N2+ (A 2Πu <- X2Σg+) excitation in the charge-exchange collision with caesium: two-electron effects at keV energies

Charge-exchange collisions of 5 keV N2+ on Cs are examined using translational spectroscopy to identify the internal energy states of the fast neutralized products. Optical pumping of the reactant, N2+ (A <- X) or Cs(2P <- 2S), is used to change selectively the electronic configurations and the reaction energy defect, while monitoring the resulting changes in the product state distribution. The N2+/Cs charge exchange yields N2 Rydberg states. A simple model of their formation would involve the transfer of the single outer electron in Cs into a Rydberg orbit built on the N2+ core. However, the present studies identify a surprising and highly efficient reaction mechanism in which the electronic configuration of the N2+ reactant is also changed during the electron transfer. This facile two-electron process, which occurs for both near-resonant reaction channels and has not been observed in other molecular charge-transfer reactions, is believed to be related to the very small energy separation between the ground and first excited electronic states of N2+. (author)