) _mAu _n^2- for m = 2, 4, and 6.