Eur. Phys. J. D 35, 177-191 (2005)
https://doi.org/10.1140/epjd/e2005-00230-6

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

Fachbereich Physik, Technische Universität, 67653 Kaiserslautern, Germany

Corresponding author: ^a hotop@physik.uni.kl.de

Received: 14 April 2005
Published online: 17 August 2005

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 –600 K, stagnation pressures –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 SF as well as SF 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 SF and SF formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for  K and shifting to lower energies with rising and a monotonical decrease towards higher energies. SF 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 SF/SF intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at  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 SF, but the F^- signals are three to four orders of magnitude lower than those for SF; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.