Table of contents

We analyse (e, 3e) processes on helium, with one fast and two slow electrons in the final channel, where strong deviations from a first-Born description were reported recently (Lahmam-Bennani et al 2001 J. Phys. B: At. Mol. Opt. Phys.34 3073–87). The four-particle continuum is described in different approximations leading up to a 6C wavefunction which takes all two-body Coulomb interactions into account. It is shown that deviations from a first-Born description arise from two different effects which are important in two different energy regions.

Normal Auger decay spectra of the 2p core hole in potassium have been measured both for molecular gas-phase and crystalline solid-state samples of KCl. While the spectrum for the latter is easily interpreted using a strictly ionic approach, the molecular spectrum gives evidence of strong additional effects. The observed differences between the two phases are discussed from the viewpoint of the ionic model, considering also the perturbation by the different environments and symmetries. The uniquely molecular effects seen in the Auger electron spectra are attributed to the sudden change in character of the valence (Cl 3p) orbital upon core-hole decay, accompanied by dynamic relaxation processes.

Fine-structure separations and M1 and E2 transition probabilities for the lowest ²P doublet of boron-like ions, from Z = 10 to 30, have been calculated using the multireference relativistic configuration interaction (MRCI) method based on the no-pair Dirac–Coulomb–Breit Hamiltonian. Analytic basis sets of Gaussian-type functions are employed to expand the upper and lower components of the Dirac four-spinors in the matrix Dirac–Fock self-consistent field and relativistic MRCI procedures.

Simultaneous target and projectile single ionization in 3.6 MeV u⁻¹ C²⁺ + He collisions is considered, within a six-body classical trajectory Monte Carlo model. Analysis of the relative azimuthal angle between the two emitted electrons allows one to discriminate the ionization contribution produced by the two-centre dynamical electron–electron interaction from that due to nucleus–electron interactions. The present calculations agree well with cross section measurements recently performed in kinematically complete experiments.

We demonstrate that a recently published quasiclassical Møller type approach (Geyer and Rost 2002 J. Phys. B: At. Mol. Opt. Phys.35 1479) can be used to overcome the problem of autoionization, which arises in classical trajectory calculations for many-electron targets. In this method, the target is stabilized dynamically by a backward–forward propagation scheme. We illustrate this refocusing and present total cross sections for single and double ionization of helium by electron impact.

This article reviews the current status of inner-shell spectroscopies of free atoms and molecules using high-resolution soft-x-ray monochromators installed in the soft-x-ray beamlines at the third-generation synchrotron radiation facilities. Beamlines and endstations devoted to atomic and molecular inner-shell spectroscopies and various types of experimental techniques, such as ion yield spectroscopy, resonant photoemission spectroscopy and multiple-coincidence momentum imaging, are described. Experimental results for K-shell excitation of Ne, O K-shell excitation of H₂O and CO₂, C K-shell excitation and ionization of CO₂ and B K-shell excitation of BF₃, obtained at beamline 27SU of SPring-8 in Japan, are discussed as examples of atomic and molecular inner-shell spectroscopies using the third-generation synchrotron radiation sources.

We have investigated the contribution that the frequency-independent magnetic term of the Breit interaction makes to electron scattering from heavy, closed-shell atoms at low and intermediate energies. We treat the Breit interaction as a perturbation to the scattering phase shifts calculated within a consistently relativistic approximation via the Hulthén–Kato scattering identity. Results are presented for scattering from both xenon and mercury. Although the Breit interaction makes a noticeable contribution to the scattering phase shift, in most cases the difference is not significant on the scale of the accuracy of current experimental measurements. However, we have identified one situation where the differences are large enough that they might be resolved in future experiments.

A simplified qualitative model based on the shake-off mechanism framework is proposed in the first Born approximation to study the triple ionization (e, 4e) process of the Li atom by fast electron impact in a coplanar geometry. The recent experiment of Wehlitz et al (1998 Phys. Rev. Lett.81 1813), on the triple photo ionization (γ, 3e) of the Li atom has stimulated us to study the above (e, 4e) process for low momentum transfer () and for high incident energy, since in the dipolar limit → 0, the results for (e, 4e) should converge to the corresponding (γ, 3e) results. The model proposed is particularly suitable for low momentum transfer as well as for asymmetric kinematics with respect to the electrons in different shells. The fully differential (spin averaged) cross sections (7DCS) with respect to the angular distributions of the shake-off electron, which in the present model is assumed to be the valence electron of the Li atom, are studied. 7DCS is also studied for the second electron ejected from the 1s shell for some selective kinematics. The peak values of the 7DCS versus incident electron energy curves for a given set of kinematics qualitatively compare well with the experimental findings of Wehlitz et al. To the best of our knowledge, this is the first theoretical work performed relating to the 7DCS calculation of the (e, 4e) process of the Li atom.

The frequency dependence of non-sequential double ionization (NSDI) of Ne is studied using a semiclassical rescattering model. It is found that the ratio (Ne²⁺/Ne⁺), calculated for a laser intensity of 1.0 × 10¹⁵ W cm⁻², increases while the wavelength decreases down to about 250 nm, the limit of the applied model. Careful investigations of the trajectories show that there are two categories of double ionization: 'instant ionization' and 'delayed ionization'. By limiting the time interval between the re-collision time and ionization time, the 'delayed ionization' is identified to give rise to the high probability of NSDI in the short wavelength region.

Dissociative electron capture accompanied by target ionization (DECI) was studied for collisions of 20 keV H₂⁺ with Ar. A three-dimensional fragment imaging technique in combination with a recoil-ion time-of-flight method was used to measure the kinetic-energy release and the dissociation angle of the H–H fragment pair arising from the DECI process. We observed that the distribution of the dissociation angle is strongly anisotropic. A three-body simulation suggests that collisions between the individual fragments and the target significantly influence the two-body dissociation of the H–H pair at impact parameters where the DECI process occurs.

We present a new method to extract momentum distributions from time-dependent wavepacket calculations. In contrast to established Fourier transformation of the spatial wavepacket at a fixed time, the proposed 'virtual detector' method examines the time dependence of the wavepacket at a fixed position. In first applications to the ionization of model atoms and the dissociation of H₂⁺, we find a significant reduction of computing time and are able to extract reliable fragment momentum distributions by using a comparatively small spatial numerical grid for the time-dependent wavefunction.

We have performed an R-matrix with pseudo-states (RMPS) calculation of electron-impact excitation in C²⁺. Collision strengths and effective collision strengths were determined for excitation between the lowest 24 terms, including all those arising from the 2s3l and 2s4l configurations. In the RMPS calculation, 238 terms (90 spectroscopic and 148 pseudo-state) were employed in the close-coupling (CC) expansion of the target. In order to investigate the significance of coupling to the target continuum and highly excited bound states, we compare the RMPS results with those from an R-matrix calculation that incorporated all 238 terms in the configuration-interaction expansion, but only the lowest 44 spectroscopic terms in the CC expansion. We also compare our effective collision strengths with those from an earlier 12-state R-matrix calculation (Berrington et al 1989 J. Phys. B: At. Mol. Opt. Phys.22 665). The RMPS calculation was extremely large, involving (N + 1)-electron Hamiltonian matrices of dimension up to 36 085, and required the use of our recently completed suite of parallel R-matrix programs. The full set of effective collision strengths from our RMPS calculation is available at the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center web site.

Total scattering cross sections of the rare gases He, Ne and Ar have been measured, for electrons in the energy range from 4 eV to 2 keV. The measurements were performed using a linear transmission device and an electron energy analyser, with an angular resolution better than 1.1 × 10⁻³ sr and with an energy resolution far below the first excitation threshold in the three atoms. The uncertainties of the experimental results are generally smaller than 3%. With a few exceptions, the present results are in good agreement with the data of other authors. Systematic deviations were found between the present results and the data available for the total scattering cross sections of He at high electron energies. The deviations increase with increasing energy and reach about 13% at 2 keV. It appears that different levels of the separation of forward-scattered electrons are responsible for these systematic deviations.

The contributions of order e⁴ to the T matrix for electron impact double ionization of atomic helium have been evaluated. The resulting fivefold differential cross sections are characterized by the interference of the amplitudes associated with the different combinations of binary interactions of the active electrons. They show strong artefacts due to neglect of high-order post-collisional interactions, but also remarkable agreement with current experimental data if one of the atomic electrons is ejected into the forward half-plane. The results allow a discussion of the validity of the 'reaction mechanism' and 'photoionization limit' approaches to the interpretation of experimental (e, 3e) data.

We apply an accurate analytical ansatz to investigate the systematics of allowed terms and the distribution of one-electron angular momenta for highly correlated triply excited states in lithium or lithium-like ions. For low principal quantum numbers the overall symmetry characterized by the total spin, angular momentum and parity of the three electrons plays an important role for the distribution over l quantum numbers. For higher principal quantum numbers, the spatial repulsion dominates over symmetry constraints, and the distribution may be compared with a polarized single-electron description.

The fragmentation dynamics of the SF₆ molecule following the excitations of S 2p electrons into unoccupied molecular orbitals has been studied using the energy-resolved electron–ion coincidence technique. Fragmentation patterns were found to depend on the particular excitation and on the electronic state of the molecular ion. The spectator resonant Auger decay at the 2p → 6a_1g resonance induces changes in the ion distributions as compared to direct photoionization. Furthermore, coincidence spectra related to the same Auger structure display different ion abundances at the 2t_2g and 4e_g shape resonances. Differences were also found in the Auger decay spectra. These findings give further support for the previously suggested many-electron character of the 4e_g shape resonance.

It is suggested that the R-matrix cross-section for Ps–Xe scattering (Blackwood J E, McAlinden M T and Walters H R J 2002 J. Phys. B: At. Mol. Opt. Phys.35 2661) is incorrect.

A numerical inaccuracy in the static-exchange calculation of Blackwood et al (2002 J. Phys. B: At. Mol. Opt. Phys.35 2661) for Ps(1s)–Xe scattering is corrected. The corrected result now conforms to the same pattern as He, Ne, Ar and Kr.

Due to a copy-and-paste error in manuscript preparation, figure 9 of [1] appeared in print with an incorrect figure caption. Specifically, all data shown in this figure correspond to an electron scattering angle of 30° and a photon energy of 80 keV. This implies that our rIPA results can be compared with the data in figure 3 of [2] (as suggested by a footnote in this paper). The conclusions of [1] remain unchanged.

We would like to thank E H Haug for bringing this mistake to our attention.

References

[1] Keller S and Dreizler R M 1997 J. Phys. B: At. Mol. Opt. Phys.30 3257 [2] Tseng H K 2002 J. Phys. B: At. Mol. Opt. Phys.35 1129

The J. Phys. B publishing team would like to apologize to the authors of the above paper. In this paper, references [42] and [43] were printed incorrectly. The correct references are:

[42] Bordenave-Montesquieu A, Gleizes A and Benoit-Cattin P 1982 Phys. Rev. A 25 245-67 [43] Bordenave-Montesquieu A et al 1987 J. Phys. B: At. Mol. Phys.20 L695-703.