Shape Resonances and Elastic Cross Sections in Electron Scattering by CF 3 Br and CF 3 I.

Trifluoroiodomethane (CF ₃ I) is one of the most appealing candidates for applications in plasma-based technologies in view of its many interesting advantages when compared to more standard gases such as trifluorobromomethane (CF ₃ Br). Low-energy electrons are prone to decomposing these molecules into reactive species, and knowledge on the collision cross sections is fundamental for modeling transport and reactivity in plasma environments. Despite many studies on electron collisions with the abovementioned molecules, there are conflicting results on the assignment of shape resonances and on the magnitudes of total cross sections. Here, we try to clarify these aspects by performing ab initio electron scattering calculations. We found integral cross sections in fair agreement with the most recent measurements, in contrast to previous reports. For each molecule, we found a σ _CX ^* resonance (antibonding between the carbon and the heavy halogen) at 1 eV in CF ₃ Br and at ∼0 eV in CF ₃ I. Furthermore, there are three shape resonances of σ _CF ^* character; two are degenerate and account for a broad feature around 6 eV and the other one appears around 9.5 eV. We also discuss the possible role of the degenerate resonance in dissociative electron attachment reactions, as well as the impact of the heavy halogen on the cross sections and on the shape resonances.