Showing papers by "Igor Bray published in 2013"
TL;DR: This work investigates the single-photon double ionization of helium at photon energies of 440 and 800 eV and observes doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy.
Abstract: We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.
48 citations
TL;DR: In this article, the authors extended the convergent-close-coupling method to positron-molecule collisions within the adiabatic (fixed-nuclei) approximation.
Abstract: The $ab$ $initio$ convergent-close-coupling method has been extended to positron-molecule collisions within the adiabatic (fixed-nuclei) approximation. Application to molecular hydrogen at energies from 0.1 to 1000 eV has yielded convergent total ionization and grand total cross sections over most of the energy range. We find that very large calculations are required for convergence, even in the case of low-energy elastic scattering, due to the effects of positronium formation. In general, the comparison with experiment is good.
29 citations
TL;DR: A novel time-dependent convergent close-coupling approach to the scattering problem that accounts for all possible orientations of the molecular target, has been developed and is applied to study single ionization of molecular hydrogen on the wide energy range from 1 keV to 2 MeV with a particular emphasis on low energies.
Abstract: Theoretical confirmation of the experimentally observed phenomenon [Knudsen et al., Phys. Rev. Lett. 105, 213201 (2010)] of target structure-induced suppression of the ionization cross section for low-energy antiproton-molecular hydrogen collisions is given. To this end a novel time-dependent convergent close-coupling approach to the scattering problem that accounts for all possible orientations of the molecular target, has been developed. The approach is applied to study single ionization of molecular hydrogen on the wide energy range from 1 keV to 2 MeV with a particular emphasis on low energies. Results for the orientation-averaged total single ionization cross section are compared with available experimental data and good agreement is found at low ( 90 keV) energies. A minor discrepancy is found within a small energy gap near the maximum of the cross section.
22 citations
11 citations
TL;DR: In this paper, the authors considered positron scattering on the alkali-metal atoms of Li, Na, and K at very low energies, where only the elastic scattering and positronium formation in the ground state are the two open channels.
Abstract: We consider positron scattering on the alkali-metal atoms of Li, Na, and K at very low energies, where only the elastic scattering and positronium formation in the ground state are the two open channels. Utilizing the recently developed two-center convergent close-coupling method [Lugovskoy, Kadyrov, Bray, and Stelbovics, Phys. Rev. A 82, 062708 (2010)] we investigate the behavior of the cross sections as the impact energy goes to zero and demonstrate their convergence. The study sets quantitative benchmarks for any rigorous theoretical treatment of the collision problems.
11 citations
TL;DR: In this article, the convergent close-coupling method was extended to investigate electron scattering from the vibrationally excited molecular hydrogen ion H${{}_{2}}^{+}], within the Born-Oppenheimer approximation.
Abstract: We have extended the convergent close-coupling method to investigate electron scattering from the vibrationally excited molecular hydrogen ion H${{}_{2}}^{+}$, within the Born-Oppenheimer approximation. Results are presented for proton-production and dissociative-ionization cross sections. The comparison with experiment is excellent across the energy range from near threshold to 1 keV.
10 citations
TL;DR: In this article, the relativistic convergent close-coupling method was applied to the calculation of high-energy electron-impact-excitation cross sections of highly charged hydrogen-like ions.
Abstract: Exact relativistic plane-wave Born (RPWB) matrix elements of the M\o{}ller interaction are incorporated in the ``analytic Born subtraction technique'' and employed in the relativistic convergent close-coupling method. Application to the calculation of high-energy electron-impact-excitation cross sections of highly charged hydrogenlike ions demonstrates the ``Bethe rise,'' an effect that is manifest in Bethe's original 1932 work on relativistic high-energy, electron-impact excitation. The result represents an improvement over Bethe's relativistic high-energy theory developed in the 1930s in that (i) both target and projectile electrons are represented relativistically with Dirac spinor wave functions and (ii) the dipole approximation plus additional assumptions are not employed in the RPWB scattering amplitude of the M\o{}ller interaction.
10 citations
TL;DR: In this paper, the double photoionization of Mg has been studied experimentally and theoretically in a kinematic where the two photoelectrons equally share the excess energy, and the observation of a symmetrized gerade amplitude, which strongly deviates from the Gaussian ansatz, is explained by a twoelectron interference predicted theoretically, but never before observed experimentally.
Abstract: The double photoionization of Mg has been studied experimentally and theoretically in a kinematic where the two photoelectrons equally share the excess energy. The observation of a symmetrized gerade amplitude, which strongly deviates from the Gaussian ansatz, is explained by a two-electron interference predicted theoretically, but never before observed experimentally. Similar to the Cooper minima in the single photoionization cross section, the effect finds its origin in the radial extent and oscillation of the target wave function.
9 citations
5 citations
TL;DR: In this article, the decay of the 2s22p 2P and 2s2p2 2D triply excited negative ion resonances in helium to singly excited n = 2 states have been measured using a magnetic angle changer with a soft-iron core.
Abstract: Excitation function measurements for the decay of the 2s22p 2P and 2s2p2 2D triply excited negative ion resonances in helium to singly excited n = 2 states have been measured. These excitation functions have been determined across the complete angular range (0–180°) using a magnetic angle changer with a soft-iron core. The convergent close-coupling method has been used to calculate the cross sections, with the underlying complexity of the problem not yet being able to be fully resolved. Agreement between the present experimental data and previous experimental data is good, with these excitation functions confirming the presence of an unusual (2s22p)2P resonance behaviour in the 21S channel at 90°, where this would not usually be expected. Resonance energy and width values have been obtained, with a mean energy for the (2s22p)2P resonance of 57.20 ± 0.08 eV and a mean width of 73 ± 20 meV, and a mean energy of the (2s2p2)2D resonance of 58.30 ± 0.08 eV and a mean width of 59 ± 27 meV. Resonant cross section and ρ2 values have been calculated across the angular range for the first time, providing angular distribution data on decay propensities for both resonances.
3 citations
TL;DR: In this article, the relativistic convergent close-coupling (RCCC) calculations of the circular light polarization Stokes parameter for atomic silver associated with the radiative decay of the electron-impact excited $(4{d}^{10}5p){\phantom{\rule{0.28em}{0ex}}}^{2}{P}_{3/2}
Abstract: We present relativistic convergent close-coupling (RCCC) calculations of the ${P}_{3}$ circular light polarization Stokes parameter for atomic silver associated with the radiative decay of the electron-impact excited $(4{d}^{10}5p){\phantom{\rule{0.28em}{0ex}}}^{2}{P}_{3/2}$ state. The results are compared with the recent measurements of Jhumka et al. [Phys. Rev. A 87, 052714 (2013)]. We find excellent agreement between the RCCC results and experiment across the full energy range of measurements. We also find that relativistic effects have a weak influence on the calculation of ${P}_{3}$ in the energy range considered.
TL;DR: In this article, a collinear scattering of a structured particle from a hard wall is studied with consideration of vibrational transitions initiated by the collision, and it is shown that this problem can be solved analytically in the framework of the source function method.
Abstract: A collinear scattering of a structured particle from a hard wall is studied with consideration of vibrational transitions initiated by the collision. It is shown that this problem can be solved analytically in the framework of the source-function method. With the use of the continuum discretization technique we are able to take into account both discrete and continuum states. No approximations of the interatomic potential is required. We illustrate our approach for the case of a hydrogen molecule bound by the realistic Morse potential. simple systems. The results by Sato and Kayanuma (13) indicate that the initial molecule ground state cannot survive after the collision when the energy difference between this state and the first-excited state goes to zero. Contrarily, Kavka et al. (9) show that an arbitrarily weakly bound molecule scattering from an arbitrarily high step potential remains in the ground state with probability equal to unity. Moreover, the molecule center of mass cannot get closer to the hard wall than ξ0 ln(V0/Ein) where ξ0 is a constant of order of the mean distance between the atoms, V0 is the potential barrier heights and Ein is the molecule impact energy. The case where the hard wall is infinitely high is particularly interesting because the molecule reflects from the surface at an infinitely large distance from the surface. In the next section we present the general analytical solution to the problem of interest for case where V0 is infinite. Our approach requires no approximations on the form of the interparticle interaction. To illustrate our theory we give a numerical example in Sec. III. In the conclusion we present the summary of the results.