Triple coincidence (e,gamma2e) experiment for simultaneous electron impact ionization excitation of helium

Taking the Convergent Close-Coupling Method beyond Helium: The Utility of the Hartree-Fock Theory

The convergent close-coupling (CCC) method was initially developed to describe electron scattering on atomic hydrogen and the hydrogenic ions such as He+. The latter allows implementation of double photoionization (DPI) of the helium atom. For more complex single valence-electron atomic and ionic targets, the direct and exchange interaction with the inner electron core needs to be taken into account. For this purpose, the Hartree-Fock (HF) computer codes developed in the group of Miron Amusia have been adapted. In this brief review article, we demonstrate the utility of the HF technique by examples of electron scattering on Li and the DPI of the H− and Li− ions. We also discuss that modern-day computer infrastructure allows the associated CCC code, and others, to be readily run directly via the Atomic, Molecular and Optical Science Gateway.

Ion-lithium collision dynamics studied with a laser-cooled in-ring target

We present a novel experimental tool allowing for kinematically complete studies of break-up processes of laser-cooled atoms. This apparatus, the 'MOTReMi,' is a combination of a magneto-optical trap (MOT) and a reaction microscope (ReMi). Operated in an ion-storage ring, the new setup enables us to study the dynamics in swift ion-atom collisions on an unprecedented level of precision and detail. In the inaugural experiment on collisions with 1.5  MeV/amu O(8+)-Li the pure ionization of the valence electron as well as the ionization-excitation of the lithium target was investigated.

Nonperturbative treatment of ionization with excitation of helium by electron impact

We present cross sections for electron-impact ionization and simultaneous ionization plus excitation of helium by electron impact. The results are obtained from a fully nonperturbative close-coupling formalism using our B-spline R-matrix approach. A large number of pseudostates in the expansion of the wave function represent the coupling to the ionization continuum. We obtain excellent agreement with the directly measured experimental cross section ratios (Bellm et al., Phys. Rev. A 75, 042704 (2007)) for ionization leaving the residual He⁺ ion in either the 1s ground state or the n = 2 (2s + 2p) excited states.

Out-of-plane (e, 2e) experiments on helium L=0, 1, 2 autoionizing levels

Out-of-plane (e, 2e) measurements are reported for the helium autoionizing levels (2s2)1S, (2p2)1D, (2s2p)1P, and for direct ionization. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L=0, 1, 2. A second-order model in the projectile-target interaction correctly reproduces the observed magnitudes of the recoil peaks, but is a factor of 2 too large in the central out-of-plane region.

Electron-impact ionization and excitation of helium to the n=1-4 ionic states

We present high-precision (e,2e) measurements and calculations for the e-He four-body Coulomb breakup problem. Cross-section ratios for ionization and excitation of the first three excited states of He+ relative to the ground state have been measured for incident energies between 112 and 319 eV. Comparing the data with predictions from a state-of-the-art hybrid distorted-wave+convergent R matrix with pseudostates (close coupling) approach shows that treating the projectile-target interaction at least to second order is crucial to obtain reasonable agreement between theory and experiment. Nevertheless, our benchmark studies reveal significant theoretical problems for the symmetric energy-sharing cases, thus indicating the need for further improvement.