Beginners Positrons Positronium
Recent Research
  • Research Highlights (2019-2010)

  • Threshold Ionization
            The near-threshold direct ionization cross-section by positron impact has been measured [1]. These pioneering data have generated extensive interest, the results being consistent with the power-law form of Wannier- type theory but with an exponent considerably smaller than predicted [2]. The disagreement has been diversely attributed to anharmonic terms in the three-particle potential around the Wannier configuration [3] or to the importance of higher values of the total angular momentum azimuthal quantum number [4]. Further theoretical investigations are expected.
    1. P. Ashley, J Moxom and G Laricchia, Phys. Rev. Lett. 77, 1250 (1996)
    2. H. Klar, J. Phys. B 14, 4165 (1981); J.M. Rost, J. Phys. B 28, 3003 (1995)
    3. W. Ihra et al, Phys. Rev. Lett. 78, 4027 (1997)
    4. N.C. Deb and D.S.F. Crothers, J. Phys. B 35, L85 (2002)

    Differential Ionization
    Triple differential Cross-Sections for Molecular H
            The first triply differential cross-sections for direct ionization by positron impact have been recently measured and have resulted in the observation of electron-capture-to-the-continuum (ECC) [1], a phenomenon first predicted for positron impact over a decade ago [2]. These are challenging measurements, achieved with beam intensities less than a billionth that of their electron-impact counterpart but valuable since all the kinematic parameters are specified.The results obtained for the energy spectrum of the electrons ejected following positron impact ionisation of molecular hydrogen at 100eV are shown in the adjacent figure where they are compared with theory [3]. The small hump, at around half of the final state kinetic energy, provides the first clear experimental evidence for the occurrence of electron capture by the positron to a low-lying continuum-state of positronium. By probing the dynamical details of the collision, this type of measurement provide a sensitive test of theory. This is exemplified by our recent results obtained at 50 eV incident positron enrgy [4]. Whilst at 100eV good agreement is found between experiment and theories [3,5], at 50eV a shift has been observed in the measured energy distribution of the ejected electrons in comparison with the theoretical calculation [5]. Since the latter does not include the molecular degrees of freedom of the target, the shift might indicate the influence of molecular excitations [4] or, as recently suggested, a significant recoil of the residual target ion [6]. Further examination of these results by both theory and experiment is anticipated. Our novel parallel-plate-analyzer with time-focusing properties will facilitate investigations at low energies [7].

    1. Á. Kövér and G. Laricchia, Phys. Rev. Lett. 80, 5309 (1998)
    2. M. Brauner and J.S. Briggs, J. Phys. B 19, L325 (1986)
    3. J. Berakdar, Phys. Rev. Lett. 81, 1393 (1998)
    4. Á. Kövér, K Paludan and G Laricchia J. Phys. B 34, L219 (2001)
    5. J. Fiol, V.D. Rodriguez and R.O. Baracchina J. Phys. B 34, 933 (2001)
    6. J. Fiol and R.E. Olson J. Phys. B: At. Mol. Opt. Phys. 35 (2002) 1173
    7. Á. Kövér and G. Laricchia, Meas. Sci. Techol. 12 1875 (2001)

    Positronium formation
            Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact have been carried out in the energy range from threshold up to ~1keV. From these and other available data, the cross-sections for positronium formation have been extracted. Structure in the positronium formation cross-section has been found. The first maximum, which occurs at an energy close to twice the first threshold energy, corresponds to Ps formation into the ground state. The position and magnitude of the second peak suggests that it may be associated with the contribution of Ps formation into excited states. Upper and lower limits on this contribution have been estimated by scaling theoretical cross-sections for He and are found to be significant [1]. Direct measurements and explicit calculations of QPs(n>1) for the heavier inert atoms would clearly be desirable.

    1. G. Laricchia, P. Van Reeth, M. Szluinska and J. Moxom J Phys B 35 2525 (2002)

    Integral Ionisation
            Positron impact ionisation (both single and double) has been studied for a number of targets over a wide range of energies 1. The total ionisation cross-section of atomic hydrogen has also been investigated in order to resolve inconsistencies among experimental and theoretical determinations of the positronium formation and direct ionisation cross-sections 3.
            Detailed measurements of the single ionisation cross-section for positron impact on He and Kr atoms have been made and compared to corresponding results for electron, proton and antiproton impact(see below). The He (low Z) results suggest that the relationship between the positron and electron cross-sections is similar to that between the protons and antiprotons, whilst for Kr the results show no such similarity. This is thought to reflect the increasing importance (for high Z targets) of the static interaction for the light projectiles 3.
    Single Ionisation Single Ionisation

    [1] Shah et al (1989)
    [2] Shah and Gilbody (1985)
    [3] Hvelplund et al (1994)
    [4] Anderson et al (1989)
    [5] Krishnakumar and Srivastava (1988)
    [6] Moxom et al (1996)
    [7] Paludan et al (1997)
    [8] DuBois and Manson (1987)
    [9] Paludan et al (1998)
    1 Kara et al, 1997
    2 Ashley et al, 1996
    3 Kara et al, 1998

    Elastic scattering
            The energy dependence of the cross-section for positrons scattering elastically at 30o from argon has been measured in the vicinity of the Ps formation threshold [1]. Together with previous experimental and theoretical results, the present data indicate the influence of channel-coupling effects on elastic scattering from Ps formation and suggest a similar effect from positron impact excitation.

    1. H. Gao, A.J. Garner, J. Moxom, G. Laricchia, A. Kövér, J Phys B32 L263 (1999)

            We have also developed a model concerning the annihilation probability of positrons in binary encounters with atoms and molecules [1]. By considering the effect of virtual positronium formation, accompanied by pick-off annihilation, the model predicts a quasi-resonant enhancement of this probability at the threshold for positronium formation. A qualitative illustration of the universal trends which, according to the model, are to be expected for different targets (as well as different thresholds) is shown opposite.

    1. Laricchia G and Wilkin C, Phys Rev Letts 79 (1997) pp2241-2244
    Ratio of the annihilation to elastic collision cross-section for He. Also 
		shown are the contributions arising from virtual Ps formation and in-flight