CHANNEL-DISTORTED BRINKMAN-KRAMERS APPROXIMATION FOR IONIZATION BY HIGHLY-CHARGED ION IMPACT

We have analysed the capture-to continuum aspects of the ionization pr ocess in ion-atom collisions in a channel-distorted, first-order appro ximation. In contrast to the standard Brinkman-Kramers approximation, we have found that the channel-distorted version correctly reproduces the ionization amplitude derived in a quasi-elastic scattering model f or electrons in the binary-encounter-peak region of the ionized electr on spectrum. Clearly, the first-order approach misses the double-scatt ering contribution, known to play a significant role at the cusp of el ectrons travelling with approximately the same velocity as the project ile, but our analysis is interesting in this region too, since it show s that die single-collision part of the cusp depends strongly upon the high-momentum tail of the Compton profile of the initial electron sta te, a feature that should receive more attention in future evaluations of higher-order theories. The present calculations are tested against experimental data in the binary-encounter region. Quantum effects due to binding in the initial state are isolated in a generalized off-ene rgy-shell factor. This factor approaches unity at the centre of the bi nary peak, but appears to shift the position of the peak and to change its shape. The theoretical shift is in reasonable accord with the exp erimental data, but it is concluded that further work is needed, with special emphasis on the validity of the single-particle model and the corresponding representation of the initial state of the target atom.