软X射线光谱的第一性原理模拟(英文)

软X射线光谱是通过核激发或去激发以探测分子、表面及各种化合物的电子结构和化学结构的有效的测量技术。本文对基于密度泛函理论描述X射线吸收、发射的各种不同过程的计算方法进行了综述。重点讨论了各种方法的基本原理、实际操作和具体应用。提供了K边X射线光电子能谱、吸收和发射光谱详细的模拟细节以及一些代表性体系的算例(包括分子、富勒烯、碳纳米管、单层石墨和DNA链)。     

Evaluation of antioxidant activity and electronic structure of aspirin and paracetamol

We present a study of electronic structure, chemical bonding, and antioxidant activity of phenolic antioxidants (aspirin and paracetamol). X-ray photoelectron and emission spectra of the antioxidants have been simulated by deMon density functional theory (DFT) calculations of the molecules. The chemical bonding of aspirin is characterized by the formation of oxygen ‘lone-pair’ π-orbitals which can neutralize free radicals and thus be related to antioxidant properties of the drug. In the case of paracetamol the additional nitrogen ‘lone pair’ is formed which can explain toxicity of the drug. We propose an evaluation method of antioxidant activity based on the relationship between experimental half-wave oxidation potential (E_p/2) and calculated ionization potentials (I_P) by the DFT calculations, and can conclude that paracetamol has the higher antioxidant activity than aspirin.     

Analysis of XPS and XES of diamond and graphite by DFT calculations using model molecules

X‐ray photoelectron and emission spectra (XPS and XES) of diamond and graphite have been analyzed by deMon density‐functional theory (DFT) calculations using the model adamantane derivative (C₁₀H₁₂(CH₃)₄) and pyrene (C₁₆H₁₀) molecules, respectively. The theoretical valence photoelectron and C Kα X‐ray emission spectra for the allotrope are in good accordance with the experimental ones. The combined analysis of the valence XPS and C Kα XES enables us to divide the valence electronic distribution into the individual contributions for pσ‐, and pπ‐bonding MOs of the diamond and graphite, respectively. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 102–108, 2001     

Soft X-ray spectroscopic study on the electronic structure of WO3 thin films fabricated under various annealing temperature and gas flow conditions

The electrical properties of WO₃ thin films vary significantly depending on the growth conditions. In this work, the influence of O₂ gas on the band gap of WO₃ thin films during growth was investigated via electronic structure characterization using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). A substantial decrease in the electrical conductivity of the WO₃ films was observed with an increase in the O₂ partial pressure during growth. Spectral differences in the peak energy and intensity were apparent for WO₃ films grown under only Ar and those grown in Ar:O₂. It is difficult to explain the acquired spectrum of WO₃ with oxygen defects through the rigid-band model in terms of the simple addition of electrons to the conduction band of WO₃. Our results show that an oxygen deficiency in WO₃ moves the conduction band to the Fermi edge.     

Electronic structure of Alq3 and Liq using soft X-ray spectroscopy and density functional theory calculation

The electronic structure of aluminum tris-8-hydroxyquinoline (Alq₃) and 8-hydroquinolatolithium (Liq) was investigated using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectral simulation with density functional theory. The chemical bonding states of Alq₃ and Liq were analyzed using XPS core-level spectra. The band gaps of Alq₃ and Liq were measured by combining the XAS and XES results. Additionally, resonant and non-resonant XES were used to measure the density of states for O sites in the molecules.     

Unsaturated surface in CO saturation

In this paper, density functional theory calculations are used to address the puzzle of CO saturation on Fe(100). Through a comprehensive modeling of possible local configurations, we would be able to figure out a structural model for the CO‐saturated Fe(100). The structural model is featured by forming a local structural motif of coupled on‐top and hollow CO molecules. In this model, the on‐top‐bonded CO molecules pertain to critical couplings in the next neighbor spacing, which exclude all couplings below and above the next neighbor spacing. Rather than forming tilted on‐top CO on the surface, the critical couplings align the on‐top CO molecules in the perpendicular orientation. The new model with the coverage of 0.5 ML yields even better X‐ray emission and absorption spectra calculated by using density functional theory and shows consistent agreement with all previous experiments. Copyright © 2017 John Wiley & Sons, Ltd.     

Crystallographic study of interaction between adspecies on metal surfaces

The interaction among adsorbed atoms and molecules (adspecies) on metal surfaces plays a decisive role in catalytic reactions. Such interaction may cause structural changes of the local adsorption geometry which, together with spectroscopic and energetic data, may afford useful physical and chemical insights into the basic mechanisms of surface processes. When the adsorption geometry of a single adspecies is considered as a function of coverage, a deeper understanding of the nature of the adsorbate-substrate bonding can be obtained. Depending on the adsorbate coverage, the magnitude of adsorbate-induced relaxations and reconstructions vary widely. Occasionally, chemisorption systems transform gradually into adsorbate-substrate compounds, such as oxides, nitrides, hydrides, and sulfides. For the case of adsorption of different adspecies, coadsorption, structural data can make a vital contribution to our understanding of reaction intermediates, the promotion effect in heterogeneously catalyzed reactions, and the formation of ultra-thin compound films.     

X‐ray photoelectron and carbon Kα emission measurements and calculations of O‐, CO‐, N‐, and S‐containing substances

X‐ray fluorescence measurements for O‐containing [polyethylene oxide, polyvinyl alcohol, polyvinyl methyl ether], CO‐containing [polyvinyl methyl ketone, polyethylene terephthalate], N‐containing [poly‐4‐vinylpyridine (P4VP), polyaniline oligomer (PAO)], and S‐containing [polyphenylene sulfide] substances are presented. Carbon Kα X‐ray emission spectra (XES) and X‐ray photoelectron spectra (XPS) are compared with our DFT calculations performed with the Amsterdam density functional (ADF) program. The combined analysis of valence XPS and carbon Kα XES allows us to determine the individual contributions from pσ‐ and pπ‐bonding molecular orbitals of the polymers. The ΔSCF calculations yield the accurate C1s core‐electron binding energies (CEBEs) for all carbon sites of the organic compound. We calculate all CEBEs of the model molecules using the ΔE _KS approach. Our simulated C1s photoelectron and C Kα emission spectra are in good agreement with our measurements. We also obtain WD (work function and the other energies) values for the polymers and PAO from the difference between calculated (gas‐phase) and measured (solid) CEBE values. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 162–172, 2007     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Improving the predictive quality of time-dependent density functional theory calculations of the X-ray emission spectroscopy of organic molecules

The simulation of X-ray emission spectra of organic molecules using time-dependent density functional theory (TDDFT) is explored. TDDFT calculations using standard hybrid exchange-correlation functionals in conjunction with large basis sets can predict accurate X-ray emission spectra provided an energy shift is applied to align the spectra with experiment. The relaxation of the orbitals in the intermediate state is an important factor, and neglect of this relaxation leads to considerably poorer predicted spectra. A short-range corrected functional is found to give emission energies that required a relatively small energy shift to align with experiment. However, increasing the amount of Hartree–Fock exchange in this functional to remove the need for any energy shift led to a deterioration in the quality of the calculated spectral profile. To predict accurate spectra without reference to experimental measurements, we use the CAM-B3LYP functional with the energy scale determined with reference to a Δself-consistent field calculation for the highest energy emission transition.