Ionization of doped helium nanodroplets: Complexes of C 60 with water clusters

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of ( H 2 O ) n H 3 O + . 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 C 60 . Charged C 60 -water complexes are predominantly unprotonated; C 60 ( H 2 O ) 4 + and ( C 60 ) 2 ( H 2 O ) 4 + appear with enhanced abundance. Another intense ion series is due to C 60 ( H 2 O ) n OH + ; dehydrogenation is proposed to be initiated by charge transfer between the primary He + ion and C 60 . The resulting electronically excited C 60 + ∗ leads to the formation of a doubly charged C 60 -water complex either via emission of an Auger electron from C 60 + ∗ , or internal Penning ionization of the attached water complex, followed by charge separation within { C 60 ( H 2 O ) 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 C 60 ( H 2 O ) n + . In addition to the loss of single water molecules, a prominent reaction channel yields bare C 60 + for sizes n = 3 , 4, or 6. Ab initio Hartree-Fock calculations for C 60 -water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C 60 + . For n = 3 , 4, or 6, fissionlike desorption of the entire water complex from C 60 ( H 2 O ) n + energetically competes with the evaporation of a single water molecule.