Excitation of the 2s and 2p states of hydrogen in fast collisions of He+ with H and H2

A modulated crossed-beam technique has been used to determine cross sections for the formation of H(2p) and H(2s) atoms in collisions of He+ with H and H2 within the energy range 5-26 keV. Measurements were based on observations of the Lyman-α radiation emitted from the crossed-beam region both in the presence of and in the absence of electric-field quenching of the 2s state. The polarization of the Lyman-α radiation has also been studied. H(2p) formation cross sections are considerably larger than those for H(2s) formation for targets of H2 and H and a general understanding of the He+-H case is possible in terms of a quasi-molecular model. (author)