Resonant X-ray Absorption Research Articles

Investigation of valence states and electronic structure of ferromagnetic double-perovskite La2MnNiO6 by using synchrotron radiation

The electronic structure of ferromagnetic double-perovskite La2MnNiO6 has been investigated by employing Mn and Ni 2p→3d resonant photoemission spectroscopy (RPES) and soft x-ray absorption spectroscopy (XAS). The Mn and Ni 2p XAS measurements show that Mn ions are formally tetravalent (3d3) and that Ni ions are formally divalent (3d8). Mn and Ni 2p→3d RPES shows that the occupied Mn 3d3 (t2g3↑) states are sharp and located around 2 eV below EF, while the occupied Ni 3d8 (t2g3↑t2g3↓eg2↑) states are broader than the Mn 3d states and they extend from EF to ∼5 eV below EF.

Site-specific electronic structure of bacterial surface protein layers

We applied resonant photoemission and X-ray absorption spectroscopy for a detailed characterization of the valence electronic structure of the regular two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602. Using this approach, we detected valence electron emission from specific chemical sites. In particular, it was found that electrons from the π clouds of aromatic systems make large contributions to the highest occupied molecular orbitals.

Size Dependence ofL2,3Branching Ratio and2pCore-Hole Screening in X-Ray Absorption of Metal Clusters

Resonant 2p x-ray absorption spectra of size-selected transition metal ions and clusters consisting of 1<or=n<or=200 atoms are reported. Remnants of atomic multiplet splitting in L(2,3)-edge x-ray absorption can be resolved up to the trimer, above which the overall line shape is already bulklike. A strong cluster size dependence of the L(2,3) branching ratio was found for titanium, vanadium, and cobalt clusters. While 3d electron delocalization increases with cluster size, the apparent 2p spin-orbit splitting remains constant within the error bars. The size dependence of the L(2,3) branching ratio can be attributed to cluster size specific screening of the 2p-3d Coulomb interaction by 3d/4s valence electrons.

Rotational molecular dynamics of laser-manipulated bromotrifluoromethane studied by x-ray absorption

We present a computational study of the rotational molecular dynamics of bromotrifluoromethane (CF(3)Br) molecules in gas phase. The rotation is manipulated with an off-resonant 800 nm laser. The molecules are treated as rigid rotors. Frequently, we use a computationally efficient linear rotor model for CF(3)Br, which we compare with selected results for full symmetric-rotor computations. The expectation value (cos(2) theta)(t) is discussed. Especially, the transition from impulsive to adiabatic alignment, the temperature dependence of the maximally achievable alignment, and its intensity dependence are investigated. In a next step, we examine resonant x-ray absorption as an accurate tool to study laser manipulation of molecular rotation. Specifically, we investigate the impact of the x-ray pulse duration on the signal (particularly its temporal resolution) and study the temperature dependence of the achievable absorption. Most importantly, we demonstrated that using picosecond x-ray pulses, one can accurately measure the expectation value (cos(2) theta)(t) for impulsively aligned CF(3)Br molecules. We point out that a control of the rotational dynamics opens up a novel way to imprint shapes onto long x-ray pulses on a picosecond time scale. For our computations, we determine the dynamic polarizability tensor of CF(3)Br using ab initio molecular linear-response theory in conjunction with wave function models of increasing sophistication: Coupled-cluster singles (CCS), second-order approximate coupled-cluster singles and doubles (CC2), and coupled-cluster singles and doubles (CCSD).

Characterization of the spatiotemporal evolution of laser-generated plasmas

We characterize the time evolution of ion spatial distributions in a laser-produced plasma. Krypton ions are produced in strong, linearly and circularly polarized optical laser fields (1014–1015 W/cm2). The Kr+ ions are preferentially detected by resonant x-ray absorption. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure ion densities as a function of time with 10 μm spatial resolution for times ≤50 ns. For plasma densities of the order of 1014 cm−3, we observe a systematic expansion of the ions outward from the laser focus. We find the expansion timescale to be independent of the plasma density though strongly dependent on the plasma shape and electron temperature. The former is defined by the laser focus, while the latter is controlled by the laser polarization state. We have developed a fluid description assuming a collisionless quasineutral plasma, which is modeled using a particle-in-cell approach. This simulation provides a quantitative description of the observed behavior and demonstrates the role of the very different electron temperatures produced by circularly and linearly polarized light. These results demonstrate the utility of this method as an in situ probe of the time and spatial evolution of laser-produced plasmas.

Ferromagnet-superconductor interfaces: the length scales of interactions

Heterostructures and superlattices consisting of the half-metal ferromagnet La0.67Ca0.33MnO3 (LCMO) and the superconductor YBa2Cu3O7 (YBCO) stacked along the c-axis of the superconductor were fabricated by pulsed laser deposition techniques. Apart from their structural characterization by x-ray diffractometry and high resolution TEM, the magnetic as well as electronic interaction at the interfaces and between the layers is studied by a variety of techniques ranging from transport measurements to magnetic, neutron diffraction and XMCD analysis. It turns out that these interaction effects occurring at several different length scales. At a short length scale of ~ 3 nm, charge transfer and/or orbital reconstruction as revealed by resonant x-ray absorption and XMCD measurements are dominant, the length scale of ~ 10 nm is the regime of self-injection of spin-polarized quasiparticles and at a scale in the 100 nm range and above magnetic interactions are present affecting the flux-line lattice of the superconductor, the domain structure of the magnetic layer as well as the interlayer coupling. The different and controversially discussed ferromagnet/superconducting interactions in YBCO/LCMO hybrids are analyzed and combined to a universal picture.

Phase imaging of magnetic nanostructures using resonant soft x-ray holography

We demonstrate phase imaging by means of resonant soft x-ray holography. Our holographic phase-contrast method utilizes the strong energy-dependence of the refractive index at a characteristic x-ray absorption resonance. The general concept is shown by using a $\mathrm{Co}∕\mathrm{Pd}$ multilayer sample which exhibits random nanosized magnetic domains. By tuning below the Co $L$-edge resonance, our quantitative and spectroscopic phase method allows high-contrast imaging of nanoscale electronic and magnetic order while increasing the probing depth and decreasing the radiation dose by an order of magnitude. The complex refractive index is quantitatively obtained through the interference between resonant and nonresonant scattering.

Electronic structure of transition metal fluorides and oxides determined by resonant X-ray absorption and X-ray emission spectroscopies

Experimental data for x-ray absorption at the transition metal L 2, 3 edge for FeF2 and CoF2 are presented. They are compared with the results of calculations that include the intra-atomic effects due to the ion atomic multiplet and also of the ligand field splitting of the 3d orbitals. Good overall agreement between experiment and theory is found. We also show resonant x-ray emission data for these compounds obtained at selected excitation energies and compare them with the results of the ligand field atomic multiplet calculation. Very good agreement between the experimental and the calculated energy splittings is found. We also found good agreement for the relative intensities of the emission peaks. This indicates that, as in the case of manganese compounds, the low energy loss peaks are due to decay into d-excited states of the ground configuration. A comparison of the emission spectra of LaMnO3, MnF2, FeF2, and CoF2 obtained at excitation energies at the top of the corresponding L 2 absorption peak is presented. This provides data for the projected d-density of states of d 4, d 5, d 6, and d 7 compounds, which are in good agreement with the results of ligand field atomic multiplet calculations.

Experimental investigation of the electronic structure ofGd5Ge2Si2 by photoemission and x-ray absorption spectroscopy

The electronic structure of the magnetic refrigerantGd5Ge2Si2 has been experimentally investigated by photoemission and x-ray absorption spectroscopy.The resonant photoemission and x-ray absorption measurements performed across theGd N4,5 and Gd M4,5 edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy(4f7) and the character of the states close to the Fermi edge. The presence ofGd 5d states in the valence band suggests that an indirect 5d exchangemechanism underlies the magnetic interactions between Gd 4f moments inGd5Ge2Si2. From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of statesdo not display clear variations. A significant change is instead detected in thephotoemission spectra at higher binding energy, around 5.5 eV, likely associated to thevariation of the bonding and antibonding Ge(Si) s bands across the phase transition.

Comparison of the electronic structure of anatase and rutileTiO2single-crystal surfaces using resonant photoemission and x-ray absorption spectroscopy

A comparison of the electronic structure of rutile (110), anatase (101), and anatase (001) single-crystal surfaces has been made using resonant photoemission and x-ray absorption spectroscopy. Under identical preparative conditions, the anatase (101) surface shows the lowest $\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}3d$ and $4sp$ hybridization in the states close to the valence-band maximum of the three surfaces. It also shows the highest concentration of surface-oxygen vacancies. The effect on the electronic structure of modifying the surface preparative route and thus the concentration of surface-oxygen vacancies is examined. The $\ensuremath{\sigma}$-antibonding $\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}3d\phantom{\rule{0.3em}{0ex}}{e}_{g}∕\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ hybridization (probed by XAS) is reduced by the removal of surface-oxygen. Photoemission shows that as the number of surface-defects is increased, the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p\text{\ensuremath{-}}\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}3d\phantom{\rule{0.3em}{0ex}}{t}_{2g}\phantom{\rule{0.3em}{0ex}}\ensuremath{\pi}$-bonding interaction is disrupted. For the anatase (101) surface it is found that as the number of surface-oxygen vacancies is increased, the $\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}3d$ and $4sp$ contributions at the valence-band maximum are reduced. We discuss the correlation between electronic structure and photocatalytic activity of the different polymorphs of ${\mathrm{TiO}}_{2}$.

Electronic structure of the charge-density-wave compoundEr5Ir4Si10

Photoemission and x-ray absorption measurements have been performed onEr5Ir4Si10, a compound that exhibits an incommensurate charge-density-wave formation below 155 K.The resonant photoemission and x-ray absorption measurements performed across the ErN54 andEr M5 edge identify the Er 4f multiplet lines, the 4f occupancy and the character of the statesclose to the Fermi edge.

The influence of radio galaxy activity on X-ray absorption lines from the intracluster medium

We present an investigation of the X-ray absorption features predicted by hydrodynamic simulations of radio galaxies interacting with the intracluster medium (ICM) of their host galaxy clusters. We show how these absorption lines can be used as a new diagnostic for the radio galaxy-ICM interactions. Such interactions have been observed in numerous systems by ROSAT, Chandra and XMM-Newton, and understanding them has implications for active galactic nucleus (AGN) feedback and galaxy formation. Starting from the hydrodynamic simulations of Reynolds, Heinz & Begelman, we calculate the properties of the highly ionized iron and oxygen lines (seen in absorption against the central AGN), predicting line shapes, equivalent widths, column densities and velocity shifts. The main effect of the jet on the absorption lines is a reduction of the line strength from that of the quiescent ICM and the introduction of some velocity structure in the line profile. We investigate whether these features are detectable with current as well as future high-resolution X-ray spectrometers. We conclude that the Chandra transmission gratings have insufficient sensitivity to detect these features with high significance, and certainly would not allow a study of the dynamics of the interaction via absorption signatures. Constellation-X, on the other hand, will allow superb constraints to be derived. We can also use this analysis to assess the idea that radio galaxy induced ICM outflows give rise to the resonant oxygen X-ray absorption lines that have been claimed as evidence for the warm-hot intergalactic medium (WHIM). We show that these detached, high-velocity oxygen absorption lines cannot result from a radio galaxy-ICM interaction, thereby strengthening the WHIM interpretation.

Diversity of electronic states of [formula omitted] revealed by soft X-ray photoemission

The electronic states of the Nd 0.5 Sr 0.5 MnO 3 , which shows various electronic phases such as paramagnetic insulator, ferromagnetic metal, and the charge-ordered insulator depending on the temperature, have been revealed by high-resolution Mn 2p–3d resonant photoemission and X-ray absorption spectroscopy. These results indicate the importance of the relationship between electronic states and the Jahn–Teller effect.

Probing the Local Group Medium toward Markarian 421 withChandraand theFar Ultraviolet Spectroscopic Explorer

We report the detection of highly-ionized gas at z~0 seen in resonant UV and X-ray absorption lines toward the z=0.03 blazar Mkn 421. A total of 13 X-ray and 3 UV lines were measured (or upper limits derived), including three lines in the OVII K-series and K\alpha transitions from neon, carbon, and nitrogen. From the three OVII lines we derive a 2\sigma Doppler parameter constraint of 24<b<55 km/s. The FUSE spectrum shows strong Galactic low--velocity OVI 1032A absorption as well as a possible weak OVI high-velocity component (HVC). The Doppler parameter of the low-velocity OVI measured with FUSE is ~3\sigma higher than that derived from the OVII line ratios, indicating that the OVII and Galactic OVI arise in different phases. This velocity dispersion, along with limits on the gas temperature and density from the X-ray line ratios (assuming a single phase with collisional ionization equilibrium plus photoionization) are all consistent with an extragalactic absorber. However, the OVII Doppler parameter is inconsistent with the high temperature required to produce the observed OVI_HVC/OVII ratio, implying that the HVC is probably not related to the OVII. In addition, the OVI_K\alpha line detected by Chandra implies a column density ~4 times higher than the 1032A absorption. Although an extragalactic absorber is fully consistent with the measured column density ratios, a Galactic origin cannot be ruled out given the uncertainties in the available data.

Spectral evidence for inherent “dead layer” formation at La 1− ySr yFeO 3/La 1− xSr xMnO 3 heterointerface

We have investigated the electronic structure of a La 0.6Sr 0.4FeO 3 (LSFO)/La 0.6Sr 0.4MnO 3 (LSMO) heterointerface using Mn 2p-3d resonant photoemission spectroscopy and X-ray absorption spectroscopy. We have found that the spectral intensity of e g↑ states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness. The spectral behavior is quite similar to that of hole-doped LSMO films, indicating hole-doping in LSMO layers. We have also found that Fe 2p X-ray absorption spectrum of a LSMO/LSFO/LSMO trilayer shows the spectral evidence for electron doping in the corresponding LSFO layer. These results indicate the occurrence of charge transfer at interfaces between LSMO and LSFO layers.

Valence of YbS under pressure: A resonant inelastic x-ray emission study

We present spectroscopic data on the electronic structure of $\mathrm{YbS}$ under pressure. From resonant x-ray emission and x-ray absorption spectra in the high-resolution partial fluorescence yield mode, we find that the $\mathrm{Yb}$ valence increases from $2.3$ at ambient pressure to $2.6$ at $360\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$. The accuracy of these estimates is enhanced by a combined analysis of emission spectra measured as a function of the incident and emitted energy. The spectral line shapes reflect the mixed character of the electronic states, intermediate between atomic and bandlike.

Doping Dependence of the Electronic Structure of La1-xNaxMnO3 by Resonant X-ray Emission and X-ray Absorption Spectroscopy

The electronic structure of La 1 - x Na x MnO 3 (x = 0, 0.08, and 0.15) thin films is investigated by resonant X-ray emission spectroscopy (RXES) and X-ray absorption spectroscopy (XAS). The excitation energy of the Mn dd transitions is derived. For different carrier doping concentrations, a significant increase in the emission of the low-energy excitations is observed. This finding is consistent with the less-pronounced structural distortion and the increased density of empty eg-like majority states in the lower part of the conduction band of the lightly doped samples.

Double resonance capacitance spectroscopy (DORCAS): A new experimental technique for assignment of X-ray absorption peaks to surface sites of semiconductor

As a new microspectroscopy for semiconductor surface analysis using an X-ray beam, double resonance capacitance spectroscopy (DORCAS) is proposed. For a microscopic X-ray absorption measurement, a local capacitance change owing to X-ray induced emission of localized electrons is detected by a microprobe. The applied bias voltage V b dependence of the capacitance also provides information on the surface density of state. The resonance of the Fermi energy with a surface level by V b control makes possible the selection of the observable surface site in the X-ray absorption measurements, i.e. site-specific spectroscopy. The double resonance of the surface site selection ( V b resonance) and the resonant X-ray absorption of the selected site (photon energy hν resonance) enhances the capacitance signal. The DORCAS measurement of the GaAs surface shows correlation peaks at hν=10.402 keV and V b=−0.4 V and hν=10.429 keV and V b=+0.1 V, indicating that these resonant X-ray absorption peaks can be assigned to two different surface sites.

X-Ray Absorption Fine Structure Measurement Using a Scanning Capacitance Microscope: Trial for Selective Observation of Trap Centers in the ∼nm Region

For site-selective analysis of trap centers in a single nano-structure of a semiconductor, a new X-ray absorption spectroscopy technique using scanning probe detection of capacitance (scanning capacitance microscope-X-ray absorption fine structure; SCM-XAFS method) is proposed. Since capacitance is sensitive to localized electrons, X-ray induced photoionization of a trap center can be detected by capacitance change of the diode structure. The X-ray photon energy dependence of the capacitance involved in a point contact diode of a GaAs sample and Au-coated Si tip of SCM indicates XAFS spectra of the trap centers in the local region. Several resonant X-ray absorption peaks with different sample bias voltage dependences are found. The correspondence of the Fermi level to the specific trap level enhances the SCM-XAFS signal from the trap center, indicating that the site-selective observation of the trap center can be realized by bias control.

Studies on Surface and Interface with X-ray Absorption Fine Structure (XAFS). Microspectroscopy of Semiconductor Surfaces by SCM-XAFS Method.

For a site-selective analysis of trap centers on semiconductor surfaces, a new X-ray absorption spectroscopy using scanning probe detection of capacitance (Scanning capacitance microscope - X-ray absorption fine structure; SCM-XAFS method) is proposed. Since capacitance is sensitive to localized electron, X-ray induced photoionization of trap center can be detected in terms of capacitance change of diode structure. The X-ray photon energy dependence of the capacitance involved in a point contact diode of GaAs sample and Au-coated Si tip of SCM indicates XAFS spectra of the trap centers in the local region. The correspondence of the Fermi level to the specific trap level enhances the SCM-XAFS signal from the trap center, resulting that the site-selective observation of the trap center can be realized by bias control. The double resonance of the surface site selection (Fermi energy resonance) and the resonant X-ray absorption of the selected site (photon energy resonance) enhances the capacitance signal, indicating that the resonant X-ray absorption peaks can be assigned to the surface sites.