Rydberg-atom collisions withSF6andCCl4at very highn

Rate constants for Rydberg-atom destruction through electron transfer in collisions between K(np) Rydberg atoms and ${\mathrm{SF}}_6$ and ${\mathrm{CCl}}_4$ have been measured for values of principal quantum number n up to n∼400 using an apparatus specifically developed for very-high-n collision studies. Analysis of the data using the free-electron model provides cross sections for electron attachment to ${\mathrm{SF}}_6$ and ${\mathrm{CCl}}_4$ at electron energies down to ∼10 μeV, which are far below those accessible using any alternate technique. The measured cross sections are inversely proportional to electron velocity and are consistent with the Wigner threshold law for s-wave attachment.