New low-energy measurements and calculation of ionization in H+–H collisions

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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−1has 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 1997J. Phys. B: At. Mol. Opt. Phys.301043, down to 0.5 keV amu−1where 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 1994Phys. Rev. Lett.7346 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 1998J. Phys. B: At. Mol. Opt. Phys.311267 in which a contribution from a radial decoupling mechanism is also included in the model.