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The probability of exciting the 2s state of hydrogen in an exchange collision between a proton and a hydrogen atom has been measured by Bayfield. Calculations are carried out which give some evidence on the mechanism responsible for the experimental observations.

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The predictions of a multichannel Landau-Zener model, which includes Taulbjerg'sf;,, factor, for single-electron capture by multicharged ions, have been compared with different kinds of experimental results (electron, x, u v and energy gain spectroscopies).A reasonable agreement has been found for the populations of individual nl sublevels of capture when the electron-core interaction prevails over the I Stark mixing and when the rotational coupling can be ignored. This model has been tested with large core ions such as Ar8+(2p6), Arx+(2p53s 'P) and with ground state (C", N5+, 0", Next) and metastable ls2s 'S (C4+, N5+) helium-like ions on various targets, below 0.5 au collision velocity.