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(vₑ,vₚ,θₑ), where ${d}³{σ}{∝}[{f({v}ₑ,{v}ₚ,{{θ}}ₑ)}{|{{{→}}{v}}ₑ{-}{{{→}}{v}}ₚ|}]d{E}ₑd{{Ω}}ₑ$, 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.