Electron capture from a directed Stark-Rydberg state: Fore-and-aft ratios

Electron capture by Cs+, K+, or Li+ from a Na Rydberg target in an extreme Stark state of n = 24, m = 0 has been measured and compared with both a cla ssical and a quantal calculation as a function of the reduced velocity <(up silon)over tilde>=0.15-1.6. Peaks and shoulders are found at <(upsilon)over tilde> = 0.25, 0.5, and 1.0 in the ratio of the measured capture cross sec tions for electronic charge polarization antiparallel and parallel to the i on-beam direction. A close-coupling calculation reveals similar features in the total capture cross sections near those same scaled velocities. A clas sical trajectory calculation attributes this structure to swapping of the e lectron between the two positive charge centers as they pass one another du ring 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 re sult in the structure near <(upsilon)over tilde> = 0.25, 0.5, and 1.0, resp ectively. [S1050-2947(98)03312-5].