Positron-impact vibrational excitation cross sections and the Born dipole model

We describe \textit{in situ} measurements of the positron- and electron-impact cross sections for vibrational excitation of the infrared-active (IR) $\nu_3$ mode in CF$_4$ [1]. These cross sections are virtually identical and agree quantitatively with the predictions of the Born dipole model (BDM), which describes the effect of long range dipole coupling. We also compare the predictions of the BDM with the other positron-impact vibrational cross sections for IR modes measured to date (CO, CO$_2$, H$_2$, and CH$_4$) [2]. The BDM contributions to the measured cross sections vary widely. However, for all molecules except H$_2$ (for which the transition dipole moment is zero), the BDM model predicts the energy dependence of these cross sections quite well. The possible significance of these results will be discussed. \newline \newline [1] J.P. Marler and C.M. Surko, \textit{Phys. Rev. A.} \textbf{72}, 062702 (2005). \newline [2] J.P. Marler, G.F. Gribakin and C.M. Surko, \textit{Nuclear Instrum. and Meth. B}, in press (2006).