NEW LOW-ENERGY MEASUREMENTS AND CALCULATION OF IONIZATION IN H-H COLLISIONS()

In response to the strong current theoretical interest in models of lo w-energy ionization in H+-H collisions, a crossed-beam coincidence cou nting technique previously used in this laboratory to obtain cross sec tions 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 McL aughlin 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, Ovchi nnikov S Y, van Eck J, Westerveld W B and Niehaus A 1994 Phys. Rev. Le tt. 73 46 in the range 1-6 keV amu(-1), but are in good accord with th eir calculations based on the hidden-crossings model including only S and T promotion. However, our measurements and calculation do not supp ort the more recent calculations by Pieksma M, Ovchinnikov 8 Y and Mac ek J H 1998 J. Phys. B: At Mol. Opt. Phys. 31 1267 in which a contribu tion from a radial decoupling mechanism is also included in the model.