On the applicability of the impulse approximation for radiative electron capture into bound and continuum states

A comparison between the impulse approximation (IA) and the strong-potential Born approximation (SPB) is carried out for radiative capture of a target electron into excited bound states (REC) and into continuum states (radiative ionisation, RI) of the projectile. It is found that in the vicinity of the peak in the photon spectrum, the deviations of the SPB from the IA are small but do not disappear either for excited states or for continuum states with low momentum kappa /sub /f. The contribution of RI to the forward peak is calculated for the systems C 6+ to He and Ne 10+ to He, showing a strong skewness towards the high-energy side. For electrons with high kappa /sub /f, large deviations between SPB and IA indicate the breakdown of an atomic description.