High-resolution Absorption Spectroscopy Research Articles

High-resolution X-ray absorption spectroscopy of iron carbonyl complexes.

We apply high-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge spectroscopy (XANES) to study iron carbonyl complexes. Mono-, bi-, and tri-nuclear carbonyl complexes and pure carbonyl complexes as well as carbonyl complexes containing hydrocarbon ligands are considered. The HERFD-XANES spectra reveal multiple pre-edge peaks with individual signatures for each complex, which could not be detected previously with conventional XANES spectroscopy. These peaks are assigned and analysed with the help of TD-DFT calculations. We demonstrate that the pre-edge peaks can be used to distinguish the different types of iron-iron interactions in carbonyl complexes. This opens up new possibilities for applying HERFD-XANES spectroscopy to probe the electronic structure of iron catalysts.

CO 近红外高分辨吸收光谱特性的实验研究

CO 近红外高分辨吸收光谱特性的实验研究

Detection and analysis of three highly excited vibrational bands of 16O3 by CW-CRDS near the dissociation threshold

The present contribution is devoted to the analysis of three extremely weak A-type bands of 16O3 recorded near 7686, 7739 and 7860cm−1i.e. only a few % below the dissociation limit, D0, at about 8560cm−1. They correspond to the most excited vibration–rotation states of ozone observed so far via high-resolution absorption spectroscopy. They were detected by high sensitivity CW-cavity ring down spectroscopy with a typical noise equivalent absorption, αmin, on the order of 2×10−10cm−1 and modelled using the effective operator approach. The derived band centres and rotational constants show a very good agreement with recent theoretical predictions based on a new ab initio potential energy surface.

Dual-comb spectroscopy based on quantum-cascade-laser frequency combs

Dual-comb spectroscopy performed in the mid-infrared-where molecules have their strongest rotovibrational absorption lines-offers the promise of high spectral resolution broadband spectroscopy with very short acquisition times (μs) and no moving parts. Recently, we demonstrated frequency comb operation of a quantum-cascade-laser. We now use that device in a compact, dual-comb spectrometer. The noise properties of the heterodyne beat are close to the shot noise limit. Broadband (15 cm(-1)) high-resolution (80 MHz) absorption spectroscopy of both a GaAs etalon and water vapour is demonstrated, showing the potential of quantum-cascade-laser frequency combs as the basis for a compact, all solid-state, broadband chemical sensor.

Site-selective high-resolution X-ray absorption spectroscopy and high-resolution X-ray emission spectroscopy of cobalt nanoparticles.

The special (macroscopic) properties of nanoparticles are mainly due to their large surface-to-volume ratio. Thus, the separate characterization of geometric and electronic properties of surface and bulk would be favorable for a better understanding of the properties of nanoparticles. Because of the chemical sensitivity of X-ray fluorescence lines, in particular those involving higher lying electronic states, high-resolution fluorescence-detected X-ray absorption spectra (HRFD-XAS) offer these opportunities. In this study, three types of wet-chemically synthesized Co nanoparticles, ∼6 nm in diameter with varying thicknesses of a protective shell, were investigated at the ID26 beamline of the European Synchrotron Radiation Facility. HRFD-XAS spectra at the Co K-edge, that is, X-ray absorption near-edge structure (HRFD-XANES) and extended X-ray absorption fine structure (HRFD-EXAFS) spectra, were recorded via detection of the Kβ1,3 fluorescence at specific energies. As these spectra are only partly site-selective due to a strong overlap of the emission lines, a numerical procedure was applied based on a least-squares fitting procedure, realized by singular value decomposition. The detailed analysis of the obtained site-selective spectra, regarding chemical composition and crystallographic phase, using measured and simulated FEFF9-based reference spectra, showed that the metallic core had mainly hexagonal close-packed structure with lattice constants matching bulk Co; the spectra for the shell could be satisfactorily fitted by a mixture of CoO and CoCO3; however, with an obvious need for at least a third compound. To obtain additional information about ligands attached to Co, valence-to-core X-ray emission spectra (VTC-XES) using the Kβ2,5 and the satellite structure Kβ″ and VTC-XANES spectra thereof were also recorded, by which the former results are confirmed. Further on, FEFF simulations indicate that a Co-N compound is a very likely candidate for the third component. The presented results clearly show that VTC-XES and HRFD-XAS are suitable tools for the detailed specification of the core and the surface of nanoparticles, in particular upon realizing "real" site-selectivity for XANES and EXAFS with a general strategy applicable to a wide range of systems.

Study of the electronic and magnetic properties as a function of isoelectronic substitution in SmFe1−xRuxAsO0.85F0.15

We have studied the electronic and magnetic properties of SmFe1−xRuxAsO0.85F0.15 (x = 0, 0.05, 0.25, 0.33, 0.5) by high-resolution x-ray absorption and x-ray emission spectroscopy. The local Fe magnetic moment (μ) tends to decrease for a small Ru substitution, but it shows a clear increase with further substitution. It appears that impurity scattering prevails in reducing the μ with small Ru substitution due to an extended Ru d-band. A nanoscale phase separation, that decouples the FeAs layers from the spacer layers, drives the increase of μ at higher Ru substitution. The results provide important information on nanoscale phase separation due to isoelectronic substitution in the active layers of iron-based 1111-superconductors and its effect on the local magnetic properties.

Photoluminescence in ZnO:Co2+ (0.01%–5%) Nanoparticles, Nanowires, Thin Films, and Single Crystals as a Function of Pressure and Temperature: Exploring Electron–Phonon Interactions

This work investigates the electronic structure and photoluminescence properties of Co2+-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co2+ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co2+ (0.01 mol %) can be related to the 4T1(P) → 4A2(F) transition of CoO4 (Td), contrary to MgAl2O4:Co2+ and ZnAl2O4:Co2+ spinels in which the red emission is ascribed to the 2E(G) → 4A2(F) transition. We show that the low-temperature emission band consists of a 4T1(P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zero-phonon line to higher energy as expected for the 4T1(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rock-salt structural phase transition, yielding a change of Co2+ coordination from 4-fold Td to 6-fold Oh . We also show that the optical properties of ZnO:Co2+ (Td) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co2+ enters into regular Zn2+ sites in low concentration systems (less than 5% of Co2+), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also supported by Raman spectroscopy.

High‐resolution absorption spectroscopy of the circumgalactic medium of the Milky Way

AbstractIn this article we discuss the importance of high‐resolution absorption spectroscopy for our understanding of the distribution and physical nature of the gaseous circumgalactic medium (CGM) that surrounds the Milky Way. Observational and theoretical studies indicate a high complexity of the gas kinematics and an extreme multi‐phase nature of the CGM in low‐redshift galaxies. High‐precision absorption‐line measurements of the Milky Way's gas environment thus are essential to explore fundamental parameters of circumgalactic gas in the local Universe, such as mass, chemical composition, and spatial distribution. We shortly review important characteristics of the Milky Way's CGM and discuss recent results from our multi‐wavelength observations of the Magellanic Stream. Finally, we discuss the potential of studying the warm‐hot phase of the Milky Way's CGM by searching for extremely weak [Fe X] λ 6374.5 Å and [Fe XIV] λ 5302.9 Å absorption in optical QSO spectra. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of the Rate Constant for the OH(X2Π) + OH(X2Π) → H2O + O(3P) Reaction Over the Temperature Range 295 to 701 K

The rate constant for the radical-radical reaction OH(X(2)Π) + OH(X(2)Π) → H2O + O((3)P) has been measured over the temperature and pressure ranges 295-701 K and 2-12 Torr, respectively, in mixtures of CF4, N2O, and H2O. The OH radical was produced by the 193 nm laser photolysis of N2O. The resulting O((1)D) atoms reacted rapidly with H2O to produce the OH radical. The OH radical was detected by high-resolution time-resolved infrared absorption spectroscopy using a single Λ-doublet component of the OH(1,0) P1e/f(4.5) fundamental vibrational transition. A detailed kinetic model was used to determine the reaction rate constant as a function of temperature. These experiments were conducted in a new temperature controlled reaction chamber. The values of the measured rate constants are quite similar to the previous measurements from this laboratory of Bahng and Macdonald (J. Phys. Chem. A 2007 , 111 , 3850 - 3861); however, they cover a much larger temperature range. The results of the present work do not agree with recent measurements of Sangwan and Krasnoperov (J. Phys. Chem. A 2012 , 116 , 11817 - 11822). At 295 K the rate constant of the title reaction was found to be (2.52 ± 0.63) × 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes both experimental scatter and an estimate of systematic errors at the 95% confidence limit. Over the temperature range of the experiments, the rate constant can be represented by k1a = 4.79 × 10(-18)T(1.79) exp(879.0/T) cm(3) molecule(-1) s(-1) with a uncertainty of ±24% at the 2σ level, including experimental scatter and systematic error.

Electrochromic performance of nanocomposite nickel oxide counter electrodes containing lithium and zirconium

Nickel oxide materials are suitable for counter electrodes in complementary electrochromic devices. The state-of-the-art nickel oxide counter electrode materials are typically prepared with multiple additives to enhance peformance. Herein, nanocomposite nickel oxide counter electrodes were fabricated via RF magnetron co-sputtering from Ni–Zr alloy and Li2O ceramic targets. The as-deposited nanocomposite counter electrodes were characterized with inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). It was found that the stoichiometry, crystal structure and electronic structure of the nickel oxide-based materials could be readily tuned by varying the Li2O sputter deposition power level. Comprehensive electrochromic evaluation demonstrated that the performance of the nickel oxide-based materials was dependent on the overall Li stoichiometry. Overall, the nanocomposite nickel oxide counter electrode containing lithium and zirconium synthesized with a Li2O deposition power of 45W exhibited the optimal performance with an optical modulation of 71% and coloration efficiency of 30cm2/C at 670nm in Li-ion electrolyte.

Oxygen Evolution Activity and Stability of Ba6Mn5O16, Sr4Mn2CoO9, and Sr6Co5O15: The Influence of Transition Metal Coordination

Several coordination motifs of cobalt and manganese ions were obtained in various transition metal oxides, which enabled different oxidation and spin states. Combined high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) confirmed the presence of coordination environments such as Co2+ in disordered prisms and Co4+/Mn4+ in face-shared octahedra. The influence of cobalt and manganese coordination on the oxygen evolution reaction (OER) activity and oxide stability in alkaline solution was studied. Under cycling, the surface of perovskites that consists of Co2+ in prisms was amorphized and the activity was similar to that of LaCoO3, which has a stable surface composed of Co3+ in octahedral coordination. These findings highlight the critical role of the electronic structure of transition metal oxides on the OER activity and stability.

Fundamental vibrational transitions of hydrogen chloride detected in CRL 2136

We would like to understand the chemistry of dense clouds and their hot cores more quantitatively by obtaining more complete knowledge of the chemical species present in them. We have obtained high-resolution infrared absorption spectroscopy at 3-4 um toward the bright infrared source CRL 2136. The fundamental vibration-rotation band of HCl has been detected within a dense cloud for the first time. The HCl is probably located in the warm compact circumstellar envelope or disk of CRL 2136. The fractional abundance of HCl is (4.9-8.7)e-8, indicating that approximately 20 % of the elemental chlorine is in gaseous HCl. The kinetic temperature of the absorbing gas is 250 K, half the value determined from infrared spectroscopy of 13CO and water. The percentage of chlorine in HCl is approximately that expected for gas at this temperature. The reason for the difference in temperatures between the various molecular species is unknown.

Determination of Bond Lengths and Electronic Structure of Cd<sub>1-x</sub>Zn<sub>x</sub>Te Ternary Alloys by Synchrotron Radiation

High-resolution synchrotron radiation x-ray absorption spectroscopy on Zn K-, Cd L3- and Te L3-edges for Cd1-xZnxTe ternary alloys with x = 0.10, 0.30, 0.50 and 0.90 are presented. A detailed analysis of the extended x-ray absorption fine structure using the IFEFFIT program, and the chemical bonds of Zn-Te are obtained, suggesting distortion of the Te sub-lattice. The x-ray absorption near-edge structure of the Zn K-, Cd L3- and Te L3-edge are investigated, and the electronic structures of Cd1-xZnxTe with various compositions are studied.

Laser Absorption Spectroscopy of the d3Πg ← c3Σu+ Transition of C2

High-resolution laser absorption spectroscopy using concentration modulation has been applied to record the near-infrared spectrum of C2 in an AC hollow-cathode discharge of acetylene/helium mixtures. The (2,1) and (1,0) vibronic bands of the d3Π(g)– c3Σ(u)+ system have been observed in the spectral region between 12,010 and 12,540 cm(–1). While the analysis of the (1,0) band was straightforward, the (2,1) band was found to be perturbed. Using the effective Hamiltonians for 3Π and 3Σ+ states, molecular constants for the vibrational levels of the c3Σ(u)+ state were retrieved in least-squares fits of the observed spectral lines. The experimental conditions, detailed analysis of the perturbations, and the determined molecular constants are reported.

Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduc- tion states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as model system for mixed valent 4f-electron materials. Using high-resolution x-ray absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f-electron materials.

Local surrounding of vanadium atoms in CuCr1 − x V x S2: X-ray absorption spectroscopy analysis

In the present work local surrounding of vanadium atoms in layered copper-chromium disulfides CuCr1 − xVxS2 is investigated using high-resolution X-ray absorption spectroscopy above vanadium K-edge. Based on experimental and theoretically simulated spectra comparison it is shown that vanadium atoms replace chromium ones even at high concentrations of vanadium and that they are in 3+ oxidation state.

Investigation of superexcited states in molecular deuterium

Abstract A high-resolution VUV laser absorption spectroscopy is used to investigate the photon-induced ion-pair formation in D2 in the spectral region from 140117.4 to 140732.0 cm−1. We report determination of photoionization efficiency in ortho-para D2 in supersonic molecular beam with energy resolution of 0.1 cm−1. The assignment of the superexcited D2 Rydberg states is done based on the quantum defect theory.

On the chemical state of Co oxide electrocatalysts during alkaline water splitting

Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.

An investigation of the electrochemical delithiation process of carbon coated α-Fe2O3 nanoparticles

The electrochemical lithiation–delithiation of iron oxide is a rather complex process, which is still not fully understood. In this study we investigated the electrochemical lithiation–delithiation mechanism of hematite by means of X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy, high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS). Since the delithiation process has been so far less investigated, particular attention was dedicated to the characterization of the chemical species that are formed during this process. The results of this investigation indicated that at the end of the delithiation process lithium iron oxide α-LiFeO2 is formed. The formation of this compound may be the explanation for the irreversible capacity loss in the first cycle, which is usually assigned to the formation of an organic gel-like layer. Based on these results a new charge–discharge mechanism of hematite in lithium-ion batteries (LIBs) is proposed and discussed.

High-resolution molybdenum K-edge X-ray absorption spectroscopy analyzed with time-dependent density functional theory

X-ray absorption spectroscopy (XAS) is a widely used experimental technique capable of selectively probing the local structure around an absorbing atomic species in molecules and materials. When applied to heavy elements, however, the quantitative interpretation can be challenging due to the intrinsic spectral broadening arising from the decrease in the core-hole lifetime. In this work we have used high-energy resolution fluorescence detected XAS (HERFD-XAS) to investigate a series of molybdenum complexes. The sharper spectral features obtained by HERFD-XAS measurements enable a clear assignment of the features present in the pre-edge region. Time-dependent density functional theory (TDDFT) has been previously shown to predict K-pre-edge XAS spectra of first row transition metal compounds with a reasonable degree of accuracy. Here we extend this approach to molybdenum K-edge HERFD-XAS and present the necessary calibration. Modern pure and hybrid functionals are utilized and relativistic effects are accounted for using either the Zeroth Order Regular Approximation (ZORA) or the second order Douglas-Kroll-Hess (DKH2) scalar relativistic approximations. We have found that both the predicted energies and intensities are in excellent agreement with experiment, independent of the functional used. The model chosen to account for relativistic effects also has little impact on the calculated spectra. This study provides an important calibration set for future applications of molybdenum HERFD-XAS to complex catalytic systems.