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We present a theoretical study of the polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) emitted from the 1s orbital of oxygen atoms of dissociating dicationic carbon monoxide CO2+. Due to the polarization average, the contribution of the direct wave of the photoelectron, which represents the largest contribution to the MFPADs, is removed, so that the PA-MFPADs clearly show the details of the scattering image of the photoelectron. As a result, it is necessary to employ an accurate theory for the theoretical analysis of the continuum state. In this study, we apply a full-potential multiple scattering theory, where the space is partitioned into Voronoi polyhedra and truncated spheres, to take into account the electron charge density outside the physical atomic spheres. We do not use the spherical harmonic expansion of the cell shape functions to avoid divergence problems. The potentials in the scattering cells are computed using the multiconfigurational second-order perturbation theory restricted active space method to take into account the influence of the core hole in the electron charge density in the final state, so that a realistic relaxation can be achieved. We show that the full-potential treatment plays an important role in the PA-MFPADs at a photoelectron kinetic energy of 100 eV. In contrast, the PA-MFPADs are not sensitive to any type of major excited states in the Auger final state. We also study the dynamics of the CO2+ dissociation. We find that the PA-MFPADs dramatically change their shape as a function of the C–O bond length.

International audience; Ballistic Electron Emission Microscope (BEEM) is a microscope to investigate Schottky barrier based on Scanning Tunneling Microscope (STM) setup. The theoretical scheme widely used for STM is mostly focusing on an electric current from the tip tunneling through the vacuum to the sample surface. However, this model is not applicable for BEEM, since in the BEEM case, electrons tunneling through the vacuum are transported in the material over a very long range. We propose a theoretical model based on the real space full potential multiple scattering theory in order to describe this transport phenomena within the one electron picture. It is analogous to the theoretical model of angle resolved photoemission, except that the electron is emitted from the tip. This framework describes the tunneling effect and the multiple scattering in the tip and the sample and between them. Moreover this theory can be applied for non-Hermitian Hamiltonian, so that the loss of electrons at the Schottky barrier can be mimicked by introducing an imaginary part in the optical potential. © 2018, Springer International Publishing AG, part of Springer Nature.

We propose a comparative and critical assessment of multiple scattering expansions. The so-called multiple scattering series expansion is much used in the description of spectroscopies at higher energies. However, it is plagued with convergence problems when operated at lower energies. We compare this method to related methods that can be found in the literature, relying both on finite and infinite expansions. After discussing the pros and cons of these methods, we establish a simple alternative to multiple scattering series expansion which has a wider and faster range of convergence. [DOI: 10.1380/ejssnt.2012.599]

We derive here a general, basis-independent, multiple scattering theory in a partitioned space. For this, we decompose a given set of atoms into non-intersecting subsets and construct the multiple scattering theory associated with these objects. We show that all the actual theories of electron spectroscopies are particular cases of our theory but that many other choices can be made to give a more adequate description of these spectroscopies. In that way, this theory is highly customizable to the problem under study.

Some important phonon effects observed in X-ray absorption and X-ray photoemission spectra are discussed on the basis of nonequilibrium Green's function theory. This theoretical framework allows us to incorporate phonon effects, such as Debye–Waller (DW) factors, Franck–Condon (FC) factors and electron–phonon interactions in a natural way. In the case of core level excitations, we can take into account the core–hole effects in lesser Green's function g and photoelectron propagation in greater Green's function g > . For the core–hole propagation we derive some formulas to describe the thermally displaced core functions: we have p components even for deep core s orbital due to the thermal motion. We should notice that the thermal fluctuation is quite small but it is already in the order of the spread of the core functions. Applying Mermin's theorem, we can calculate the thermal average of the hole propagator g : here an important ingredient is the Debye–Waller factor used in X-ray and neutron diffraction. For the pre-edge structures, the intensity associated with forbidden electric dipole transition is sensitive to the temperature compared with allowed electric quadrupole transition. We also discuss the FC and their interference, which have negligible contribution to pre-edge intensity and energy shift. The quasi-particle energy is also influenced by the core displacement which can be responsible for the peak shift of the pre-edges. We also discuss the changes of the photoelectron angular distributions caused by the thermal atomic vibration.

We propose here a systematic and straightforward way to include point-group symmetries into the theory of photoelectron diffraction. In addition to reducing the number of scattering path operators that have to be calculated, this approach permits as well as to derive a new algorithm to minimize the number of paths that have to be explored in the series expansion cluster method by making use of path degeneracies.

This paper presents a general theory of electron energy loss spectroscopy based on a recently developed basis-independent multiple-scattering formalism. This approach allows to establish the cross-section in a physically transparent way. An application to the particular case of spherical waves in a real space representation is made to show the flexibility of the method.

In some cases, high energy photoelectron diffraction peaks corresponding to well defined crystallographic directions show a splitting that cannot be explained by the traditional interpretation. Such an effect occurs in the O 1s 0° peak in MgO(001). Using cluster calculations, we demonstrate that this effect is induced by close neighbours via an inter-chain process and explain why it does not occur for the Mg 2s peak.

International audience; Methodological advances in multiple scattering theory (MST) in both wave and Green's function versions are reported for the calculation of electronic ground and excited state properties of condensed matter systems with an emphasis on core-level photoemission and absorption spectra. Full-potential MST is reviewed and extended to non-local potentials. Multichannel MST is reformulated in terms of the multichannel density matrix whereby strong electron correlation of atomic multiplet type can be accounted for in both ground and excited states.

Angle-resolved X-ray photoelectron spectroscopy (XPS) has been used to characterise chemically etched GaAs(001) versus epitaxially grown surfaces. The measured amplitude of X-ray photoelectron diffraction (XPD) modulation is then an excellent probe of the local structural environment around the emitter atom in the near-surface substrate. The observed modulations are here compared to the results of single- and double-scattering cluster calculations with a fine agreement. Moreover, because of our choice of the emission plane and taking into account the similarity in the scattering factors of Ga and As atoms, XPD effects can be almost eliminated from the angular variation of the fractional peak ratio IAs(IAs + IGa) which is then easily related to the arsenic concentration profile CAs(z). A best-fitting procedure is used to select the experimental concentration depth profile which characterises the surface under study. Samples exposed to different technological treatments, like ion implantation, chemical etching and annealing have been studied and compared. The concentrations of As atoms at the surface for As-capped, MBE-grown (001) samples after annealing around 350°C and for chemically etched GaAs(001) wafers indicate the same type of surface (As-terminated) as far as composition and structure are concerned.

We have studied the growth and magnetic properties of thin Fe-Ge films synthesized (codeposited at room temperature and postannealed at $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$) on Ge(111) wafers versus stoichiometry. Morphology and crystal structure have been investigated in situ by means of scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron diffraction and ex situ with x-ray diffraction. The magnetic properties were characterized ex situ by conventional polar and longitudinal magneto-optical Kerr effect and transverse biased initial inverse susceptibility and torque measurements. It is found that the growth is epitaxial for Ge content up to $\ensuremath{\sim}48\text{ }\text{at}\text{.}\text{ }\mathrm{%}$ ($\ensuremath{\sim}{\text{Fe}}_{1.1}\text{Ge}$ composition). In particular, the film is homogeneous and flat and adopts a crystalline structure of hexagonal symmetry derived from the $\text{B}{8}_{2}$ $({\text{Ni}}_{2}\text{In})$ structure over a wide stoichiometry range extending from ${\text{Fe}}_{2}\text{Ge}$ to ${\text{Fe}}_{1.1}\text{Ge}$. The epitaxial orientation between the Ge substrate and the germanide layer is $(0001)\text{Fe-Ge}\ensuremath{\parallel}(111)\text{Ge}$ with $[11\overline{2}0]\text{Fe-Ge}\ensuremath{\parallel}[\overline{1}10]\text{Ge}$. We found however that the surface periodicity and the out-of-plane lattice parameter $c$ evolve within this stoichiometry range and two distinct stoichiometry regimes appear on both sides of a critical stoichiometry $(\ensuremath{\sim}{\text{Fe}}_{1.5}\text{Ge})$. Indeed, from ${\text{Fe}}_{2}\text{Ge}$ to ${\text{Fe}}_{1.5}\text{Ge}$ the surface periodicity is $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $c$ continuously decreases with Fe content, whereas from ${\text{Fe}}_{1.5}\text{Ge}$ to ${\text{Fe}}_{1.1}\text{Ge}$ the surface periodicity is $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ and $c$ remains constant. These features have been interpreted as a clear fingerprint of a minor transformation of the crystalline structure but without any change in symmetry. This structural order transformation is discussed in relation to previous results reported in the case of macroscopic single-crystal Fe-Ge ingots. On both sides of the wide $[{\text{Fe}}_{2}\text{Ge},{\text{Fe}}_{1.1}\text{Ge}]$ composition range the layer is no more homogeneous. More precisely, for higher Fe content the film contains both the above mentioned ${\text{Ni}}_{2}\text{In}$-derived phase and a Fe-richer phase (probably bcc Fe) whereas for higher Ge content the layer is amorphous. Magnetic characterization showed in particular that the homogeneous ${\text{Ni}}_{2}\text{In}$-derived epilayers are ferromagnetic with a Curie temperature that varies drastically with the stoichiometry, rising up to a high ${T}_{\text{C}}$ value of $\ensuremath{\sim}450\text{ }\text{K}$ for the Fe-rich ${\text{Fe}}_{1.9}\text{Ge}$ composition. Finally, whatever the stoichiometry, the magnetic easy axis of the homogeneous phase lies in the film plane and a small uniaxial anisotropy is superimposed on a sixfold order one that results from the hexagonal symmetry of the crystallographic structure.

International audience; We introduce a powerful alternative expansion method to perform multiple scattering calculations. In contrast to standard MS series expansion, where the scattering contributions are grouped in terms of scattering order and may diverge in the low energy region, this expansion, called correlation expansion, partitions the scattering process into contributions from different small atom groups and converges at all energies. It converges faster than MS series expansion when the latter is convergent. Furthermore, it takes less memory than the full MS method so it can be used in the near edge region without any divergence problem, even for large clusters. The correlation expansion framework we derive here is very general and can serve to calculate all the elements of the scattering path operator matrix. Photoelectron diffraction calculations in a cluster containing 23 atoms are presented to test the method and compare it to full MS and standard MS series expansion.

International audience; We have developed a generalization of the multiple-scattering formalism to deal with Auger-photoelectron coincidence spectroscopy APECS in the solid state. We have merged the exact atomic treatment of the angular correlations between the two electrons and the single-particle approach, on which the multiplescattering description of condensed matter relies. This allows the recovering, even in extended systems, of the entangled form of the electron-pair wave function characterizing the coincidence angular diffraction pattern. In the atomic limit our formalism correctly reproduces the cross section, as calculated within the statistical-tensors approach, usually employed in atomic physics. We have then performed numerical calculations for the Ge100 L3M45M45 APECS and compared the results with previous experiments. We found that, in the given geometry, the diffraction patterns in coincidence with different directions of the photoelectron keep little memory of the atomic anisotropy. We speculate on the conditions to be fulfilled in order to enhance the atomic-orbital sensitivity in APECS through solid-state diffraction effects.

International audience; We present a unitary cluster approach to the calculation of several electron and photon spectroscopies, ranging from core and valence level photoelectron diffraction and absorption to electron, Auger and anomalous diffraction. Electron energy loss and Auger-photoelectron coincidence spectroscopies can also be treated in the same frame. This approach is based on multiple-scattering theory with a complex optical potential of the Hedin-Lundqvist type and is valid for all electron kinetic energies. Similarities and differences between these diffraction techniques are examined and cluster size convergence is discussed in connection with the electron mean free path. Applications to selected problems are presented to illustrate the method, both for structural and electronic analysis.

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect., 13 pages, 8 figures

International audience; Black phosphorus (BPh) is a layered material with strong in-plane anisotropy of its structural and electronic properties; in spite of the great potential of BPh for conceptually new devices in optoelectronics and plasmonics, its fundamental electronic excitations have not yet been fully elucidated. In order to discriminate collective (plasmons) and single-particle (inter band transitions) excitations, we investigate the energy-loss distribution of P 1s photoelectrons in hard X-ray photoelectron spectra of BPh over a wide energy range. The energy-loss function (ELF), averaged over the principal directions of the BPh crystal, has been retrieved by using a Fourier Transform analysis to eliminate multiple inelastic scattering events. At low loss energies (1-8 eV), weak unresolved energy loss peaks are well described by DFT calculated inter band transitions, showing some anisotropy in the dielectric function ε(ω,q) tensor of BPh. At high loss energies, the ELF is dominated by the collective excitation of valence electrons with a peak energy at 20.1±0.2 eV, and weak anisotropy is found in the DFT calculated Im(-1/ε) tensor. The anomalously small peak energy (9.0±0.5 eV) of a weak surface plasmon resonance is attributed either to low surface electron density in the terminal phosphorene layer or to some anisotropic surface plasmon propagation.