Low-energy electron attachment to SF6 at sub-meV resolution using a tunable laser photoelectron method -: art. no. 042706

Electron attachment to sulfur hexafluoride is studied using a photoelectron source of ultralow-energy electrons with sub-meV resolution. Vacuum-ultraviolet laser radiation produced through nonlinear-optical techniques is used to photoionize xenon at and above its P-2(1/2)o ionization limit. The resulting photoelectrons interact with sulfur hexafluoride admixed to the xenon. The electron energy is continuously scanned from 0 to 127 meV, with a resolution of 0.1 meV at threshold. Computational modeling indicates that the attachment cross section is well described by the Wigner threshold law at electron energies less than about 5 meV. Data at higher energies are best described by the Wigner s-wave form, or by a Klots form with beta =0.228. Cusps are observed at the omega (6), omega (1), and omega (3) vibrational modes of SF6. While the omega (1) and omega (3) cusps have their maxima at the spectroscopic energies of 97.1 and 117.6 meV, respectively, the omega (6) cusp has its maximum at 46 meV, nearly 3 meV higher than its spectroscopic value of 43.2+/-0.4 meV. Possible explanations are considered, such as a low-l angular-momentum barrier, an overtone mode of SF6 excited by the collision, or clustering. The definitive cause of this shift is not understood.