s-wave threshold in electron attachment: Observations and cross sections in CCl4 and SF6 at ultralow electron energies

Measurements are reported for the dissociative attachment process e^-(ɛ)+CCl₄-->Cl^-+CCl ₃ and the attachment process e^-(ɛ)+SF₆-->SF₆ ^- at electron energies ɛ of below threshold to 140 milli-electron-volts, and at resolutions of 6 and 8 meV full width at half-maximum, respectively. Resolution-limited narrow structure is observed in both molecules at energies below 10 meV, similar to structure reported earlier in attachment to 2-C₄F₆ and CFCl₃. We ascribe this structure to the divergence of the attachment cross section in the limit ɛ,l-->0. At the present limits of resolution, the attachment cross sections σ_A(ɛ) at the lowest energies are consistent with the Wigner threshold law for s-wave attachment in which σ_A(ɛ)~ɛ^-1/2. Comparisons are given with experimental collisional-ionization (high-Rydberg-state attachment) results, electron-swarm unfolded cross sections, and earlier threshold photoionization data. Comparisons are also made for SF₆ with several theoretical calculations of the thermal attachment cross section.