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High resolution study of anion formation in low-energy electron attachment to SF6 molecules in a seeded supersonic beam

  • M. Braun
  • S. Barsotti
  • S. Marienfeld
  • E. Leber
  • J. M. Weber
  • M.-W. Ruf
  • H. HotopEmail author
Fundamental Processes in the Gas Phase

Abstract.

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF6 molecules in helium carrier gas, nozzle temperatures T0= 300–600 K, stagnation pressures p0= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF6 molecules at high energy resolution. Using the standard LPA method, the yield for SF6- as well as SF5- and F- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF6- and SF5- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF6- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF5- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T0= 300 K and shifting to lower energies with rising T0) and a monotonical decrease towards higher energies. SF5- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF6 molecules with rising pressure. The SF5-/SF6- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF5- spectra are used as thermometers for the internal temperature of the SF6 molecules in the seeded supersonic beam which (at p0= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F- formation is similar to that for SF6-, but the F- signals are three to four orders of magnitude lower than those for SF6-; in view of the rather high endothermicity of F- formation the origin of the F- signals is discussed in some detail.

Keywords

Anion Formation Energy Width Stagnation Pressure Electron Attachment High Energy Resolution 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005

Authors and Affiliations

  • M. Braun
    • 1
  • S. Barsotti
    • 1
  • S. Marienfeld
    • 1
  • E. Leber
    • 1
  • J. M. Weber
    • 1
  • M.-W. Ruf
    • 1
  • H. Hotop
    • 1
    Email author
  1. 1.Fachbereich Physik, Technische UniversitätKaiserslauternGermany

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