Showing papers by "Igor Bray published in 2007"
TL;DR: In this paper, the convergent close-coupling formalism was applied to describe direct double photoionization (DPI) of the valence $n{s}^{2}$ shell of alkaline-earth-metal atoms.
Abstract: We apply the convergent close-coupling formalism to describe direct double photoionization (DPI) of the valence $n{s}^{2}$ shell of alkaline-earth-metal atoms: beryllium $(n=2)$, magnesium $(n=3)$, and calcium $(n=4)$. We consider the range of photon energies below the onset of resonant and Auger ionization processes where the subvalent and core electrons can be treated as spectators. By comparing alkaline-earth-metal atoms with helium, we elucidate the role of the ground state and final ionized state correlations in DPI of various quasi-two-electron atoms.
33 citations
TL;DR: It is found that convergent S-wave model cross sections are obtained only if complete expansions are utilized on both the atomic and the positronium centers, and it is suggested that, in the model and full calculations, the separate contributions to the breakup cross section from both centers become equal at threshold.
Abstract: We consider the positron-impact ionization (breakup) of atomic hydrogen utilizing the full and $S$-wave model calculations, concentrating on the near-threshold energy region. Unlike the corresponding electron-impact case, the $S$-wave model does support the Wannier-like threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)]. It is found that convergent $S$-wave model cross sections are obtained only if complete expansions are utilized on both the atomic and the positronium centers. Furthermore, we suggest that, in the model and full calculations, the separate contributions to the breakup cross section from both centers become equal at threshold.
28 citations
TL;DR: In this article, a joint experimental and theoretical investigation of superelastic electron scattering from laser-excited Cs atoms in the (6p){sup 2}P{sub 3/2} state is presented.
Abstract: We present results from a joint experimental and theoretical investigation of superelastic electron scattering from laser-excited Cs atoms in the (6p){sup 2}P{sub 3/2} state. Comparison of the measured pseudo-Stokes parameters P{sub 1}, P{sub 2}, and P{sub 3} and the total degree of polarization P{sup +} for incident energies of 5.5 eV and 13.5 eV, respectively, with theoretical predictions based upon a nonrelativistic convergent close-coupling method and a 24-state semirelativistic Breit-Pauli R-matrix approach indicates that driving channel coupling to convergence for these observables is more important than accounting for relativistic effects.
9 citations
TL;DR: The quadrupole channel of two-photon double ionization of He exhibits two distinctly different modes of correlated motion of the photoelectron pair as discussed by the authors, the center-of-mass mode and the relative motion mode.
Abstract: The quadrupole channel of two-photon double ionization of He exhibits two distinctly different modes of correlated motion of the photoelectron pair. The kinematics of the mode associated with the center-of-mass motion favors large total momenta maximized at parallel emission where the interelectron repulsion is strong. In contrast, the mode associated with the relative motion favors large relative momenta maximized at antiparallel emission where the interelectron repulsion is relatively weak. This difference in the interelectron repulsion allows for much wider angular correlation width in the relative motion mode as compared to the center-of-mass mode.
8 citations
01 Nov 2007
TL;DR: In this paper, the convergent close-coupling (CCC) method is applied to studies of two-photon double ionization (TPDI) of He.
Abstract: We review recent applications of the convergent close-coupling (CCC) method to studies of two-photon double ionization (TPDI) of He. In a weak-field regime, the electron-photon interaction can be treated within the lowest order perturbation theory (LOPT) whereas the electron-electron interaction is included in full. The intermediate states of the target can either be represented by a discrete set of B-splines in a box or summed over with an average weight (closure approximation). In a non-perturbative regime, we solve the time-dependent Schr?dinger equation on a square-integrable basis and project this solution on a set of CCC final states. In both regimes, we are able to calculate reliably the total intgrated and full differential TPDI cross-sections. Besides purely numerical results, we introduce a convenient analytical parametrization of the TPDI amplitude in the manner similar to single photon double ionization. Aided with this parametrization, we observe two distinctly different modes of correlated motion of the photoelectron pair. We also derive the angular anisotropy parameters and the recoil ion momentum distribution for TPDI of He. The latter can be compared with recent experimental observations.
7 citations
TL;DR: In this paper, the angular anisotropy of the recoil ion in two-photon double ionization (TPDI) of helium has been analyzed using convergent-close-coupling.
Abstract: We present convergent-close-coupling (CCC) calculations of the angular anisotropy parameters {beta}{sub 2},{beta}{sub 4} and the recoil ion momentum distribution d{sigma}/dp in two-photon double ionization (TPDI) of helium. In a stark contrast to single-photon double ionization (SPDI), where the {beta}{sub 2} parameter varies widely changing the angular distribution from isotropic to nearly dipole for slow and fast photoelectrons, respectively, the {beta} parameters for TPDI show very little change. The angular distribution of the recoil ion is fairly isotropic in TPDI as opposed to a strong alignment with the polarization of light in SPDI.
6 citations
TL;DR: In this paper, triple-differential cross sections (TDCSs) for electron-impact ionization of He(1s{sup 2}){sup 1}S leading to He{sup +}(1 s) are calculated using the highly accurate convergent close-coupling (CCC) method for incident projectile energies of 268.6 and 112.6 eV, with at least one of the outgoing electrons having an energy of 44 eV.
Abstract: Triple-differential cross sections (TDCSs) for electron-impact ionization of He(1s{sup 2}){sup 1}S leading to He{sup +}(1s) are calculated using the highly accurate convergent close-coupling (CCC) method for incident projectile energies of 268.6 and 112.6 eV, with at least one of the outgoing electrons having an energy of 44 eV. These results are used to obtain absolute TDCSs from the recent experimental data [Bellm et al., Phys. Rev. A 75, 042704 (2007)] for TDCS ratios of ionization with no excitation to ionization with excitation resulting in He{sup +}(n=2,3,4). The TDCSs can be used as comparison benchmarks in future studies, and already allow us to test the accuracy of the TDCSs obtained from the hybrid distorted-wave+R-matrix (close-coupling) model, which was fairly successful in predicting the ratios, using CCC for n=1 and experimental results for n=2,3,4.
6 citations
TL;DR: In this article, the authors presented fully differential measurements of 730-eV electron-impact single ionization of the ground state of helium with 205- or 100eV outgoing electrons.
Abstract: We present fully differential measurements of 730-eV electron-impact single ionization of the ground state of helium with 205- or 100-eV outgoing electrons. Internormalized data are obtained for coplanar geometries with the fast electron detected at IA =6°, 9°, and 12°. The data are compared, where possible, with the corresponding data of Catoire [J. Phys. B 39, 2827 (2006)] and the convergent close-coupling theory. An improved agreement is found between the present measurements and the theory.
4 citations
01 Nov 2007
TL;DR: In this article, the authors considered the near-threshold behavior for positron-impact ionization (breakup) of hydrogen and used the two-centre convergent close-coupling method.
Abstract: The near-threshold behavior for positron-impact ionization (breakup) of hydrogen is considered. The two-centre convergent close-coupling method is used. We find that convergence in the cross section is only practically obtained if two near-complete expansions are used, one centred around H and the other around Ps. The contribution to the breakup cross section becomes the same for both centres as the threshold is approached. The calculations are found to be in good agreement with the threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)].
01 Jan 2007
TL;DR: In this article, the available experimental and theoretical cross section data for inelastic collision processes of ground (3s) and excited (3p, 4s, 3d, 4p, 5s, 4d,4f) state Na atoms with electrons, protons, and multiply charged ions have been collected and critically assessed.
Abstract: The available experimental and theoretical cross section data for inelastic collision processes of ground (3s) and excited (3p, 4s, 3d, 4p, 5s, 4d, 4f) state Na atoms with electrons, protons, and multiply charged ions have been collected and critically assessed. In addition to existing data, electron-impact cross sections, for both excitation and ionization, have been calculated using the convergent close-coupling approach. In the case of proton-impact the database was enlarged by new atomic-orbital close-coupling calculations. Both electron-impact and proton-impact processes include excitation from the ground state and between excited states (n = 3 5). For electron-impact also ionization from all states is considered. In the case of proton-impact electron loss cross sections (the sum of ionization and singleelectron charge transfer) are given. Well-established analytical formulae used to t cross sections, earlier published