Electron-stimulated desorption of thin epitaxial films of KBr grown on (100)InSb

We have measured time-of-flight (TOF) distributions of Br atoms desorbed from thin (less than 1000 Å) epitaxial films of KBr on (100) InSb with a 2 keV electron beam. Although the general structure of the TOF spectra was similar to that obtained previously for the thick crystals, both the fast and the slow (thermal) components of the distribution were strongly dependent on the film thickness. We argue that this dependence is due to two different diffusion processes involved in the transport of the primary excitation products from the bulk to the surface. By measuring the velocity resolved ESD yield for films of various thicknesses, we determined that a diffusion length of the carriers responsible for the thermal ESD component varied from 30 to 700 Å with temperature in the range 20–300°C. In contrast, for the non-thermal desorption we found the carrier diffusion length of about 140 Å which did not depend significantly on the temperature.     

Determination of the K adsorption site on Fe(1 1 0) with XSW

We have used X-ray standing waves (XSW) in near normal incidence to determine the K–Fe bond length and the adsorption site of K at the saturation coverage at room temperature on the Fe(1 1 0) surface. Three different scattering geometries were used to enable the determination of the adsorption site by triangulation. From the results we conclude that the potassium atoms adsorb in a distorted hexagonal overlayer. The Fe–K distance, as determined from the measurements in the (2 2 0) Bragg reflection, is 3.4±0.2 Å. The long bridge site seems to be the preferred adsorption site for the potassium atoms in the distorted hexagonal overlayer. This geometry not only fits all the XSW data, but also explains all spots in the LEED pattern without the need to introduce multiple scattering. Comparison of the measured and simulated XSW data, based on the distorted hexagonal overlayer, enables a more accurate determination of the Fe–K bond length to 3.36±0.14 Å. This corresponds to a potassium hard sphere radius of r_K=2.12±0.14 Å. This radius is among the largest reported for potassium on a metal, which is attributed to the high coverage and coordination of the K atoms in this overlayer arrangement.     

Dimer elongation on the monohydride-covered Ge/Si(100) surface

Using transmission ion channeling, we have made the first measurement of the Ge dimer geometry for the monohydride-covered Ge/Si(100)-2×1 surface. Comparison of calculated angular scans with experimental angular scans near the 100 and 110 directions has resulted in a measured Ge dimer bond length of 2.8 Å, which is 8% longer than the corresponding dimer bond length reported for Ge on Si(100) in the absence of H. This elongation is similar to that reported for Si dimers on the Si(100) surface. Also, relative to the (100) surface plane, the dimers change from tilted without H to untilted with H.     

Atomic and electronic structure of the corundum (0001) surface: comparison with surface spectroscopies

The electronic structure and geometry of the Al-terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 Å, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS He I) and metastable impact electron spectra (MIES) is given using the calculated total density of states of the slab and the projections to the atoms, atomic orbitals, and He 1s floating atomic orbital at different positions above the surface. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms. The good agreement of simulated and experimental UPS and MIES spectra supports the correctness of calculated surface relaxation.     

Surface and subsurface distortions of the Au{110}-(1×2) structure

The reconstruction of the Au{110}-(1×2) missing-row surface has been studied by means of the new scattering and recoiling imaging spectrometry (SARIS) technique. The three-dimensional focusing patterns observed for scattering of 4 keV He⁺, Ne⁺ and Ar⁺ ions are highly sensitive to the structure of both the surface and subsurface layers. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) were used to simulate the scattering patterns. Using an R-factor comparison of the experimental and simulated images, it is demonstrated that SARIS is sensitive to changes of the order of 0.02 Å in the structural parameters of this Au surface. These parameters involve interlayer spacings, row pairing and row buckling in the first-through fifth atomic layers. Results for the shallow surface layers are in general agreement with the those of previous studies. The new results include structural parameters for the deeper subsurface layers and the observation of an oscillatory behavior of the layer spacings which is damped towards deeper layers.     

Adsorption position of oxygen on the Pt(111) surface

The adsorption position of oxygen on the clean Pt(111) surface has been determined by means of the transmission channeling technique. Oxygen adsorbs in a well ordered p(2 × 2) overlayer structure at temperatures 200 T 350 K. From an analysis of the angular scans along the [111], [110] and [100] axial directions it is concluded that the O atoms are adsorbed in the fcc three-fold hollow site exclusively at a height of 0.85 ± 0.06 Å above the Pt surface layer. From a narrowing of the [111] angular O scan, the O RMS displacement parallel to the surface is found to be 0.16 ±0.03 Å.     

Electronic structure investigation of the Ag(1 1 0)(n×1)O surfaces––new photoemission results for an old problem

We have used angle-resolved photoelectron spectroscopy to investigate the occupied antibonding electron states of the Ag(1 1 0)(n×1)O surface along different directions in the surface Brillouin zone. We present experimental evidence that several earlier results obtained along (along the Ag–O chains) contain admixtures from contamination, most probably from carbonate-like contributions. New results are obtained along and (perpendicular to the chains). These data indicate that the n=2 structure is stabilized by repulsive electronic interaction between neighbouring chains, which diminishes drastically for n=3 and disappears almost completely at n4. This observation points to a strain field within the substrate which stabilizes the geometry between n=3 (interchain distance 8.7 Å) and n=8 (23.1 Å). Its existence is indirectly seen in the n-dependence of the surface phonon energies at , which can be explained quantitatively by umklapp-processes induced by the lateral periodicity of the strain field. We compare our photoemission results for (2 × 1)O with available surface band structure calculations.     

A multiple-scattering approach to X-ray photoelectron diffraction

We present an alternative method for multiple scattering calculations of X-ray photoelectron diffraction from periodic surfaces. The technique uses a time-reversed RHEED wave function as the final state in a one-step model of the photoemission process. We apply the method to the Ni(001)c(2×2)S system and find an excellent agreement with the experimental data for vertical height of the S overlayer of 1.35±0.05 Å above the top Ni layer. Our calculations display a much better agreement with the experimental data than has been previously reported.     

高光洁度玻璃基片的表面散射和体散射测量

 提出了一种利用总积分散射（TIS）测量K9玻璃基片表面散射和体散射的实验方法。首先采用磁控溅射技术在基片表面沉积厚度为几十nm的金属Ag薄膜,然后将基片的表面和体区分开考虑,通过TIS测得了基片上下表面的均方根粗糙度, 进而求得基片的总散射和表面散射,最后计算得到了体散射。分别利用TIS和原子力显微镜（AFM）测量了3个样品上表面所镀Ag膜的均方根粗糙度,两种方法所得的均方根粗糙度的数值相差不明显,差值分别为0.08,0.11和0.09 nm, 表明TIS和AFM的测量结果相一致。利用该方法测得3块K9玻璃基片的总散射分别为6.06×10^-4,5.84×10^-4和6.48×10^-4,表面散射介于1.25×10^-4~1.56×10^-4之间,由此计算得到的体散射分别为3.10×10^-4,3.30×10^-4和3.61×10^-4。     

Measurement of sample temperatures under magic-angle spinning from the chemical shift and spin-lattice relaxation rate of 79Br in KBr powder

Accurate determination of sample temperatures in solid state nuclear magnetic resonance (NMR) with magic-angle spinning (MAS) can be problematic, particularly because frictional heating and heating by radio-frequency irradiation can make the internal sample temperature significantly different from the temperature outside the MAS rotor. This paper demonstrates the use of (79)Br chemical shifts and spin-lattice relaxation rates in KBr powder as temperature-dependent parameters for the determination of internal sample temperatures. Advantages of this method include high signal-to-noise, proximity of the (79)Br NMR frequency to that of (13)C, applicability from 20 K to 320 K or higher, and simultaneity with adjustment of the MAS axis direction. We show that spin-lattice relaxation in KBr is driven by a quadrupolar mechanism. We demonstrate a simple approach to including KBr powder in hydrated samples, such as biological membrane samples, hydrated amyloid fibrils, and hydrated microcrystalline proteins, that allows direct assessment of the effects of frictional and radio-frequency heating under experimentally relevant conditions.     

In-situ observation of an ordered sulfate adlayer on Au(100) electrodes

An ordered adlayer of sulfate or bisulfate has been found on the unreconstructed Au(100) surface in 0.1 M H₂SO₄ by in-situ STM. The ordered adlayer is formed concomitantly with lifting of the (hex)-reconstruction. The new structure consists of sulfate/bisulfate rows that run parallel to the edges of the islands that were created during the (hex)→(1×1) transition. The distance between the adions within the rows is 4.0±0.3 Å, and that between rows 10.4±0.7 Å, corresponding to a superstructure with a surface coverage of about 0.2.     

LEED structure determination of the Ni(1 1 1)(√3×√3)R30°-Sn surface

Quantitative low energy electron diffraction has been used to determine the structure of the Ni(1 1 1)(√3×√3)R30°-Sn surface phase. The results confirm that the surface layer comprises a substitutional alloy of composition Ni₂Sn as previously found by low energy ion scattering (LEIS), and also shows that there is no stacking fault at the substrate/alloy interface as has been found in (√3×√3)R30°-Sb surface alloys on Ag and Cu(1 1 1). The surface alloy layer is rumpled with the Sn atoms 0.45 ± 0.03 Å higher above the substrate than the surrounding Ni atoms. This rumpling amplitude is almost identical to that previously reported on the basis of the LEIS study. Comparison with similar results for Sn-induced surface alloy phases on Ni(1 0 0) and Ni(1 1 0) shows a clear trend to reduced rumpling with reduced surface atomic layer density, an effect which can be rationalised in terms of the different effects of valence electron charge smoothing at the surface.     

LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

Structural properties of diamond fine particles and clusters prepared by detonation and decomposition of TNT

The thermal parameter B for three different particle sizes of diamond samples (bulk powder 1–4 μm, fine particle 144–195 Å and cluster 55–61 Å) was determined by the grazing incidence X-ray diffraction method. The values of B were found to be in the range 0.50–0.70 Å² for particles in the size range 195–55 Å and 0.27 Å² for 1–4 μm. All of them are larger than that of diamond bulk. A clear size dependence of B, increasing with decreasing particle size, was found. By analysing X-ray diffraction data at several temperatures the magnitude of B was found to be due to B_S (static part) instead of B_T (dynamic part). The average B_S values obtained were 0.04 Å², 0.19 Å² and 0.27 Å² for bulk powder, fine particle and cluster samples respectively. Ultrahigh resolution transmission electron microscope (TEM) observation confirmed the presence of strain, distortion, roughness and dislocation lines in many particles. TEM images of particles indicate that the clusters were not spherical in shape; they were mostly cubiform and some were truncated prism-like polyhedral. The present study reveals that the B_S component is responsible for the large B value in diamond fine particles and clusters. No clear surface local atomic distortion was found in the particles.     

Temperature dependence of Pt(111) surface relaxation

A careful investigation of the Pt(111) surface has been carried out, using the MeV heliumscattering technique. The anomalously large surface relaxation effect reported previously has been identified as an experimental artifact resulting from an unexpectedly large surface damage effect at low temperature. Optimum conditions have been established for minimizing the main experimental sources of error: background subtraction, radiation damage, and deviations from the Rutherford scattering law. Using these optimum conditions, a series of scattering measurements has been made over the temperature range 40–300 K. At all temperatures, we observe a significant anisotropy in the 〈111〉 angular scans, indicating an outward relaxation of the Pt(111) surface plane. By comparing this observed anisotropy with a set of Monte Carlo simulations, a value of 0.03 ? 0.01 Å (i.e. 1.3 ? 0.4%) is obtained for the surface relaxation. The temperature dependence of the surface peak also indicates that the enhanced vibrational amplitude of the surface atoms is not nearly as large as had been derived previously from high-temperature LEED studies.     

Relaxation of the (1 1 1) surface of δ-Pu and effects on atomic adsorption: An ab initio study

The computational formalism of the full-potential all-electron linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method has been employed to study the relaxation of the δ-Pu(1 1 1) surface and the consequent effects for atomic adsorption of C, N, and O atoms on this surface. The underlying theoretical principle is the generalized gradient approximation to density functional theory (GGA-DFT) and the surface was modeled by a five-layer slab with a (2×2) surface unit cell. Upon relaxation of the slab, the interlayer separation between the surface and the subsurface layers expanded by 7.1% with respect to the bulk interlayer separation, while the separation between the subsurface and central layers expanded by 0.4%. To study adsorption on the surface, the adatoms were allowed to approach the surface at four high symmetry adsorption sites, namely, the top, bridge, hollow FCC, and hollow HCP sites, the adlayer structure corresponding to a coverage of 0.25 of a monolayer in all cases. The hollow FCC adsorption site was found to be the most stable site for C and N with chemisorption energies of 6.420 and 6.549 eV, respectively, while the hollow HCP adsorption site was found to be the most stable site for O with a chemisorption energy of 7.858 eV. The respective distances of the C, N, and O adatoms from the surface were found to be 1.22, 1.09, and 1.22 Å. The work function and net magnetic moments, respectively, increased and decreased in all cases upon chemisorption compared with the bare δ-Pu(1 1 1) surface. The electronic structure of the interactions between the adsorbates and the substrate is discussed in detail.     

Charge ordering in Eu3S4 determined by the valence-difference contrast of synchrotron X-ray diffraction

Synchrotron X-ray diffraction study for single crystals of Eu₃S₄ has revealed that a Th₃P₄-type structure transforms to a charge-ordered one at T_c=188.5 K. The crystal structures of Eu₃S₄ at T=300, 180 and 160 K were determined in the least-squares refinements with the Mo K intensity data. The valence-difference contrast method was applied at the L_II absorption edge of Eu, utilizing a large difference in anomalous scattering factors between Eu²⁺ and Eu³⁺. The cation distribution of Eu²⁺ and Eu³⁺ was determined by crystal-structure analyses based on the intensity data collected at two wavelengths of λ=1.6312 and 1.6298 Å.The least-squares structural refinements suggest that the most plausible atomic arrangement is [Eu³⁺]^4a[Eu²⁺Eu³⁺]^8dS₄. The charge-ordering scheme is that a half of Eu³⁺ ions occupy the whole 4a sites in the crystal structure, while the remaining half of Eu³⁺ ions mix with Eu²⁺ in the 8d sites. The scheme is also supported by the energy dependence of Bragg intensities for 400 and 004 reflections.     

Four-fold hollow site for S in a Ni(100) raft on NiO(100)

The reaction of H₂S with NiO(100) has been studied by polarization-dependent surface EXAFS. The results evidence reduction of the selvedge to form a Ni raft having S in four-fold sites with a S–Ni bond length of 2.21±0.02 Å. The Ni–Ni in-plane distance is 2.77±0.09 Å, representing a 6±4% contraction compared to that in NiO(100).     

The structure of ammonia molecules on MgO(100) surfaces from monolayer to bulk condensation

The structure of ND₃ molecules adsorbed on MgO (100) surfaces has been studied by neutron diffraction within the 10–80 K temperature range and at 0.7, 1 and 2.3 monolayer coverage. The neutron spectra suggest that the monolayer presents a short range order with a hcp packing of ammonia molecules, a coherence length of 25 ± 2 Å and a nearest neighbour distance of 3.61 ± 0.04 Å The molecular spacing remains the same between 10 and 80K that we interpret as small higher order commensurate islands. Above one monolayer coverage, bulk crystallites form on top of the first monolayer.