Vibrational resonance and threshold effects in inelastic electron collisions with methyl iodide molecules

A joint experimental and theoretical study of low-energy electron collisions with methyl iodide molecules with emphasis on dissociative attachment (DA) over the range 0-170 meV has been carried out. Below the onset for the symmetric C-I stretch (3 = 1) vibrational excitation, a prominent narrow peak is observed followed by a cusp at threshold. On approach to the 3 = 2 onset, the DA cross section shows a rapid drop terminated by a cusp at threshold. The experimental findings are interpreted by an R-matrix calculation which includes non-local effects and involves realistic model potentials for the neutral molecule and negative ion. Mediated by the combined effects of dipolar and polarization attraction in the electron CH3I interaction, the peak below the 3 = 1 onset is identified as a vibrational (`nuclear-excited') Feshbach resonance which so far may represent the clearest example of its kind. The higher vibrational onsets are free of such resonances. The effects of initial vibrational excitation are discussed and found to be significant. Predictions are made for elastic (-wave) and vibrationally inelastic cross sections and structure at the 3 = 1 onset is discussed. Absolute DA cross sections and DA rate coefficients for Maxwellian electron gases with variable electron temperature (for the fixed gas temperature TG = 300 K) are presented.