J. A. Ludlow

Double photoionization of atoms and ions confined in charged fullerenes

The double photoionization of a helium atom and a singly charged lithium positive ion enclosed in charged Cq60 fullerenes is studied using the time-dependent close-coupling method. Continuum raising and lowering effects are seen in the shift of the double-ionization threshold of the confined atom or ion. Confinement resonances in the double-photoionization cross section, previously observed for helium confined inside a neutral fullerene, persist for helium confined in both positively and negatively charged fullerenes and are found to be independent of the charge on the fullerene. The double photoionization of a confined singly positively charged lithium ion is found to exhibit similar behaviour to a confined neutral atom, with confinement resonances present in the double-photoionization cross section for confinement in neutral, positively and negatively charged fullerenes. In addition, the confinement resonances are shown to be sensitive to the width of the fullerene with the resonances having a smaller amplitude for larger fullerene widths.

Electron-impact double ionization of magnesium

Theory and experiment are compared for the electron-impact double ionization of Mg. Direct ionization cross sections, involving the simultaneous ionization of both 3s electrons, are calculated using a non-perturbative time-dependent close-coupling method. Indirect ionization cross sections, involving the ionization of either a 2p or 2s electron followed by autoionization, are calculated using a perturbative time-independent distorted-wave method. At low energies the direct ionization cross sections are found to be in good agreement with experiments, while at the higher energies the indirect ionization cross sections are also found to be in good agreement with experiments.

Electron-impact ionization of ground and metastable neon

Electron-impact ionization of neutral neon continues to be of interest both from a fundamental and an applied perspective. Non-perturbative calculations are required for the electron-impact ionization of ground and excited states of neon. R-matrix with pseudostates (RMPS) calculations for the electron-impact ionization of the 1s22s22p6 ground state of Ne at higher incident electron energies are now made possible by the further partitioning of each individual LS partial wave across a greater number of processors. Furthermore, instead of a long sequence of parallel matrix diagonalizations, every partial wave diagonalization is now carried out concurrently. The ground-state cross section is found to be in excellent agreement with recent experimental measurements. The previous work of Ballance et al (2004 J. Phys. B: At. Mol. Opt. Phys. 37 4779), for the ionization of the 1s22s22p53s excited state of Ne, has been extended to examine the ionization of the 1s22s22p54s and 1s22s22p55s excited states. Very good agreement is found between R-matrix with pseudostates and time-dependent close-coupling calculations for the 1s22s22p54s excited state. The non-perturbative cross sections for low nl are used to scale semi-classical impact parameter cross sections to high nl. Ionization cross sections for the ground and all excited states will enable the accurate determination of the generalized collisional-radiative ionization rate coefficient for neutral Ne in finite plasmas.

Single photoionization with excitation and double photoionization of He@C36, He@C60 and He@C82

In anticipation of future experimental investigations of two-electron processes in endohedral atoms, the double photoionization and single photoionization with excitation of helium confined in the fullerenes C36, C60 and C82 have been studied using the time-dependent close-coupling method. It is found that both He@C60 and He@C82 display strong confinement resonances in the double photoionization cross section, whereas for He@C36, the confinement resonances are suppressed. For single photoionization leaving the He+ ion in its ground state, we found not only that the magnitudes of the cross sections for He@C36, He@C60 and He@C82 are similar to the case of helium but that confinement resonances are also apparent. For single photoionization with excitation to the n = 2 shell, the endofullerene cross sections showed a reduction as compared to bare He atoms, while for photoionization with excitation to the n = 3 shell, the cross sections for the endofullerenes are enhanced, with a particularly strong enhancement evident for He@C36.

Electron-impact double ionization of B+

Time-dependent close-coupling and time-independent distorted-wave methods are used to calculate the electron-impact double-ionization cross section for the 1s22s2 ground configuration of the B+ atomic ion. The direct double-ionization cross section is calculated using the non-perturbative close-coupling method between the direct double-ionization threshold of 63.1 eV and the 1s ionization threshold of 218.4 eV. The indirect single ionization–autoionization cross section is calculated using the perturbative distorted-wave method between the 1s ionization threshold of 218.4 and 750 eV. The double-ionization cross section calculated using the two methods is compared with a crossed-beam experiment over the entire energy range.

Electron-impact ionization of Xe24 +

Configuration-average distorted-wave calculations are carried out for the electron-impact single ionization of Xe24 +. Contributions are included from direct ionization of the 3s, 3p, 3d and 4s subshells and from indirect ionization via 3s → nl, 3p → nl and 3d → nl excitations followed by autoionization. Branching ratios are found for single versus double ionization of the 3s and 3p subshells and for autoionization versus radiative decay of all 3l → nl excitations. Additional distorted-wave and R-matrix calculations find resonant-capture double-autoionization contributions to be quite small. The total ionization cross section for Xe24 + is found to be dominated by indirect excitation–autoionization contributions, especially near the single-ionization threshold. An approximate 15% reduction in the total ionization cross section is found due to the radiative decays included in the branching ratios.

A large-scale R-matrix calculation for electron-impact excitation of the Ne2 +, O-like ion

The five JΠ levels within an np2 or np4 ground-state complex provide an excellent testing ground for the comparison of theoretical line ratios with astrophysically observed values, in addition to providing valuable electron temperature and density diagnostics. The low-temperature nature of the line ratios ensures that the theoretically derived values are sensitive to the underlying atomic structure and electron-impact excitation rates. Previous R-matrix calculations for the O-like Ne ion, Ne2 +, exhibit spurious structure in the cross sections at higher electron energies, which may affect Maxwellian averaged rates even at low temperatures. Furthermore, there is an absence of comprehensive excitation data between the excited states that may provide newer diagnostics to complement the more established lines discussed in this paper. To resolve these issues, we present both a small-scale 56-level Breit–Pauli calculation and a large-scale 554-level R-matrix intermediate coupling frame transformation calculation that extends the scope and validity of earlier JAJOM calculations both in terms of the atomic structure and scattering cross sections. Our results provide a comprehensive electron-impact excitation data set for all transitions to higher n-shells. The fundamental atomic data for this O-like ion are subsequently used within a collisional radiative framework to provide the intensity line ratios across a range of electron temperatures and densities of interest in astrophysical observations.

Breit?Pauli R-matrix electron-impact excitation calculations along the argon isonuclear sequence

Electron-impact excitation of argon ions is a vital component in the modelling of fusion tokamak experiments. Currently the available electron-impact excitation rates for the argon isonuclear sequence are a collection of isolated calculations carried out within various theoretical scattering models. With the development of our parallel suite of Breit?Pauli codes and the accessibility of massively parallel architectures, there was the opportunity to calculate the entire argon isonuclear sequence and provide a single, comprehensive and complete excitation data set. These data will subsequently be archived as Maxwellian averaged rate coefficients. We report on Breit?Pauli R-matrix electron-impact excitation calculations that have been performed for Ar3+, Ar4+, Ar5+, Ar7+, Ar8+, Ar10+, Ar11+, Ar12+, Ar13+, Ar14+ and Ar17+. Together with existing R-matrix calculations, this completes the generation of excitation data for the argon isonuclear sequence.

Double photoionization of helium including quadrupole radiation effects

Non-perturbative time-dependent close-coupling calculations are carried out for the double photoionization of helium including both dipole and quadrupole radiation effects. At a photon energy of 800 eV, accessible at current synchrotron light sources, the quadrupole interaction contributes around 6% to the total integral double photoionization cross section. The pure quadrupole single energy differential cross section shows a local maximum at equal energy sharing, as opposed to the minimum found in the pure dipole single energy differential cross section. The sum of the pure dipole and pure quadrupole single energy differentials is insensitive to non-dipole effects at 800 eV. However, the triple differential cross section at equal energy sharing of the two ejected electrons shows strong non-dipole effects due to the quadrupole interaction that may be experimentally observable.

Investigating the static dipole polarizability of noble-gas atoms confined in impenetrable spheres and shells

The static dipole polarisability of noble gas atoms confined by impenetrable spheres and spherical shells is studied using the B-spline random phase with exchange approximation. The general trend in dipole polarisabilities across the noble gas sequence shows a decrease in the dipole polarisability as the volume of the confining impenetrable sphere is reduced and a large increase in the dipole polarisability for confinement by impenetrable spherical shells as the inner shell radius is increased.