Evidence for Second-Born-Approximation Contributions to Continuum-Electron Capture by Positive Ions

We describe a unique indicator for the effect of second—Born-approximation terms in the secondary-electron spectra of continuum-electron capture by positive ions. Specifically, a discontinuity in the function $f(v_e,v_p,{\theta }_e)$, where $d^3\sigma \propto \left[\frac{f(v_e,v_p,{\theta }_e)}{|{\overrightarrow{\mathrm{v}}}_e-{\overrightarrow{\mathrm{v}}}_p|}\right]d{E}_{e}d{\Omega }_e$, indicates second—Born-approximation contributions. Three different experiments are analyzed with respect to this indicator. In all cases we find strong evidence for the discontinuity and hence the presence of second—Born-approximation terms.