Ion–molecule reactions of ammonia clusters with C60 aggregates embedded in helium droplets

Helium nanodroplets are co-doped with C60 and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C60 units. The high mass resolution of Δm/m ≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C60 aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated Cx(NH3)n+ ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C60(NH3)n+ and C60(NH)n−1NH4+ become about equal. Dehydrogenated C60NH2+ ions are relatively abundant; their formation is attributed to a transient doubly charged C60–ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He+ ion and C60. The abundance of CxNH3+ and CxNH4+ ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH3 or NH4 from the presumably weakly bound CxNH3+ and CxNH4+ ions is at odds with the lack of enhancement in the abundance of C120+ and C180+. Mass spectra of C60 dimers complexed with water complement a previous study of C60(H2O)n+ recorded at much lower mass resolution.