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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)
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Differential Ionization
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        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)
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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)
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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.
[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
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        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)
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Annihilation
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        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
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