Anatoli Kheifets

Complete photo-fragmentation of the deuterium molecule

All properties of molecules—from binding and excitation energies to their geometry—are determined by the highly correlated initial-state wavefunction of the electrons and nuclei. Details of these correlations can be revealed by studying the break-up of these systems into their constituents. The fragmentation might be initiated by the absorption of a single photon, by collision with a charged particle or by exposure to a strong laser pulse: if the interaction causing the excitation is sufficiently understood, the fragmentation process can then be used as a tool to investigate the bound initial state. The interaction and resulting fragment motions therefore pose formidable challenges to quantum theory. Here we report the coincident measurement of the momenta of both nuclei and both electrons from the single-photon-induced fragmentation of the deuterium molecule. The results reveal that the correlated motion of the electrons is strongly dependent on the inter-nuclear separation in the molecular ground state at the instant of photon absorption.

Electronic Band Structure of Beryllium Oxide

The energy–momentum resolved valence band structure of beryllium oxide has been measured by electron momentum spectroscopy (EMS). Band dispersions, bandwidths and intervalence bandgap, electron momentum density (EMD) and density of occupied states have been extracted from the EMS data. The experimental results are compared with band structure calculations performed within the full potential linear muffin-tin orbital approximation. Our experimental bandwidths of 2.1 ± 0.2 and 4.8 ± 0.3 eV for the oxygen s and p bands, respectively, are in accord with theoretical predictions, as is the s-band EMD after background subtraction. Contrary to the calculations, however, the measured p-band EMD shows large intensity at the Γ point. The measured full valence bandwidth of 19.4 ± 0.3 eV is at least 1.4 eV larger than the theory. The experiment also finds a significantly higher value for the p-to-s-band EMD ratio in a broad momentum range compared to the theory.

Time delay in valence-shell photoionization of noble-gas atoms

We use the nonrelativistic random-phase approximation with exchange to perform calculations of valence-shell photoionization of Ne, Ar, Kr, and Xe from their respective thresholds to photon energy of 200 eV. The energy derivative of the complex phase of the photoionization matrix elements is converted to the photoelectron group delay that can be measured in attosecond streaking or two-photon sideband interference experiments. Comparison with reported time-delay measurements in Ne and Ar at a few selected photon energies is made. Systematic mapping of time delay across a wide range of photon energies in several atomic targets allows to highlight important aspects of fundamental atomic physics that can be probed by attosecond-time-delay measurements.

Electronic band structure of magnesium and magnesium oxide: experiment and theory

Electron momentum spectroscopy (EMS) has been used to measure the valence band electronic structure of thin magnesium and magnesium oxide films. The band structures have also been calculated within the linear muffin-tin orbital (LMTO) approximation. The free-electron-like parabola characteristic of metallic solids was observed for magnesium with a bandwidth of approximately 6 eV, in agreement with previous measurements. The inclusion of energy broadening due to finite hole-lifetime effects and a Monte Carlo simulation of multiple scattering events gives good agreement between calculated and measured band structures. However, we measure a much higher intensity due to plasmon excitation compared with the simulated intensity. Upon oxidation the valence structure splits into two distinct, less dispersive bands typical of an ionic solid. Intensity due to plasmon excitation was almost completely absent in the experimental spectra for magnesium oxide. The LMTO calculation reproduces the overall structure and dispersion range of the oxide. The measured and calculated energy gap between upper and lower valence bands and their relative intensities do not agree quantitatively. This discrepancy may be due to a contribution of magnesium s states to the predominantly oxygen p states in the upper band.

A comparative experimental and theoretical investigation of the electron-impact double ionization of He in the keV regime

We determine, both experimentally and theoretically, the fully resolved fivefold differential cross section (5DCS) of double ionization of helium by 5.6 keV electron impact. Symmetric energy sharing between the two ejected electrons is investigated at the excess energy of 8 and 20 eV with 0.22 and 0.24 au momentum transfer, respectively. Absolute 5DCS are determined by normalizing the experimental data to the well established single-ionization cross sections. The calculation is performed by using the convergent close-coupling method for the interaction between the two slow ejected electrons, together with the first Born approximation for the interaction of the fast incident electron with the atom. Experimental and theoretical 5DCS tend to agree in shape but disagree in magnitude by factors of three and 14 for the 20 and 8 eV excess energies, respectively. The small momentum transfer invites absolute comparison of the present electron-impact double-ionization results with the corresponding double-photoionization experiment and theory. Theoretically, the momentum transfer is sufficiently close to zero to show the scaling between the two scattering processes. This smallness of the momentum transfer also makes the calculated 5DCS nearly invariant with respect to simultaneous inversion of the momenta of the two ejected electrons.

Measurements of relative photoemission time delays in noble gas atoms

We determine relative photoemission time delays between valence electrons in different noble gas atoms (Ar, Ne and He) in an energy range between 31 and 37 eV. The atoms are ionized by an attosecond pulse train synchronized with an infrared laser field and the delays are measured using an interferometric technique. We compare our results with calculations using the random phase approximation with exchange and multi-configurational Hartree-Fock. We also investigate the influence of the different ionization angular channels.

Absolute triple differential cross sections for photo-double ionization of helium - experiment and theory

We have measured absolute triple differential cross sections for photo-double ionization of helium at 20 eV excess. The measurement covers the full ranges of energy sharing and emission angles of the two photoelectrons. We compare our data for selected geometries with the convergent close-coupling (CCC) calculations as well as 2SC calculations by Pont and Shakeshaft and 3C calculations by Maulbetsch and Briggs. In terms of the absolute magnitude and the trend in the shapes of the triple differential cross section for different geometries we find good agreement of the CCC and published 2SC calculations with our measurement, though differences with respect to the observed shape of individual patterns still exist.

Application of the CCC method to the calculation of helium double-photoionization triply differential cross sections

The convergent close-coupling (CCC) formalism is employed to calculate the fully resolved triply differential cross section (TDCS) for helium double photoionization. This is the first ab initio calculation of the kind where all three gauges of the electromagnetic interaction produce TDCS within a few per cent of each other. Comparison is made with the experimental data in the range of photon energies 10 - 53 eV above the threshold, both for equal and unequal energy sharing. The present calculations agree very well, both in shape and where available in magnitude, with the experimental data obtained at the BESSY storage ring (Berlin, Germany) (Schmidt et al 1996 J. Electron Spectrosc. Relat. Phenom. 79 279 and references therein). In addition, excellent agreement is found with the calculations of Pont and Shakeshaft (1995 Phys. Rev. A 51 R2676).

DWBA-G calculations of electron impact ionization of noble gas atoms

We perform calculations of electron impact ionization of noble gas atoms within the distorted wave Born approximation (DWBA) corrected by the Gamow factor (G) to account for the post-collision interaction. We make an extensive comparison with experimental data on He 1s2, Ne 2s2, 2p6 and Ar 3p6 under kinematics characterized by large energy transfer and close to minimum momentum transfer from the projectile to the target. For all atoms, good agreement between theory and experiment is achieved. In the case of Ar, the disagreement of experimental data with theory reported earlier by Catoire et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 2827) is reconciled.

Determination of the energy-momentum densities of aluminium by electron momentum spectroscopy

The energy-resolved momentum densities of thin polycrystalline aluminium films have been measured using electron momentum spectroscopy (EMS), for both the valence band and the outer core levels. The spectrometer used for these measurements has energy and momentum resolutions of around 1.0 eV and 0.15 atomic units, respectively. These measurements should, in principle, describe the electronic structure of the film very quantitatively, i.e. the dispersion and the intensity can be compared directly with theoretical spectral momentum densities for both the valence band and the outer core levels. Multiple scattering is found to hamper the interpretation somewhat. The core-level intensity distribution was studied with the main purpose of setting upper bounds on these multiple-scattering effects. Using this information we wish to obtain a full understanding of the valence band spectra using different theoretical models of the spectral function. These theoretical models differ significantly and only the cumulant expansion calculation that takes the crystal lattice into account seems to describe the data reasonably well.