Probing the valence character of O 1s-->Rydberg excited O2 by participator Auger decay measurements and partial ion yield spectroscopy following x-ray absorption

The valence character of O 1s-->Rydberg excited O2 is investigated by means of participator Auger decay spectroscopy, performed at selected photon energies across the K-shell resonance region, and by means of partial ion yield x-ray absorption spectroscopy. For several of the excitation energies studied, the authors find substantial sigma*(4Sigmau-, 2Sigmau-) valence character being mixed with nssigma and npsigma (4Sigmau-, 2Sigmau-) Rydberg states. An experimental indication of a coupling between the channels associated with quartet and doublet ion cores is considered and discussed. New spectroscopic constants are derived for the singly ionized X 2Pig state of O2 based on the observation of at least 20 vibrational sublevels.     

Angular distribution of different vibrational components of the X and B states reached after resonant Auger decay of core-excited H2O: experiment and theory

Vibrationally resolved spectra have been obtained for the lowest-lying cationic states X (2)B(1), A (2)A(1), and B (2)B(2) of the water molecule reached after participator resonant Auger decay of core-excited states. The angular distribution has been measured of the first four vibrational components of the X state in the photon energy regions including the O 1s-->4a(1) and the O 1s-->2b(2) core excitations, and for different portions of the vibrational envelope of the B state in the photon energy region including the O 1s-->2b(2) core excitation. For the X state, a large relative spread in beta values of the different vibrational components is observed across both resonances. For the B state, a very different trend is observed for the high binding energy side and the low binding energy side of the related spectral feature as a function of photon energy. A theoretical method based on the scattering K matrix has been used to calculate both the photoabsorption spectrum and the beta values, by taking both interference between direct and resonant photoemission and vibrational/lifetime interference into account. The numerical results show qualitative agreement with the trends detected in the experimental values and explain the conspicuous variations of the beta values primarily in terms of coupling between direct and resonant photoemission by interaction terms of different sign for different final vibrational states.     

Mechanism of anion formation in C 1s-->pi*-excited carbon dioxide

The mechanism of anion formation from core-excited carbon dioxide is investigated in the C 1s-excitation regime (280-340 eV), where negative-ion-positive-ion coincidences and negative-ion-positive-ion-positive-ion coincidences experiments are performed. O(-) formation occurs efficiently upon C 1s-->pi(*)-excitation (290.7 eV) from the singly charged cation CO(2) (+). This anion is measured in coincidence with the singly charged atoms C(+) and O(+). The formation of atomic cations is accompanied by a substantial kinetic energy release. This suggests that an intermediate CO(++) is formed together with O(-), where the dication decays via fission. The results are discussed in terms of an anion formation mechanism from core-excited molecules considering previous experimental results.     

Combined X-ray absorption spectroscopy and density functional theory examination of ferrocene-labeled peptides

A combination of soft X-ray absorption spectroscopy (XAS) measurements and StoBe density functional theory (DFT) calculations has been used to study the electronic structures of the ferrocene-labeled peptides Fc-Pro(n)-OBz (n = 1-4). Excellent agreement between the measured and the simulated data is observed in all cases, and the origin of all major spectral features was assigned. The breaking of the degeneracy of the ferrocene 3e(2u)-like unoccupied molecular orbital under the influence of a substituent attached to a Cp ring was observed experimentally. The influence of the bonding environment on the O 1s and N 1s XAS spectra was examined. A corrected assignment of one of the major features in the Fe 2p XAS spectra of ferrocene is proposed and supported by the DFT simulations, as well as the measured spectra.     

Hydrogen bonding of water in 3-methylpyridine studied by O 1s X-ray emission and absorption spectroscopy

O 1s X-ray emission and X-ray absorption spectroscopy is applied to probe hydrogen bonding of water (D(2)O) in 3-methylpyridine. Owing to element selectivity of X-ray spectroscopies the electronic structure of water in the binary mixture was observed selectively. Based on the observed spectral changes associated with hydrogen bonding in O 1s X-ray emission and X-ray absorption spectra, we have investigated the hydrogen bond of the mixture sample over a wide range of D(2)O concentrations (X(D(2)O) = 0.02-1.0) at room temperature under atmospheric pressure.     

Fluorescence and metastability of N2O2+: theory and experiment

State-selective mass spectrometry has revealed one conclusive and another probable metastable state of the N2O2+ dication, assigned respectively as 1 3Pi at 38.5 eV and 2 3Pi at 42.5 eV. Photon coincidence experiments confirm that dissociation of 1 3Pi is preceded by a fluorescent transition to X 3Sigma- and also indicate that an identical mechanism occurs for 2 3Pi. Highly correlated MRCI calculations are performed at a range of N2O2+ geometries, from which both N-N and N-O bond stretching curves are generated. Substantial barriers along both coordinates are observed for 1 3Pi and 2 3Pi, although the increasing density of states at higher energy may allow spin-orbit or vibronic predissociation for 2 3Pi. Fragment emissions derived from N2O+ and N2O2+ are analyzed with the aid of glass filters, from which NO (X 2Pi<--A 2Sigma+) and vibrationally excited N2+ (X 2Sigmag+<--B 2Sigmau+) transitions are deduced.     

Core localization and sigma* delocalization in the O 1s core-excited sulfur dioxide molecule

Electron-ion-ion coincidence measurements of sulfur dioxide at discrete resonances near the O 1s ionization edge are reported. The spectra are analyzed using a model based upon molecular symmetry and on the geometry of the molecule. We find clear evidence for molecular alignment that can be ascribed to symmetry properties of the ground and core-excited states. Configuration interaction (CI) calculations indicate geometry changes in accord with the measured spectra. For the SO(2) molecule, however, we find that the localized core hole does not produce measurable evidence for valence localization, since the transition dipole moment is not parallel to a breaking sigma* O-S bond, in contrast to the case of ozone. The dissociation behavior based upon the CI calculations using symmetry-broken orbitals while fixing a localized core-hole site is found to be nearly equivalent to that using symmetry-adapted orbitals. This implies that the core-localization effect is not strong enough to localize the sigma* valence orbital.     

Formation of solid solution and its effect on lithium insertion schemes for advanced lithium-ion batteries: X-ray absorption spectroscopy and X-ray diffraction of LiCoO2, LiCo1/2Ni1/2O2 and LiNiO2

The X-ray absorption spectra (XAS) of LiCoO₂, LiCo_1/2Ni_1/2O₂ and LiNiO₂ were examined together with X-ray diffraction (XRD). Co and Ni K-edge XANES spectra of LiCo_1/2Ni_1/2O₂ are quite similar to that of LiCoO₂ or LiNiO₂, suggesting that electronic states of Co and Ni in LiCo_1/2Ni_1/2O₂ are Co³⁺ and Ni³⁺. Analytical results of Co and Ni K-edge EXAFS oscillations on the first coordination shell of nickel and cobalt ions in LiCo_1/2Ni_1/2O₂ indicate that the local environment around the targeted species is the same as that in LiCoO₂ or LiNiO₂. Since there is no doubt about the crystal and electronic structures of LiCoO₂ and LiNiO₂, the results indicate that LiCo_1/2Ni_1/2O₂ consists of low-spin states of Co³⁺ and Ni³⁺ distributed at equivalent positions in triangular lattice of sites forming homogeneous transition metal oxide layers. Thus, XAS complements XRD in describing solid solution LiCo_1/2Ni_1/2O₂ of LiCoO₂ and LiNiO₂. The electrochemical behaviors of LiCoO₂, LiCo_1/2Ni_1/2O₂ and LiNiO₂ are also restated and the effects of the formation of solid solution on the change in lattice dimension during topotactic electrochemical reactions are discussed.     

Structural environment of uranium (VI) species sorbed onto ZrP2O7: X-ray absorption spectroscopy study

The structure of the surface complex formed during the sorption of UO₂²⁺_aq ion onto the selected phosphate solid has been investigated mainly using X-ray absorption spectroscopy. Samples were prepared by batch experiments. L_III U edge measurements have shown that uranyl ions are sorbed on the phosphate surface as a mononuclear bidentate inner-sphere complex.     

O2 reduction on graphite and nitrogen-doped graphite: experiment and theory

An experimental and theoretical study of electroreduction of oxygen to hydrogen peroxide is presented. The experimental measurements of nitrided Ketjenblack indicated an onset potential for reduction of approximately 0.5 V (SHE) compared to the onset potential of 0.2 V observed for untreated carbon. Quantum calculations on cluster models of nitrided and un-nitrided graphite sheets show that carbon radical sites formed adjacent to substitutional N in graphite are active for O2 electroreduction to H2O2 via and adsorbed OOH intermediate. The weak catalytic effect of untreated carbon is attributed to weaker bonding of OOH to the H atom-terminated graphite edges. Substitutional N atoms that are far from graphite sheet edges will be active, and those that are close to the edges will be less active. Interference from electrochemical reduction of H atoms on the reactive sites is considered, and it is shown that in the potential range of H2O2 formation the reactive sites are not blocked by adsorbed H atoms.     

An X-ray absorption spectroscopy investigation of the formation of FeCo alloy nanoparticles in Al2O3 xerogel and aerogel matrixes

Extended X-ray absorption fine structure and X-ray absorption near-edge structure techniques were used to study in detail the structural characteristics of FeCo-Al2O3 nanocomposite xerogels and aerogels. The formation of bcc FeCo alloy, which cannot be assessed unambiguously by X-ray diffraction, dispersed within the alumina matrix was evidenced in the final samples obtained by heat treatment at 800 degrees C in reducing atmosphere. Aerogel samples reduced below 800 degrees C still present a fraction of oxidized metal together with the bcc alloy. The investigation of the xerogels and aerogels calcined at increasing temperature indicates that Fe(III) and Co(II) ions are present and they are located in the tetrahedral sites of the spinel structure of the matrix (gamma-Al2O3); moreover, the precursor of the spinel is more ordered in the aerogel sample than the xerogel sample.     

Polarization dependent resonant x-ray emission spectroscopy of D2O and H2O water: assignment of the local molecular orbital symmetry

The polarization dependence of the split two peaks in the lone-pair region in the x-ray emission spectra has been determined at several different excitation energies for both D(2)O and H(2)O water. In contrast to predictions based on a narrow range of local water structures where the two peaks would be of different molecular orbital symmetry and arise from, respectively, intact and dissociated molecules, we show that the two peaks in the lone-pair region are both of lone-pair 1b(1) orbital symmetry. The results support the interpretation that the two peaks appear due to fluctuations between two distinct different main structural environments.     

Advanced X-ray absorption and emission spectroscopy: in situ catalytic studies

Knowledge of the structure of catalysts is essential to understand their behavior, which further facilitates development of an active, selective, and stable catalyst. Determining the structure of a functioning catalyst is essential in this regard. The structure of a catalyst is prone to change during the catalytic process and needs to be determined in its working conditions. In this tutorial review, we have summarized studies done at synchrotron radiation facilities that illustrate the capability to determine catalyst structure using X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). These studies aim at facilitating the determination of the dynamic structure-performance relationships during a catalytic process.     

Multi-electron spectroscopy: Auger decays of the argon 2s hole

Auger decay of an inner shell hole is an efficient way to create multiply charged ions in the gas phase. We illustrate this with the example of the argon 2s decay, and show that multi-electron coincidence spectroscopy between the 2s photoelectron and all released Auger electrons leads to a complete reconstruction of the Ar 2s decay cascade. Spectra of the intermediate and final Ar(n+) states are obtained and are compared with a theoretical model.     

Fifth-order Raman spectroscopy of liquid benzene: experiment and theory

The heterodyned fifth-order Raman response of liquid benzene has been measured and characterized by exploiting the passive-phase stabilization of diffractive optics. This result builds on our previous work with liquid carbon disulfide and extends the spectroscopy to a new liquid for the first time. The all-parallel and Dutch Cross polarization tensor elements are presented for both the experimental results and a finite-field molecular dynamics simulation. The overall response characteristics are similar to those of liquid carbon disulfide: a complete lack of signal along the pump delay, an elongated signal along the probe delay, and a short-lived signal along the time diagonal. Of particular interest is the change in phase between the nuclear and electronic response along the probe delay and diagonal which is not seen in CS2. Good agreement is achieved between the experiment and the finite-field molecular dynamics simulation. The measurement of the low-frequency Raman two-time delay correlation function indicates the intermolecular modes of liquid benzene to be primarily homogeneously broadened and that the liquid loses its nuclear rephasing ability within 300 fs. This rapid loss of nuclear correlations indicates a lack of modal character in the low-frequency motions of liquid benzene. This result is a validation of the general nature of the technique and represents an important step forward with respect to the use of nonlinear spectroscopy to directly access information on the anharmonic motions of liquids.     

Sulfur K-edge X-ray absorption spectroscopy and density functional theory calculations on superoxide reductase: role of the axial thiolate in reactivity

Superoxide reductase (SOR) is a non-heme iron enzyme that reduces superoxide to peroxide at a diffusion-controlled rate. Sulfur K-edge X-ray absorption spectroscopy (XAS) is used to investigate the ground-state electronic structure of the resting high-spin and CN- bound low-spin FeIII forms of the 1Fe SOR from Pyrococcus furiosus. A computational model with constrained imidazole rings (necessary for reproducing spin states), H-bonding interaction to the thiolate (necessary for reproducing Fe-S bond covalency of the high-spin and low-spin forms), and H-bonding to the exchangeable axial ligand (necessary to reproduce the ground state of the low-spin form) was developed and then used to investigate the enzymatic reaction mechanism. Reaction of the resting ferrous site with superoxide and protonation leading to a high-spin FeIII-OOH species and its subsequent protonation resulting in H2O2 release is calculated to be the most energetically favorable reaction pathway. Our results suggest that the thiolate acts as a covalent anionic ligand. Replacing the thiolate with a neutral noncovalent ligand makes protonation very endothermic and greatly raises the reduction potential. The covalent nature of the thiolate weakens the FeIII bond to the proximal oxygen of this hydroperoxo species, which raises its pKa by an additional 5 log units relative to the pKa of a primarily anionic ligand, facilitating its protonation. A comparison with cytochrome P450 indicates that the stronger equatorial ligand field from the porphyrin results in a low-spin FeIII-OOH species that would not be capable of efficient H2O2 release due to a spin-crossing barrier associated with formation of a high-spin 5C FeIII product. Additionally, the presence of the dianionic porphyrin pi ring in cytochrome P450 allows O-O heterolysis, forming an FeIV-oxo porphyrin radical species, which is calculated to be extremely unfavorable for the non-heme SOR ligand environment. Finally, the 5C FeIII site that results from the product release at the end of the O2- reduction cycle is calculated to be capable of reacting with a second O2-, resulting in superoxide dismutase (SOD) activity. However, in contrast to FeSOD, the 5C FeIII site of SOR, which is more positively charged, is calculated to have a high affinity for binding a sixth anionic ligand, which would inhibit its SOD activity.     

Distinguishing the early and late transition states and exploring the validity of sigma --> sigma*#, sigma# --> sigma*, and sigma --> pi*(C=O) concepts in diastereoselection from NBO analysis

Natural bond orbital (NBO) analysis of several early TSs does not support the sigma --> sigma*# hypothesis. The sigma --> pi*(C=O) interaction controls the carbonyl pyramidalization that, in turn, controls the pi-selectivity of a nucleophilic addition. In contrast, late TSs are devoid of sigma --> pi*(C=O) interactions, and they benefit from sigma --> sigma*# interactions that control pi-selectivity. The evidence in favor of Anh-Felkin's sigma# --> sigma* hypothesis is weak. The electron-withdrawing sigma(C-F) in the 2-fluoropropanal-LiCN TS did not align anti to the incipient bond even though there was complete conformational freedom. The initial guess for the TS in which sigma(C-F) was held anti to sigma# optimized to what had lost the said geometrical relationship. Furthermore, in the TS for axial addition of LiCN to 2-ax-F-cyclohexanone, the net sigma --> sigma*# interaction was considerably larger than the net sigma# --> sigma* interaction. The relative TS energies require that the equatorial addition of LiCN to 2-ax-F-cyclohexanone be favored over the axial addition in good compliance with the available experimental results.     

High resolution photofragment translational spectroscopy of the near UV photolysis of indole: dissociation via the 1pi sigma* state

The fragmentation dynamics of indole molecules following excitation at 193.3 nm, and at a number of different wavelengths in the range 240 < or = lambda(phot) < or = 286 nm, have been investigated by H Rydberg atom photofragment translational spectroscopy. The longer wavelength measurements have been complemented by measurements of excitation spectra for forming parent and fragment ions by two (or more) photon ionisation processes. Analysis identifies at least three distinct contributions to the observed H atom yield, two of which are attributable to dissociation of indole following radiationless transfer from the 1pi pi* excited states (traditionally labelled 1L(b) and 1L(a)) prepared by UV single photon absorption. The structured channel evident in total kinetic energy release (TKER) spectra recorded at lambda(phot) < or = 263 nm is rationalised in terms of N-H bond fission following initial pi* <-- pi excitation and subsequent coupling to the 1pi sigma* potential energy surface via a conical intersection between the respective surfaces--thereby validating recent theoretical predictions regarding the importance of this process (Sobolewski et al., Phys. Chem. Chem. Phys., 2002, 4, 1093). Analysis provides an upper limit for the N-H bond strength in indole: D0(H-indolyl) < or = 31,900 cm(-1). Unimolecular decay of highly vibrationally excited ground state molecules formed by internal conversion from the initially prepared 1pi pi* states is a source of (slow) H atoms but their contribution to the TKER spectra measured in the present work is dwarfed by that from H atoms generated by one or more (unintended but unavoidable) multiphoton processes.