Dielectric constants of adsorbed CO on Cu and Ag

Adsorption of CO on evaporated Ag and Cu films was studied by ellipsometry and resistivity measurements. Changes in ellipsometric angles δΔ and δψ due to adsorption of CO were analyzed by an improved linear approximation of the stratified layer model of adsorption. Adsorption of CO on Cu induced a 23% increase in the relative resistivity change (δR/R) which was proportional to δψ, while adsorption of CO on Ag induced a 1% increase without the proportionality. The dielectric constant of CO adsorbed on Ag is $\text{?}{}_1\text{= 2.2?0.7i}$ at λ = 1152 nm in comparison to ?₁ = 2.1 for gas phase CO; that of CO on Cu is $\text{?}{}_1\text{= ?4.8 ? 8.5i}$, which is consistent with the prediction by the Bennett and Penn theory. The large difference in $\text{}\text{?}$ge₁, of adsorbed CO on Ag and Cu is understood in terms of the energy level of the 2 π molecular orbital of adsorbed CO relative to the Fermi level. A possibility of adlayer plasmon excitation is discussed.     

Thin film quantum size effects: I. The effect of defect structure at the vacuum/film interface

The quantum size oscillations observed in the transmitted current for very low energy electrons normally incident on thin epitaxial (111) films of Cu and Ag on W(110) are sensitive to the structure of both the vacuum/film and the film/substrate interface. We report here the contribution of the vacuum/film interface as a function of oxygen adsorption, ion bombardment damage and annealing temperature. We find that the quantum size oscillations are always reduced in amplitude with increasing disorder on an atomic scale at the film surface. Random oxygen adsorption generally suppresses the quantum size effect (QSE) structure. The QSE amplitude is also significantly reduced by defect structure in the form of a random, irregular array of monatomic steps at the film surface. Using the LEED (00) beam width as an index of surface roughness, we find that the amplitude of the quantum size oscillations decreases linearly with surface step atom density. The QSE amplitude is reduced by a factor of two for step atom densities as low as 6.5%, and nearly extinguished for a step atom density of 12%. The QSE structure provides a relative indication of surface roughness at least as sensitive as LEED beam broadening and work function change measurements. We conclude that a relatively well-ordered and uniform film surface is a necessary (but not sufficient) condition for the observation of quantum size structure.     

The influence of crystallographic steps on coadsorption: CO---O/Ni(119)

On low index nickel surfaces, repulsive interactions between atomic oxygen and CO drive the phase separation of these species into oxygen-rich and CO-rich islands. Because these adsorbates interact differently with crystallographic steps, the size and the structure of these islands are modified on stepped surfaces. We have monitored coadsorption-induced changes in the distribution of CO with IRRAS, observing six different CO stretching bands which are assigned to distinct local chemisorption environments. When oxygen fully saturates sites along the step edge, the steps are completely blocked from CO adsorption and virtually all the CO population on the terraces shifts from atop to bridge sites. This terrace site shift is similarly accomplished by atomic oxygen chemisorbed at terrace sites. From these coadsorption-induced changes in CO site distributions, constrained by the 10 Å terrace width, we conclude the through-metal O---CO interaction responsible for this CO site shift must be operative over a range of 5 Å. At θ_o = 0.18 ML, when oxygen occupies, but does not fully saturate the step edge, a new CO adsorption site is created, with a characteristic frequency of 1750 cm⁻¹. This new site is assigned to CO bonded to kinks along the step edge based upon its low intensity ( geometric kink density), enhanced binding strength and sensitivity to oxygen coverage. At higher oxygen coverages, compression of the CO adlayer is observed, with CO shifting to asymmetric bridge sites. As saturation coverage is approached, CO occupies weakly bound sites in close proximity ( 3 Å) to O adatoms, with high characteristic frequencies of 2100 cm⁻¹.     

氧在Nb(110)表面吸附的第一性原理研究

通过第一性原理赝势平面波方法研究了氧在Nb(110)表面的吸附性质随覆盖度变化规律. O在Nb(110)表面最稳定吸附位是洞位,次稳定吸附位是长桥位. 在长桥位吸附时, O诱导Nb(110)表面功函数随覆盖度的增加而几乎线性增加;但当O在洞位吸附时, 与干净Nb表面相比, 覆盖度为0.75 ML和1.0 ML时功函数增加, 而覆盖度为0.25 ML和0.5 ML时功函数减小.通过对面平均电荷密度分布和偶极矩变化的讨论, 解释了由吸附导致功函数复杂变化的原因.通过对表面原子结构和态密度分析, 讨论了O在Nb表面吸附时引起表面原子结构变化以及O和Nb(110)表面原子的相互作用.     

The adsorption of oxygen on the stepped Pt(S)-[9(111) × (111)] face

The adsorption of oxygen on the stepped Pt(S)-[9(111) × (111)) face has been studied by flash desorption, LEED and AES. On adsorbing oxygen the (1 × 1) LEED pattern of the clean face was transformed into a (2 × 2) pattern. A lower limit of the initial sticking coefficient of 0.06 and a saturation coverage of approximately 0.5 monolayer were determined. The flash desorption spectra exhibited two not completely resolved desorption maxima. From the peak temperatures the activation energies of desorption were estimated to be 41 and 49 kcal/mole. Under the same experimental conditions some experiments were done on a smooth (111) Pt face. However, the results did not differ significantly from those obtained on the stepped surface. In addition on the smooth (111) face the adsorption of oxygen activated in a high frequency discharge was studied. Oxidation was not observed beyond the chemisorption layer which is formed from molecular oxygen.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.     

The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.     

The adsorption and decomposition of NO on Pd(110)

The sticking probability of nitric oxide (NO) on Pd(110) and the relative selectivity of the surface to nitrogen (N₂) and nitrous oxide (N₂O) production has been measured as a function of coverage and as a function of surface and gas temperatures using a molecular beam. It is found that, at low temperatures (<440 K), molecular adsorption occurs with an initial sticking probability of 0.40 ± 0.02, rising quickly to a maximum of about 0.48 ± 0.02 as coverage increases before falling towards saturation. Following adsorption at 170 K four distinct adsorption sites can be identified by subsequent TPD. Hence, if beaming occurs at a temperature above the TPD peak due to a given site, then that site cannot be populated and the saturation coverage is found to be reduced. At higher temperatures (440–650 K) the sticking probability is seen to decrease continuously as a function of coverage. At a given NO uptake, the sticking probability falls with temperature indicating that the rate of NO desorption is significant in this temperature range. In addition, dissociation occurs leading to the desorption of nitrogen and nitrous oxide leaving only oxygen adatoms on the surface. The oxygen adatoms poison further reaction but can be cleaned off, even at the lowest temperature at which dissociation occurs, by hydrogen or carbon monoxide. At the low temperature end of this range more nitrous oxide is produced than nitrogen but this ratio falls with temperature until, above 600 K, there is 100% selectivity to the production of nitrogen which we propose is due to the low lifetime of molecular NO on the surface. However, at such high temperatures, reaction only occurs on a few sites probably located at the few step edges present on the crystal.     

Influence of oxygen adsorption on the surface potential of a metal oxide semiconductor

Analytical expressions describing dependences of the surface density of adsorbed oxygen ions and energy band bending in the subsurface region of a metal oxide semiconductor on the oxygen concentration that consider not only the process of neutral gas particle adsorption, but also their charge transfer at the expense of electron capture from the conduction band are presented. It is demonstrated that the heat of oxygen ion absorption is equal to the sum of the heat of neutral particle adsorption and the energy gap between the Fermi level and the level of the oxygen ion on the semiconductor surface. When the adsorption equilibrium is established, an analytical expression describing the time dependence of the energy band bending can be obtained only for small change of the oxygen concentration in the gas mixture.     

Oxygen induced reconstruction of (h11) and (100) faces of copper

A LEED and AES study on oxygen adsorption on Cu(100) and (h11) faces with 5 h 15 has been performed under various adsorption conditions (220 K T 670 K and 1 × 10⁻⁸ P 6 × 10⁻⁵ Torr of oxygen). The dependence of adsorption temp on the oxygen surface superstructures is pointed out. At least, three oxygen surface states exist on a Cu(100) face. For low temperature exposures to oxygen, under conditions of slow surface diffusion, on the (100) face, two oxygen surface phases exist: a “four spots” and a c(2 × 2) superstructure, both observed even at saturation coverage; on all the stepped faces, a c(2 × 2) appears and no faceting is observed. For high temperature exposures, on the (100) face, two oxygen superstructures are observed, a “four spots” followed by a (2√2 × √2)R45° at higher coverages; on all the stepped faces, surface diffusion is activated and oxygen induced faceting occurs. The appearance of faceting is associated with the onset of the formation of the (2√2 × √2)R45° structure on the (100) face. The oxygen induced faceting and the oxygen surface meshes are reversible with coverages. At saturation coverage, a non-reversible surface transition between the c(2 × 2) and (2√2 × √2)R45° superstructures is observed at 420 ± 20 K. The importance of impurity traces on the surface meshes is emphasized. Oxygen coverage at saturation is independent of the studied faces and adsorption temperature. Faceting occurs at a critical coverage value, whatever the stepped faces and adsorption temperature are. Models of the oxygen structure on the (h10) stepped faces are discussed.     

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO₂ film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.     

基于动态光谱法的多波长脉搏血氧饱和度测量

目前临床上血氧饱和度的无创检测主要基于双波长的脉搏血氧饱和度测量原理,但其检测精度仍然需要进一步的探究和完善。近年来,国内外的很多研究者采用三波长甚至八波长的方法测量血氧饱和度,从某种程度上减小了误差。动态光谱法作为一种新型血液成分无创检测方法,能消除受试者个体差异和测量环境等的影响,在血红蛋白浓度的无创检测研究中取得了很好的效果。基于动态光谱法对多波长下脉搏血氧饱和度检测进行了研究：对60名重症监护患者进行动态光谱采集以及动脉抽血分析血氧饱和度值;采用高灵敏度光纤光谱仪,采集受试者指端透射多波长下的光谱信息;以单拍提取法提取波长范围为606.44~987.55 nm的动态光谱;以动脉血气分析中血氧饱和度值为参考真值,建立血氧饱和度与多波长动态光谱数据的组合间隔偏最小二乘校正模型;得到预测集的相对误差为±0.017 6,而两波长测量装置监护仪上得到的数据相对误差为±0.116 4。结果表明：利用高灵敏度光纤光谱仪采集多波长光谱信息,用动态光谱法进行数据预处理,进行多波长血氧饱和度检测,有效降低了血氧饱和度的测量误差。     

Interactions du rhénium à haute température avec N2O sous basse pression

Starting at room temperature, N20 adsorption on rhenium proceeds dissociatively. Oxygen atoms remain on the surface while nitrogen molecules are desorbed. The overall process is characterized by an initial sticking coefficient value equal to 0.3 at 298 °K. In stationary conditions, and in a higher temperature range (> 1200°K) rhenium trioxide and oxygen atoms are the reaction products, depending on oxygen coverage on the surface. When the oxygen coverage is low, atomization, characterized by a reactive sticking probability of 0.2 is the only observable process. All the results are consistent with a model, previously proposed for the system oxygen-rhenium and oxygen-transition metals. The main differences in reaction rates between rhenium and oxygen or N₂O are interpreted in terms of saturation coverages.     

Adsorption of oxygen on silver single crystal surfaces

The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10^?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10^?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.     

First-principles study on anatase TiO2（101） surface adsorption of NO

In this paper, the stable structure and the electronic and optical properties of nitric oxide （NO） adsorption on the anatase TiO2 （101） surface are studied using the plane-wave ultrasoft pseudopotential method, which is based on the density functional theory. NO adsorption on the surface is weak when the outermost layer terminates on twofold coordinated oxygen atoms, but it is remarkably enhanced on the surface containing O vacancy defects. The higher the concentration of oxygen vacancy defects, the stronger the adsorption is. The adsorption energies are 3.4528 eV （N end adsorption）, 2.6770 eV （O end adsorption）, and 4.1437 eV （horizontal adsorption）. The adsorption process is exothermic, resulting in a more stable adsorption structure. Furthermore, O vacancy defects on the TiO2 （101） surface significantly contribute to the absorption of visible light in a relatively low-energy region. A new absorption peak in the low-energy region, corresponding to an energy of 0.9 eV, is observed. However, the TiO2 （101） surface structure exhibits weak absorption in the low-energy region of visible light after NO adsorption.     

H2S在氧化石墨烯表面的吸附和分解的理论研究

用周期性密度泛函方法对H₂S在氧化石墨烯(GO)上的吸附和分解进行了理论计算, 讨论了H₂S和GO上的羟基和环氧基团的反应过程．结果表明,反应过程是通过H₂S或-SH上的H转移使得GO的环氧基开环和羟基氢化,当GO相反面存在羟基时有助于环氧基团的开环和羟基氢化反应．H₂S在GO上吸附和分解到S原子的反应机理中引入了相应的中间态,计算两次脱氢过程能垒分别为3.2和10.4 kcal/mol,第二个H原子的转移是GO还原过程的速率决定步骤．结果还表明GO上的羟基和环氧基团有助于加强S原子和石墨烯的结合．     

The influence of nonpolar organics adsorption on the electrochemical behaviour of powdered activated carbon electrodes in aqueous electrolytes

Adsorption and electrochemical studies were carried out on three activated carbon samples first oxidized, then heat-treated under vacuum (at 180, 500 and 900 °C). The investigations were performed with aqueous electrolyte (Na₂HPO₄ and H₃PO₄) solutions containing selected nonpolar organics (benzene and n-hexane). Adsorption measurements were carried out on solution with a wide range of organics concentration (up to saturation point). Cyclovoltammetric curves of powdered electrodes prepared from the activated carbon samples were recorded for the organics in saturated solutions. The electric double layer capacities of the anodic and cathodic parts were estimated, and the surface anodic and cathodic charge was calculated both in absence and presence of organics in the electrochemical systems. The relative surface charge (in relation to systems without organics) was found to decrease with a reduction in the concentration of surface oxygen-containing groups. Other physicochemical parameters characterizing the degree of surface oxidation (total oxygen concentration, primary water adsorption centres) were also taken into consideration. The correlation between adsorption capacity towards the nonpolar organic compounds (obtained from adsorption isotherms) and change of surface charge was analyzed.     

Oxygen adsorption on the aluminum (111) surface by low-energy current image diffraction: A new approach

Scanning an aluminum (111) surface with low-energy electrons (20–200 eV), diffraction patterns in the speciment current image were observed to change upon exposure to oxygen. In particular at a primary beam energy of 20.8 eV relative to the vacuum, the pattern of the clean surface includes both hexagonal and trigonal features. Upon exposure to oxygen the hexagonal part of the image fades continuously so that by 290 L of oxygen only the feature of trigonal symmetry remains. Since oxygen is known from low-energy electron diffraction (LEED) and surface extended X-ray adsorption fine structure (SEXAFS) to form an ordered ad-layer on the (111) surface of aluminum, this suggests that current image diffraction (CID) can also be used to investigate adsorption.     

Photoemission study of the chromium(111) surface interacting with oxygen

The interaction of the Cr(111) surface with O₂ was studied by means of X-ray and UV photoemission and also work function measurements. A strong oxygen adsorption was found even at very low exposures, suggesting a high sticking coefficient. Previous treatments of the clean surface such as argon-ion bombardment or annealing result in significant changes of the surface structure reflected on work function and adsorption kinetics. No work function change was observed in the initial stage of adsorption, ruling out a model of chemisorption on top. In this range the sticking coefficient remains also constant, supporting a model of rapid regeneration of the genuine surface sites and incorporation of oxygen into the lattice. But in contrast with non transition metals like Cs or Sr, oxygen absorbed at room temperature in Cr, remains essentially in the topmost layers of the surface. At room temperature this initial stage of oxygen incorporation is followed by chemisorption on the corrosion film obtained when the uppermost layers are saturated with oxygen. The oxide layer has a stoichiometry close to Cr₂O₃ at saturation, but the detailed electronic structure depends on previous thermal treatments. Exposures at room temperature lead to a thin (about 9 Å), probably amorphous corrosion layer with a maximum work function change Δφ = +0.9 eV. Adsorption followed by heating at 500° C results in a much thicker corrosion film with a limiting work function decrease of Δφ = ?1.2 eV. The XP and UP spectra differ significantly in both cases and suggest a Fermi level shift of nearly 1 eV connected with oxygen adsorption on the Cr₂O₃ surface. The thickness of the corrosion film may be further increased by heating at 500°C in oxygen. The usual XPS spectra of bulk chromium sesquioxide are then clearly observed.     

O2在Mo(001)表面吸附的第一原理研究

采用第一原理方法计算了O2分子在 Mo(001) 表面的吸附,得到了吸附构型的各种参数,并且计算了O2分子在 Mo(001) 表面4个位置（顶位,桥位,穴位垂直,穴位平行）吸附后的能量,结果表明在顶位吸附能最高。通过对O2分子在 Mo(001) 表面吸附的原子轨道电荷分布与态密度图的分析可以看出在吸附过程中主要是O原子的2p轨道电子与钼的4s和4d轨道电子的相互作用。