Laser Photoelectron Attachment to Molecules in a Skimmed Supersonic Beam: Diagnostics of Weak Electric Fields and Attachment Cross Sections Down to20μeV

Combining a tunable single mode laser–atomic beam photoelectron source with a skimmed supersonic beam target we have carried out ultrahigh resolution studies of threshold electron attachment to ${\mathrm{SF}}_6$ molecules. Monitoring ${\mathrm{SF}}_6^{-}$ formation around the onset for field ionization of high $n$ Rydberg atoms, residual electric fields are diagnosed and reduced to levels around 0.01 V/m, allowing the first study of free electron attachment cross sections $\sigma \left(E\right)$ for energies down to about $20\mu \mathrm{eV}$ at energy widths as low as $20\mu \mathrm{eV}$. The present results for ${\mathrm{SF}}_6^{-}$ production conclusively demonstrate the convergence towards the limiting $s$-wave attachment behavior $\sigma \propto E^{-1/2\mathrm{}}$ at energies below 1 meV.