Positron impact ionization

Fig. 5.6. Depiction of near-threshold (a) electron impact ionization and (b) positron impact ionization.

Ashley, Moxom and Laricchia (1996) measured the positron impact-ionization cross section in helium and found that its energy dependence up to 10 eV beyond the threshold was quite accurately represented by a power law, as in equation (5.8), but with the exponent having the value 2.27 rather than Klar s value of 2.651. This discrepancy prompted Ihra et al. (1997) to extend the Wannier theory to energies slightly above the ionization threshold using hidden crossing theory. They derived a modified threshold law of the form... 

A variety of approximation methods have been applied to positron impact ionization and, as with other scattering processes, the most attention has been given to atomic hydrogen and helium. The first Born... 

Fig. 5.8. The triple differential cross section for positron impact ionization of atomic hydrogen, expressed as a function of the energy of the ejected electron. The scattered positron and electron both emerge in the direction of the incident...

The first studies of positron impact ionization were based around the TOF systems originally developed for total cross section measurements see section 2.3 and, for example, Griffith et al. (1979b), Coleman et al. (1982), Mori and Sueoka (1984, 1994), though only the latter workers, as described in section 5.1, claimed to be able to distinguish between ionization and excitation using the TOF technique. 

Fig. 5.10. Apparatus, not to scale, of Jones et al. (1993) for positron impact ionization of atomic hydrogen. Squares with crosses, Helmholtz coil shaded rectangles, stainless steel shielding black rectangles in beam line, lead shielding.

Fig. 5.11. Positron impact ionization of atomic hydrogen. Experiment , Jones...

Fig. 5.12. Positron impact ionization of helium gas. Experiment a, Fromme et al. (1986) o, Moxom, Ashley and Laricchia, (1996) , Jacobsen et al. (1995b) ...

Fig. 5.13. Positron impact ionization for neon (left) and argon (right) gases. Experimental data for neon , Kara et al. (1997a) , Knudsen et al. (1990) , Jacobsen et al. (1995b). Experimental data for argon o, Moxom, Ashley and Laricchia (1996) , Jacobsen et al. (1995b) , Knudsen et al. (1990). Theory for...

The first reported study of the behaviour of double differential cross sections for positron impact ionization was that of Moxom et al. (1992) these workers conducted a search for electron capture to the continuum (ECC) in positron-argon collisions. In this experiment electrons ejected over a restricted angular range around 0° were energy-analysed to search for evidence of a cusp similar to that found in heavy-particle collisions (e.g. Rodbro and Andersen, 1979 Briggs, 1989, and references therein), which would be the signature of the ECC process. 

Brauner, M. and Briggs, J.S. (1991). Structures in differential cross sections for positron impact ionization of hydrogen. J. Phys. B At. Mol. Opt. Phys. 24 2227-2236. 

Threshold law for positron impact ionization of atoms. Phys. Rev. Lett. 78 4027-4030. 

Kara, V. (1999) Positron impact ionization studies. Ph.D. thesis, University of London. 

Raith, W. (1987). Positron-impact ionization and positronium formation. In Atomic Physics with Positrons, eds. J.W. Humberston and E.A.G. Armour (Plenum Press) pp. 1-14. 

The process, which leads to the liberation of a target electron, following the impact of a charged particle on an atom or a molecule, is one of the most important and fundamental phenomena studied in the field of atomic collisions. The interest in the ionization process goes back many decades, and it has been studied intensively since then. The largest volume of experimental data has been obtained for impact of electrons and protons, but a great deal of information has also been collected for impact of heavier ions. Recently, the electron data have been surveyed by for example Tawara et al. [4.1] and the proton data by Rudd et al. [4.2]. Since then, it has become possible to measure ionization cross sections for antiproton and positron impact. Some of the experimental measurements of positron-impact ionization were reviewed by Charlton and Laricchia [2.12]. From comparisons between data obtained with equivelocity p", p , e", and e , a substantial amount of new information on the ionization process has become available. In this section, we will review the new results obtained for the single-ionization cross section,... 

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