Electron ionization of superfluid helium nanodroplets doped with C60 and small molecules

Helium droplets were co-doped with C60 and ammonia, water or carbon dioxide and the resulting cluster size distributions of the produced cationic complexes are presented. Mass spectra obtained by electron ionization were recorded with high mass resolution of m/m ~ 6000 (FWHM). Series of C60(H2O)n, C60(NH3)n and C60(CO2)n complexes for n up to 20 molecules attached to the C60 were observed among other cations. For C60-water cluster ions, the resulting size distribution revealed magic numbers for n=4 and n=7 water molecules attached to the fullerene. Ionic C60-ammonia complexes showed an enhanced stability in the cluster size distribution for n=4 ammonia molecules attached as well. However, no intensity anomalies were observed for larger n. For the nonpolar carbon dioxide, no magic numbers were found in the cluster size distribution of C60(CO2)n. These findings suggest that single carbon dioxide molecules are homogeneously distributed on the C60; in contrast to ammonia and water, where clusters are attached to the fullerene. In addition, dehydrogenation and protonation reactions of water and ammonia clusters on C60 were observed in the present study.