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

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 8 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.