Negative-ion contribution to threshold peaks observed in vibrational excitation spectra of molecules by electron impact

The contribution of negative ions, formed by dissociative attachment, to threshold peaks reported in the vibrational excitation spectra of some molecules is discussed. It is concluded that negative ions formed in the energy range of the vibrational levels of a neutral molecule with no kinetic energy and a large cross section contribute significantly to threshold peaks or simulate threshold peaks in the vibrational excitation functions of this molecule. Using a crossed beam electron impact spectrometer with two 127 degrees electrostatic energy filters and a quadrupole mass spectrometer following the scattered particles analyser, it is shown that the narrow peaks close to threshold reported by Rohr (1979) in the vibrational cross section of HBr for v=3 up to v=5 are due to Br - ions. For the v=1 and v=2 vibrational cross sections, the threshold peak contains about 5% and 40% of Br - ions respectively. It is suggested that, because of the very large cross section for SF 6 - formation, the threshold peak observed in the excitation function of the nu 1 mode of SF 6 may be mostly due to SF 6 - ions.