Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Oxygen adsorption on an alkali metal-covered Ni(100) surface

The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of θ_O^cwhose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(100), which suggests that alkali metals cause no change in the character of the Ni-O bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.     

Hydrogen adsorption on Ni (100)

The adsorption of hydrogen on the (100) plane of nickel at room temperature has been investigated using the technique of flash desorption spectroscopy. It is shown that no variation in the adsorption enthalpy of 23.1 kcal/mole occurs during the chemical cleaning of the surface by repeated oxidation and reduction. The number of adsorption sites does however increase to 3.3×10¹⁴/cm² during this process. Determination of the partition functions of the adsorbed species and of the activated complex indicates that the hydrogen atoms are localised on specific adsorption sites but that greater liberty exists in the activated complex. Finally the experimental desorption spectra may be described using a model with a repulsive interaction of 400 cal/mole between nearest neighbours.     

Chalcogen adsorption on Ni(100)

The successive stages of the oxidation of Ni(100) have been investigated by angle-resolved uv photoemission. The adsorption spectra are very similar to that measured previously for the same surface saturated with sulphur. Results are interpreted using recent theoretical calculations. It is found that this interpretation can be extended to the other chalcogens adsorbed on Ni(100). When increasing oxygen exposures, photoemission spectra have shown a continuity of the electronic character in the oxidation process.     

On the surface phonons due to oxygen adsorption on Ni (100)

Assuming that an oxygen adsorbed atom is linked to four Nickel atoms by central harmonic forces, the surface phonons energies are calculated in the case of P 2 × 2 and C 2 × 2 structures. The comparison to measured low-energy electron losses indicates that the Ni-O bond strength in the P 2 × 2 adsorbed structure is twice that of the C 2 × 2. This result gives some information concerning the potential shape near equilibrium position.     

Strong dependence of the (100) surface composition of A 50-50 iron-cobalt monocrystalline alloy with ion bombardment and annealing

A decomposition of the overlapping iron and cobalt Auger transitions, using pure element experimental spectra, has been applied to the Auger analysis of a 50-50 iron-cobalt alloy. We have found that a linear combination of the pure element spectra reproduces quite accurately the alloy spectra in a wide range of surface composition. Using this method a high depletion in iron upon argon bombardment is observed and different recovering temperatures are found for iron and cobalt.     

Studies of adsorption and electron-induced dissociation of Fe(CO)5 on Si(100)

The adsorption and electron-stimulated decomposition properties of Fe(CO)₅ on Si(100) have been investigated. At substrate temperatures in the range 77–300 K, no chemisorption states are populated to a detectable extent. Physisorption at 77 K is observed, with a sticking probability of ≈ 0.2; TDS indicates that the heat of adsorption is constant at 33 ± 4 kJ mol⁻¹ both in the monolayer and multilayer regimes and the desorption kinetics are suggestive of island formation. Prominent electron-stimulated desorption (ESD) and decomposition effects have been investigated. Fe(CO)₅ multilayers decompose by rapid ESD of iron carbonyls, whilst monolayer decomposition involves the evolution of O and CO species in a stepwise manner from the parent molecule. Cross-sections for the processes occurring are evaluated; the electron stimulated rate of FeCO bond cleavage is demonstrated to be more rapid than the rate of escape of oxygen containing entities from the surface and this is explained in terms of bond orientation effects.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni₂MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.     

CO adsorption on alkali-metal covered Ni(100)

The effect of preadsorbed alkali metal atoms Na, K and Cs on CO adsorption on Ni(100) has been studied using Auger spectroscopy and thermal desorption. It was found that the presence of alkali metals causes an appearance of several more tightly bound states in the CO thermal desorption spectra. The observed difference in carbon and oxygen Auger peak line shape on a bare and alkali modified Ni(100) is indicative that the presence of alkali adatoms induces CO decomposition on the Ni(100) surface. The fraction of dissociated CO increases with the amount of alkali adatoms present. At the same overlayer coverage the dissociation probability increases in the sequence Na, K, Cs. A comparison of the strength of the promoting effect on CO dissociation with the changes in the surface electron density in the presence of alkali adatoms has shown that at low overlayer coverages the electronic factor plays a major role in explaining the action of the surface modificators.     

Temperature induced asymmetric effects in the surface extended energy loss fine structure of Ni(100)

Electron energy loss spectra have been detected on a Ni(100) surface above the M_{2, 3} core edge at several different temperatures, between 80 and 1300K. We find a strong temperature dependence of the SEELFS signal. In particular we find a strong attenuation of the signal on increasing the temperature (especially the part originating from higher coordination shells). We find also an apparent contraction of the nearest neighbour distance obtained by the Fourier transform technique. We present a model calculation by which we tentatively interpret the apparent contraction as originating from an artifact introduced by the experimental integration limits of the Fourier transform. This in turn is due to an asymmetric pair distribution function of atoms in the surface region.     

原子分子在Ni(100)表面吸附的第一性原理研究

本文采用密度泛函理论,结合周期性平板模型,通过对原子H、N、O、S和C,分子CO、N2、NH3、NO,以及自由基CH3、CH、CH2、OH在Ni(100)表面吸附的研究,比较了它们的吸附能,稳定吸附位点,吸附结构及扩散能垒等信息.这些吸附质与表面结合能力从小到大依次是N2NH3COCH3NOHOHCH2CNSONCHC.在所有的原子中,O原子倾向于吸附在桥位,而其余的原子则倾向于吸附在空位.除N2之外的分子吸附物(CO、NO、NH3),最佳吸附位点均为四重空位,而N2的最稳定吸附位置为顶位.对于自由基吸附物(CH、CH2、CN、OH)而言,它们倾向于吸附在四重空位,而CH3则稳定吸附在桥位.     

原子分子在Ni(100)表面吸附的第一性原理研究

本文采用密度泛函理论,结合周期性平板模型,通过对原子H、N、O、S和C,分子CO、N2、NH3、NO,以及自由基CH3、CH、CH2、OH在Ni(100)表面吸附的研究,比较了它们的吸附能,稳定吸附位点,吸附结构及扩散能垒等信息. 这些吸附质与表面结合能力从小到大依次是N2相似文献   

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.