Close-coupling calculations of Ca^- formation by charge transfer from Rydberg atoms

We formulate the semiclassical close-coupling method for a theoretical description of ion-pair formation in Rydberg-atom–ground-state-atom collisions at thermal energies and apply it to the process Ca(4s²)+Ne^*(n)→Ca^-(4s²4p)+Ne⁺. Our results for the formation of an isolated negative-ion state are very close to those obtained previously using the decay model, although the results for S states are somewhat higher, since the decay model underestimates the survival probability for the ion pair. After the inclusion in our close-coupling calculations of the two fine-structure components of Ca^-, we find that the cross section increases substantially (by about a factor of 2) because of correlation between probabilities for population of two fine-structure sublevels.