Unoccupied Valence Research Articles

The nanoscale structure and unoccupied valence electronic states in FeSe1-xTex chalcogenides probed by X-ray absorption measurements.

We have studied the nanoscale structure and unoccupied electronic states in FeSe1-xTex by a combined analysis of Se K, Te L1 and Fe K-edges X-ray absorption measurements. Extended X-ray absorption fine structure (EXAFS) results show that iron-chalcogen (Fe-Se and Fe-Te) distances in ternary FeSe1-xTex are similar to those measured for binary FeSe and FeTe. The local Fe-Se/Te distances determined by different absorption edges fit well in the characteristic Z-plot of random alloys, providing unambiguous support to the inhomogeneous nanoscale structure of the ternary FeSe1-xTex system. X-ray absorption near-edge structure (XANES) spectra reveal a gradual evolution of the unoccupied valence electronic states as a function of Te-substitution in FeSe1-xTex. The Fe 3d-Se 4p/Te 5p hybridization is found to decrease with Te-substitution, accompanied by an increase in unoccupied Se 4p states and a decrease in unoccupied Te 5p states. The results are discussed in the frame of local inhomogeneity in the FeSe1-xTex system driven by random alloying of Se/Te atoms.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory.

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

Solvation dependence of valence electronic states of water diluted in organic solvents probed by soft X-ray spectroscopy.

Knowledge of hydrogen bonds is important in a wide range of scientific fields. However, information on the electronic state of hydrogen bonded molecules in liquids and solutions is still limited. We applied X-ray emission and absorption spectroscopy to observe the electronic state around oxygen of water and investigate the solvent dependence of occupied and unoccupied valence electronic states of water in acetonitrile, 3-methylpyridine, and ethylenediamine at room temperature under atmospheric pressure. Based on the direct comparison of the spectra, the pronounced variation of the unoccupied 2b2 state of water in acetonitrile and 3-methylpyridine was assigned to the difference of the dipole-dipole interaction and the donor hydrogen bond. For ethylenediamine solution, an enhancement of the peak structure in the post-edge region of X-ray absorption spectra and an evident shoulder structure on the higher energy side of the occupied 1b1 state in X-ray emission spectra were observed. Although ethylenediamine has two amino groups that can form the hydrogen bonds as a proton donor, the obtained results indicate that the amino groups of ethylenediamine also act as a proton acceptor in the hydrogen bond between water molecules. A systematic peak intensity variation of the pre-edge peak corresponding to the lowest unoccupied molecular orbital, 4a1, was also found through the comparison of three organic solvents. Since the peak intensity tends to decrease with increasing number of hydrogen bonds, the results can be interpreted by the covalency of hydrogen bonds.

Rapid bond rearrangement in core-excited molecular water

The angular anisotropy of fragments created in the dissociation of core-electron excited water molecules is studied to probe the correlation between fragmentation channels, kinematics and molecular geometry. We present fragment kinetic measurements for water molecules where the inner-shell oxygen electron is excited to the unoccupied 4a1 and 2b2 valence molecular orbitals. The kinematics of individual fragmentation channels are measured using fully three-dimensional momentum imaging of fragments. The results show that the geometry of the molecule and the kinetic energy of fragments are strongly coupled in the atomisation process. In addition we identify a fragmentation process arising from bond rearrangement evidenced by the H2(+)-O(+) ion pair which is accessible for resonant excitation of the 1s electron. In all of the two-body fragmentation processes the dissociation takes place along the potential-energy surface, while atomisation reveals both dissociation along the potential surface and Coulomb explosion. The angular distribution of fragments suggests that the bond rearrangement is very rapid; likely on a sub 10 fs time scale.

The Hydrogen Bond of Water from the Perspective of Soft X-Ray Spectroscopy

Its importance for life and its unusual properties keep water within the focus of ongoing research; this focus especially applies to water in the liquid phase. Scientists agree that the hydrogen-bond network, which is formed by interactions between the water molecules, is key for understanding the anomalies of water. However, a better understanding of the structure of this network, as well as its dynamics, must yet be established. Soft X-ray spectroscopy allows the investigation of the local electronic structure of water by probing the occupied and unoccupied valence molecular orbitals. In this Focus Review, we present soft-X-ray-based techniques, their development in terms of liquid spectroscopy, and recent studies on the hydrogen-bond network of liquid water.

Theoretical Investigation of Paramagnetic NMR Shifts in Transition Metal Acetylacetonato Complexes: Analysis of Signs, Magnitudes, and the Role of the Covalency of Ligand–Metal Bonding

Ligand chemical shifts are calculated and analyzed for three paramagnetic transition metal tris-acetylacetonato (acac) complexes, namely high-spin Fe(III) and Cr(III), and low-spin Ru(III), using scalar relativistic density functional theory (DFT). The signs and magnitudes of the paramagnetic NMR ligand chemical shifts are directly related to the extent of covalent acac oxygen-to-metal σ donation involving unoccupied metal valence d(σ) acceptor orbitals. The role of delocalization of metal-centered spin density over the ligand atoms plays a minor secondary role. Of particular interest is the origin of the sign and magnitude of the methyl carbon chemical shift in the acac ligands, and the role played by the DFT delocalization error when calculating such shifts. It is found that the α versus β spin balance of oxygen σ donation to metal valence d acceptor orbitals is responsible for the sign and the magnitude of the ligand methyl carbon chemical shift. A problematic case is the methyl carbon shift of Fe(acac)(3). Most functionals produce shifts in excess of 1400 ppm, whereas the experimental shift is approximately 279 ppm. Range-separated hybrid functionals that are optimally tuned for Fe(acac)(3) based on DFT energetic criteria predict a lower limit of about 2000 ppm for the methyl carbon shift of the high-spin electronic configuration. Since the experimental value is based on a very strongly broadened signal it is possibly unreliable.

Unoccupied states in Cu and Zn octaethyl-porphyrin and phthalocyanine

Copper and zinc phthalocyanines and porphyrins are used in organic light emitting diodes and dye-sensitized solar cells. Using near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the Cu 2p and Zn 2p edges, the unoccupied valence states at the Cu and Zn atoms are probed and decomposed into 3d and 4s contributions with the help of density functional calculations. A comparison with the N 1s edge provides the 2p states of the N atoms surrounding the metal, and a comparison with inverse photoemission provides a combined density of states.

Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts

We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt-Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L(2)-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.

Realization of an atomic sieve: Silica on Mo(1 1 2)

The adsorption of Pd, Ag and Au atoms on a porous silica film on Mo(1 1 2) is investigated by scanning tunneling microscopy and density functional theory. While Pd atoms are able to penetrate the holes in the silica top-layer with virtually no barrier, Ag atoms experience an intermediate barrier value and Au atoms are completely unable to pass the oxide surface. The penetration probability does not correlate with the effective size of the atoms, but depends on their electronic structure. Whereas Pd with an unoccupied valence s-orbital has a low penetration barrier, Ag and Au atoms with occupied s-states experience a substantial repulsion with the filled oxide states, leading to a higher barrier for penetration. In the case of Ag, the barrier height can be temporally lowered by promoting the Ag 5s-electron into the support. The Mo-supported silica film can thus be considered as a primitive form of an atomic sieve whose selectivity is controlled by the electronic structure of the adatoms.

Electronic structure ofMn12derivatives on the clean and functionalized Au surface

We present a detailed study on the electronic properties of monolayers of ${\mathrm{Mn}}_{12}$ derivatives chemically grafted on clean as well as on functionalized Au(111) surfaces. Scanning tunneling microscopy and x-ray photoelectron spectroscopy were employed to ensure the successful monolayer deposition. Unoccupied and occupied valence band states in the electronic structure of ${\mathrm{Mn}}_{12}$-clusters were probed by means of x-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES) at the Mn $2p\text{\ensuremath{-}}3d$ absorption edge, respectively. XAS measurements reveal a significant difference between the Mn oxidation states of ${\mathrm{Mn}}_{12}$ cores bound to the Au surface compared with the single crystal environment. Direct deposition of ${\mathrm{Mn}}_{12}$ derivatives onto the Au surface leads to a strong fragmentation of ${\mathrm{Mn}}_{12}$ cores. An appropriate combination of ${\mathrm{Mn}}_{12}$ cluster and substrate functionalization leads to formation of ${\mathrm{Mn}}_{12}$ monolayers with a large fraction of ${\mathrm{Mn}}_{12}$ cores retaining their structural integrity upon surface deposition. By extracting the Mn $3d$ partial density of states from the RPES spectra a very good agreement with previously reported $\mathrm{LDA}+U$ calculations on ${\mathrm{Mn}}_{12}$ for $U=4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ [Boukhvalov et al., J. Electron Spectrosc. Relat. Phenom. 137--140, 735 (2004)] was found, showing the importance of electron correlation effects in ${\mathrm{Mn}}_{12}$ systems.

Inner-shell spectroscopy by the Gaussian and augmented plane wave method

We present an approach for calculating near-edge X-ray absorption spectra at the density functional theory level, which is suited for condensed matter simulations. The method is based on the standard solution of the all-electron KS equations with a modified core-hole potential, which reproduces the relaxation of the orbitals induced by the promotion of the core electron to an unoccupied valence level. The all-electron description of the charge density is based on the Gaussian and augmented plane wave formalism. The reliability of the proposed method is assessed by comparing the computed spectra of some small molecules in the gas phase to the experimental spectra reported in literature. The sensitivity of the computed spectra to the local environment, i.e. the specific bonds formed by the absorbing atom or the presence of hydrogen bonds, open promising perspective for this technique as a predictive tool in the investigation of a more complex system of an unknown structure. The straightforward extension of the method to condensed matter is demonstrated by the calculation of the C K-edge in diamond.

Surface electronic properties of H‐terminated diamond in contact with adsorbates and electrolytes

AbstractA comprehensive summary of surface electronic properties of undoped hydrogen terminated diamond covered with adsorbates or in electrolyte solutions is given. The formation of a conductive layer at the surface is characterized using Hall effect, conductivity, contact potential difference (CPM), scanning electron microscopy (SEM), and cyclic voltammetry experiments. Data are from measurements on homoepitaxially grown CVD diamond films with atomically smooth hydrogen terminated surfaces. The data show that due to electron transfer from the valence band into empty states in of the electrolyte, a highly conductive surface layer is generated. Holes propagate in the layer with mobilities up to 350 cm2/Vs. The sheet hole density in this layer is in the range 1011 to 5 × 1012 cm−2, and dependents on pH of the electrolyte or adsorbate. This has been utilized to manufacture ion sensitive field effect transistors (ISFET) from diamond. The drain source conductivity of single crystalline CVD diamond is pH dependent, with about 66 mV/pH, which is in reasonable agreement with the Nernst law. Due to strong coulomb repulsion between positive ions in the electrolyte and the H+‐surface termination of diamond, an enlarged tunneling gap is established which prevents electronic interactions between the electrolyte and diamond. This is a “virtual gate insulator” of diamond ISFETs. Application of potentials larger than the oxidation threshold of +0.7 V (pH 13) to +1.6 V (pH 1) gives rise to strong leakage currents and to partial surface oxidation. In addition, the electronic interaction of diamond with redox couples is characterized using cyclic voltammetry experiments. The results are well described by the transfer doping model which accounts for the specific properties of undoped diamond immersed in electrolyte solutions are covered simply by adsorbates. In addition, numerical solutions of the Schrödinger and Poisson equations are used to show that the density of state distribution of unoccupied valence band states in case of a perfect interface would be governed by two‐dimensional properties (2D). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Inverse photoemission spectroscopy of multiwall carbon nanotubes

Multiwall Carbon Nanotubes (MWCNTs) were synthesized by Chemical Vapor Deposition (CVD). Two different procedures were used to grow MWCNT films roughly, aligned in the direction normal to the SiO2/Si(111) substrate. Inverse Photoemission Spectroscopy measurements, on these samples, show the existence of resonances which could be traced back to a flat graphene sheet. The unoccupied valence band is fairly similar to that shown by graphite except by an additional intensity in the vicinity of the Fermi level. This resonance could be interpreted both as tubes tips end effects or van Hove singularities in the density of states.

Hybrid exchange-correlation functional for core, valence, and Rydberg excitations: Core-valence-Rydberg B3LYP

The core-valence-Rydberg Becke's three-parameter exchange (B3)+Lee-Yang-Parr (LYP) correlation functional (CVR-B3LYP) is proposed as a means to improve descriptions of Rydberg excitations of core-valence B3LYP (CV-B3LYP). CV-B3LYP describes excitations from both core and occupied valence orbitals to unoccupied valence orbitals with high accuracy but fails to describe those to Rydberg orbitals. CVR-B3LYP, which adopts the appropriate portions of Hartree-Fock exchange for unoccupied valence and Rydberg regions separately, overcomes the disadvantage of CV-B3LYP. Numerical assessment confirms that time-dependent density functional theory calculations with CVR-B3LYP succeed in describing not only core excitations but also Rydberg excitations with reasonable accuracy.

Carbon 1s Excitation Spectroscopy of Propyne, Trifluoropropyne, and Propargyl Alcohol

The C 1s excitation spectra of propyne (HC2CH3), 3,3,3-trifluoropropyne (HC2CF3), and propargyl alcohol (HC3CH3OH) have been studied using synchrotron radiation and ion time-of-flight mass spectrometry. Discrete peaks below the carbon 1s ionization thresholds are compared and assigned, aided in part by ab-initio calculations incorporating an explicit C 1s hole. Calculated C 1s ionization potentials are in good agreement with previously reported experimental values. Calculated absolute excitation energies consistently underestimate the transition term values, but calculated relative excitation energies and intensities are in good agreement with the experimental results. The spectra are dominated by intense C 1s --> pi transitions. In the case of propyne, C 1s excitations from each of the three chemically inequivalent carbon atoms are observed. The effect of electronegative substitution is found to be different for the C 1s --> Rydberg transitions than for transitions to unoccupied valence levels, with Rydberg transition energies shifting with changes in the C 1s ionization potentials but valence transition energies showing only small changes with electronegative substitution. The C 1s (3a1,4a1) --> pi (6e) transitions of trifluoropropyne are shifted to lower energy relative to propyne even though the electronegative fluorine atoms cause a significant shift to higher energy in the corresponding C 1s IPs.

Photoemission and Near-Edge X-Ray Absorption Fine Structure Studies of the Bacterial Surface Protein Layer ofBacillussphaericusNCTC 9602

The electronic structure of the regular, two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602 has been examined by photoemission (PE) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both the O 1s and the N 1s core-level PE spectra show a single structure, whereas the C 1s core-level spectrum appears manifold, suggesting similar chemical states for each oxygen atom and also for each nitrogen atom, while carbon atoms exhibit a range of chemical environments in the different functional groups of the amino acids. This result is supported by the element-specific NEXAFS spectra of the unoccupied valence electronic states, which exhibit a series of characteristic NEXAFS peaks that can be assigned to particular molecular orbitals of the amino acids by applying a phenomenological building-block model. The relative contributions of the C-O, C-N, and C-C bond originating signals into the C 1s PE spectrum are in good agreement with the number ratios of the corresponding bonds calculated from the known primary structure of the bacterial surface protein. First interpretation of the PE spectrum of the occupied valence states is achieved on the basis of electronic density-of-states calculations performed for small peptides. It was found that mainly the pi clouds of the aromatic rings contribute to both the lowest unoccupied and the highest occupied molecular orbitals.

Photoemission Studies of Polythiophene and Polyphenyl Films Produced via Surface Polymerization by Ion-Assisted Deposition

Conducting polymer films are grown by mass-selected, hyperthermal thiophene ions coincident on a surface with a thermal beam of organic monomers of either alpha-terthiophene (3T) or p-terphenyl (3P) neutrals. Mass spectrometry and X-ray photoelectron spectroscopy previously verified polymerization of both 3T and 3P by 200 eV C(4)H(4)S(+) during surface polymerization by ion-assisted deposition (SPIAD). The electronic structure of these films are probed here by ultraviolet photoelectron spectroscopy (UPS) and polarized near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and compared with similar spectra of evaporated films. The conducting polymer films formed by SPIAD display new valence band features resulting from a reduction in both their band gap and barrier to hole injection, which are calculated from the occupied and unoccupied valence band states measured by UPS and NEXAFS. These changes in film electronic structure result from an increase in the electron conjugation length and other changes in film structure induced by SPIAD.

Electronic Structure of Regular Bacterial Surface Layers

We report photoemission and near-edge x-ray absorption fine structure measurements of the occupied and unoccupied valence electronic states of the regular surface layer of Bacillus sphaericus, which is widely used as the protein template for the fabrication of metallic nanostructures. The two-dimensional protein crystal shows a semiconductorlike behavior with a gap value of approximately 3.0 eV and the Fermi energy close to the bottom of the lowest unoccupied molecular orbital. We anticipate that these results will open up new possibilities for the electric addressability of biotemplated low-dimensional hybrid structures.

Resonant OKα emission spectra of CuGeO 3 single-crystal

The electronic structure of CuGeO 3 has been studied using soft X-ray absorption and emission spectroscopy. The unoccupied and occupied valence states are reflected by O 1s absorption and OKα emission spectra, respectively. An absorption pre-peak appearing at 531 eV in O 1s absorption spectrum indicates the localized nature of the unoccupied O 2p admixture to Cu 3d states. A large band gap is observed in the absorption–emission spectrum. Some changes of spectral profile of the OKα emission spectra are attributed to the existence of the inequivalent oxygen sites.

The improved virtual orbital-complete active space configuration interaction method, a “packageable” efficient ab initio many-body method for describing electronically excited states

We describe a computationally efficient ab initio many-body method that can be used as a “packageable approximation” for computing excited state properties for small to large molecular systems, including those of multiconfigurational character. The method is based on first order multi-reference many-body perturbation theory (MR-MBPT), where the unoccupied valence orbitals are obtained by using an extension of Huzinaga’s improved virtual orbital (IVO) generation technique. Because the method employs a complete active space (CAS) which contains singly, doubly, and higher excited state configurations with respect to the zeroth order ground state configuration, the approach (IVO-CASCI) is capable of providing a more accurate description of the excited states than the widely used packageable configuration interaction with singles (CIS) at a fraction of computational labor. Moreover, unlike the CASSCF approach this IVO-CASCI method does not require iterations and therefore is more computationally efficient and free of the convergence problems that sometimes plague CASSCF calculations with increasing size of the CAS. Excited state energies are compared with energies from the widely used CIS, MCSCF, and CASSCF methods for the C2H+, C2H, CaOH, cyclic-C3H, and porphin molecules. The computed IVO-CASCI transition energies are generally more accurate than the CASSCF. For example, our energies are comparable to CIS energies for CaOH and porphin, while the C2H+, C2H, and C3H IVO-CASCI transition energies are more accurate than the CASSCF and CIS energies.