Low-energy behavior of exothermic dissociative electron attachment

We discuss two models for electron attachment to molecules: the Vogt-Wannier model for capture into a polarization well and the resonance model for dissociative attachment. The Vogt-Wannier model is generalized for the case of a target with a permanent dipole moment, and results are presented for dissociative attachment to CH₃I. It is shown that the resonance theory should incorporate in this case a weakly bound dipole-supported state of CH₃I^-, whereas the generalized Vogt-Wannier theory gives a reasonable estimate for the cross section in the meV and sub-meV region. The Vogt-Wannier model is also applied to the process of attachment to SF₆, CCl₄, and C₆₀. In the first case the s-wave capture model provides a satisfactory description of the experimental data for energies below the first vibrational excitation threshold, whereas for CCl₄ it underestimates the attachment cross section by a factor of 2 in the sub-meV region. For C₆₀ we suggest that electron attachment is dominated by s-wave capture in the region below 2 meV and by p-wave capture in the energy range above 4 meV. Our model reproduces data for Rydberg electron and free-electron attachment observed in beam experiments. It is, however, at variance with the strong rise of the attachment rate coefficients with electron temperature observed in flowing afterglow--Langmuir probe measurements.