LASER PHOTOELECTRON ATTACHMENT TO MOLECULES IN A SKIMMED SUPERSONIC BEAM - DIAGNOSTICS OF WEAK ELECTRIC-FIELDS AND ATTACHMENT CROSS-SECTIONS DOWN TO 20 MU-EV

Combining a tunable single mode laser-atomic beam photoelectron source with a skimmed supersonic beam target we have carried out ultrahigh r esolution studies of threshold electron attachment to SF6 molecules. M onitoring SF6- formation around the onset for field ionization of high n Rydberg atoms, residual electric fields are diagnosed and reduced t o levels around 0.01 V/m, allowing the first study of free electron at tachment cross sections sigma(E) for energies down to about 20 mu eV a t energy widths as low as 20 mu eV. The present results for SF6- produ ction conclusively demonstrate the convergence towards the limiting s- wave attachment behavior sigma proportional to E-(1/2) at energies bel ow 1 meV. [S0031-9007(98)06711-8].