title: Ionization of doped helium nanodroplets: complexes of C60 with water clusters.
abstract: 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.
authors.affiliation: Institut für Ionenphysik und Angewandte Physik and Center for Molecular Biosciences Innsbruck, Leopold Franzens Universität, 6020 Innsbruck, Austria.
authors.forename: S
authors.initials: S
authors.lastname: Denifl
authors.forename: F
authors.initials: F
authors.lastname: Zappa
authors.forename: I
authors.initials: I
authors.lastname: Mähr
authors.forename: A
authors.initials: A
authors.lastname: Mauracher
authors.forename: M
authors.initials: M
authors.lastname: Probst
authors.forename: J
authors.initials: J
authors.lastname: Urban
authors.forename: P
authors.initials: P
authors.lastname: Mach
authors.forename: A
authors.initials: A
authors.lastname: Bacher
authors.forename: D K
authors.initials: DK
authors.lastname: Bohme
authors.forename: O
authors.initials: O
authors.lastname: Echt
authors.forename: T D
authors.initials: TD
authors.lastname: Märk
authors.forename: P
authors.initials: P
authors.lastname: Scheier
mesh_terms: D017666:Deuterium Oxide / Q000737:chemistry; D004583:Electrons; D037741:Fullerenes / Q000737:chemistry*; D006371:Helium / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D011789:Quantum Theory; D014867:Water / Q000737:chemistry*
chemical_list: D037741:Fullerenes; D011522:Protons; D014867:Water; D006371:Helium; D017666:Deuterium Oxide; C069837:fullerene C60