Systematic comparison of positron - and electron - impact excitation of the 3 vibrational mode of CF

Absolute measurements are presented for the excitation of the 3 vibrational mode in CF4 by positron and electron impact from 0.1 to 2 eV. To minimize systematic differences, these measurements were made using the same trap-based electron or positron beam, associated experimental apparatus, and procedures. Unlike other vibrational excitation cross sections studied to date, the near-threshold cross section for the 3 vibrational mode in CF4 is similar, both in magnitude and shape, for positrons and electrons. Comparison of the cross sections with an analytic Born dipole model yields good agreement, while comparison of this model with other measured positron-impact vibrational cross sections indicates that the contribution of this long-range dipole coupling varies widely. The maximum value of the cross section in CF4 is the largest of any positron-impact vibrational excitation cross section measured to date. This provides a likely explanation of the observation that CF4 is very effective when used as a buffer gas to cool positron gases and plasmas.