Comment on the asymmetry in the cusp of the differential cross section for charge capture to the continuum

For electron capture from noble-gas atoms to the continuum of energetic bare projectiles an asymmetry has recently been observed in the cusp of the differential cross section at $v_e=v$ (where $v_e$ and $v$ are electron and projectile velocities, respectively). It is shown that the Oppenheimer-Brinkman-Kramers approximation can provide an asymmetry if terms linear in $\overrightarrow{\mathrm{k}}={\overrightarrow{\mathrm{v}}}_e-\overrightarrow{\mathrm{v}}$ are kept in the expansion of the outgoing Coulomb wave. In contrast to the predictions of the second Born approximation, the resulting shape is independent of the projectile charge.