J. H. McGuire

Cross-section ratio of double to single ionization of helium by Compton scattering of 40-100-keV x rays

We have measured the ratio of cross sections for double to single ionization of helium by Compton scattering, R{sub C}={sigma}{sub C}{sup ++}/{sigma}{sub C}{sup +}, at photon energies of 40, 80, and 100 keV using cold target recoil-ion momentum spectroscopy. Comparison with calculations involving highly correlated initial states and approximate final states with and without final-state correlations, represented by 3C and 2C wave functions respectively, shows that the influence of final-state correlations persists to very high photon energies. A comparison with recent charged-particle data is made. {copyright} {ital 1999} {ital The American Physical Society}

Interaction of atomic systems with X-ray free-electron lasers

The planned construction of an X-ray free-electron laser (XFEL) will provide new opportunities for research in various areas of physics, chemistry and biology. The proposed design of the XFELs at DESY (Deutsches Elektronen-Synchrotron) and SLAC (Stanford Linear Accelerator Center) is built on the concept of a fourth-generation synchrotron source and will provide an intense pulse (I(0) approximately 10(16) W cm(-2), tau(gamma) approximately 100 fs) for photon wavelengths down to 1 A. Some guidelines for applications of these sources pertaining to atomic physics are presented here. Issues such as the onset of strong photon-field effects, multiple ionization and hollow-atom formation are analyzed. Attention is especially given to studying the interaction with rare-gas atoms, for which some numerical estimates are provided.

Excitation of autoionizing states of helium by proton and antiproton impact

A method for evaluating the excitation by particles of charge of autoionizing states of helium is presented which includes interference with degenerate single ionization continuum states both before and after excitation. The decay into the final continuum state may be influenced by Coulomb interaction with the projectile in the final state. These effects influence the terms which may be observed experimentally. Calculations using this method are presented and compared with other calculations and experimental data.

Total cross sections for transfer ionization in fast ion-helium collisions

The effects of electron correlation and second-order terms on theoretical total cross sections of transfer ionization in collisions of the helium atom with fast H + ,H e 2+ and Li 3+ ions are studied and reported. The total cross sections are calculated using highly correlated wavefunctions with expansion of the transition amplitude in the Born series through the second order. The results of these calculations are in sensible agreement with experimental data.

Differential cross sections in antiproton and proton-helium collisions

Cross sections are presented for the first time for antiproton-helium collisions at an energy of 945 keV differential in longitudinal electron and recoil-ion momenta. The longitudinal momentum distributions for antiproton impact are compared with 1 MeV proton-helium collision. The electron and the recoil-ion momentum distributions for antiprotons agree with those for protons to within 10%. We did not observe a difference between antiproton impact and proton impact. A comparison with CDW and CTMC theories is presented.

Projectile electron excitation and loss in ion‐atom collisions

The contribution from the nucleus‐electron and the electron‐electron interactions to the electron loss process by ions passing through neutral targets is reviewed with particular emphasis on recent theoretical concepts and experimental techniques which have been developed in this field. One important conclusion of these studies, namely the connection between the electron‐loss process and electron‐ion ionization phenomena, is also discussed.

Time Ordering Dynamics in Ionization‐Excitation of Helium to HeII (2p) Magnetic Sublevels Using Extreme Ultraviolet Spectropolarimetry

Time ordering of interactions in dynamic quantum multi‐electron systems provides a constraint that connects the time evolution of different electrons in ion‐atom collisions. When spatial correlation between electrons is combined with time ordering, the time evolution operators for different electrons become correlated. We report in this study evidence for quantum time correlation between electrons by comparing full second‐Born calculations, which include both spatial and temporal electron correlation, to recent magnetic sublevel cross section measurements corresponding to simultaneous ionization‐excitation of He (1s2) 1So to He+ (2p) 2Po levels by proton impact. The experimental substates cross sections σ0 and σ1 for ML =0, ±1 are determined by combining corresponding total extreme ultraviolet (EUV) cross sections with our recent polarization fraction measurements. Such results have important applications for the understanding of quantum phenomena like time entanglement.

How do electrons communicate about time

We show that time correlation between electrons requires that the Dyson time ordering operator, T, differs from its uncorrelated value and spatial electron-electron correlation be present. In this paper we decompose T into an uncorrelated term, Tunc, plus a correlated term, Tcor=T∼Tunc, which leads to time correlation in time dependent external interactions. Effects of time correlation between electrons can be observed. Two examples are presented. In transfer ionization the time correlation operator incoherently changes the shape of an electron-electron Thomas peak. In double excitation the influence of Tcor in amplitudes for coherently interfering pathways changes resonance intensities and profiles.

Magnetic sublevel population in 1s-2p excitation of helium by fast electrons and protons

We report experimental and theoretical results for the magnetic sublevel population of the helium atom in collisions with fast (vi = 3-9 au) electrons and protons. Cross sections for excitation of magnetic sublevels with M = 0 and ±1 have been obtained using polarization measurements of emitted radiation in combination with differential cross sections. Calculations have been carried out using the expansion of the transition amplitude in the Born series over the projectile-target interaction through the second order. Results of calculations are in agreement with experimental data. We find that the particle-antiparticle Z± difference exceeds the statistical error of measurement up to collision velocities vi≈6 au for excitation of sublevels with M = 0.

Ionization-excitation of H- and He by Compton scattering

We report calculations for exclusive ionization-excitation Compton cross sections, both total and differential in energy transfer, for n = 3-5 of the negative hydrogen ion and the helium atom. We find that, unlike the cross section for n = 1 and 2, the exclusive cross sections for n≥3 are not very sensitive to atomic parameters such as geometric size and electron correlation. We discuss differences and similarities of the cross sections when different initial-state wavefunctions are used, and compare with the Compton cross section for collision of a photon with a free electron. Our calculations have been carried out for photon energies from 6 to 60 keV.