Adsorption of helium on isolated C60 and C70 anions

Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C-60(-) and C-70(-) anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1x1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C-60(-) or 65 to C-70(-). For C-60, the anomaly coincides with the one observed for cationic complexes but for C-70 it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium-fullerene complexes.