title: Ionization of methane clusters in helium nanodroplets.
abstract: The electron ionization of helium droplets doped with methane clusters is investigated for the first time using high-resolution mass spectrometry. The dominant ion products ejected into the gas phase are the unprotonated (CH(4))(n)(+) cluster ions along with the protonated ions, CH(5)(+)(CH(4))(n-1). The mass spectra show clear evidence for magic numbers, which are broadly consistent with icosahedral shell closings. However, unusual features were observed, including different magic numbers for CH(5)(+)(CH(4))(n-1) (n=55, 148) when compared to (CH(4))(n)(+) (n=54, 147). Possible interpretations for some of these differences are proposed. Products of the type [C(2)H(x)(CH(4))(n)](+), which result from ion-molecule chemistry, are also observed and these too show clear magic number features. Finally, we report the first observation of (CH(4))(n)(2+) dications from methane clusters. The threshold for dication survival occurs at n≥70 and is in good agreement with a liquid droplet model for fission of multiply charged ions. Furthermore, we present evidence showing that these dications are formed by an unusual two-step mechanism which is initiated by charge transfer to generate a monocation and is then followed by Penning ionization to generate a dication.
authors.affiliation: Institute for Ion Physics und Applied Physics, University of Innsbruck, Innsbruck, Austria.
authors.forename: Christian
authors.initials: C
authors.lastname: Leidlmair
authors.forename: Peter
authors.initials: P
authors.lastname: Bartl
authors.forename: Harald
authors.initials: H
authors.lastname: Schöbel
authors.forename: Stephan
authors.initials: S
authors.lastname: Denifl
authors.forename: Shengfu
authors.initials: S
authors.lastname: Yang
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Ellis
authors.forename: Paul
authors.initials: P
authors.lastname: Scheier