Helium scattering from metal surfaces: potential determination and crystallography

We describe a method for determining the interaction potential between a helium atom and a metal surface from diffraction data. Results are presented for He/Ni(110) and He/Cu(110). For He/Cu(110) the potential 3s in good agreement with a first principles calculation but this is not true for He/Ni(110). We also show that a surface relaxation of 10% produces measurable changes in intensities.     

Analyses of Ne-diffraction data from transition-metal surfaces based on charge-density calculations

We evaluate the proportionality constant β for neon in the Esbjerg-Nørskov potential V_R(r) = βϱ(r) from experimental data on Ni(110), Ni(113), Cu(110) and Pd(110). Our calculations using overlapping atomic charge densities require that β be material and surface-dependent. Although Ne diffraction is more sensitive to details of corrugation shapes than He, we found it to be insensitive to normal relaxations of the topmost layers.     

Adsorption an Einkristall-Oberflächen von Cu/Ni-legierungen. I

The adsorption of O₂ and CO on the (110) face of a Cu/Ni alloy (55 at% Cu) has been studied by means of low energy electron diffraction (LEED), Auger electron spectroscopy, work function measurements, and flash desorption. A comparison with the behavior of Cu(110) and Ni(110) is made. It is shown that the height of an Auger peak is proportional to the surface concentration of the corresponding species and that the surface composition of the alloy is identical with the composition of the bulk. Adsorption of oxygen leads to the formation of an ordered 2 × 1 structure, as is the case for Cu(110) and Ni(110). Further exposure causes disordered adsorption in contrast to the pure components where c6 × 2 respectively 3 × 1 structures are formed. Oxygen increases the work function of Cu and Cu/Ni by about 0.25 eV whereas for Ni the increase is > 1 eV. CO is not irreversibly adsorbed on Cu at 25°C, but forms a stable 1 × 1 structure on Ni(110). With the alloy two ordered phases (2 × 1 and 2 × 2) are observed. The flash desorption spectrum shows three maxima which are similar to the binding states of CO on Ni(110) and Ni(100). The results are discussed in view of the electronic structure of Cu/Ni alloys and the parameters influencing the configuration of adsorbed particles.     

The role of HH interactions in the formation of ordered structures on Ni and Pd single crystals

The interaction between H adatoms on a surface is calculated within the embedded cluster model of chemisorption. The model is first applied to the case of two H atoms on a free electron surface. The interaction energy is found to be an oscillatory function of the H-H separation R_ab. Application of the free electron model to the problem of chemisorption on transition metal surfaces leads to unphysical results with the prediction of formation of ordered H overlayers which are not observed in LEED experiments. We next include the l = 2 TM muffin tins. Results for H adsorption on the low index faces of Ni and Pd substrates are presented. Graphitic structures are predicted for the (111) faces of both Ni and Pd with the H atoms occupying both types of three-fold hollow sites on the surface. This agrees with the results of LEED experiments for H/Ni(111). Comparison with experiment is not possible in the case of H/Pd(111) owing to the lack of low temperature studies for that system. Zig-zag chains with the H atoms adsorbed in sites of three-fold coordination on alternate sides of the TM(110) rows are predicted for both Ni and Pd. This is in agreement with the results of He diffraction experiments for H/Ni(110). No structure determination has been done for H/Pd(110). Adsorption in the four-fold centre sites for H on the (100) faces of Ni and Pd is found to be unfavourable. The H atoms are expected to adsorb in sites of three-fold symmetry below the (100) surface for H on Pd with formation of a c(2 × 2) structure in agreement with the LEED observations. For H/Ni(100) the H atoms are believed to adsorb above the surface, away from the centre site and to bond to two surface Ni atoms. No short-range ordered structures are predicted in this case.     

Metastable helium atom stimulated desorption of H+ Ion

H+ desorption induced by the impact of metastable helium atoms has been found for H(2)O/Na/Ni(110) coadsorption systems. The measurements were carried out using a time-of-flight technique and a pulsed-discharge type metastable helium atom ( He(*)) source. It is concluded that the H+ desorption by He(*) is induced by a hole created on the valence levels via the Auger deexcitation of He(*). The H+ desorption by He(*) may be understood within the framework of the Menzel-Gomer-Readhead model.     

镍单晶膜上氧化镍的配位生长取向关系

对在膜面为(110),(001)和(111)的镍单晶膜上生成的氧化镍取向进行了电子衍射分析,除在(111)和(001)镍膜上肯定了前人已发现的氧化镍与镍的平行取向关系外,还在(110)和(001)镍单晶膜上发现了(111)_NiO∥(001)_Ni,〈110〉_NiO∥〈110〉_Ni取向关系以及〈110〉_NiO∥〈110〉_Ni氧化镍纤维织构。在所有氧化镍与镍的取向关系中均有〈110     

A quantitative ion-scattering study of the Ni(110) surface during the early stages of oxidation

The adsorption of oxygen on clean Ni(110) has been studied at room temperature and at 475 K by Rutherford backscattering, using the effects of channeling and blocking, and lowenergy electron diffraction. At both temperatures successive LEED structures are formed at low oxygen coverage (?0.5 monolayer). With increasing oxygen content stoichiometric NiO is formed on top of the Ni(110) surface, at room temperature as an amorphous layer and at 475 K as patches of crystalline oxide, oriented with the NiO(100) planes parallel to the Ni(110) surface plane. At 475 K the nickel atoms in the interface region between oxide and substrate are displaced over a thickness of less than 2 monolayers. Based on the measurement of the oxide composition as function of coverage we suggest a modification of the island growth model as proposed by Holloway and Hudson for the Ni(100) and (111) surfaces.     

Adsorption of carbon monoxide on Ni(110) above atmospheric pressure investigated with surface X-ray diffraction

The crystallographic structure of CO adsorbed on Ni(110) has been investigated with x-ray diffraction at ambient pressures of CO ranging from 2.3 to 10(-10) bars. At room temperature, the high pressure and vacuum structures are identical. Above room temperature ( approximately 130 degrees C), the high pressure of CO induces a restructuring of the Ni substrate that develops strained (111) microfacets.     

The effect of an external electric field on the survival of surface-produced negative ions

Resonant charge exchange between slow atomic particles and a partially coated metal surface, in the presence of a homogeneous electric field, is considered. The survival probabilities are calculated for H^- ions, emerging from Cs-coated W and K-coated Ni(110) in fields of strengths 10^-4, 10^-5 and 0 au.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

注氢Fe-1wt%Ni合金中的辐照诱导偏析

利用离子加速器注氢和高压电子显微镜的电子辐照方法,研究了Fe-1wt%Ni铁素体钢中Ni合金元素的辐照诱导偏析现象．实验结果表明,Ni与空位、氢结合成复合体,其扩散机制为逆Kirkendall效应,即Ni的扩散方向与空位的扩散方向一致．在空洞附近富集的Ni会沿{110}平面共格析出．在特定的入射方向的电子衍射图象中,能够观察到由于这种平面共格析出引起的衍射条纹． 关键词：     

Spin reorientation and structural relaxation of atomic layers: pushing the limits of accuracy

The correlation between an ad-layer-induced spin reorientation transition (SRT) and the ad-layer-induced structural relaxation is investigated by combined in situ surface x-ray diffraction and magneto-optical Kerr-effect experiments on Ni/Fe/Ni(111) layers on W(110). The Fe-induced SRT from in-plane to out-of-plane, and the SRT back to in-plane upon subsequent coverage by Ni, are each accompanied by a small lattice relaxation of at most 0.002 A. Such a small strain variation excludes a magnetoelasticity driven SRT, and we suggest the interface anisotropy as a possible driving force.     

Surface stress of Ni adlayers on W(110): the critical role of the surface atomic structure

Puzzling trends in surface stress have been reported experimentally for Ni/W(110) as a function of Ni coverage. In order to explain this behavior, we have performed a density-functional-theory study of the surface stress and atomic structure of the pseudomorphic and several different possible 1 × 7 configurations for this system. For the 1 × 7 phase, we predict a different, more regular atomic structure than previously proposed based on surface x-ray diffraction. At the same time, we reproduce the unexpected experimental change of surface stress between the pseudomorphic and 1 × 7 configuration along the crystallographic surface direction which does not undergo density changes. We show that the observed behavior in the surface stress is dominated by the effect of a change in Ni adsorption/coordination sites on the W(110) surface.     

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.     

Formation of bcc-Ni thin film on GaAs(100) substrate and phase transformation from bcc to fcc

Ni thin films are prepared on GaAs(100) single-crystal substrates at room temperature by using an ultra-high vacuum radio-frequency magnetron sputtering system. The growth behavior and the crystallographic properties are studied by in-situ refection highenergy electron diffraction and pole-figure X-ray diffraction. In an early stage of film growth, a metastable bcc Ni(100) single-crystal film is formed on GaAs(100) substrate, where the bcc structure is stabilized through hetero-epitaxial growth. With increasing the film thickness, fcc crystals coexist with the bcc(100) crystal. High-resolution cross-sectional transmission electron microscopy shows that the film consists of a mixture of bcc and fcc crystals and that a large number of planar faults exist parallel to the fcc(111) close-packed plane. The results indicate that the bcc structure starts to transform into fcc structure through atomic displacement parallel to the bcc{110} close-packed planes.     

A classical path approximation for diffractive surface scattering

The well-known classical path approximation is applied to a calculation of diffraction intensities in the scattering of atoms from a rigid crystal with a soft interaction potential. A general expression is derived for the diffraction intensities which can be applied to potentials with several higher-order terms in the Fourier series. For an uncorrugated Morse potential with a first-order exponential corrugation term an analytic solution is obtained which is compared with the infinite order suddent (IOS) approximation calculations for Ne/W(110) and He/LiF(100). Both approximations are very accurate for the weakly corrugated Ne/W system. For He/LiF the present approximation is more accurate than the sudden (IOS) approximation and has the added advantage of providing an analytic solution. Several improvements are suggested.     

Effects of adsorbates on the formation of doubly excited He atoms during impact of He2+ ions on Fe(110) and Ni(110) surfaces

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with projectile energies between 74 eV and 124 eV with an Fe(110) and a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage with oxygen for both surfaces. For a controlled O₂ adsorption on the Fe(110) and Ni(110) surfaces we can explain the resulting changes in the electron spectra by the modification of the measured work functions of the target surfaces. In terms of thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures, we present an alternative interpretation of similar previous studies by another group, where the local electron spin polarization of Fe(110) and Ni(110) surfaces was deduced from changes in the electron spectra as function of target temperature.     

Theoretical study of aging effects on LaNi_5 tritium storage

1 Introduction LaNi5 intermetallic compounds, used in storage, separation and purification of hydro- gen and hydrogen isotopes, are known for a high-density storage, easy activation, low pressure, and rapid and reversible absorption and desorption[1-15]. Tritium as an impor- tant material in the nuclear industry is radioactively decaying into 3He with a half-life of 12.3 years. With time, there are many changes in the electronic and physical properties of the tritide for the deposition of 3He…     

Hydrogen chemisorption on Ni(110) by high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectra of hydrogen-covered Ni(110) surfaces both at 100 and 300 K are presented. The adsorbed sites of hydrogen atoms are discussed.High-resolution electron energy loss spectra of hydrogen covered Ni(110) surfaces have been studied. Tentative models for the adsorbed sites of hydrogen atoms are as follows: (1) For the (2 × 1)-H surface, hydrogen is adsorbed in the three-coordinated sites of the rudimentary (111) face of the unreconstructed Ni(110) substrate. (2) For the low-temperature (1 × 2)-H surface, hydrogen is adsorbed in the three-coordinated sites and, probably, in the two-fold hollow sites of the distorted Ni(110) substrate. (3) For the room-temperature (1 × 2)-H surface, hydrogen is disorderedly adsorbed in the three-coordinated, two-fold hollow and short-bridge sites and, possibly, in the octahedral sites of the distorted Ni(110) substrate. Some of the unresolved problems in the above assignments are summarized: (1) Strictly, the three-coordinated sites above are somewhat different from those discussed in the molecular-beam diffraction study [5]. (2) For the low-temperature (1 × 2)-H surface, the loss associated with hydrogen in the two-fold hollow sites is apparently not observed. (3) Intensity changes of the three losses for the room-temperature (1 × 2)-H surface with increasing hydrogen pressure (Fig. 2) are not well understood.     

Pb液滴在Ni基底润湿铺展行为的分子动力学模拟

采用分子动力学方法研究了Pb液滴在Ni(100)晶面、(110)晶面和(111)晶面的铺展润湿行为. 结果表明: Pb液滴在Ni(100)及(111)基底上的润湿铺展现象呈各向同性, 而在Ni(110)基底上的润湿铺展现象呈明显的各向异性, 且这种各向异性源于Ni(110)晶面点阵结构上Pb原子沿不同晶向的扩散机制及速度的明显差异; Pb液滴在Ni(111)晶面上铺展时, 未发生表面合金化, 液滴铺展动力学描述近似满足 R² ∝ t, 而液滴在(100)晶面和(110)晶面上铺展时表面产生合金化现象, 铺展动力学关系近似满足 R⁴ ∝ t, 且液滴在(100)晶面上的铺展速度高于(110)晶面上的铺展速度. 关键词： 分子动力学 润湿各向异性 铺展膜 扩散机制