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A crossed-beam technique has been used to determine cross sections for Balmer alpha emission from hydrogen atoms excited by Ht, He* and He'+ impact within the energy range 2.5-100 keVamu-'. Results have been compared where possible with Balmer alpha cross sections derived from theoretical predictions of cross sections for excitation of the n = 3 rublevels of H.

In response to the strong current theoretical interest in models of low-energy ionization in H+–H collisions, a crossed-beam coincidence counting technique previously used in this laboratory to obtain cross sections in the range 9–1500 keV amu−1 has been successfully adapted to extend the measurements downwards in energy from 9 to 1.25 keV amu−1. In addition, we have extended the triple-centre calculations of McLaughlin B M, Winter T G and McCann J F 1997 J. Phys. B: At. Mol. Opt. Phys. 30 1043, down to 0.5 keV amu−1 where agreement with experiment is found to be satisfactory. Our new measurements and calculation are at variance with relative cross sections measured by Pieksma M, Ovchinnikov S Y, van Eck J, Westerveld W B and Niehaus A 1994 Phys. Rev. Lett. 73 46 in the range 1–6 keV amu−1, but are in good accord with their calculations based on the hidden-crossings model including only S and T promotion. However, our measurements and calculation do not support the more recent calculations by Pieksma M, Ovchinnikov S Y and Macek J H 1998 J. Phys. B: At. Mol. Opt. Phys. 31 1267 in which a contribution from a radial decoupling mechanism is also included in the model.

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