Binary-encounter electrons observed at 0° in collisions of 1–2-MeV/amu ${\mathrm{H}}^{+}$, ${\mathrm{C}}^{6+}$, ${\mathrm{N}}^{7+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{F}}^{9+}$ ions with ${\mathrm{H}}_2$ and He targets

The energy distribution of binary-encounter electrons (BEE) produced in collisions of 1–2 MeV/amu ${\mathrm{H}}^{+}$ and bare C, N, O, and F ions with ${\mathrm{H}}_2$ and He gas targets is reported at 0° with respect to the beam direction. These electrons result from ionization of the target due to hard collisions with the projectile and can thus be considered to be produced in a process analogous to elastic scattering of a free electron from a highly charged ion. An impulse-approximation (IA) model has been developed to describe this process in which ‘‘quasifree’’ target electrons undergo 180° Rutherford scattering in the projectile frame. The measured BEE double-differential production cross sections for bare ions were well described by this model and were found to scale with ${\mathit{Z}}_{\mathit{p}}^2$ and ${\mathit{E}}_{\mathit{p}}^{\mathrm{-}(\mathrm{\sim }2.6–2.7)}$ where ${\mathit{Z}}_{\mathit{p}}$ and ${\mathit{E}}_{\mathit{p}}$ are the charge and energy of the projectile, respectively. An energy shift of the BEE below 4t, where t is the cusp electron energy, is observed and is also predicted by the IA treatment. A plane-wave Born approximation (PWBA) calculation for BEE production is also found to be in overall agreement with our data. However, the energy shift of the BEE peak could not be fully accounted for within this PWBA calculation.