SiC外延层表面化学态的研究

用高分辨X射线光电子能谱仪(XPS)和傅里叶变换红外(FTIR)光谱仪研究了SiC外延层表面的组分结构. XPS宽扫描谱,红外掠反射吸收谱及红外镜面反射谱的解析结果说明SiC外延层表面是由Si—O—Si和Si—CH₂—Si聚合体构成的非晶SiC_xO_y:H. SiC外延层表面的化学态结构为Si(CH₂)₄,SiO(CH₂)₃,SiO₂(CH₃)₂,SiO₃(CH₃),Si—Si,游离H₂O,缔合OH,Si—OH,O和O₂. 根据化学态结构和元素电负性确定了化学态的各原子芯电子束缚能顺序,并与XPS窄扫描谱拟合结果相对比,建立了化学态与其束缚能的对应关系,进而用Si(CH₂)₄的实际C 1s束缚能值进行校正,确定了各化学态的束缚能. 结果发现,除了SiC_xO_y(x=1,2,3,4,x+y=4)的Si 2p束缚能彼此不同外,其C 1s和O 1s彼此也不相同,其中SiO₂(CH₃)₂和SiO₃(CH₃)的C 1s束缚能与CH_m和C—O中C 1s的相近,对此从化学态结构,元素电负性和邻位效应进行了解释. 关键词： SiC 化学态 XPS FTIR     

甲烷分子电子碰撞电离和解离的实验研究

本文利用反应显微成像技术(reaction microscope)研究了54 eV电子入射甲烷分子导致的电离解离过程,详细分析了电离解离产生的CH⁺₂,CH⁺,C⁺离子碎片的动能分布情况.实验结果表明,该入射能量下产生CH⁺₂,CH⁺,C⁺离子碎片主要贡献来自2a₁内价轨道电子的直接电离过程产生的离子态(2a< 关键词： 反应显微成像谱仪 电离解离 能量沉积 动能分布     

Thermochemistry,bond energies and internal rotor barriers of methyl sulfinic acid,methyl sulfinic acid ester and their radicals

Sulfur–Oxygen containing hydrocarbons are formed in oxidation of sulfides and thiols in the atmosphere, on aerosols and in combustion processes. Understanding their thermochemical properties is important to evaluate their formation and transformation paths. Structures, thermochemical properties, bond energies, and internal rotor potentials of methyl sulfinic acid CH₃S(?O)OH, its methyl ester CH₃S(?O)OCH₃ and radicals corresponding to loss of a hydrogen atom have been studied. Gas phase standard enthalpies of formation and bond energies were calculated using B3LYP/6‐311G (2d, p) individual and CBS‐QB3 composite methods employing work reactions to further improve accuracy of the ${\Delta} _{{\bf f}} H_{{\bf 298}}^{{\bf o}} $ . Molecular structures, vibration frequencies, and internal rotor potentials were calculated. Enthalpies of the parent molecules CH₃S(?O)OH and CH₃S(?O)OCH₃ are evaluated as ?77.4 and ?72.7 kcal mol^?1 at the CBS? QB3 level; Enthalpies of radicals C^?H₂? S(?O)? OH, CH₃? S^?(?O)₂, C^?H₂? S(?O)? OCH₃ and CH₃? S(?O)? OC^?H₂ (CBS‐QB3) are ?25.7, ?52.3, ?22.8, and ?26.8 kcal mol^?1, respectively. The CH₃C(?O)O—H bond dissociation energy is of 77.1 kcal mol^?1. Two of the intermediate radicals are unstable and rapidly dissociate. The CH₃S(?O)? O. radical obtained from the parent CH₃? S(?O)? OH dissociates into methyl radical (${\bf CH}_{{\bf 3}}^{{\bf .}} $ ) plus SO₂ with endothermicity (ΔH_rxn) of only 16.2 kcal mol^?1. The CH₃? S(?O)? OC^?H₂ radical dissociates into CH₃? S^?=O and CH₂=O with little or no barrier and an exothermicity of ?19.9 kcal mol^?1. DFT and the Complete Basis Set‐QB3 enthalpy values are in close agreement; this accord is attributed to use of isodesmic work reactions for the analysis and suggests this combination of B3LYP/work reaction approach is acceptable for larger molecules. Copyright © 2010 John Wiley & Sons, Ltd.     

Catalytic effect of water,water dimer,water trimer,HCOOH, H2SO4, CH3CH2COOH and HN(NO2)2 on the isomerisation of HN(NO2)2 to O2NNN(O)OH: a mechanism study

In this article, the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH without and with catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂) have been investigated theoretically at the CBS-QB3 level of theory. Our results show that the catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄ and CH₃CH₂COOH) shows different positive catalytic effects on reducing the apparent activation energy of the isomerisation reaction processes. Such different catalytic effects are mainly related to the number of hydrogen bonds and the size of the ring structure in X (X = H₂O, (H₂O)₂ and (H₂O)₃)-assisted transition states, as well as different values of pK_a for H₂SO₄, HCOOH and CH₃CH₂COOH. Very interesting is also the fact that H₂SO₄-assisted reaction is the most favourable for the hydrogen transfer from HN(NO₂)₂ to O₂NNN(O)OH, due to the smallest pK_a (?3.0) value of H₂SO₄ than H₂O, HCOOH, H₂SO₄ and CH₃CH₂COOH, and also because of the largest ∠X???H???Y (the angle between the hydrogen bond donor and acceptor) involved in H₂SO₄-assisted transition state. Compared to the self-catalysis of the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH, the apparent activation energy of H₂SO₄-assisted channel also reduces by 9.6 kcal?mol^?1, indicating that H₂SO₄ can affect the isomerisation of HN(NO₂)₂ to O₂NNN(O)OH, most obvious among all the catalysts H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂.     

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH₃OH), dimethyl ether (CH₃OCH₃) formaldehyde (H₂CO), acetaldehyde [H(CH₃)CO], and acetone [(CH₃)₂CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH₃OH and H(CH₃)CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mulliken's theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.     

UPS measurements of molecular energy level of condensed gases

There have been numerous attempts to use ultraviolet photoemission spcctroscopy (UPS) to monitor the chemical states of adsorbed gas molecules on metal surfaces. To interpret the data correctly, one has to determine the effect of photoemission on the measured energy levels of the molecule. We have measured the UPS spectra of seven gases (C₆H₆, C₅H₅N, CH₃OH, C₂H₅OH, H₂CO, H₂O, NH₃) condensed on a LN₂ cooled MoS₂ substrate at hv = 21.2 eV. The inertness of the MoS₂ substrate assures that no strong chemical bonding exists between the substrate and adsorbed molecules. For each gas, the spectrum of the condensed phase is similar to the corresponding spectrum of the gas phase except all the energy levels are shifted up by the same amount. This shift ranges from 1 to 1.65 eV for the gases studied. The energy shift is attributed to the dielectric screening of the hole produced during the optical excitation.     

Adsorption of water and methanol on GaAs(110) surfaces studied by ultraviolet photoemission

UV photoemission spectroscopy (UPS) with He 1 radiation (hν = 21.2 eV) has been used to study the interaction of H₂O and CH₃OH with GaAs(110) surfaces prepared by cleavage in ultrahigh vacuum (UHV). For H₂O two molecularly adsorbed phases can be distinguished at 300 K: at low coverage H₂O is chemisorbed by its oxygen lone-pair orbital to the surface whereas for higher exposures a less perturbed species which resembles more a “physisorbed” or condensed H₂O layer is found. At 180 K only the less perturbed species can be identified. Also CH₃OH is chemisorbed molecularly at lower coverage with its oxygen end to the GaAs surface. For higher exposures two additional emission bands are observed which are interpreted as due to the methoxy radical CH₃O resulting from a partial decomposition of CH₃OH.     

A first-principles study on the adsorption behaviour of methanol and ethanol over C59B heterofullerene

In the present study, the adsorption behaviour of methanol (CH₃OH) and ethanol (C₂H₅OH) molecules over heterofullerene C₅₉B surface is studied by density functional theory calculations. This heterofullerene is obtained from C₆₀ by substituting a carbon atom with a boron atom and relaxing self-consistently the structure to the local minimum. The adsorption of CH₃OH and C₂H₅OH on the C₅₉B is exothermic and the relaxed geometries are stable. The CH₃OH and C₂H₅OH adsorption can also induce a change in the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy gap of the nanocage. The dehydrogenation pathways of CH₃OH and C₂H₅OH via O–H and C–H bonds scission are also examined. The results indicate that O–H bond scission is the most favourable pathway on the C₅₉B surface.     

CH3OH在ZnO(0001)表面的光催化解离

本文利用266 nm波长的激光及程序升温脱附的方法研究了甲醇在ZnO(0001)表面的光催化反应. TPD结果显示部分的CH₃OH以分子的形式吸附在ZnO(0001)表面,而另外一部分在表面发生了解离. 实验过程中探测到H₂,CH₃^·,H₂O,CO,CH₂O,CO₂和CH₃OH这些热反应产物. 紫外激光照射实验结果表明光照可以促进CH₃OH/CH₃O^·解离形成CH₂O,在程序升温或光照的过程中它又可以转变为HCOO^-. CH₂OH_Zn与OH_ad反应在Zn位点上形成H₂O分子. 升温或光照都能促进CH₃O^·转变为CH₃^·. 该研究对CH₃OH在ZnO(0001)表面的光催化反应机理提供了一个新的见解.     

The catalytic effects of H2CO3, CH3COOH,HCOOH and H2O on the addition reaction of CH2OO + H2O → CH2(OH)OOH

The addition reaction of CH₂OO + H₂O → CH₂(OH)OOH without and with X (X = H₂CO₃, CH₃COOH and HCOOH) and H₂O was studied at CCSD(T)/6-311+ G(3df,2dp)//B3LYP/6-311+G(2d,2p) level of theory. Our results show that X can catalyse CH₂OO + H₂O → CH₂(OH)OOH reaction both by increasing the number of rings, and by adding the size of the ring in which ring enlargement by COOH moiety of X inserting into CH₂OO···H₂O is favourable one. Water-assisted CH₂OO + H₂O → CH₂(OH)OOH can occur by H₂O moiety of (H₂O)₂ or the whole (H₂O)₂ forming cyclic structure with CH₂OO, where the latter form is more favourable. Because the concentration of H₂CO₃ is unknown, the influence of CH₃COOH, HCOOH and H₂O were calculated within 0–30 km altitude of the Earth's atmosphere. The results calculated within 0–5 km altitude show that H₂O and HCOOH have obvious effect on enhancing the rate with the enhancement factors are, respectively, 62.47%–77.26% and 0.04%–1.76%. Within 5–30 km altitude, HCOOH has obvious effect on enhancing the title rate with the enhancement factor of 2.69%–98.28%. However, compared with the reaction of CH₂OO + HCOOH, the rate of CH₂OO···H₂O + HCOOH is much slower.     

The electronic spectrum of chloromethanol ClCH2OH in comparison with isomeric methylhypochlorite CH3OCl

Large-scale multi-reference configuration interaction calculations are employed to differentiate in the electronic spectrum between chloromethanol ClCH₂OH and its isomer methylhypochlorite CH₃OCl by certain fingerprints. The first dipole-allowed transitions of the CH₃OCl spectrum are obtained at 4.13 eV(1¹A″ ← X¹A¹) and 5.50eV (2¹ A′ ← X¹A′). in reasonable agreement with prior calculations (4.24eV and 5.58eV) and in accordance with experimental measurements of 4.02 eV and 5.27 eV. For the yet to be observed spectrum of ClCH₂OH the corresponding transitions are predicted at higher energies of 7.32eV for 1¹A″ ← X¹A′ and 7.95eV for 2¹A′ ← X¹A′ and 7.98eV for 2¹A″ ← X¹A′.     

50—5000 eV电子被C4H8O,C5H10O2, C6H5CH3和C4H8O2散射的总截面

使用电子被C, H和O原子散射总截面的实验数据, 利用修正后的可加性规则计算了能量为50—5000eV的电子被4个复杂大分子C₄H₈O, C₅H₁₀O₂, C₆H₅CH₃和C₄H₈O₂散射的总截面, 并将计算结果与实验结果及其他理论计算结果进行了比较. 结果表明, 即 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Two-dimensional condensation of water and alcohols on NaF

On the surface of NaF the adsorption isotherms of H₂O, D₂O, CH₃OH, C₃H₃OH and 1-C₃H₇OH as well as the infrared spectra of H₃O, D₂O, dilute HDO, CH₃OH and CH₃OD were measured. The adsorption temperatures of H₃O (253–308 K) were within the phase transition region where two phases of low and high density coexist, while those of CH₃OH, C₂H₅OH and 1-C₃H₃OH were yet within a super-critical region. The entropy of the 2D condensed H₂O on NaF was found to be 14.0 cal K^?1 mol^?1, which suggests that the condensed phase of water on NaF is liquid-like. The OD stretching band of dilute HDO in the 2D condensed water gives a maximum adsorption at ca. 2530 cm^?1 with a half width of ca. 150 cm^?1, being in good agreement with that in liquid water. Comparison of the integrated absorbance of the D₂O bending mode with that of the OD stretching mode suggests that the cluster size of the 2D condensed water on NaF decreases with increasing temperature. The 2D critical temperature and the occupied areas of these adsorbates enable us to conclude that the compatibility of the molecular size with the surface lattice is not important in the occurrence of the 2D condensation of the hydrogen-bonding molecules on NaF and that adsorbed molecules are randomly oriented on the surface to the extent similar to that in 3D liquid state.     

Measurement of mass attenuation coefficients in some Cr,Co and Fe compounds around the absorption edge and the validity of the mixture rule

The total mass attenuation coefficients for elements Cr, Co and Fe and compounds CrCl₂, CrCl₃, Cr₂(SO₄)₃K₂SO₄·24H₂O, CoO, CoCl₂, Co(CH₃COO)₂, FePO₄, FeCl₃·6H₂O, Fe(SO₄)₂NH₄·12H₂O were measured at different energies between 4.508 and 14.142 keV using secondary excitation method. Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr were chosen as secondary exciters. 59.5 keV γ-rays emitted from a ²⁴¹Am annular source were used to excite a secondary exciter and K_α(K-L₃, L₂) lines emitted by the secondary exciter were counted by a Si(Li) detector with a resolution of 160 eV at 5.9 keV. It was observed that mixture rule method is not a suitable method for determination of the mass attenuation coefficients of compounds, especially at an energy that is near the absorption edge. The obtained values were compared with theoretical values.      

全维高精度势能面上F+CH3OH反应的产物振动态分布

甲醇与氟原子之间的抽氢反应可以生成HF和CH₃O、CH₂OH自由基等产物. 该反应在环境化学、燃烧化学、辐射化学和星际化学中都非常重要. 基于之前构建的全维高精度势能面,本文采用准经典轨线方法研究了该典型反应的动力学. 特别是使用正则模式分析方法确定了多原子产物CH₃O和CH₂OH的振动态分布. 研究发现,当反应物处于振转基态时,CH₃O和CH₂OH主要分布在基态. 当反应物CH₃OH的OH伸缩模式激发为第一激发态时,产物CH₂OH的OH伸缩模式、扭转模式、H₂CO 面外弯曲模式及其组合会被有效激发. 在两条通道中,可用能量大部分都流入HF的振动能和产物的平动能,而自由基产物CH₃O或CH₂OH只得到非常少的能量,与实验结果一致,这也表明了自由基的旁观者性质.     

Pyrolysis of diethyl carbonate: Shock-tube and flow-reactor measurements and modeling

Shock-tube and flow-reactor experiments were used to study the thermal decomposition of diethyl carbonate (C₂H₅OC(O)OC₂H₅; DEC). The formation of CO₂, C₂H₄, and C₂H₅OH was measured with gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) behind reflected shock waves. The same products were also detected by GC/MS in flow reactor experiments. All experiments combined span a temperature range of 663–1203 K at pressures between 1.0 and 2.0 bar. Time-resolved species concentration profiles from HRR-TOF-MS and product compositions from GC/MS measurements were simulated applying a detailed reaction mechanism for DEC combustion. A master-equation analysis was conducted based on computed energies from G4 calculations. Quantum chemical calculations confirm that DEC primarily decomposes by six-center elimination, C₂H₅OC(O)OC₂H₅ → C₂H₄ + C₂H₅OC(O)OH (1a), followed by rapid decomposition of the alkoxy acid, C₂H₅OC(O)OH → C₂H₅OH + CO₂ (1b). Measured DEC decomposition rate constants k(T) at p ≈ 1.5 bar can be represented by the Arrhenius equation k(T) = 10^13.64±0.12 exp(?204.24±1.95 kJ/mol/RT) s ^{? 1}. Theoretical predictions for k_1a were in good agreement with experimentally derived values. The theoretical analysis also included dipropyl carbonate (C₃H₇OC(O)OC₃H₇; DPC) decomposition and the reactivities of DEC and DPC are compared and discussed in the context of reactivity of dialkyl carbonates under pyrolytic conditions.     

Townsend-Koeffizient α für Alkoholdämpfe,Luft und Argon

Measurements of the first Townsend coefficient for the vapours of CH₃OH, C₂H₅OH, C₃H₇OH(n), C₃H₇OH(iso) are described. The experimental arrangement is tested by measuring the known values of air and argon.     

Relative intensities in x-ray photoelectron spectra. Part VI. The spectra of He(I), He(II), Y Mζ and Zr Mζ

The 2s- and 2p-electron photoionization cross-sections at photon energies up to 190 eV have been calculated, using the RPAE method for averaged configurations of the C, N, O and Ne atoms. The RPAE method ensures a more accurate relation between the cross-sections, 2s/2p, than that obtained using the Hartree—Fock method. Within the framework of the Gelius—Siegbahn model, but with the use of theoretical atomic cross-sections, we have calculated the photoionization cross-sections for He(I), He(II), Y Mζ, Zr Mζ for CH₄, C₂H₆, C₃H₈, C₂H₄, C₂H₂, NH₃, H₂O, CN^?, N₂, CO, CO₂, N₂O and NO₂^? molecules. For CO, N₂, CO₂, N₂O and H₂O molecules, a comparison is made between the theoretical and experimental cross-sections for hν < 60 eV. The calculated absolute and relative values of the molecular-orbital cross-sections are in reasonable agreement with experiment, especially at hν ? 40 eV. The calculations correctly reproduce the change in intensities under the transition He(I) → He(II). We have shown that our calculations have a significant advantage over those performed using the PW and OPW approximations. It is shown for NO, N₂, CO, H₂O, CH₄, NH₃ and N₂O molecules that the total photoionization cross-section calculated taking into account the real structure of the molecular orbitals is in better agreement with the experimental photoabsorption cross-section than is the sum of the cross-sections for the atoms in a molecule.     

Energies and chemical shifts of the sulphur 1s level and the KL2L3(1D2) Auger line in H2S,SO2 and SF6

The sulphur 1s binding energies and KL₂L₃(¹D₂) Auger energies have been measured in gaseous H₂S, SO₂ and SF₆. The experimental data, including the chemical shifts, are compared with various theoretical ab initio results. Theoretical and experimental values agree within 1-2 eV for the chemical shift and the binding energy of the 1s level, provided in the latter case relaxation, relativistic and correlation corrections are applied. Likewise, Shirley's method²⁰, which uses empirical energies, predicts the Auger energies satisfactorily. The measured S 1s binding energies are 2478.5(1) eV, 2483.7(1) eV and 2490.1(1) eV, and KL₂L₃(¹D₂) Auger energies are 2098.7(1) eV, 2095.5(2) eV, 2092.6(1) eV for H₂S, SO₂ and SF₆, respectively. The chemical shift for the 1s electron is found to be greater than for the 2s or 2p electron and in better accord with the prediction of the potential model. Data suggest the molecular relaxation energy to be small compared with the atomic relaxation energy.     

Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy

Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH₃ bond in anisole (C₆H₅OCH₃) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C₆H₅O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C₅H₆) and cyclopentadienyl radicals (C₅H₅). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10^?3–10^?2) are CH₃, CH₄, C₂H₂, C₂H₄, C₂H₆, CH₂O, C₅H₅ (cyclopentadienyl radical), C₅H₆ (cyclopentadiene), C₆H₆ (benzene), C₆H₅OH (phenol), and C₆H₅CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C₆H₅CHO), (2) formation of phenol (C₆H₅OH), and (3) unimolecular decomposition of anisole to phenoxy (C₆H₅O) and CH₃ radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C₅H₅ and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.