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ABSTRACT: Aromatic molecules are central components of model systems for molecular electronics, with C60 one of the most studied. Upon adsorption on (metallic) substrates a splitting of the frontier orbitals is commonly observed, with a strong dependence on substrate material, but little dependence on substrate structure. We report the detailed photoelectron angle dependence of C60/Al(110) over a wide range of energy, finding a strong remnant molecular character. In particular, certain HOMO-derived suborbitals couple strongly, and others weakly, with the metal, which results in final state charging for those weakly coupled. C 1s data correlate well with the assignments made on this basis, as does the comparison of ground state partial densities-of-states (PDOS) to photoelectron spectra. Detailed analysis of the PDOS supports a rough division into surface-near and surface-far components, in agreement with the molecular picture. The component spectral widths are attributed to intramolecular vibrational coupling, which is suggested to aid in the electronic decoupling of certain suborbitals from the substrate, facilitating the observed final state charging.
10/2010;
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Johan Gråsjö,
Egil Andersson,
Johan Forsberg,
Emad F Aziz,
Barbara Brena,
Christian Johansson, Joseph Nordgren,
Laurent Duda,
Joakim Andersson,
Franz Hennies,
Jan-Erik Rubensson,
Per Hansson
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ABSTRACT: Oxygen K absorption and emission spectra of water molecules confined in dodecyltrimethyl ammonium chloride micelle structures are presented. The local electronic structure of the water molecules is found to be dramatically different from the electronic structure of water molecules in the gas-phase as well as in liquid water. Hybridization with states of the ions in the surrounding ions is directly observed, and evidence for stabilization of the water molecules relative to molecules in bulk water is found.
The Journal of Physical Chemistry B 07/2009; 113(24):8201-5. · 3.70 Impact Factor
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ABSTRACT: Resonantly excited soft x-ray emission spectra of liquid water have been measured at two different angles relative to the polarization direction of the exciting radiation. The results demonstrate that the emission is much more isotropic than expected for the free water molecule but that the trends in the remaining anisotropy qualitatively agree. It is found that the two sharp oxygen peaks at high emission energy correspond to states of different symmetry.
Phys. Rev. B. 04/2009; 79(13).
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Emad F Aziz,
Johan Gråsjö,
Johan Forsberg,
Egil Andersson,
Johan Söderström,
Laurent Duda,
Wenhua Zhang,
Jinglong Yang,
Stefan Eisebitt,
Christel Bergström,
Yi Luo, Joseph Nordgren,
Wolfgang Eberhardt,
Jan-Erik Rubensson
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ABSTRACT: Via fluorescence yield (FY) and resonant inelastic scattering spectroscopy in the soft X-ray range we find that soft X-rays induce formation of N2 molecules in solid NH4Cl and in related compounds. The nitrogen molecules form weak bonds in NH4Cl, so that a substantial fraction of the molecules remains in the sample. From measurements of the FY as a function of exposure and temperature, the rates for the photochemical processes are estimated. At elevated temperatures (363 K), several nitrogen atoms are removed from the sample per incoming photon. At lower temperatures (233 K), the rate is reduced to around 0.02 nitrogen atoms for each incoming photon. Virtually all these atoms form N2 molecules which are bound in the sample. The generality and implications of these results are briefly discussed.
The Journal of Physical Chemistry A 11/2007; 111(39):9662-9. · 2.95 Impact Factor
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ABSTRACT: The electronic structure of pol(ethylene oxide) (PEO) in a thin (<1 mu) film sample was experimentally probed by X-ray emission spectroscopy. Both nonresonant and resonant X-ray emission spectra were simulated by using density functional theory (DFT) applied to four different models representing different conformations in the polymer. Calculated spectra were compared with experimental results for the PEO film. It was found that the best fit was obtained with the polymer conformation in PEO electrolytes from which the salt (LiMF6, M = P, As, or Sb) had been removed. This conformation is different from the crystalline bulk polymer and implies that film casting, commonly used to form electrolytes for Li polymer batteries, induces the same conformation in the polymer not depending upon the presence of salt.
The Journal of Physical Chemistry B 10/2007; 111(40):11658-61. · 3.70 Impact Factor
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ABSTRACT: Recent developments in the field of ultra-soft x-ray emission spectoscopy have opened up several new possibilities for research in atomic, molecular and solid state physics. Monochromatized synchrotron radiation has become available for selective excitation of ultra-soft x-ray fluorescence spectra, recorded in high resolution. Substantial advancements have taken place in the experimental techniques used to record highly resolved emission spectra of free molecules, excited by electron beam. This progress is discussed with regard to the new experimental developments and to current research in various fields. Some lines of direction, along which ultra-soft x-ray spectroscopy is likely to develop further are pointed out.
Physica Scripta 02/2007; 1990(T31):103. · 1.20 Impact Factor
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ABSTRACT: We explore the notion of using a transition metal compound as a bridge between the free and the surface adsorbed species for the interpretation of core electron spectroscopies. Calculations for this purpose are carried out for carbon and oxygen X-ray absorption, X-ray emission and X-ray core electron shake-up spectra for Cr(CO)6, and the results are compared with previous calculations on free CO and on CO-copper clusters. A detailed analysis of spectra is made with some previous assignments being revised. Experimental non-resonant and resonant X-ray emission spectra are presented.
Physica Scripta 03/2006; 59(2):138. · 1.20 Impact Factor
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ABSTRACT: We have applied x-ray emission spectroscopy and density functional theory (DFT) to study the chemical and electronic structures of liquid methanol. The x-ray emission spectra at carbon and oxygen K edges of methanol in different hydrogen-bonded clusters are simulated. It is shown that hydrogen bonding strongly influences the spectral profile of O K emission, but not the C K emission. The methanol chain and ring conformations show a distinct difference in their electronic structures. The molecular orbitals of chains are strongly localized, whereas for the ring structures they show strong delocalization characteristics and behaviorlike covalent π orbitals in a conjugated system. A comparison of experimental spectra and DFT calculations suggests that liquid methanol comprises combinations of rings and chains of methanol molecules linked with hydrogen bonds and is dominated by structures with the size of six and eight molecules.
Phys. Rev. B. 03/2005; 71(10).
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ABSTRACT: Soft x-ray absorption (SXAS) and emission (SXES) spectroscopies were applied to study the nitrogen bonding structure in magnetron sputtered CNx thin films. By comparing with calculated spectra of N in different model systems, N in three main bonding environments can be identified: (i) C�N bonds, with a sharp SXAS peak at 399.5 eV, (ii) pyridine-like N (i.e., N bonded to two C atoms), with an x-ray absorption resonance at ∼398.5 eV, and (iii) N substituted in graphite, possibly with one sp3 carbon as a neighbor (SXAS energy ∼401 eV). These bondings are present in all CNx films analyzed; however, as shown earlier, the relative intensities between the peaks may vary with the growth conditions. Differences in the coordination of the nearest or second nearest C neighbors only cause slight changes in the peak positions and spectrum shape. © 2001 American Institute of Physics.
Applied Physics Letters 12/2001; 79(26):4348-4350. · 3.84 Impact Factor
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ABSTRACT: Advanced nano and micro particulate thin films of 3d transition and post-transition metal oxides consisting of nanorods and microrods with parallel and perpendicular orientation with respect to the substrate normal, have been successfully grown onto various substrates by heteronucleation, without template and/or surfactant, from the aqueous condensation of solution of metal salts or metal complexes (aqueous chemical growth). Three-dimensional arrays of iron oxide nanorods and zinc oxide nanorods with parallel and perpendicular orientation are presented as well as the oxygen K-edge polarization dependent x-ray absorption spectroscopy (XAS) study of anisotropic perpendicularly oriented microrod array of ZnO performed at synchrotron radiation source facility.
MRS Proceedings. 12/2000; 704.
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ABSTRACT: Large arrays of perpendicularly oriented anisotropic nanoparticles of ferric oxyhydroxide (Akaganeite, β-FeOOH) and oxide (Hematite, α-Fe2O3) of typically 3-5 nm in diameter, self-assembled as bundles of about 50 nm in diameter and of up to 1 μm in length have been successfully grown onto polycrystalline substrates without template and/or surfactant by heteronucleation from an aqueous solution of ferric salts and their optical and electronic properties investigated.
MRS Proceedings. 12/1999; 635.
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The Journal of Chemical Physics 08/1997; 107(7):2699-2699. · 3.33 Impact Factor
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ABSTRACT: Although resonant x-ray scattering of molecules fulfills strict electronic symmetry selection rules, as now firmly proven by spectra of diatomic molecules, the accumulated body of data for polyatomic molecules indicates that an apparent breaking of these rules represents the common situation rather than the exception. The CO2 molecule provides a good example of symmetry breaking, with the oxygen x-ray emission spectra showing strong intensity for transitions that are forbidden by the parity selection rule. We present time-independent and time-dependent theories for frequency-dependent resonant x-ray scattering beyond the Born–Oppenheimer approximation in order to explore under what circumstances one can anticipate symmetry breaking in the spectra of polyatomic molecules. The theory starts out from the Kramers–Heisenberg dispersion relation and is generalized for vibrational degrees of freedom and for nonadiabatic coupling of the electronic (vibronic) states, including the frequency dependency of the scattering cross section. Different limiting cases and few-level models are considered. The symmetry breaking is proven to be the result of pseudo-Jahn–Teller-like vibronic coupling between near-degenerate core-excited states. Thus vibronic interaction over the antisymmetric vibrational mode between the “bright” 1σg−12πu1 and “dark” 1σu−12πu1 intermediate states of CO2 allows transitions otherwise forbidden. The measurements and theory demonstrate that the symmetry-selective character of the resonant x-ray emission is strongly frequency dependent. The strong intensity of “dipole-forbidden” transitions in the π∗ oxygen K spectrum at resonance is reduced monotonically with the detuning of the excitation energy from resonance, and the spectra become “symmetry purified.” Simulations with full vibronic coupling predict this feature of the x-ray scattering experiment and a few-level model explains the energy dependence of the symmetry selection and the symmetry purification at large detuning energies in the limit of narrowband photon excitation. © 1997 American Institute of Physics.
The Journal of Chemical Physics. 02/1997; 106(9):3439-3456.
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ABSTRACT: For soft x‐ray fluorescence spectroscopy with selective energy excitation, it is desirable to have maximum flux at moderate resolution. A plane grating monochromator with varied line spacing can be used in the convergent beam of an ellipsoidal focusing mirror to provide moderate resolution with a large collection angle and minimum number of reflecting surfaces. We determine the design parameters for a varied‐line‐space grating monochromator to provide light in the 200–1000 eV range. The results of modeling are discussed with respect to the optimal choices of monochromator for our experimental requirements. A novel mechanical design is proposed for constructing the monochromator in a compact form with the minimum number of moving parts and alignments. © 1995 American Institute of Physics.
Review of Scientific Instruments 03/1995; · 1.37 Impact Factor
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ABSTRACT: Ab initio Hartree-Fock calculations of relative X-ray transition probabilities for core hole states of N2, CO, H2O and NH3 have been carried out and compared with the respective high-resolution soft X-ray spectra. The same one-determinental wavefunctions were employed for both initial and final states and the dependence of the X-ray transition moments on the choice of orbitals and of basis set parameters was investigated. In particular, orbitals optimized for a transition state were tested. The use of the one-center intensity model as a guide for the assignment of second row X-ray spectra was justified at bothab initio and semiempirical (CNDO) levels of approximation. The breakdown of the MO-picture for inner-valence electrons is demonstrated in the X-ray spectrum of N2 and the analogy with the corresponding photoelectron bands is pointed out.
Theoretical Chemistry Accounts 05/1981; 58(2):111-119. · 2.16 Impact Factor
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Tanel Käämbre,
Joachim Schiessling,
Lisbeth Kjeldgaard,
Limin Qian,
Ingrid Marenne,
James N. O’Shea,
Joachim Schnadt,
Dennis Nordlund,
Chris J. Glover,
Jan-Erik Rubensson,
Petra Rudolf,
Nils Mårtensson, Joseph Nordgren,
Paul Brühwiler
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ABSTRACT: We present C 1s x-ray absorption, x-ray emission, and resonant inelastic x-ray scattering (RIXS) spectra of single-phase crystalline K3C60. The comparison to valence-band photoelectron spectra from the same sample facilitates identification of the contribution from surface and bulk electronic states in the latter. Bulk-sensitive techniques show that the valence bands of K3C60 and pure C60 are characterized by spectral features of similar width, in agreement with the predictions of band-structure calculations. Symmetry selectivity in the RIXS process allows us to assign peaks in the C 1s absorption spectrum, demonstrating a close correspondence with pure C60 also in the conduction band. The symmetry selectivity is as pronounced in K3C60 as in pure C60, indicating that the local C60 symmetry is not appreciably affected by the K doping, either in the ground state or intermediate state, on the time scale of 6 fs.
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ABSTRACT: Magnetron-sputtered carbon nitride thin films with different structures and compositions were analyzed by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), as well as X-ray emission spectroscopy (XES). In all techniques, the carbon spectra are broad and featureless with little variation depending on growth conditions. The nitrogen spectra, on the other hand, show more distinct features, providing a powerful tool for structural characterization. By comparing the experimental spectra with calculations on different model systems, we are able to identify three major bonding structures of the nitrogen—N1: nitrile (CN) bonds; N2: Pyridine-like N, i.e., N bonded to two C atoms; and N3: graphite-like N, i.e., N bonded to three C atoms as if substituted in a graphitic network, however, possibly positioned in a pentagon and/or with sp3 carbon neighbors. The presence of N2 and N3 are best detected by XPS, while N1 is better detected by NEXAFS. The calculated XES spectra also give good indications how valence band spectra should be interpreted. Films grown at the higher temperatures (≥350 °C) show a pronounced angular dependence of the incoming photon beam in NEXAFS measurements, which suggests a textured microstructure with standing graphitic basal planes, while amorphous films grown at low temperatures show isotropic properties.
Thin Solid Films.
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The Journal of Physical Chemistry C. 114(43):18686-18692.
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ABSTRACT: Resonantly excited carbon and oxygen x-ray-emission spectra of gaseous carbon monoxide are presented. Emission spectra obtained with selective excitation to the π* valence orbital and to various Rydberg levels are compared to satellite-free nonresonant spectra. Screening effects caused by the excited electron, creating energy shifts and intensity variations in the resonant spectra compared to the nonresonant spectra, are observed, as well as an angular dependence of the resonantly excited spectra. The experimental spectra are compared to simulated spectra where the vibronic part is computed by means of a lifetime-vibrational interference formalism. The electronic intensities are analyzed by a separate-state, self-consistent-field method and a formalism for resonant inelastic x-ray scattering, focusing on screening and angular dependence.
Phys. Rev. A. 55(1).
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ABSTRACT: The applications of resonant soft X-ray emission spectroscopy on a variety of carbon systems have yielded characteristic fingerprints. With high resolution monochromatized synchrotron radiation excitation, resonant inelastic X-ray scattering has emerged as a new source of information about electronic structure and excitation dynamics. The selectivity of this excitation, in terms of energy and polarization, has also facilitated studies of emission anisotropy. Various features observed in resonant emission spectra have been identified and studied. In the ordered wide-band systems (e.g., diamond and graphite), momentum conservation of resonant inelastic X-ray scattering has been observed, and the aspect of using this for studies of band dispersion is pointed out. In the molecule-like systems, such as C60, C70, benzene and derivatives, conjugated polymers, symmetry selection rules have been established, and deviations from the symmetry selection rules observed have been interpreted in terms of dynamical symmetry breaking. In the studies of π-conjugated polymers, it has been found that the benzene-ring has a strong identity as a building block in some of the conjugated polymers. Isomeric dependence was able to be clarified in the studies of polypyridine. The doping induced protonation effect has been observed as the depletion of the upmost occupied valence band in the doped polyaniline.
Journal of Electron Spectroscopy and Related Phenomena.
