Ionization of doped helium nanodroplets: Complexes of $C60$ with water clusters

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of $(H2O)nH3O+$⁠. Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with $C60$⁠. Charged $C60$-water complexes are predominantly unprotonated; $C60(H2O)4+$ and $(C60)2(H2O)4+$ appear with enhanced abundance. Another intense ion series is due to $C60(H2O)nOH+$⁠; dehydrogenation is proposed to be initiated by charge transfer between the primary $He+$ ion and $C60$⁠. The resulting electronically excited $C60+∗$ leads to the formation of a doubly charged $C60$-water complex either via emission of an Auger electron from $C60+∗$⁠, or internal Penning ionization of the attached water complex, followed by charge separation within ${C60(H2O)n}2+$⁠. This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of $C60(H2O)n+$⁠. In addition to the loss of single water molecules, a prominent reaction channel yields bare $C60+$ for sizes $n=3$⁠, 4, or 6. Ab initio Hartree–Fock calculations for $C60$-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to $C60+$⁠. For $n=3$⁠, 4, or 6, fissionlike desorption of the entire water complex from $C60(H2O)n+$ energetically competes with the evaporation of a single water molecule.