ATTACHMENT OF ELECTRONS TO MOLECULES AT SUBMILLIELECTRONVOLT RESOLUTION

Using a novel laser photoelectron method, we have measured electron attachment to neutral molecules with sufficiently high energy resolution to investigate the limiting E-1/2 behaviour of the cross section, predicted for s-wave attachment at electron energies E approaching zero. Our results for SF6, obtained with an effective energy width of about 0.2 meV in the energy range 0-200 meV, show that the E-1/2 dependence is only reached at very low energies (less-than-or-similar-to meV) and that the cross section decreases more strongly towards higher energies, exhibiting a pronounced downward cusp at the threshold for nu(1) vibrational excitation of SF6. At the lowest energies, the experimental rate constant is compatible with the theoretical value for capture due to the e(-)-SF6 polarization potential.