Negative ion formation through dissociative electron attachment to the group IV tetrabromides: Carbon tetrabromide, silicon tetrabromide and germanium tetrabromide

As a part of our efforts to characterize the electron attachment reactions of the group IV tetrahalides, XY₄ (X = C, Si, Ge and Y = F, Cl, Br) we here follow up on our previous study on XF₄ and report electron attachment to the tetrabromides: CBr₄, SiBr₄ and GeBr ₄ in the incident electron energy range from about 0 to 10 eV. The formation of Br^-, XBr₃ ^-, XBr₂ ^- and Br₂ ^- is observed from all these compounds, and additionally the molecular anion is observed from SiBr ₄. The main DEA contributions from CBr₄ and GeBr ₄ are observed through a low lying resonance, which we assign as a T₂ shape resonance associated with a single occupation of the t ₂ symmetry LUMO+1 of these molecules. This resonance is also apparent in the ion yield from SiBr₄, but there the main contributions are the molecular anion and Br^- in a narrow energy range peaking at 0 eV. We attribute these 0 eV contributions to a vibrational Feshbach resonance associated with the a₁ symmetry LUMO. Further resonances are observed, for all the compounds investigated here, in the energy range between 3 and 6 eV. To establish the thermochemical thresholds for the individual channels we have conducted DFT calculations from which we report the threshold values, bond dissociation energies and electron affinities for individual fragments and their neutral precursors. These values are compared to values in the literature where available, and discussed in the context of our experimental results.