301 research outputs found
Mechanisms of positron annihilation on molecules
The aim of this work is to identify the mechanisms responsible for very large
rates and other peculiarities observed in low-energy positron annihilation on
molecules. The two mechanisms considered are: (i) Direct annihilation of the
incoming positron with one of the molecular electrons. This mechanism dominates
for atoms and small molecules. I show that its contribution to the annihilation
rate can be related to the positron elastic scattering cross section. This
mechanism is characterized by strong energy dependence of the annihilation rate
at small positron energies if a low-lying virtual level or a weakly bound state
exists for the positron. (ii) Resonant annihilation, which takes place when the
positron undergoes resonant capture into a vibrationally excited quasibound
state of the positron-molecule complex. This mechanism dominates for larger
molecules capable of forming bound states with the positron. For this mechanism
the energy-averaged annihilation rate is proportional to the level density of
the positron-molecule complex, which is basically determined by the spectrum of
molecular vibrational states populated in the positron capture. For
room-temperature positrons this mechnism can produce annihilation rates up to 5
orders of magnitude greater than the first one. Its contribution is inversely
proportional to the positron momentum at small positron energies.Comment: 28 pages, 5 figures, REVTeX, scheduled for publication in Phys. Rev.
A, Feb 200
-ray spectra and enhancement factors for positron annihilation spectra with core-electrons
Many-body theory is developed to calculate the -spectra for positron
annihilation with valence and core electrons in the noble gas atoms. A proper
inclusion of correlation effects and core annihilation provides for an accurate
description of the measured spectra [Iwata \textit{et al.}, Phys. Rev. Lett.
{\bf 79}, 39 (1997)]. The theory enables us to calculate the enhancement
factors , which describe the effect of electron-positron
correlations for annihilation on individual electron orbitals . We find
that the enhancement factors scale with the orbital ionization energy
(in electron-volt), as ,
where ~eV, ~eV and .Comment: 5 pages, 5 figure
Effect of dipole polarizability on positron binding by strongly polar molecules
A model for positron binding to polar molecules is considered by combining
the dipole potential outside the molecule with a strongly repulsive core of a
given radius. Using existing experimental data on binding energies leads to
unphysically small core radii for all of the molecules studied. This suggests
that electron-positron correlations neglected in the simple model play a large
role in determining the binding energy. We account for these by including
polarization potential via perturbation theory and non-perturbatively. The
perturbative model makes reliable predictions of binding energies for a range
of polar organic molecules and hydrogen cyanide. The model also agrees with the
linear dependence of the binding energies on the polarizability inferred from
the experimental data [Danielson et al 2009 J. Phys. B: At. Mol. Opt. Phys. 42
235203]. The effective core radii, however, remain unphysically small for most
molecules. Treating molecular polarization non-perturbatively leads to
physically meaningful core radii for all of the molecules studied and enables
even more accurate predictions of binding energies to be made for nearly all of
the molecules considered.Comment: 26 pages, 18 figure
Many-body theory calculations of positron binding to negative ions
A many-body theory approach developed by the authors [Phys. Rev. A 70, 032720
(2004)] is applied to positron bound states and annihilation rates in atomic
systems. Within the formalism, full account of virtual positronium (Ps)
formation is made by summing the electron-positron ladder diagram series, thus
enabling the theory to include all important many-body correlation effects in
the positron problem. Numerical calculations have been performed for positron
bound states with the hydrogen and halogen negative ions, also known as Ps
hydride and Ps halides. The Ps binding energies of 1.118, 2.718, 2.245, 1.873
and 1.393 eV and annihilation rates of 2.544, 2.482, 1.984, 1.913 and 1.809
ns, have been obtained for PsH, PsF, PsCl, PsBr and PsI, respectively.Comment: 19 pages, 13 figures, submitted to International Review of Atomic and
Molecular Physic
Multiphoton detachment from negative ions: new theory vs experiment
In this paper we compare the results of our adiabatic theory (Gribakin and
Kuchiev, Phys. Rev. A, accepted for publication) with other theoretical and
experimental results, mostly for halogen negative ions. The theory is based on
the Keldysh approach. It shows that the multiphoton detachment rates and the
corresponding n-photon detachment cross sections depend only on the asymptotic
behaviour of the bound state radial wave function. The dependence on the
exponent is very strong. This is the main reason for the disagreement with some
previous calculations, which employed bound state wave functions with incorrect
asymptotic forms. In a number of cases our theoretical results produces best
agreement with absolute and relative experimental data.Comment: 9 pages, Latex, IOP style, and 3 figures fig1.ps, fig2.ps, fig3.ps,
submitted to J. Phys.
Ps-atom scattering at low energies
A pseudopotential for positronium-atom interaction, based on electron-atom
and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr
and Ps-Ar scattering are calculated. This approach allows us to extend the
Ps-atom cross sections, obtained previously in the impulse approximation [Phys.
Rev. Lett. 112, 243201 (2014)], to energies below the Ps ionization threshold.
Although experimental data are not available in this low-energy region, our
results describe well the tendency of the measured cross sections to drop with
decreasing velocity at a.u. Our results show that the effect of the
Ps-atom van der Waals interaction is weak compared to the polarization
interaction in electron-atom and positron-atom scattering. As a result, the Ps
scattering length for both Ar and Kr is positive, and the Ramsauer-Townsend
minimum is not observed for Ps scattering from these targets. This makes Ps
scattering quite different from electron scattering in the low-energy region,
in contrast to the intermediate energy range from the Ps ionization threshold
up to a.u., where the two are similar.Comment: 26 pages, 10 figure
Similarity between positronium-atom and electron-atom scattering
We employ the impulse approximation for description of positronium-atom
scattering. Our analysis and calculations of Ps-Kr and Ps-Ar collisions provide
theoretical explanation of the similarity between the cross sections for
positronium scattering and electron scattering for a range of atomic and
molecular targets observed by S. J. Brawley et al. [Science 330, 789 (2010)].Comment: 10 pages, 3 figure
Coherent and stochastic contributions of compound resonances in atomic processes: Electron recombination, photoionization and scattering
In open-shell atoms and ions, processes such as photoionization, combination
(Raman) scattering, electron scattering and recombination, are often mediated
by many-electron compound resonances. We show that their interference
(neglected in the independent-resonance approximation) leads to a coherent
contribution, which determines the energy-averaged total cross sections of
electron- and photon-induced reactions obtained using the optical theorem. In
contrast, the partial cross sections (e.g., electron recombination, or photon
Raman scattering) are dominated by the stochastic contributions. Thus, the
optical theorem provides a link between the stochastic and coherent
contributions of the compound resonances. Similar conclusions are valid for
reactions via compound states in molecules and nuclei
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