N. Wassdahl

Soft x-ray emission spectroscopy using monochromatized synchrotron radiation (invited)

Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flippe...

The adsorption structure of glycine adsorbed on Cu(110); comparison with formate and acetate/Cu(110)

The molecular orientation of an ordered monolayer of glycine adsorbed on Cu(110) has been studied using X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS), X-ray Photoelectron Diffraction (XPD), Low-Energy Electron Diffraction (LEED) and theoretical calculations. In particular, the NEXAFS results are discussed in terms of the spectra of the related molecules ammonia (NH3), formate (HCOO), and acetate (CH3COO) on Cu(110). Whereas the latter two molecules chemisorb in similar geometries, glycine is found to assume a very different chemisorption geometry. Formate and acetate bond through two equivalent oxygen atoms with the molecular plane oriented nearly perpendicular to the surface, aligned along the [110]-azimuth. In the case of adsorbed glycine (NH2CH2COO), the azimuthal orientation is still present, i.e. the bonding oxygen atoms are aligned along the [110]-azimuth, but the molecule is found to bend towards the surface. A second chemisorption bond is formed at the nitrogen end of the molecule, involving copper atoms in the neighboring [110]-row. We therefore have the interesting case of a chemisorption bond involving different functional groups in the same molecule.

The electronic structure and surface chemistry of glycine adsorbed on Cu(110)

We present a combined density functional theory and x-ray emission spectroscopy study of the bonding and chemistry of glycine (NH2CH2COOH) chemisorbed on Cu(110). The amino acid deprotonates upon adsorption. The adsorbate exhibits a rich surface chemistry leading to several intermediate adsorption structures. The most stable geometry is found to involve both the carboxylic and amino functional end groups in the bond. This structure appears only after annealing to 400 K, which in the present work is attributed to a removal of surface or subsurface hydrogen from the metal. Comparison with experimental x-ray emission and near edge x-ray absorption fine structure (NEXAFS) spectra provide a detailed picture of the electronic structure for the most stable structure. This allows conclusions to be drawn regarding the covalent interaction of the adsorbate system. When combined with theoretical calculations addressing, e.g., the electrostatic adsorbate–substrate interaction, a complete picture of the surface chemic...

Beamline I511 at MAX II, capabilities and performance

The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The per

Ammonia adsorbed on Cu(110): An angle resolved x-ray spectroscopic and ab initio study

We present a study of a monolayer of ammonia (NH3) adsorbed on Cu(110) using core level spectroscopies in combination with ab initio calculations based on density functional theory. In particular, x-ray emission spectroscopy has been applied, providing an unsurpassed view of the electronic structure of NH3 upon adsorption. The saturated NH3 monolayer, Θ∼0.4 ML, is found to induce strong adsorbate–adsorbate interaction, causing the molecules to tilt on the surface. Based on the angular distribution of the x-ray emission (XE) spectra, we have been able to estimate a mean tilt angle from the surface normal of 40°–45° for the saturated monolayer; the accompanying theoretical calculations for up to three NH3 molecules on a Cu21 all-electron cluster model support a tilted structure due to adsorbate–adsorbate dipole, and possibly hydrogen bonding, interactions. Since the creation of a core hole on the nitrogen atom site in the intermediate state of the XE process does not affect the symmetry of the molecule, a s...

The bonding of simple carboxylic acids on Cu(110)

We present a study of the bonding of formate (HCOO) and acetate (CH3COO) chemisorbed on Cu(110) using core level spectroscopies in combination with theoretical calculations. For the first time, we apply x-ray emission spectroscopy (XES) to these systems. When XES is used in conjunction with x-ray absorption spectroscopy (XAS) and ab initio calculations, new information about the electronic interaction in the adsorbate–substrate system is provided. In particular, we have used the azimuthal orientation of the COO–surface bond on the (110) surface, to make a complete partition into x, y, and z orbital contributions. The surface bond is found to be predominantly ionic. For the case of adsorbed formate, the covalent bonding is dominated by 6a1/7a1, (σ)-type, frontier orbitals, interacting with the Cu valence band. The resulting hybrid orbitals form a distribution of states that cross the Fermi level. The contribution from adsorbate π-type orbitals is small. The chemical bond formation of adsorbed acetate is ve...

Radiative decay of multiply excited core hole states in H2O

A high resolution x‐ray emission spectrum of water vapor has been recorded, using a 7 keV electron beam and a 10 m grazing incidence spectrometer. The spectrum is interpreted in terms of dipole transition moments between CI wave functions built from separately optimized molecular orbitals. The principal features of the spectrum are reproduced considering transitions in the singly and doubly ionized species, giving rise to main lines and Wentzel–Druyvesteyn satellites, respectively.

Tunable-excitation soft X-ray fluorescence spectroscopy of high-Tc superconductors: an inequivalent-site seeing story

Abstract The study of high- T c superconductors and related compounds by means of soft X-ray fluorescence spectroscopy with monochromatic photon excitation is reviewed. It is shown that this spectroscopy provides a powerful tool for probing the local electronic structure at inequivalent O sites. The states in the valence band belonging to different sites are selectively studied by virtue of large variations in the O 1 s X-ray absorption cross-section for these sites at certain excitation energies as well as by taking advantage of 1 s chemical shifts between the sites. The character of unoccupied states is analyzed by monitoring the changes in the shape of X-ray fluorescence spectra with varying energies of incident photons. Issues are addressed, such as symmetry of doping-induced states and the origin of charge-carriers with respect to inequivalent sites, as well as the influence of electron–electron interaction.

Symmetry Selection in Polarized Resonant X-Ray Emission Spectroscopy in La2CuO4.

Experimental and theoretical investigations are made for the polarization dependence in the resonant X-ray emission spectra due to the Cu 3 d →2 p electronic transition under the Cu 2 p →3 d resonant excitation (Cu 2 p -RXES) in La 2 CuO 4 . It is shown that the polarization dependence of the Cu 2 p -RXES can be deduced from the spectra obtained under the various geometrical configurations of the crystal axis, the polarization direction of the incident beam, and the direction of the detector. The calculated results are in fairly good agreement with the experimental results.

Energy dependence of Cu L2,3 satellites using synchrotron excited x-ray-emission spectroscopy

This thesis addresses the electronic structure of molecules and solids using resonant X-ray emission and photoemission spectroscopy. The use of monochromatic synchrotron radiation and the improved performance of the instrumentation have opened up the possibility of detailed analyses of the response of the electronic systems under interaction with X-rays. The experimental studies are accompanied by numerical ab initio calculations in the formalism of resonant inelastic scattering. The energy selectivity has made it possible for the first time to study how the chemical bonds in a molecule break up during resonant inelastic X-ray scattering. In the conjugated polymer systems, the element selectivity of the X-ray emission process made it possible to probe the different atomic elements separately. The X-ray emission technique proved to be useful for extracting isomeric information, and for measuring the change in the valence levels at different degrees of doping. In this thesis, spectral satellite features in transition metals were thoroughly investigated for various excitation energies around a core-level threshold. By measuring the relative spectral intensity of the satellites it was possible to extract information on the partial core-level widths. Using the nickel metal system as an example, it was shown that it is possible to probe the different core-excited states close toshake-up thresholds by measuring the relative spectral intensity variation of the Auger emission.Resonant photoemission measurements showed unambiguous evidence of interference effects. Theseeffects were also thoroughly probed using angle-dependent measurements. The combination of X-rayemission and absorption were useful for studying buried layers and interfaces due to the appreciable penetration depth of soft X-rays. X-ray scattering was further found to be useful for studying low-energy excited states of rare earth metallic compounds and transition metal oxides.