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

Abstract

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 C_x(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 C_xNH₃⁺ and C_xNH₄⁺ 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 C_xNH₃⁺ and C_xNH₄⁺ 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.