s-wave threshold in electron attachment: Observations and cross sections inCCl4andSF6at ultralow electron energies

Measurements are reported for the dissociative attachment process $e^{\mathrm{-}}$(ε)+${\mathrm{CCl}}_4$→${\mathrm{Cl}}^{\mathrm{-}}$+CCl ${}_3{}^{}{}_{}{}^{}$ and the attachment process $e^{\mathrm{-}}$(ε)+${\mathrm{SF}}_6$→${\mathrm{SF}}_6$ ${\mathrm{}}^{\mathrm{-}}$ at electron energies ε 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-${\mathrm{C}}_4$ ${\mathrm{F}}_6$ and ${\mathrm{CFCl}}_3$. We ascribe this structure to the divergence of the attachment cross section in the limit ε,l→0. At the present limits of resolution, the attachment cross sections ${\sigma }_A$(ε) at the lowest energies are consistent with the Wigner threshold law for s-wave attachment in which ${\sigma }_A$(ε)∼${\epsilon }^{\mathrm{-}1/2}$. 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 ${\mathrm{SF}}_6$ with several theoretical calculations of the thermal attachment cross section.