title: Electron ionization of different large perfluoroethers embedded in ultracold helium droplets: effective freezing of short-lived decomposition intermediates.
abstract: BACKGROUND
  Electron ionization of three perfluoroethers (PFEs), C(6)F(14)O(3), C(8)F(18)O(4), and C(10)F(20)O(5), is studied in the gas phase and when the molecules are embedded in ultracold helium (He) droplets. The molecules investigated are model compounds for perfluoropolyethers used as lubricants in technical applications. The present study gives insight into possible radiolysis pathways upon radiation exposure.
  
  
  METHODS
  The experiments utilized a crossed electron/droplet beam apparatus consisting of a He droplet source and pick-up chamber combined with a commercial time-of-flight mass spectrometer. The doped droplets were ionized by electron ionization at 70 eV.
  
  
  RESULTS
  The He environment strongly affects the ionization patterns in the way that both the molecular ion M(+) and high-mass fragment ions formed by the loss of light neutral species such as F([M-F](+)), or CF(3)OCF(2) ([M-CF(3)OCF(2)](+)), etc., became strong signals in the mass spectrum. These signals were not or only barely visible in the gas-phase experiment and were identified as short lived (< µs) dissociation intermediates which in the gas phase immediately decomposed into lower-mass fragment ions.
  
  
  CONCLUSIONS
  Ionic fragmentation intermediates are frozen and subsequently stabilized in the He environment. Helium droplets can hence be viewed as a cryogenic laboratory transforming short-lived decomposition intermediates into stable fragment ions appearing as strong signals in the mass spectrum.
authors.affiliation: Institut für Ionenphysik und Angewandte Physik, Leopold Franzens Universität Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria.
authors.forename: P
authors.initials: P
authors.lastname: Bartl
authors.forename: K
authors.initials: K
authors.lastname: Tanzer
authors.forename: C
authors.initials: C
authors.lastname: Mitterdorfer
authors.forename: S
authors.initials: S
authors.lastname: Karolczak
authors.forename: E
authors.initials: E
authors.lastname: Illenberger
authors.forename: S
authors.initials: S
authors.lastname: Denifl
authors.forename: P
authors.initials: P
authors.lastname: Scheier