Theoretical and experimental studies of the structure and dynamics of the CaF2(1 1 1) surface

The structure and dynamics of the CaF₂(1 1 1) surface were investigated by means of low-energy electron diffraction (LEED) and molecular dynamics (MD) simulations at 300 K. LEED beam intensities were recorded as a function of electron energy and were analyzed with the tensor LEED approach. Positions as well as mean square amplitudes of the ions in the first layers were fitted to the experimental I(E) curves. According to both LEED and MD, the CaF₂(1 1 1) surface structure is similar to the bulk-terminated structure with only small relaxation of the outermost ions. Moreover, both methods show an enhancement of vibrational amplitudes in the outermost F-Ca-F triple layer.     

Structure and thermal decomposition of para- and ortho-xylene on Pt(111): a HREELS, LEED and TPD study

High resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy and low energy electron diffraction (LEED) were used to investigate the low temperature adsorption structure and subsequent thermal decomposition of p- and o-xylene on Pt(111). Both xylenes adsorb intact on Pt(111) at 245 K with the aromatic ring oriented parallel to the surface. At about 350 K, both p- and o-xylene begin thermal decomposition by partial dehydrogenation of the methyl groups. Further decomposition pathways are markedly different for the two xylenes. For p-xylene, the remaining aliphatic and aromatic CH bonds break at about 550 K, leaving CH and C₂H species on the surface. In contrast, the aliphatic groups of o-xylene completely dehydrogenate at about 440 K, leaving a fragment with the aromatic ring oriented nearly perpendicular to the surface.     

Low-energy electron diffraction study of the phase transition of Si(001) surface below 40 K

The phase transition of Si(001) surface below 40 K was studied by low-energy electron diffraction (LEED). The temperature dependence of the intensities and widths of the quarter order diffraction spots and LEED intensity versus electron energy curves (I-V curves) were obtained in the temperature region from 20 to 300 K. While the spot intensities of the quarter order spots decrease and the widths broaden, the I-V curves do not change so much below 40 K. This clearly shows that a phase transition occurs from an ordered phase above 40 K to a disordered phase below 40 K.     

O在预覆盖K的Ag(110)表面的共吸附及其性质

本文用X射线光电子能谱(XPS),紫外光电子能谱(UPS),电子能量损失谱(EELS)和低能电子衍射(LEED)研究了O与预覆盖K的Ag(110)表面相互作用及其性质。在低覆盖度K下,发现有两种O的吸附态,经鉴别为溶解到表面下的O^2-和表面上吸附的O^x-增加K的覆盖度,出现分子状态的吸附物O₂^δ-,它与表面下存在的K相联系。XPS和UPS均清楚地显示出对应于三种不同吸附态的光电子发射峰。Ag(110)表面预覆盖K后的粘滞系数大大增加。K和O的共吸附引起它们彼此向Ag(110)表面下的溶解。LEED实验结果表明,清洁Ag(110)表面覆盖单层K原子后衍射图形从(1×1)变到(1×2),再吸附O后表面吸附层结构变为(2×1)。另外,结合UPS和EELS测量初步考察了O/K/Ag(110)共吸附系统的电子结构。本文还提出了一个共吸附模型来解释这些现象。 关键词：     

Energy-dependent cancellation of diffraction spots due to surface roughening

The low-energy electron diffraction (LEED) pattern of the step-kinked Pt{531} surface at 200 K shows energy-dependent cancellation of diffraction spots over unusually large energy ranges, up to 100 eV. This cannot be reproduced theoretically when a flat surface geometry is assumed. A relatively simple model of roughening, however, involving 0.25 ML of vacancies and adatoms leads to very good agreement with the experiment. The cancellation of intensities within a very narrow range of adatom or vacancy coverages is caused by the interference of electrons emerging from different heights but similar local environments. This is a rare example where the energy dependence of integrated LEED spot intensities is dramatically affected by the long-range arrangement of atoms.     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

Charge trapping in NiO surfaces during LEED observations

Time dependent LEED intensities have been observed for a NiO(100)-Cl surface. Measurements of this effect as a function of temperature, beam current and beam energy are reported. Associated observations of the current to ground and the electron loss spectrum are used to suggest that electron trapping is occurring in the crystal surface due to the adsorbed Cl. Excitation from these trapped states could result in decaying LEED beam intensities.     

Formation of patchy surface overlayers alkali adsorption and alkali carbon monoxide and oxygen coadsorption on Ru (0001) and Ru(10\overline 1 0)

The retarding-potential and LEED methods are used for obtaining information about the structure and uniformity of alkali (K and CS) and alkali-coadsorbate (CO and O) layers on Ru(0001) and Ru surfaces at 300 K. It was established that for alkali layers on the anisotropic Ru surface and for mixed coadsorbate layer the shape of the retarding potential curves, used for the work function measurements, becomes anomalous which indicates coexistence of patches of different work function on the surface. LEED data on these patchy surfaces showed the formation of a variety of ordered structures depending on the coadsorbate coverages. On the basis of the existing theory a simple mathematical simulation was performed in an attempt to interpret the observed changes in the retarding potential curves. The possible changes of this curves, induced by the formation of patches with varying contribution to the total retarding potential signal and the advantage of the retarding field method for determination the uniformity of surface overlayers are discussed.     

Perspectives for surface structure analysis with low energy electron diffraction

The two main methods for surface structure determination, X-ray diffraction and low energy electron diffraction, are briefly compared and two areas are discussed where the application and further development of LEED seems promising. One field is the measurement of thermal vibration with LEED and the second is the analysis of substrate induced distortions in adsorbed organic molecules. As a test case for the analysis of thermal vibrations the results of a temperature dependent LEED I(V) analysis of Cu(1 1 0) is presented showing that LEED is sensitive enough to measure anisotropic vibration amplitudes. As example for organic molecules the results of a LEED I(V) analysis of thiouracil on Ag(1 1 1) are presented and compared to the results of a previous X-ray study. The differences in the structural details can be related to the different diffraction geometries of surface X-ray diffraction and LEED.     

Adsorption of CO on a pseudomorphic palladium monolayer on Mo(110)

Adsorption of CO on a Pd monolayer (ML) supported on Mo(110) has been studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). Three ordered CO substructures denoted as are observed with LEED. The binding energy of C0 on the 1.0 ML Pd/Mo(110) surface is reduced by 12 kcal/mol relative to the Pd(111) surface, consistent with previous results for supported palladium monolayers on other substrates. Two vibrational states of C0 are observed near 1950 and 2050 cm⁻¹, with the feature at the lower wavenumber having the smaller binding energy.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

Order-disorder transition for corrugated Au layers

Atomic-scale structure of the growth of a gold film on (1 1 2) plane of Mo single crystal was investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) up to two monolayers (ML) of gold coverage. Both LEED and STM results establish that Au grows on Mo(1 1 2) in a layer-by-layer mode, for at least the first two monolayers. A number of ordered structures are formed and both the first and second layers adopt the Mo(1 1 2) 1 × 1 surface structure upon completion. For some gold layers on Mo(1 1 2), notably the 1.66 monolayer 3 × 1 and 1.75 monolayer 4 × 1 gold overlayers, we find evidence of a phase transition associated with increasing disorder in gold layers with structural corrugation and anisotropic band structure. The signature of this phase transition, at temperatures in the range of 400-500 K, is a sharp decrease in the overlayer effective Debye temperature.     

Evidence for subsurface atomic displacements of the GaAs(110) surface from LEED/CMTA analysis

The low-energy electron diffraction constant momentum transfer average (LEED/CMTA) technique is applied to the GaAs(110) surface. In addition to verifying our earlier model of the top layer atomic displacements, atomic displacements in the first subsurface layer, opposite in direction to those in the top layer, are suggested. Best estimates for atomic positions in the surface and first subsurface layers are given.     

Relaxation and thermal vibrations at the NaF(100) surface

The relaxation and the thermal vibrations of the NaF(100) surface are investigated in the temperature range between 25 K and 230 K by means of low-energy electron diffraction (LEED) and a subsequent I(V) structure analysis based on the tensor LEED approach (TLEED). According to the experiments, the NaF(100) surface is not significantly relaxed and has the ideal truncated bulk structure. The thermal vibrational amplitudes of the ions in the topmost layer are significantly enhanced compared to the bulk by a factor of 1.35 ± 0.15 and are equal within the error-bars for Na⁺ and F^? ions. Moreover, the relaxation and the dynamics of the NaF(100) surface are investigated using periodic density functional theory (DFT) calculations using pseudopotentials. In agreement with the experimental findings, the calculated relaxation of the NaF(100) surface is weak with static shifts of the ions of 0.01 Å to 0.02 Å. In the topmost layer, the Na⁺ ions are predicted to be slightly inward shifted, whereas the F^? ions are outward shifted, in accordance to predictions of previous shell-model calculations. A Born Oppenheimer molecular dynamics (BO-MD) simulation of the dynamics at the NaF(100) surface leads to a smaller enhancement of thermal motions of the ions at the surface compared to the experiment.     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

Structure and composition of chemically prepared and vacuum annealed InSb(0 0 1) surfaces

The InSb(0 0 1) surfaces chemically treated in HCl-isopropanol solution and annealed in vacuum were studied by means of X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and electron energy-loss spectroscopy (EELS). The HCl-isopropanol treatment removes indium and antimony oxides and leaves on the surface about 3 ML of physisorbed overlayer, containing indium chlorides and small amounts of antimony, which can be thermally desorbed at 230 °C. The residual carbon contaminations were around 0.2-0.4 ML and consisted of the hydrocarbon molecules. These hydrocarbon contaminations were removed from the surface together with the indium chlorides and antimony overlayer. With increased annealing temperature, a sequence of reconstructions were identified by LEED: (1 × 1), (1 × 3), (4 × 3), and (4 × 1)/c(8 × 2), in the order of decreasing Sb/In ratio. The structural properties of chemically prepared InSb(0 0 1) surface were found to be similar to those obtained by decapping of Sb-capped epitaxial layers.     

Inversion of atom diffraction intensities for surface structural determination

Atom diffraction from surfaces has proved to be a very useful method for structural studies on surfaces, particularly for incommensurate layers and hydrogen adlayers which are not easily analyzed with LEED. We have developed a rapid method to obtain the surface corrugation function directly from the measured intensities within the hard wall model. This avoids a tedious search in parameter space to determine surface structures and is valid for surfaces whose corrugation amplitudes are below the Rayleigh limit. We will discuss the method and its sensitivity to experimental errors in determining the diffraction intensities as well as its applicability when only a fraction of the allowed diffraction beams are experimentally accessible.     

On Auger electron spectroscopic measurements of ZnCl2 —coal interaction

By combining electron stimulated desorption (ESD) with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function change (Δφ) measurements the information content of ESD with regard to surface structure and composition is examined, using the surface systems O/W(100) and O/W(110). Although it is not possible to separate the local interaction from the ion escape phase, the comparison of the ESD results with Information derived from LEED, AES and Δφ and the use of simple models of the local interaction gives a rather detailed picture of the location and environment of adsorbed atoms which provides a reasonably reliable basis for the interpretation of UPS spectra of adsorption layers.ESD is extremely sensitive to adsorbed layers. The fact that the ion signal depends not only on coverage but also on the structure and structure-dependent properties of the adsorbate makes on the one hand coverage determination difficult if not impossible, on the other hand opens the door to structure analysis. The potential for obtaining structure information can be easily assessed by comparison with electron probe results.In comparison with other ion probes such as ion scattering spectroscopy and secondary ion mass spectroscopy, ESD is at present the most promising ion probe method for obtaining information on the location of adsorbed atoms from angular and energy distribution measurement (ESDIAD and ESDIED). This is clearly seen by the comparison with the structural data derived from LEED, AES and Δφ measurements for the complex system O/W(100). The consistency of the data obtained with ESD and electron probe techniques lends strong support to the simple models on which the analysis of the ESD results from chemisorbed layers are based. The comparison of ESD results from the system O/W(100) at high coverage and from O/W(110) with 0⁺ ion emission from oxides shows, however, that caution is in place when assigning ESD features to atoms chemisorbed on the metal surface. Without a careful analysis of the ion energy, threshold and/or cross-section such ions cannot be distinguished from ions produced by dissociation of oxides which may be present on the surface only in small quantity. These ions usually are not related to the chemisorbed species which covers most of the surface and therefore dominates the signals seen with (nearly) all other surface probes.If the consistency of LEED, AES, Δφ and ESD data for O/W(100) is not fortuitous, then ESD has already given some important feed-back to the electron probe techniques: the structural models derived from vibrational ELS spectra have to be revised. Increasing accumulation of experimental data and deepening of the theoretical understanding of the physical processes involved in ion emission will have to show how much further information complementary to that from electron probes can be obtained from ion probes.     

The structure of the Pt(100) surface

The atomic arrangement in the first two layers of the clean Pt(100) surface is described from a Fourier analysis of low energy electron diffraction (LEED) intensities. The Fourier transform and sensible platinum-platinum bond lengths require that the top layer is corrugated, the corrugation “step” being approximately 0.4 Å. The structure of the first layer is partly described by a hcp rearrangement from the bulk fcc structure. The rearrangement is related to geometrical and electronic properties of the metals. Some qualitative and quantitative comments are made on the general value of Fourier methods in surface crystallography.