AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

Adsorption and decomposition of NO on K-deposited Pd(1 1 1)

The adsorption and decomposition of NO on a K-deposited Pd(1 1 1) surface were investigated using X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programmed desorption. For the K-deposited Pd(1 1 1) surface, two different NO adsorption sites were observed in addition to the Pd site. On the clean Pd(1 1 1) surface, the adsorption of NO was purely molecular and reversible, but on the K-deposited surface, the adsorbed NO decomposed at two different temperatures, 530 and 610 K. These results indicate that the NO adsorption and decomposition sites were newly created by the deposition of K onto the Pd(1 1 1) surface.     

The adsorption of NO on an oxygen pre-covered Pt(1 1 1) surface: in situ high-resolution XPS combined with molecular beam studies

Adsorption of NO on a Pt(1 1 1) surface pre-covered with a p(2 × 2) atomic oxygen layer has been studied in situ by high-resolution X-ray photoelectron spectroscopy and temperature-programmed XPS using third-generation synchrotron radiation at BESSY II, Berlin, combined with molecular beam techniques and ex situ by low energy electron diffraction and temperature-programmed desorption. O 1s XP spectra reveal that an ordered p(2 × 2)-O layer dramatically changes the adsorption behavior of NO as compared to the clean surface. The atomic oxygen occupies fcc hollow sites, and therefore blocks NO adsorption on these sites, which are energetically preferred on clean Pt(1 1 1). As a consequence, NO populates on-top sites at low coverage. At 110 K for higher coverages, NO can additionally adsorb on hcp hollow sites, thereby inducing a shift of the O 1s binding energy of atomic oxygen towards lower energies by about 0.25 eV. The bond strength of the hcp hollow NO species to the substrate is weakened by the presence of atomic oxygen. A sharp p(2 × 2) LEED pattern is observed for NO adsorption on the oxygen pre-covered surface, up to saturation coverage. The total saturation coverage of NO on Pt(1 1 1) pre-covered with varying amounts of oxygen (below 0.25 ML) decreases linearly with the coverage of oxygen. The initial sticking coefficient of NO is reduced from 0.96 on clean Pt(1 1 1) to 0.88 on a p(2 × 2) oxygen pre-covered surface.     

Adsorption of NO on unreduced and reduced CeO2 surfaces: A plane-wave DFT study

The adsorption of NO on the (1 1 1) and (1 1 0) surfaces of ceria (CeO₂) was studied using projector-augmented wave (PAW) method based density-functional theory within the generalized gradient approximation (GGA). Several adsorption sites for NO on the stoichiometric surfaces are found, all with weak molecule-surface interaction. The adsorption on the reduced surfaces is much stronger. The O-ends of the adsorbed NO molecules fill the oxygen vacancies and the N-O bonds are elongated. If two such adsorbed NO molecules, residing at neighbouring sites, meet, their N-ends will form a strong N-N bond with little or no barrier. This is an intermediate step towards dissociation of free N₂ which is calculated to be strongly thermodynamically driven.     

NO在稀有金属钇表面的吸附行为

以往的理论在预测六方结构(HCP)金属的表面能时,计算值与实验值存在较大误差.鉴于此,本文首先用一种较为合理的方法精准地预测了稀有金属钇(Y)(0001)面的表面能,计算值(1.141 J/M²)与实验值(1.125 J/M²)吻合的很好.随后,系统研究了NO小分子在Y(0001)面不同位置(空位、桥位和端位)的吸附行为.结果表明:空位(H1)表现出了良好的吸附能力,吸附能超过了5eV,同时N-O键长伸长量超过了24%,此时,NO分子几乎平行地吸附于Y(0001)表面.所有的吸附位置的N-O分子伸长量范围为0.2?-0.42?.这种伸长量明显超过了NO在其它金属表面时的计算结果.     

NO对未燃尽炭吸附汞影响的机理研究

本文应用量子化学密度泛函理论B3PW91方法,研究了NO对飞灰中未燃尽炭吸附汞的影响机理。建立了含有NO的未燃尽炭固体表面簇模型,计算得到了单质汞在含有NO的未燃尽炭表面的不同吸附位、吸附构型和吸附能。不同吸附方式中,NO平行吸附在未燃尽炭表面时最稳定。NO对单质汞在飞灰未燃尽炭表面的吸附的影响机理比较复杂,与烟气中NO的含量有关。研究结果表明量子化学的理论计算是揭示汞等痕量元素的吸附机理的一种有效方法。     

Adsorption behavior and reaction properties of NO and CO on Rh(1 1 1)

Reactions between NO and CO on Rh(1 1 1) surfaces were investigated using infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. NO adsorbed on the fcc, atop, and hcp sites in that order, whereas CO adsorbed initially on the atop sites and then on the hollow (fcc + hcp) sites. The results of experiments with NO exposure on CO-preadsorbed Rh(1 1 1) surfaces indicated that the adsorption of NO on the hcp sites was inhibited by preadsorption of CO on the atop sites, and NO adsorption on the atop and fcc sites was inhibited by CO preadsorbed on each type of site, which indicates that NO and CO competitively adsorbed on Rh(1 1 1). From a Rh(1 1 1) surface with coadsorbed NO and CO, N₂ was produced from the dissociation of fcc-NO, and CO₂ was formed by the reaction of adsorbed CO with atomic oxygen from dissociated fcc-NO. The CO₂ production increased remarkably in the presence of hollow-CO. Coverage of fcc-NO and hollow-CO on Rh(1 1 1) depended on the composition ratio of the NO/CO gas mixture, and a gas mixture with NO/CO ? 1/2 was required for the co-existence of fcc-NO and hollow-CO at 273 K.     

Gamma holography from multiple scattering

Since the introduction of heterodyne methods for synchrotron radiation (Cousesement et al. in Phys. Rev. B 54:16003, 1996; Callens et al. in Phys. Rev. 67:104423, 2003) one observes interferences between two scattering amplitudes; the scattering amplitude of resonant nuclei in a reference sample and the scattering amplitude of nuclei in the sample under investigation. Theses interferences can easily been observed as resonances in velocity spectra when one uses a time integrated method. They can also been observed as quantum beats, when one would use the time differential method. For both methods it is important that one uses a reference sample and therefore both methods disserved the name “heterodyne methods.” As theses interferences are a product of two scattering amplitudes, the amplitude of a wave scattered form the investigated sample can be known with its phase. But it is assumed that the reference wave is known in advance by a proper choice of the reference sample. At first sight it is very likely that multiple scattering would add more complexity but in this paper it is claimed that on the contrary it provide a bonus, especially for single crystals. It provokes only a line broadening and a line shift of the resonances in the velocity spectra (or a change in the damping and frequency of the quantum beats when the time spectra are registered). Moreover these changes in the line shapes can easily be measured and they provide all the information needed to reconstruct a 3-D picture of the atomic arrangement of resonant nuclei and moreover they distinguish between different hyperfine sites. The method may be more practical for measurements on synchrotron radiation but it does also apply to velocity spectra obtained from resonant scattering with strong sources. The use of radioactive sources suffer from the disadvantage of poorer statistics or much longer accumulation times but they enjoy the advantage to be table-top and at-home experiments. As strong sources are now commercially available this possibility to measure not only the hyperfine fields but also the corresponding crystal structure could give a renewed impetus to the investigations with Moessbauer spectrometry, with “at home and table top” instrumentation.     

NO adsorption and diffusion on unreconstructed Pt{1 0 0} surface. A density functional theory investigation

Ab initio density functional theory was used to investigate the adsorption and diffusion of a single NO molecule on the unreconstructed Pt{1 0 0}-(1 × 1) surface. To our knowledge this is the first theoretical study of the NO diffusion activation energy on the Pt{1 0 0} surface. The most stable adsorption position for NO corresponds to the bridge site with the axis of the molecule perpendicular to the surface. The bond of the NO molecule to the surface is through the N-atom. We found that there is a low adsorption energy when the NO molecule is bonded through the O-atom and the axis is perpendicular to the surface, for the three high symmetry sites investigated. NO diffusion between bridge-hollow sites, bridge-atop sites, and hollow-atop sites was also investigated. The barrier for NO diffusion is 0.41 eV, which corresponds to the energy difference between the bridge and hollow sites. This value is around 15% of the highest adsorption energy found on this surface. NO stretch frequencies are also calculated for the three high symmetry sites investigated.     

磁场和温度对束缚磁极化子有效质量的影响

研究强、弱耦合情形下,库仑场中束缚磁极化子的性质.采用改进的线性组合算符方法研究束缚磁极化子的振动频率和有效质量的温度依赖性,对RbCl晶体进行数值计算,结果表明:在强耦合情形下,束缚磁极化子的振动频率随温度的升高和磁场的增强而增加;有效质量随温度的增加而增加,但随磁场的增强而减少.     

An investigation of the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1}

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1} at room temperature. RAIRS experiments show that no new species are formed when NO is adsorbed onto a Pt{2 1 1} surface that has been pre-dosed with oxygen and no species are lost from the spectra, compared to spectra recorded for NO adsorption on the clean Pt{2 1 1} surface. However pre-dosed oxygen atoms do influence the frequency and intensity of several of the observed infrared bands. In stark contrast, pre-dosed O has a large effect on the TPD spectra. In particular N₂ and N₂O desorption, seen following NO adsorption on the clean Pt{2 1 1} surface, is completely inhibited. This effect has been assigned to the blocking of NO dissociation by the pre-adsorbed O atoms. A new NO desorption peak, not seen for NO adsorption on the clean Pt{2 1 1} surface, is also observed in TPD spectra recorded following NO adsorption on an oxygen pre-dosed Pt{2 1 1} surface.     

New travelling wave solutions for combined KdV-mKdV equation and （2＋1）-dimensional Broer- Kaup- Kupershmidt system

Some new exact solutions of an auxiliary ordinary differential equation are obtained, which were neglected by Sirendaoreji {\it et al in their auxiliary equation method. By using this method and these new solutions the combined Korteweg--de Vries (KdV) and modified KdV (mKdV) equation and (2+1)-dimensional Broer--Kaup--Kupershmidt system are investigated and abundant exact travelling wave solutions are obtained that include new solitary wave solutions and triangular periodic wave solutions.     

NO adsorption on the Rh(110) surface: kinetics and composition of the adlayer studied by fast XPS

The adsorption and dissociation of NO on the Rh(110) surface were studied by synchrotron radiation X-ray photoemission spectroscopy at temperatures in the range 210–370 K. The O 1s or N 1s spectra were collected every 14 s while the surface was continuously exposed to a steady NO gas pressure. The difference in the binding energies for the atomic oxygen (O 1s ≤530.2 eV), atomic nitrogen (N 1s 397.2 eV) and molecular upright bonded NO molecules (O 1s ≥531.0 eV and N 1s 400 eV) allowed us to distinguish these surface species and to follow the evolution of the adsorbate layer. In addition to these dominating surface species a new species, characterized by O 1s binding energy of 530.7 eV and N 1s binding energy similar to that of the atomic nitrogen, was detected within a narrow coverage range. This state is tentatively assigned to a “lying down” NO bonding configuration, detectable at the timescale of the measurements. The uptake plots, constructed using the integrated intensity of the deconvoluted O 1s and N 1s spectra, are used to elucidate the effect of the reaction temperature and surface coverage and composition on the kinetics of dissociative and molecular NO adsorption of Rh(110).     

用Xα-DV方法研究CO,NO在Ⅷ族过渡金属表面的化学吸附(Ⅱ)——NO分子在Pd(111)表面的化学吸附

本文用X_α-DV方法计算了NO在Pd(111)表面化学吸附问题。得到了它的电子结构,包括分子丛轨道能量本征值谱、态密度、电荷转移等等结果。在计算中特别考虑了NO之间的相互作用,所得总态密度与实验UPS十分相符,从而支持了LEED所示的几何结构,决定了NO的吸附高度为1.27?,并得知吸附于Pd表面的NO分子之间的相互作用十分重要。从理论上探讨了NO分子在Pd表面吸附时的活化作用。计算了NO分子各个轨道上的占有数,发现其电荷转移情况与CO在过渡金属表面吸附的情况相似。另外,还发现NO的吸附对Pd的价电子能带无重大影响。 关键词：     

NO adsorption and thermal behavior on Pd surfaces. A detailed comparative study

The adsorption and thermal behavior of NO on ‘flat’ Pd(111) and ‘stepped’ Pd(112) surfaces has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and electron stimulated desorption ion angular distribution (ESDIAD) techniques. NO is shown to molecularly adsorb on both Pd(111) and Pd(112) in the temperature range 100–373 K. NO thermally desorbs predominantly molecularly from Pd(111) near 500 K with an activation energy and pre-exponential factor of desorption which strongly depend on the initial NO surface coverage. In contrast, NO decomposes substantially on Pd(112) upon heating, with relatively large amounts of N₂ and N₂O desorbing near 500 K, in addition to NO. The fractional amount of NO dissociation on Pd(112) during heating is observed to be a strong function of the initial NO surface coverage. HREELS results indicate that the thermal dissociation of NO on both Pd(111) and Pd(112) occurs upon annealing to 490 K, forming surface-bound O on both surfaces. Evidence for the formation of sub-surface O via NO thermal dissociation is found only on Pd(112), and is verified by dissociative O₂ adsorption experiments. Both surface-bound O and sub-surface O dissolve into the Pd bulk upon annealing of both surfaces to 550 K. HREELS and ESDIAD data consistently indicate that NO preferentially adsorbs on the (111) terrace sites of Pd(112) at low coverages, filling the (001) step sites only at high coverage. This result was verified for adsorption temperatures in the range 100–373 K. In addition, the thermal dissociation of NO on Pd(112) is most prevalent at low coverages, where only terrace sites are occupied by NO. Thus, by direct comparison to NO/Pd(111), this study shows that the presence of steps on the Pd(112) surface enhances the thermal dissociation of NO, but that adsorption at the step sites is not the criterion for this decomposition.     

ESCA measurement of surface atom oxidation states on chemically modified Mo(100) surfaces

Quantitative surface atom oxidation states are measured in a conventional ESCA spectrometer. Spectra that solely reflect the chemical nature of the surface atoms are generated from a linear combination of spectra measured at two take-off angles. Proper selection of the multiplicative constants results in removal of bulk atom effects from the measured spectra. A simple method which is based on photoelectron attenuation lengths is used to calculate these coefficients. The surface Mo atom oxidation numbers for a series of chemically modified Mo(100) surfaces have been obtained by this procedure. Charge transfer effects resulting from the adsorption of electronegative surface modifiers (B, C, O and CO) are observed. At constant adatom coverage, the Mo oxidation number is linearly dependent on the adatom electronegativity. For a series of partially oxidized Mo(100) surfaces the extent of charge transfer to the metal atoms is not a simple function of the surface oxygen coverage. A model for the initial oxidation of Mo(100) is presented which is in agreement with previous structural and electronic state studies. At oxygen coverages below one monolayer, a chemisorbed oxygen phase exists which produces a constant Mo oxidation number of 1.3. At higher coverages, a rapid increase in the Mo surface atom charge indicates the formation of a surface oxide layer. The electron deficient Mo surface atoms are acidic sites for adsorption of gas phase Lewis bases. The relative strengths of the acidic sites correlate with the metal atom oxidation numbers. This result suggests that the observed adsorption selectivity and reactivity of these surfaces can be discussed in a traditional acid-base context.     

The adsorption of hydrogen on iron; A surface orbital modified occupancy — bond energy bond order calculation

A Surface Orbital Modified Occupancy — Bond Energy Bond Order (SOMO-BEBO) model calculation of hydrogen adsorption on iron is presented. This calculation represents a novel approach to the CFSO-BEBO method in that the calculation is correlated in a consistent way with the thermal desorption spectra of the hydrogen-iron system. Heats of molecular adsorption calculated are ?32.88, ?35.68 and ?49.57 kJ/mol for the iron (110), (100), and (111) surfaces, respectively. Heats of dissociative adsorption calculated are ?54.40, ?75.30 and ?87.90 kJ/mol for the three states on the iron (111) surface; ?51.21 and ? 73.62 kJ/mol for the two states on the iron (100) surface; and ?63.78 kJ/mol for the one state on the iron (110) surface. Activation energies for dissociative adsorption were found to be small or zero for the iron (111) surface while non-zero activation energies of 49.27 and 45.05 kJ/mol were calculated for the iron (100) and (110) surfaces, respectively. The FeH single-order bond energy has been calculated to be 298.2 kJ/mol. The radius of the hydrogen surface atom has been estimated to be 1.52 × 10^?10 m consistent with the expected size of an H^? ion. The elimination of certain surface sites for molecular adsorption as a result of the ferromagnetism of iron is suggested by the calculation. The reason for the absence of well defined LEED patterns for hydrogen adsorption on the iron (111) and (100) surfaces [Bozso et al., Appl. Surface Sci. 1 (1977) 103] is explained on the basis of the size of the H^? surface ion. The adsorption of hydrogen on the iron (110) surface is consistent with a relatively stable, small-sized H⁺₂ surface ion giving, therefore, a regular LEED pattern and a positive surface potential upon adsorption of hydrogen on this surface.     

Photoelectron spectroscopy of methyl,ethyl and isopropyl alcohol adsorbed on the (110) surface of Cu

Methyl, ethyl and isopropyl alcohol were allowed to adsorb on a clean (110) face of a copper crystal at room temperature, and photoelectron spectra were obtained using He(I) (21.2 eV) radiation. The spectra of the adsorbed species differed sufficiently from those of the free molecules to indicate a chemical change on adsorption. In accordance with studies in the literature using flash-desorption and energy-loss spectroscopy, it is suggested that the observed spectra are those of the corresponding alkoxy radicals formed as a result of dehydrogenation. In the case of the methoxy radical, calculations were carried out for a cluster of nine copper atoms and the results shown to be consistent with the experimental data. Results were also obtained on the rates of adsorption and desorption and on the dependence of the spectra on the polar angle between the normal to the surface and the photoelectron direction. Finally, results were obtained on the extent of adsorption by means of X-ray photoelectron spectroscopy.     

里德堡NO分子在强磁场中的回归谱

利用分子轨道理论,结合一个新的模型势,我们给出了里德堡NO分子中激发态电子所受到的作用势的解析表达式.利用分子的闭合轨道理论和分区自洽的迭代方法,首次从理论上计算了NO分子在强磁场中的回归谱.通过和A.Matzkin等人给出的NO分子在强磁场中的实验谱的比较,表明我们的计算是正确的.