Laser Photoelectron Attachment to Molecules in a Skimmed Supersonic Beam: Diagnostics of Weak Electric Fields and Attachment Cross Sections Down to<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>20</mml:mn><mml:mi/><mml:mi mathvariant="italic">μ</mml:mi><mml:mi>eV</mml:mi></mml:math>

Abstract

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 ${\mathrm{SF}}_{6}^{\ensuremath{-}}$ 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 $\ensuremath{\sigma}(E)$ for energies down to about $20\ensuremath{\mu}\mathrm{eV}$ at energy widths as low as $20\ensuremath{\mu}\mathrm{eV}$. The present results for ${\mathrm{SF}}_{6}^{\ensuremath{-}}$ production conclusively demonstrate the convergence towards the limiting $s$-wave attachment behavior $\ensuremath{\sigma}\ensuremath{\propto}{E}^{\ensuremath{-}1/2}$ at energies below 1 meV.