ELECTRON CAPTURE FROM A DIRECTED STARK-RYDBERG STATE : FORE-AND-AFT RATIOS

Electron capture by ${\mathrm{Cs}}^{+},$ ${\mathrm{K}}^{+},$ or ${\mathrm{Li}}^{+}$ from a $\mathrm{Na}$ Rydberg target in an extreme Stark state of $n=24, m=0$ has been measured and compared with both a classical and a quantal calculation as a function of the reduced velocity $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.15\ensuremath{-}1.6.$ Peaks and shoulders are found at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.25,$ 0.5, and 1.0 in the ratio of the measured capture cross sections for electronic charge polarization antiparallel and parallel to the ion-beam direction. A close-coupling calculation reveals similar features in the total capture cross sections near those same scaled velocities. A classical trajectory calculation attributes this structure to swapping of the electron between the two positive charge centers as they pass one another during the collision. The total classical capture cross section of a Rydberg atom having an upstream-directed charge distribution is shown to be the sum of five-and-greater-swap, three-swap, and one-swap contributions, which result in the structure near $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.25, 0.5,$ and 1.0, respectively.