Search

International audience; We have derived a new Young’s formula for the polarization-averaged molecular frame photoelectron angular distributions (PA-MFPADs), as a bond length ruler for dissociating hetero-diatomic molecules. The relative errors against numerical results were within 5 %. © 2022 The Author(s).

International audience; A number of renormalization schemes for improving the convergence of multiple scattering series expansions are investigated. Numerical tests on a small Cu(111) cluster demonstrate their effectiveness, for example increasing the rate of convergence by up to a factor 2 or by transforming a divergent series into a convergent one. These techniques can greatly facilitate multiple scattering calculations, especially for spectroscopies such as photoelectron diffraction, Auger electron diffraction, low energy electron diffraction , where an electron propagates with a kinetic energy of hundreds of eV in a cluster of hundreds of atoms.

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; A brief derivation of the cross section in the independent particle approximation for some of the spectroscopies treated in the msspec program package is presented. We solve the related Schrödinger equation with a complex energy-dependent effective potential in the framework of multiple scattering theory to write the cross-section for photoemission and photoabsorption in a physically transparent way that provides insight in their interpretation and analysis. Relativistic corrections are also implemented. In order to be able to apply this theory to a wide variety of systems we use a kind of all-purpose optical potential, depending only on the local density of the system under investigation, and discuss its merits and drawbacks. A Green’s function approach is shown to be necessary to write the photoabsorption cross section in the case of complex potential. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; In this introductory chapter, we develop the scattering theory necessary to understand the theoretical models underlying the multiple scattering codes. First, the elementary theory is presented and it is then formalized to introduce the different operators whose matrix elements are computed in the codes. Then, we extend the theory to the case of a collection of potentials, i.e. multiple scattering. Finally we outline the way cross-sections can be derived from the multiple scattering framework and give some practical examples. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; We derive a general spherical wave multiple scattering expression of the EELS cross-section. Contrarily to most of the previous theoretical frameworks, this approach treats all the electrons involved on an equal footing with respect to multiple scattering. The main point in our results is that the cross-section depends not only on Coulomb integrals on the absorbing atom, but also on contributions from all the other atoms. We show that these external contributions should be restricted to neighbouring atoms. © 2018, Springer International Publishing AG, part of Springer Nature.

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.

International audience; We present a scattering theoretic approach to the calculation of the cross-section of (e,2e) impact spectroscopy where all the electrons involved are treated within the real space multiple scattering framework. This approach is particularly suited to the reflection geometry at low kinetic energies, with the ejection of a core-level electron. In this case, we expect (e,2e) spectroscopy can be turned into an extremely sensitive surface structure probe. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; X-ray Photoelectron Spectroscopy (XPS) used in quantitative chemical analysis of solid surfaces requires subtraction of a broad background, arising from various energy loss mechanisms, to obtain reliable core level peak intensities. Besides single electron excitation, collective electron oscillations (plasmons) can be excited in the bulk and at the surface. Photoelectron energy loss spectroscopy (XPS-PEELS) is a non-destructive tool useful for both process control and thin film metrology. This review emphasizes its versatility to elucidate material research issues. The energy loss function (ELF) is useful for thin film growth optimization since it gives insight in valence electron density, hardness, optical band gap and interface properties such as adhesion and wetting. XPS-PEELS also provides depth and width of implanted atom profiles in solid targets, e.g. Ar nanobubbles in Al. Special emphasis is given to the retrieval of electronic properties from XPS-PEELS data. Since the ELF, < Im[-1/(q, )] >q, is related to the q-averaged dielectric function, q, the latter can be obtained by taking into account multiple bulk and surface plasmon excitations. This task is rather simple in wide band gap materials where the ELF and the no-loss peak are clearly separated, as illustrated by amorphous silicon, amorphous carbon or Al oxide data. In contrast, in metals or small band gap materials, the broad asymmetric photoemission peak overlaps the ELF and low energy features in the ELF may be lost. A Fourier Transform (FT) method is proposed to analyze PEELS data, with the objective of retrieving such low energy excitations, e.g. inter band transitions. This FT method is compared with an empirical method based on a smooth cutoff of the zero-loss peak, using PEELS data obtained from Al2O3. Current developments of a quantum mechanical theory are crucial to obtain the respective contributions of intrinsic and extrinsic plasmon excitation (along with their interference) and to assess some approximations performed in classical treatments.

International audience; We present an interface package, called ES2MS, for passing self-consistent charge density and potential from Electronic Structure (ES) codes To Multiple Scattering (MS) codes. MS theory is based on the partitioning of the space by atomic-size scattering sites, so that the code provides the charge densities and potentials for each scattering site. For pseudo potential codes, the interface solves Poisson equation to construct the all-electron potential on the radial mesh which is used to solve the transition operators (T-matrix) and Green’s functions in MS codes. We show the algorithm of the interface and the example for X-ray absorption spectra of graphene.

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.

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; In this paper we present a single-particle scattering approach for the angular correlation between a photoelectron and the subsequent Auger electron from atomic targets. This method is proposed as an alternative approach with respect to the usual density matrix formalism, since it is more convenient for extension to the solid state case. Such an extension is required by the great progress made in the field of coincidence spectroscopy in condensed matter systems. We derived a tensor expression for the cross section and an equivalent expression in terms of convenient angular functions has been treated for the case of linearly polarized light. Numerical calculations are performed for the L3M2,3M2,3 transition in argon, in the single configuration Dirac-Fock scheme. Results are compared with experimental data for different final angular momentum states of the doubly charged ion and for different kinematical conditions.