Electronic band structure of calcium oxide

We employed electron momentum spectroscopy (EMS) to measure the bulk electronic structure of calcium oxide. We extracted the electron momentum density (EMD), density of occupied states (DOS), band dispersions, bandwidths and intervalence bandgaps from the data. The results are compared with calculations based on the full potential linear muffin-tin orbital(FP-LMTO) approximation. While the bandwidths of 0.6±0.2 and 1.2±0.1 eV for the s- and p-bands, respectively, and their dispersions agree well with the LMTO calculation, the relative intensity of the two bands is at odds with the theory. The measured intervalence bandgap at the Γ-point of 16.5±0.2 eV is larger by 2.1 eV than that from the LMTO calculation. The experimental bandwidth of the Ca 3p semi-core level of 0.7±0.1 eV agrees with the LMTO prediction. The measured bandgap between this level and the s-band is 3.6±0.2 eV. The Ca 3s-3p level splitting is in excellent agreement with the literature. © 2004 Elsevier B.V. All rights reserved

Interpreting Attoclock Measurements of Tunnelling Times

Resolving in time the dynamics of light absorption by atoms and molecules, and the electronic rearrangement this induces, is among the most challenging goals of attosecond spectroscopy. The attoclock is an elegant approach to this problem, which encodes ionization times in the strong-field regime. However, the accurate reconstruction of these times from experimental data presents a formidable theoretical challenge. Here, we solve this problem by combining analytical theory with ab-initio numerical simulations. We apply our theory to numerical attoclock experiments on the hydrogen atom to extract ionization time delays and analyse their nature. Strong field ionization is often viewed as optical tunnelling through the barrier created by the field and the core potential. We show that, in the hydrogen atom, optical tunnelling is instantaneous. By calibrating the attoclock using the hydrogen atom, our method opens the way to identify possible delays associated with multielectron dynamics during strong-field ionization.Comment: 33 pages, 10 figures, 3 appendixe

Electronic band structure of beryllium oxide

Atomic and Molecular Physics Laboratories, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia. 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

Mechanism of the low-ejection-energy (e,2e) reaction on a graphite surface

We develop a theoretical model to describe a slow electron ejection from a crystal by electron impact at a moderate incident energy. The electron impact ionization is considered within the first Born approximation. The projectile is treated as a plane wave whereas the target electron initial and final states are described by the bulk one-electron wave functions in the momentum space representation. To allow the ionized electron to escape from the crystal the final state in the bulk of the crystal is matched in energy and a parallel component of momentum by a plane wave in the vacuum. This theoretical model is used to simulate the binding-energy spectra obtained by the grazing-angle reflection mode (e,2e) reaction on the surface of highly oriented pyrolytic graphite

Mechanism of the low-ejection-energy (e,2e) reaction on a graphite surface RID C-9131-2009 RID G-7348-2011

We develop a theoretical model to describe a slow electron ejection from a crystal by electron impact at a moderate incident energy. The electron impact ionization is considered within the first Born approximation. The projectile is treated as a plane wave whereas the target electron initial and final states are described by the bulk one-electron wave functions in the momentum space representation. To allow the ionized electron to escape from the crystal the final state in the bulk of the crystal is matched in energy and a parallel component of momentum by a plane wave in the vacuum. This theoretical model is used to simulate the binding-energy spectra obtained by the grazing-angle reflection mode (e,2e) reaction on the surface of highly oriented pyrolytic graphite

Momentum distribution and valence-band reconstruction in graphite by grazing incidence (e,2e) spectroscopy RID C-9131-2009 RID G-7348-2011

The capability of grazing angle (e,2e) experiments to map momentum distribution is demonstrated.: This is an application of reflection (e,2e) experiments as a unique twofold spectroscopy, binding energy and momentum distribution, for solid surfaces. The reported experiments have been performed on highly oriented pyrolytic graphite at about 300-eV incident energy, grazing angle, and for ejected electron energies ranging from 3.7 to 30 eV. These experiments, interpreted in the framework of the plane-wave impulse approximation, yield the binding energy vs momentum dispersion curves of valence electron states as well as the individual pi-state momentum density. Both results compare favorably with calculated graphite band structure. [S0163-1829(98)06404-2]

Direct double ionization of the Ar+ M shell by a single photon

Direct double ionization of the Ar+(3p-1) ion by a single photon is investigated both experimentally and theoretically. The photon-ion merged-beams technique was employed at the Advanced Light Source in Berkeley, USA, to measure absolute cross sections in the energy range from 60 to 150 eV. In this range, three contributions to the double ionization of Ar+ are to be expected: the removal of two 3p electrons, of a 3s and a 3p electron, and of two 3s electrons. Among the possible mechanisms leading to double ionization, the TS1 (two-step one) process dominates in the near-threshold region. In TS1, a photoelectron is ejected and, on its way out, knocks out a secondary electron. This two-step mechanism is treated theoretically by multiplying the calculated cross section for direct single photoionization of a given subshell with the calculated (e,2e) ionization probability for the ejected photoelectron to knock off a secondary electron. The calculated cross section is in very good agreement with the experiment

Direct double ionization of the Ar+ M shell by a single photon

Direct double ionization of the Ar+(3p-1) ion by a single photon is investigated both experimentally and theoretically. The photon-ion merged-beams technique was employed at the Advanced Light Source in Berkeley, USA, to measure absolute cross sections in the energy range from 60 to 150 eV. In this range, three contributions to the double ionization of Ar+ are to be expected: the removal of two 3p electrons, of a 3s and a 3p electron, and of two 3s electrons. Among the possible mechanisms leading to double ionization, the TS1 (two-step one) process dominates in the near-threshold region. In TS1, a photoelectron is ejected and, on its way out, knocks out a secondary electron. This two-step mechanism is treated theoretically by multiplying the calculated cross section for direct single photoionization of a given subshell with the calculated (e,2e) ionization probability for the ejected photoelectron to knock off a secondary electron. The calculated cross section is in very good agreement with the experiment