Ejection of quasi-free-electron pairs from the helium-atom ground state by single-photon absorption.

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.

Evidence for a T-shape break-up pattern in the triple photoionization of Li.

We examine the angular distributions of all three electrons ionized from Li by a single photon near the triple ionization threshold using a fully quantum-mechanical treatment. We find strong evidence for a T-shape break-up pattern at a 5 eV excess energy as previously predicted by quasiclassical simulations [A. Emmanouilidou and J.

Strong molecular alignment dependence of H2 electron impact ionization dynamics.

Low-energy (E(0) = 54 eV) electron impact single ionization of molecular hydrogen (H(2)) has been investigated as a function of molecular alignment in order to benchmark recent theoretical predictions [Colgan et al., Phys. Rev.

Angular distributions for the complete photofragmentation of the Li atom.

We explore the complete breakup of the Li atom after absorption of a single photon, the purest example of the so-called four-body Coulomb problem. The resulting strongly correlated three-electron continuum is investigated by calculating the angular distributions of the ionized electrons using advanced close-coupling techniques. We find that the distributions are dominated by the Coulomb interactions between the electrons, that multiple break-up processes can be identified, and that the complex dynamics of the fragmentation process are evident for most scattering geometries.

Contribution of near threshold states to recombination in plasmas.

Resonance states in atoms or ions at low energies can control the rates of important plasma processes (e.g., dielectronic recombination).

Controlling two-electron threshold dynamics in double photoionization of lithium by initial-state preparation.

Double photoionization (DPI) and ionization-excitation (IE) of Li(2s) and Li(2p), state-prepared and aligned in a magneto-optical trap, were explored in a reaction microscope at the free-electron laser in Hamburg (FLASH). From 6 to 12 eV above threshold (homega = 85, 91 eV), total as well as differential DPI cross sections were observed to critically depend on the initial state and, in particular, on the alignment of the 2p orbital with respect to the VUV-light polarization, whereas no effect is seen for IE. The alignment sensitivity is traced back to dynamical electron correlation at threshold.

Differential cross sections for the ionization of oriented H2 molecules by electron impact.

A nonperturbative close-coupling technique is used to calculate differential cross sections for the electron-impact ionization of H2 at an energy of 35.4 eV. Our approach allows cross sections for any orientation of the molecule with respect to the incident electron beam to be analyzed.

Physical interpretation of the "kinetic energy release" effect in the double photoionization of H(2).

A physical interpretation is given for the variation with internuclear separation of the fully differential cross section for double photoionization of H2. This effect is analyzed in a geometry where the fourbody interaction is completely probed. Excellent agreement is found between experiment and time-dependent close-coupling theory after convoluting the latter over the relevant solid angles.

Fully correlated electronic dynamics for antiproton impact ionization of helium.

We present total cross sections for single and double ionization of helium by antiproton impact over a wide range of impact energies from 10 keV/amu to 1 MeV/amu. A nonperturbative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the ionized electrons. Excellent agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections at high impact energies, whereas for lower impact energies, some discrepancies with experiment are found.

Triple differential cross sections for the double photoionization of H2.

Triple differential cross sections arising from the break up of the H2 molecule by a single photon are presented. The time-dependent close-coupling technique is used to calculate differential cross sections for various geometries. Excellent agreement is found between current work and recent exterior complex-scaling calculations, confirming, for the first time, the absolute magnitude of the triple differential cross sections.

Lattice calculations of the photoionization of Li.

Calculations are presented for the double photoionization (with excitation) and triple photoionization of the Li atom. The motion of all three electrons is treated equally by solving the time-dependent Schrödinger equation in nine dimensions. A radial lattice is used to represent three of the nine dimensions, while a coupled channel expansion is used to represent the other six dimensions.

Collisional-radiative study of lithium plasmas.

The sensitivity of lithium plasma models to the underlying atomic data is investigated. Collisional-radiative modeling is carried out with both the Los Alamos and ADAS suite of codes. The effects of plane-wave Born, distorted-wave, and nonperturbative R -matrix with pseudostates and time-dependent close-coupling electron impact atomic data on derived plasma quantities such as the ionization balance and radiated power are studied.

Observation of trielectronic recombination in Be-like Cl ions.

Recombination involving the core excitation of two electrons, which may be termed trielectronic recombination, has been experimentally identified for the first time. Using Cl13+ ions circulating in the TSR heavy-ion storage ring, we have observed surprisingly strong low-energy trielectronic recombination resonances, comparable to the dielectronic process. At higher electron-ion collision energies, trielectronic recombination is suppressed due to the autoionization of the triply excited intermediate state into excited final states.

Time-dependent close-coupling calculations of dielectronic capture in He.

We report on the first time-dependent close-coupling calculation of dielectronic capture into a doubly excited state of a two-electron atom. An incoming electron is represented by a Gaussian wave packet which collides with singly ionized helium in its ground state. The close-coupling equations describe the propagation of the total compound wave function on a two-dimensional radial lattice.

Core-excited resonance enhancement in the two-photon complete fragmentation of helium.

A time-dependent close-coupling method is used to calculate, for the first time, fully differential cross sections for the complete fragmentation of helium by two photons. Surprising differences in the magnitude of the total-integral cross sections are found in comparisons with other calculations. These differences are found to be due to a core-excited resonance enhancement of the two-photon process for both single and double ionization.

Benchmark nonperturbative calculations for the electron-impact ionization of Li(2s) and Li(2p).

Three independent nonperturbative calculations are reported for the electron-impact ionization of both the ground and first excited states of the neutral lithium atom. The time-dependent close-coupling, the R matrix with pseudostates, and the converged close-coupling methods yield total integral cross sections that are in very good agreement with each other, while perturbative distorted-wave calculations yield cross sections that are substantially higher. These nonperturbative calculations provide a benchmark for the continued development of electron-atom experimental methods designed to measure both ground and excited state ionization.

Fine structure continuum cross section ratios in the two-photon ionization of rubidium using elliptically polarized light.

Using fully relativistic perturbation theory we report fine structure continuum cross section ratios for the two-photon ionization of rubidium under elliptically polarized light. The choice of light polarization and wavelength matches the recent complete experiments on rubidium reported by Wang and Elliott [Phys. Rev.

Deep-core dielectronic-capture resonances in the electron-impact ionization of heavy atomic ions.

Experimental measurements and theoretical calculations are carried out for the electron-impact ionization of Sm(12+). The low energy region of the single ionization cross section for Sm(12+) is found to be dominated by contributions from the indirect process of excitation autoionization. At about 1.