Application of high energy ion channeling to GaAs(110), Au-GaAs(110) and Pd-GaAs(110)

The first application of high energy ion channeling to the atomically clean GaAs(110) surface and metal-GaAs interfaces is reported. Questions of sample preparation, background correction and computer simulation are addressed. It is found that the Ga and As atoms at the clean surface are laterally displaced ? 0.1 Å from the ideal bulk-like sites. The implications of this result to current LEED models are discussed. Au overlayers, deposited at room temperature, do not seem to produce lateral displacements of the substrate for coverages below ≈ 5 monolayer (ML). However, ≈ 0.9 ML of the substrate are expanded or contracted upon Au deposition; this process is completed at a coverage of 0.5 ML. Neither an indication of any order in the Au film is found, nor seems a significant (? 5%) fraction of Au atoms to occupy substitutional sites. In contrast, room-temperature deposition of Pd disorders the substrate substantially, without threshold coverage, even at very small film thicknesses.     

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.     

High-resolution photoemission from Ag(100), Ag(110), and Ag(111)

Energy distribution curves of photoelectrons emitted normal to (100), (110), and (111) faces of silver have been obtained at photon energies of 21.22, 16.85, and 11.83 eV. The results are compared with Christensen's relativistic band structure calculation of bulk silver yielding a close correspondence between experiment and theory. A surface state in the L gap immediately below the Fermi level is identified.     

Cu(100),(111),(110)表面晶格振动和力常数

采用嵌入原子方法得到了描述Cu基态相互作用性质的半经验函数;推导了表面面间力常数的计算公式,并计算了Cu(100),(111),(110)表面面间力常数,由此揭示了原子间相互作用在体内和在表面附近的区别,以及它们在不同表面结构中的差异。采用递推方法计算了相应的表面振动的投影态密度,与电子能量损失谱(EELS)所得到的实验结果符合得较好。 关键词：     

Ferromagnetism in the thermodynamically stable monolayer Fe(110) on W(110), coated by Ag

Ferromagnetic order in the thermodynamically stable, pseudomorphic monolayer Fe(110) on W(110), coated by Ag, was studied in situ in UHV using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). Films near the monolayer coverage, prepared at 475 K, consist of nearly independent monolayer and double-layer patches. The properties of monolayer patches are nearly independent of the mean film thickness resulting in excellent conditions to determine the true monolayer properties. The Curie temperature is reduced toT _{c, mono}= 282 K = 0.27 T_c,bulk, the ground state hyperfine field is reduced toB _hf(0)=11.9 T = 0.35B _hf,bulk(0) and the magnetic moment per atom is enhanced to(0) = 2.53 _B=1.14(0)_bulk. Remanent magnetization is detected forT 260 K=0.92T _{c, mono}, square loop magnetization forT 230 K=0.82T _{c, mono}. Unusual properties of the phase transition are detected by the combination of both experimental techniques.     

Steady state shapes of periodic profiles on (110), (100), and (111) surfaces of nickel

Periodic surface profiles with amplitudes of ≦0.4 μm and periodicities of 4–20 μm were prepared on Ni(110), (100), and (111) single crystal surfaces. These crystals were annealed in ultra-high vacuum (UHV) at 1073–1327 K after they had been cleaned by Ar ion bombardment and investigated by Auger electron spectroscopy. The geometry of the profiles was studied in UHV by laser diffraction and outside the vacuum by interference microscopy. The profiles have sinusoidal shapes on Ni(110) but trapezoidal shapes on both the (100) and (111) surfaces. This type of faceting can be understood on the basis of the anisotropic surface energy of Ni, with cusps at the (100) and (111) orientations. Model calculations show in the case of anisotropic surface energy that periodic profiles develop facets which correspond to the low surface energy orientations (close-packed surfaces).     

Angular distributions of H2 desorbed from the (100), (110), and (111) faces of copper crystals

The angular distributions of hydrogen molecules desorbed from copper have been measured for single-crystal surfaces of three different orientations: (100), (110), and (111). The surfaces were cleaned to the degree that the impurities were below the level of detection by Auger electron spectroscopy. The angular distributions were all significantly more peaked at the surface normal than the distribution corresponding to diffuse emission, i.e., where the molecular flux is proportional to cos θ, where θ is the angle measured from the surface normal. We have characterized the observed angular distributions by fitting the emperical expression cos^dθ to the data, where d is an adjustable parameter. The results are: d ≈ 5 for (100); d ≈ 2.5 for (110); d ≈ 6 for (111). The distributions are nearly symmetric about the azimuthal angle between the plane of detection and the principal axes of the surface lattice. Possible models are considered.     

Planar channeling of helium ions along (100), (110) and (111) MgO

Planar channelling of 1, 1.5 and 2 MeV ⁴He⁺ ions along (100), (110) and (111) MgO have been studied experimentally using Rutherford backscattering. Values of the half angle $\text{ψ}{}^{\mathrm{P}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, shoulder half angle $\text{ψ}{}^{\mathrm{s}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and surface minimum yield x^P_min have been determined for channelling with respect to the two sublattices. Agreements and discrepancies with existing theories are discussed.     

The interacting states (030), (110), and (011) of H216O

A fit of 382 rotational levels of the three vibrational states (030), (110), and (011) of H₂¹⁶O has been performed using 51 effective constants. The Fermi-type interaction between (030) and (110) and the Coriolis-type interaction between (110) and (011) as well as between (030) and (011) are taken into account. The part of the Hamiltonian which is diagonal in the vibrational quantum numbers is a Watson-type Hamiltonian. Considering the wide spread of J and K_a values, the general agreement between experimental and calculated levels is satisfactory. A comparison with the results relative to the states (020), (100), and (001) is given.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C₂D₂ spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.     

Elastic leed intensity-energy studies of clean (001), (110) and (111) nickel surfaces

Elastic low energy electron diffraction (LEED) intensity-energy (I-E) measurements for clean (001), (110), and (111) nickel surfaces were obtained at room temperature. Surface composition was monitored by Auger spectroscopy. I-E data from 15 to 220 eV were obtained at normal incidence for the non specular beams and for the specular beams at incidence angles from 4° to 20° on the 0° and 45° azimuths of (001), on the 0° and 90° azimuths of (110), and on the 0° azimuth of (111) nickel. Normalization of the data was performed electronically during data acquisition. Intensities were calibrated with the use of a shielded, biased Faraday collector. The effects of instrumental and experimental uncertainties were examined and minimized to obtain intensities accurate to ± 15 %, energy scales accurate to ± 0.35 eV, and incident and azimuthal angles accurate to ± 0.25° and ± 1.0° respectively.All nickel surfaces have I-E spectra which are characteristic of strong multiple scattering. Angular evolution features for (001) and (110) spectra may be correlated with intraplanar resonances associated with the onset of propagating beams. Only the (001) surfaces were found to have pronounced, sharp resonance features associated with surface barrier resonances and inelastic loss processes. Kinematic analysis of the Lorenzian-shaped I-E peaks on all surfaces in consistent with surface expansion using either an energy-dependent or a constant inner potential of 10.75 ± 0.5 eV. The widths of these same peaks on all surfaces were found to vary as $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ above 40 eV and $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ below.     

Surface states calculation for the (100), (110) and (111) faces of Si

Using essentially the Heine and Jones model of the band structure for diamond-type semiconductors and a 3-dimensional surface state secular equation, we have calculated the bands of surface states for the (100), (110) and (111) orientations of the surface plane of ideal Si. Our bands for the (110) surface are very similar to those obtained by Jones. Based on these bands of surface states, we can explain reasonably both optical and electrical measurements on the (111) surface.     

Interaction of carbon dioxide with Fe(110), stepped Fe(100) and Fe(111)

The adsorption of CO₂ on single crystal surfaces of Fe(110), regularly stepped Fe(110) and Fe(111) in the temperature range between 77 and 340 K was studied by means of He(I) UPS and measurements of the change in work function. The smooth Fe(110) face proved to be completely inactive with respect to CO₂ adsorption. On a stepped Fe(110) and an Fe(111) face CO₂ is adsorbed at 77 K in the form of a linear molecule and in the form of a species the nature of which is not yet clarified. This latter form is predominant at 140 K. With increasing temperature decomposition into CO and O and finally into C and O takes place.     

Diffusion en surface de l'argent sur les plans (001), (111), (110) et des surfaces vicinales du cuivre

The diffusion coefficients of silver on singular and vicinal surfaces of copper have been measured using an oxidation method to determine the concentration profiles. The diffusion was carried out under ultra high vacuum at low temperature (250 to 500 °C) with a low concentration of the diffusing species (less than 0.5 monolayers). Comparison of the results with the terrace-ledge-kink model shows that the diffusion occurs along the ledges and also explains the diffusion on the vicinal surfaces. The differences between this model and the diffusion observed on the singular surfaces are explained by the presence of separated ledge loops on these surfaces.