Experimentally observed orientation of C60F18 molecules on the nickel single crystal (100) surface

The angular dependence of near edge X-ray absorption fine structure (NEXAFS) spectra has been obtained in the vicinity of carbon and fluorine 1s absorption edges in a monolayer film of polar fullerene fluoride (C₆₀F₁₈) molecules on a Ni(100) substrate. The fine structure of the spectra has been identified according to experimental data via calculations based on the density functional theory, and the angular dependence of the spectra has been explained. The orientations of structural molecular fragments are determined from the angular dependence of the NEXAFS spectra. It is demonstrated that the electric dipole moment of a C₆₀F₁₈ molecule is oriented along the normal to the substrate surface with an accuracy of 5°.     

A leed-aes study of the structure of sulfur monolayers on the Mo(100) crystal face

The formation of ordered phases of sulfur on the molybdenum (100) crystal face has been studied by Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Thermal Desorption Spectroscopies (TDS). Sulfur was deposited from a S₂ molecular flux streaming out of an Ag₂S containing electrochemical cell inside the UHV chamber. The use of a controlled flux of S₂ allowed the careful determination of saturation values for the monolayer, as well as the formation of multilayers of sulfur. This allowed the calibration of Auger intensities in terms of sulfur coverage. Various ordered structures, c(2 × 2), (1 × 2), $\text{2}\text{1}\text{?}\text{1}\text{1}$ and c(2 × 4), were observed by LEED for different values of the S coverage. Real space models for these structures are proposed that satisfy the coverage values observed and place sulfur atoms only on high symmetry four-fold sites on the (100) molybdenum surface.     

Surface states on the (100) plane of a sphalerite-type crystal

The surface states of an ideal (100) plane of a sphalerite-type crystal were studied in the one-electron approximation; both the plane formed by electronegative atoms (>0) and that consisting of electropositive atoms (<0) were investigated. The interpretation of surface states whose energy lies in the gap is the same as for the diamond-like lattice. The influence of electro-negativity or electropositivity of the surface on the position of these states in the gap was investigated and possible consequences of these states joining one of the energy bands of volume states were pointed out. The results of this paper can be used to discuss correlation effects on the (100) plane of a diamond-like crystal.     

Vibrational states of the Cu(100) surfaces with nickel adlayers

The vibrational spectra of the Cu(100) surface covered with one or two Ni monolayers are calculated using the embedded-atom method. The surface relaxation, the dispersion of surface phonons, and the polarization of vibrational modes of the adsorbate and the substrate are discussed in detail. The theoretical results are in good agreement with experimental data and can be used for their interpretation. The changes observed in the interatomic interactions upon application of nickel adsorbates to the copper substrate are considered.     

Theoretical vibrational frequencies for NHX and CHX reactive intermediates on nickel(100) and nickel(111) surfaces

Theoretical studies have been performed on CH_X and NH_X species chemisorbed on threefold and fourfold sites of nickel clusters. From this work we have obtained geometries and vibrational frequencies that can be compared with experimental data concerning these species chemisorbed on Ni(100) and Ni(111) surfaces. These results can be used to help assign the observed losses in EELS spectra for CH on the Ni(111) surface.     

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the $\text{(}\text{14}\text{1}\text{1}\text{5}\text{)}$ reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).     

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

The initial stages of the interaction of oxygen gas with a clean Ni (100) surface have been studied by a combination of LEED, AES, work function change and ion bombardment sectioning techniques. The reaction could be divided into three reaction regions: a fast dissociative chemisorption leading to surface structures based on the initial nickel interatomic spacing and resulting in an oxygen coverage of approximately 0.4 monolayers; a rapid oxidation leading to epitaxial NiO, two layers thick ; and a final slow thickening of bulk NiO. The first two regions were dependent only upon oxygen exposure. The third region was observed only at high gas-phase oxygen pressures or very low surface temperatures. Kinetics analyses are developed to explain the rate of oxygen chemisorption and the rate of oxide nucleation and growth.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Surface states of a semi-infinite diamond crystal limited by the (100) plane

Two kinds of surface states with energies lying in the gap between the valence and the conductive zone, were found for a semi-infinite diamond crystal, limited by the (100) plane, using the MO LCAO method, in which the molecular orbits are expressed as linear combinations ofsp ³ hybrid orbits of carbon atoms. One of these zones of surface states is completely occupied, the other completely unoccupied. The nature of these surface states is discussed and it is shown that the (100) surface of the diamond behaves similarily as if each surface atom had a free electron pair.     

A thermal desorption study of thiophene adsorbed on the clean and sulfided Mo(100) crystal face

The adsorption of thiophene (C₄H₄S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H₂ and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H₂ desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θ_s = 0.5, at which point thiophene adsorption is entirely reversible.     

Energetics and vibrational states for hydrogen on Pt(111)

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first-principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68 meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.     

Surface states on the copper (111) face

Using Chodorow's potential in the bulk region and a z-dependent potential in the vacuum region, the surface states on Cu(1 11) were investigated. Fitting of a potential parameter to the well known energy of the surface state at $\text{Γ}$ resulted in a reasonable shape of the potential and in an effective mass of the surface state band in accordance with experiment. The state recently observed at $\text{M}$ about 2 eV below E_F could not be described with the present model.     

Magnetic structure of a RbDy(WO4)2 single crystal

The results of measuring the temperature and field dependences of the magnetization of a RbDy(WO₄)₂ single crystal in the temperature range from 4.2 to 50 K and in magnetic fields up to 1.6 T are presented. The energies of the exchange and dipole-dipole interactions are estimated. The magnetic structure of the ground state is determined. Fiz. Tverd. Tela (St. Petersburg) 41, 672–676 (April 1999)     

Spectroscopy of phonon and vibronic states of YbAl3(BO3)4 single crystal

The polarized Raman and reflection spectra of a single crystal YbAl₃(BO₃)₄ at room temperature were studied. Raman active vibrational modes A ₁, E _TO, and E _LO are identified. In the Raman spectrum, we detected an intense line at a frequency of 1018 cm⁻¹, which refers to internal vibrations of the BO₃ group and is known to be promising for use in amplifiers based on stimulated Raman scattering. From the simulation of reflection spectra by the method of dispersion analysis the frequencies of A ₂ vibrational modes were determined. Intense bands observed in the low-temperature transmission spectra in the range of f-f transitions in the Yb³⁺ ion are attributed to electron-phonon transitions. The Raman lines are compared with electron-phonon lines in the transmission spectrum.