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

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.