吡嗪分子在硅(100)面吸附构型的XPS和NEXAFS谱理论研究

本文利用X射线谱研究了吡嗪(C₄H₄N₂)分子共价吸附于硅(100)面的几种吸附构型的几何结构和电子结构. 利用密度泛函理论结合团簇模型,对预测的吸附结构的碳K壳层(1s)X射线光电子能谱(XPS)和近边X射线吸收精细结构(NEXAFS)谱进行了模拟. 计算结果阐明了XPS和NEXAFS谱与不同吸附构型的对应关系. 与XPS谱相比,NEXAFS谱对所研究的吡嗪/硅(100)体系的结构有明显的依赖性,可以很好地用于结构鉴定. 根据碳原子的分类,研究了在NEXAFS光谱中不同化学环境下碳原子的光谱成分.     

Orientation of adsorbed ethylene oxide on Ni(110) by X-ray photoelectron diffraction

The adsorption of ethylene oxide on Ni(110) was studied at 95 K and monolayer coverage by angle-resolved X-ray photoelectron spectroscopy. A slow radiation-induced decomposition at hv = 1486.7 eV to most likely methoxy was noted. The orientation of the adsorbed ethylene oxide was determined by measuring forward scattering enhancements in the O 1s intensity distribution. Peaks in polar (θ) as well as azimuthal (φ) scans occurred at four angular positions in 2π above the surface: (θ = 54°, φ = 36°, 144°, 216°, 324°). These positions were evaluated to yield the tilt angle of the molecule at 48°_relative to normal, and the COC bond angle of adsorbed C₂H₄O of about 57°. The molecule is tilted towards the [001] and [001̄] directions (two domains), with a mirror plane in the [001] azimuth.     

Inner shell excitation and dissociation of condensed formamide

Carbon and nitrogen K-shell excitation and dissociation of condensed formamide at 96 K were studied by near edge X-ray absorption fine structure (NEXAFS) recorded by total electron yield (TEY), total ion yield (TIY) and photon-stimulated desorption (PSD) yield of H⁺ measurements. It was found that electronic transitions from the C 1s or the N 1s to the σ¹ (CH and/or NH) enhanced ion yields of H⁺ from the CH and/or NH functional groups. This selective dissociation indicates that the corresponding unoccupied molecular orbital has an antibonding character of the CH and/or NH. To investigate the molecular orientation of condensed formamide, incidence-angle-dependent TEY-NEXAFS spectra were measured. The CNO plane of the adsorbed formamide molecule is determined to be tilted away from the surface by an averaged angle of about 42°.     

多重散射团簇方法对吸附系统SO2/Ag(110)的理论分析

利用多重散射团簇方法(MSC)对吸附系统SO₂/Ag(110)的S原子K边X射线吸收精细结构谱(NEXAFS)作了理论分析.研究表明,覆盖度为0.5时,吸附的SO₂的S—O键长比气体状态时增长了(0.014±0.006)nm,OSO键角减小了15°±5°;SO₂分子的S原子处于芯位,但两个O原子处于不对称的位置;分子平面与(110)的夹角约为52°,同时分子平面相对衬底表面法线有一小角度的倾斜.MSC计算证实了该吸附系统存在一介于π^{关键词： X射线吸收精细结构 2/Ag(110)')" href="#">吸附系统SO₂/Ag(110) 多重散射团簇方法}     

Geometry of acetylene chemisorption on Ni(100) investigated by leed intensity analysis: The c(2 × 2) structure

Results from LEED dynamical calculations performed on the Ni(100) c(2 × 2)C₂H₂ structure, produced by adsorption of C₂H₂ at about 273 K and 1.5 L exposure, are reported. Among the several model geometries tried, the most favoured one appears to be a distorted acetylene molecule with the midpoint of the CC bond (1.20 Å long) placed above the fourfold hollow site at a vertical distance 2.02 Å from the topmost Ni layer and with the CC axis itself tilted by 50° with respect to the surface normal in the [011] direction. In this geometry the NiC distance is 2.2 Å.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

Internal hydrogen bond,torsional motion,and molecular properties of 2-methoxyethylamine by microwave spectroscopy: Methyl barrier to internal rotation for 2-methoxyethanol

The microwave spectra of four substituted isotopic species of 2-methoxyethylamine (NH₂, NHD, NDH, ND₂) have been assigned. The molecule is found to exist in a gauche form with an intramolecular hydrogen bond of the NH?O type. The four possible sets of the amino hydrogen r_s corrdinates give different H?H distances, probably because the -NH₂ group is involved in large amplitude vibrations and because of changes in the heavy atom positions arising from the deuteration of the hydrogen bond. For the most abundant species many vibrational states have been analyzed and assigned to the two possible CO torsions in the molecule. A value V₃ = 3150 ± 50 cal/mol was found for the methyl torsional barrier and V₁ = 9 ± 3 kcal/mol for the other CO torsional barrier. A third set of observed vibrational satellites is probably assignable to the CC torsion. The determination of the dipole moment and of the quadrupole coupling constants gave values which were not in good agreement with those predicted from nonhydrogen bonded molecules. In addition a value V₃ = 3100 ± 100 cal/mol was calculated for the CH₃ torsional barrier in the related 2-methoxyethanol, using previous experimental data (Canad. J. Chem.50, 1149–1156 (1972)).     

多壁碳纳米管束储氢机理的X射线吸收谱研究

采用X射线吸收精细结构光谱探索性地研究了多壁碳纳米管束.在多壁碳纳米管束不同入射角的X射线吸收精细结构光谱中，观察到C—H σ^*共振峰强度随入射角的变化而发生变化.在常温常压下出现C—H键可能与多壁碳纳米管束中存在缺陷有关，缺陷数量越大C—H σ^*共振峰的强度越大.光谱中C—C π^*和C—C σ^*共振峰强度的变化趋势都不同于C—H σ^*共振峰，这有力地证明了在常温常压条件下氢原子是吸附在多壁碳纳米 关键词： X射线吸收精细结构光谱 碳纳米管 储氢 化学吸附     

SO2/Ni(111)吸附系统局域结构的多重散射团簇理论研究

用多重散射团族方法对SO₂/Ni(111)吸附系统的S原子ls近边X射线吸收精细结构进行了详细的计算和分析,求得吸附系统的局域结构。研究结果证实了SO₂分子在Ni(111)表面是平铺吸附的,其最佳吸附位为fcc三度芯位。吸附分子的S—O键长比气体状态时增长了(0.007±0.002)nm,分子键角∠OSO减小了10°,SO₂距衬底的吸附高度为(0.20±0.02)nm。理论计算与两组实验结果进行了直接比较。 关键词： 局域吸附结构 X射线吸收精细结构 2/Ni(111)')" href="#">吸附系统SO₂/Ni(111) 多重散射团簇方法     

Phenylacetylene chemisorbed on Pt (111), reactivity and molecular orientation as probed by NEXAFS Comparison with condensed multilayer and polyphenylacetylene

Near-edge X-ray absorption spectroscopy (NEXAFS) has been used to investigate the reactivity of phenylacetylene (PA) towards a Pt(111) surface and its molecular orientation. Spectra have been taken in the photon energy region 280–320 eV at both grazing and normal X-ray beam incidence angles with respect to the sample surface. NEXAFS K-edge spectra have been recorded for PA adsorbed in the monolayer as well as in the multilayer regime and, for comparison, the spectrum of the PA related polymer, polyphenylacetylene (PPA), has also been measured. The assignment of the π* and σ* resonances has been made on the basis of the chemical structure and interpreted with the help of ab initio calculations. Both the ionisation potentials for the C 1s electrons and the theoretical C K-edge absorption spectra have been calculated for all the six inequivalent carbon atoms of PA and of phenylethylene (PE) which is a model molecule for PPA. From the analysis of the data we have indication for an interaction involving mainly the CC triple bond of the acetylenic functional group of PA that undergoes opening and leads to the formation of a di-sigma bond with Pt, while the benzene CC bonds seem not affected. A polarization dependence has been evidenced for the π* resonances of PA adsorbed in monolayer condition indicating a molecular orientation of the phenyl ring π* orbitals between 37° and 34° relative to the normal to the crystal surface.     

Investigating the fine structure of near-edge X-ray absorption in the molecular spectra of C60F18 adsorbed on a single nickel crystal

Angular dependences of the fine structure of near-edge X-ray absorption (NEXAFS) of carbon C1s spectra are obtained for a monolayer film of C₆₀F₁₈ polar molecules on a Ni (100) substrate. The fine structure and angular dependences of these spectra are interpreted using calculation data obtained by the density functional method upon fitting NEXAFS spectra by the set of trial functions. It is shown that during deposition, the dipole moment of molecules is oriented perpendicular to the surface and fluorine atoms are the ones closest to the surface.     

Spectroscopic studies of folic acid adsorbed on various metal substrates: does the type of substrate play an essential role in temperature dependence of spectral features?

Folic acid is an essential nutrient involved in biosynthetic processes and cell growth. From an analytical point of view, folic acid can be used as an active part of sensors for substances with affinity to folic acid molecules. In biological environment, sensors can be exposed to temperatures that differ from room temperature. Thus, it is important to describe the influence of temperature on adsorbed molecules, especially on orientation of molecules towards the metal surface and on stability of adsorbed layer. Surface‐enhanced Raman scattering spectroscopy is a useful tool for investigation of architecture of molecular layers adsorbed on metal surfaces because the spectral features change with varying orientation of molecules towards the surface, as well as with changes in interactions among adsorbed molecules. In this study, folic acid was adsorbed on electrochemically prepared Au and Ag substrates, and both these substrates were exposed to temperature changes according to the temperature program consisting of stabilization, the substrate temperature at 10°C, at 50°C and back at 10°C. Decomposition of adsorbed folic acid at 50°C can be excluded on both metals, though the metal (Ag or Au) influences the arrangement of adsorbed molecules and its temperature‐induced changes. In the case of Ag substrate, significant and irreversible increase of certain Stokes band intensities is observed, whereas in the case of Au substrate, the band intensity increase is negligible. Experimental spectra were analyzed using principal component analysis and supplemented with calculated data for folic acid molecule adsorbed on small metal cluster using density functional theory. Moreover, it was proved that changes of surface plasmon resonance and collision frequency of Ag substrate are negligible in the studied temperature interval. Slight re‐orientation of adsorbed folic acid molecules was identified as the main source of band intensity variation in the spectra of folic acid adsorbed on Ag. Copyright © 2014 John Wiley & Sons, Ltd.     

Fast leed intensity measurements from Ni(100)c(2 × 2)CO

The Ni(100)c(2 × 2)CO surface structure has been investigated by very fast LEED intensity measurements using a computer controlled television method. It turns out that the intensity spectra are strongly influenced by intolerably long measuring times during which the primary electron beam impinges onto the surface. The spectra have been taken within 16 sec at 100 K immediately after termination of the adsorption process for all beams simultaneously. They are compared with other measurements and with Pendrys model calculations for a CO molecule bonded linearly on top of a Ni atom with straight molecular axis normal to the surface. Using the r-factor formalism for theory-experiment comparison the bond length results to be 1.15 ± 0.1 Å for CO and 1.80 ± 0.1 Å for NiC. This is in agreement with the results of other methods and removes some discrepancies with those of earlier LEED experiments.     

A tilted precursor for CO dissociation on the Fe(100) surface

Near edge X-ray absorption fine structure (NEXAFS) has been used to study the molecular orientation of the α₃ state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45° ± 10° with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 ± 0.02 Å in the α₃ state, relative to the other molecularly adsorbed CO states on this surface.     

Structural studies of formate and methoxy groups on the Cu(100) surface using NEXAFS and SEXAFS

Local chemisorption geometries of formate (HCO₂) and methoxy (CH₃O) groups on Cu(100) were examined by means of surface extended (SEXAFS) and near-edge X-ray absorption fine structure (NEXAFS) measurements above the O K edge. At 300 K the oxygen of the formate group are equivalent and asymmetrically located near the four-fold hollow site yielding two CuO distances between 2.31 and 2.45 Å. These distances are at least 0.3 Å longer than typical CuO distances of surface and bulk compounds due to a Cu-C steric interaction. The CO bonds and the OCO angle of the formate are estimated to be $\text{1.27 ± 0.04}\text{A}\text{?}$ and 127 ± 7°, respectively. At 200 K the methoxy group has a CuO distance of $\text{1.97 ± 0.05}\text{A}\text{?}$ and the CO axis is tilted with respect to the surface normal. The exact chemisorption site of the methoxy goup could not be determined, but the atop site is ruled-out.     

Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.     

The oscillator strengths for C1s and O1s excitation of some saturated and unsaturated organic alcohols,acids and esters

Electron energy loss Spectroscopy (ISEELS) under dipole scattering conditions is used to obtain the carbon and oxygen K-shell oscillator strength spectra of methanol (CH₃OH), propanol (CH₃CH₂CH₂OH), propenol (CH₂=CHCH₂OH), propargyl alcohol (HC≡CCH₂OH), propanoic acid (CH₃CH₂COOH), acrylic acid (CH₂=CHCOOH) and propiolic acid (HC≡CCOOH). A detailed interpretation of these spectra is presented, along with a comparison with the NEXAFS spectra of multilayers of these molecules adsorbed on a Si(111) surface, as recently reported by Outka et al. (Surf. Sci., 185 (1987) 53). Good agreement is found between the multilayer NEXAFS and the gas phase ISEEL spectra, except for the carboxylic acids which differ dramatically in the discrete portion of the O1s spectrum. Possible origins for this difference are discussed. The C1s and O1s spectra of methyl formate (HCOOCH₃) are also reported and interpreted in comparison with the spectra of formic acid and methanol.     

Identification of species formed after pyridine adsorption on iron,cobalt, nickel and silver electrodes by SERS and theoretical calculations

The adsorption of pyridine (py) on Fe, Co, Ni and Ag electrodes was studied using surface‐enhanced Raman scattering (SERS) to gain insight into the nature of the adsorbed species. The wavenumber values and relative intensities of the SERS bands were compared to the normal Raman spectrum of the chemically prepared transition metal complexes. Raman spectra of model clusters M₄(py) (four metal atoms bonded to one py moiety) and M₄(α‐pyridil) where M = Ag, Fe, Co or Ni were calculated by density functional theory (DFT) and used to interpret the experimental SERS results. The similarity of the calculated M₄(py) spectra with the experimental SERS spectra confirm the molecular adsorption of py on the surface of the metallic electrodes. All these results exclude the formation of adsorbed α‐pyridil species, as suggested previously. Copyright © 2009 John Wiley & Sons, Ltd.     

c(2×2)-(K+CO)在Co{1010}表面吸附的NEXAFS研究

对CO和K共吸附在CO{1010}表面的系统进行了O的1s NEXAFS理论研究,证实吸附分子CO的键长比气体状态时增长约0.01nm.用MSC(多重散射团簇)理论计算谱与实验结果比较发现：在原π峰右侧出现了一个新的弱结构.证明了此结构是由于K通过钴衬底对CO的间接作用造成的,它具有π共振的性质. 关键词：     

Adsorption model determination of N2O/Ag(1 1 0) by theoretical studies of near-edge X-ray absorption fine structure

The nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of the N₂O adsorbed on Ag(1 1 0) have been studied by the multiple-scattering cluster (MSC) and self-consistent field (SCF) DV-Xα methods. Two adsorption models, in which the N₂O molecule attached to the Ag substrate through the central nitrogen (N_C) atom and the terminal nitrogen (N_T) atom, respectively, have been checked up thoroughly. The MSC calculation and the R-factor analysis show that the N₂O molecule is attached to the Ag substrate through the terminal nitrogen atom with the adsorption height h = 3.4 ± 0.1 Å. In the overlayer the N₂O molecules arrange themselves into a tilted chain due to the interaction between the cations and the anions in the molecules. The physical cause of the resonances in the NEXAFS spectra mentioned above has been discussed by the DV-Xα method, which confirms the MSC calculations.