Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

Low energy electron spectroscopy of CO in the 10.600–14.400 eV energy loss range

Excitation spectra of CO have been obtained at low electron impact energy in the 10.600–13.400 eV energy loss range for scattering angles from 10 to 120°, with a 35 meV experimental resolution. The angular behaviour of the observed peaks is used to discriminate singlet-singlet and singlet-triplet transitions. Previously calculated Rydberg states are observed, in particular the triplet analogue of the F¹Σ⁺ state. A new high energy valence triplet state is identified; the first observed vibrational level is at 11.595 eV and the vibrational spacing is 90 meV. Upper levels are strongly affected by predissociation.     

The NO + CO reaction on Pt(100)

The interaction of NO with CO and with H₂ on Pt(100) was studied by temperature programmed desorption (TPD), isothermal desorption mass spectrometry, and low energy electron diffraction (LEED), TPD of NO and CO coadsorbed at 120 K yields almost complete reaction with both N₂ and CO₂ products desorbing as sharp, simultaneous peaks at ≈ 410 K. with full widths at half maximum as narrow as 3 K. Isothermal desorption mass spectrometry yields N₂ and CO₂ rates that exhibit a maximum with time. Both experiments indicate that the reaction mechanism is autocatalytic. Annealing NO-CO adlayers formed at 120 K to temperatures above 300 K causes the subsequent N₂ and CO₂ TPD peaks to broaden.'TPD of NO coadsorbed with H₂ yields sharp N₂ and H₂O product peaks that closely resemble the N₂ and CO₂ peaks observed in the NO + CO reaction. LEED experiments during TPD and isothermal desorption showed that the (1 × 1) → hex substrate phase transformation sometimes accompanies desorption of N₂ and CO₂. The TPD and isothermal desorption results can be fit by two simple models: chemical autocatalysis, in which an intermediate chemical species participates in a “chain propagation” reaction, and structural autocatalysis, which involves the formation of a reactive intermediate structure involving Pt atom displacements.     

Chemisorption of CO on the unreconstructed Pt(100) surface

The chemisorption of CO on the clean, unreconstructed Pt(100)-1 × 1 surface was investigated by LEED and XPS. Three LEED patterns, c(2 × 2), (√2 × 3√2) R45° and c(4 × 2), were observed with increasing CO exposure and structure models corresponding to these LEED patterns were proposed. The absolute coverage of CO was determined by combining the O(1s) XPS data with coverage information derived from LEED. The maximum CO coverage thus obtained was θ = 0.75 and the initial sticking coefficient was determined to be s₀ = 0.6. This coverage calibration can also be utilized for other oxygen containing molecules by comparing the corresponding O(1s)it peak intensities.     

Explosive production of CO2 from (NO + CO)/Pd(100)

Explosive CO₂ production followed by the gradual N₂ production has been observed by TDS as a result of the NO-CO reaction on the Pd(100) surface which does not reconstruct. The location and width of the CO₂ desorption peak have been found to be almost independent of the total coverage. For this autocatalytic reaction, the vacancy requirement model is considered to be valid. Change in the CO₂ production by varying the NO coverage (with the CO coverage fixed) has been also studied, from which information on the island formation is derived.     

Electron energy loss spectrum of cyanogen on Pt (100)

Electron energy loss spectra for adsorbed cyanogen on Pt(100) are presented, and discussed in terms of possible models suggested by other techniques. Adsorbate induced loss features are found at 11 and 14 eV, and these are associated with levels below the Fermi level as observed in ultra-violet photoemission. As in the case of CO adsorption losses in the adsorbed phase occur at energies significantly larger than in the gas phase, indicating an upward shift of the final 2π¹ state due to mixing with metal orbitais. Thermal desorption studies of C₂N₂ on Pt clearly resolve α and β phases, but there is some controversy over whether the β phase involves mainly single CN units bonding to the metal, whether C₂N₂ is molecularly adsorbed, or whether paracyanogenlike structures form at the surface. The electron spectroscopic evidence is examined, and is shown on balance to support adsorption in some molecular form.     

The electron energy loss spectrum of EuO(100)

Energy losses of 200 eV to 2 keV electrons reflected from a disordered EuO(100) crystal show a bulk plasmon loss consistent with just less than six “quasi free” electrons per EuO unit, and 5p → nd resonance losses above the 5p threshold. The ratio of intensity of the 4d¹⁰ 4fⁿ⁰ → 4d⁹ 4fⁿ⁺¹ “giant resonance” loss at 142 eV to the corresponding direct recombination feature varies with energy, while the direct recombination and related Auger channels show similar energy dependence.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

Low energy adsorbate vibrational modes observed with inelastic helium atom scattering: CO on Pt(111)

We report on new low energy loss and gain features which appear in the time-of-flight distributions of He atoms scattered from CO adsorbed on Pt(111). On the basis of calculated normal mode energies we assign the 6 meV features to the excitation of the CO vibrational mode corresponding to a hindered translation of the upright molecule parallel to the surface. The energy and intensity of the 6 meV mode as a function of coverage, sample temperature and scattering conditions are investigated. At high coverages, concurrent with the appearance of a well ordered (4 × 2) overlayer structure, the 6 meV mode is observed to broaden and shift in energy to about 7.4 meV. The dispersion curve measured at high coverages is flat suggesting that there is no direct coupling between hindered translations of adjacent CO molecules.     

The adsorption of CO on Pt(111) studied by infrared-reflection-adsorption spectroscopy

The adsorption of CO on Pt(111) between 85K and 300K has been studied by infrared-reflection-absorption spectroscopy together with TPD and LEED. The intensity of the absorption band due to the CO stretch of the linear species shows a maximum at the formation of the (√3 × √3)R30° LEED pattern followed by a minimum at the c(4×2) structure during the adsorption of CO at low temperatures (150K). The absorption band due to the C-O stretch of the bridging species appears only after the formation of the (√3 × √3)R30° pattern and reaches maximum intensity at the c(4×2) structure. Adsorption of CO to higher coverages (corresponding to the compression structures) broadens and shifts this absorption band. At higher temperatures (150K) a third peak is observed at 40cm⁻¹ below the peak due to the bridging species and is attributed to adsorption in the three-fold sites. At 300K both peaks in this region are very broad. The intensity data differs from that measured with EELS (ref.1) and favors a “faultline” structure of the type proposed by Avery (ref.2). Together with the additional information from bandwidths it is possible to distinguish between the various structural models. The results obtained here may also be important in explaining data from other systems such as CO/Cu.     

Identification of surface vibrations on clean and oxygen covered Pt(111) surfaces with high resolution electron energy loss spectroscopy (EELS)

Clean and oxygen covered {111} recrystallized Pt surfaces were studied by EELS after surface preparation at 150≤T≤165OK. The clean surface shows Stokes as well as anti-Stokes lines of surface phonons at ±195^?1. Adsorption of small amounts of (<10^?2 monolayers) of O₂ or H₂ leads to substrate-derived phonon losses at ±380cm^?1. Oxygen exposure at different pressures, times and temperatures leads to atomic and/or molecular adsorption as well as oxide-related features which have been identified by EELS.     

High-resolution electron energy-loss spectroscopy at epitaxially grown GaAs(100)

High-Resolution Electron Energy-Loss Spectroscopy (HREELS) is shown to be a very sensitive tool to investigate the space-charge regime of n-respectively p-type semiconductors. The most simple model we applied to fit experimental spectra is based on a step-like distribution of free carriers with the Drude dielectric response function. In this case, the dispersion of surface plasmon excitations is neglected, but it is considered in the Thomas-Fermi and the Debye-Hückel models. We use these models to fit HREELS-spectra, obtained from heavily Si-doped GaAs(100), which was grown by Molecular Beam Epitaxy (MBE). A comparison shown that the Drude model overestimates both the free-carrier concentration and the plasmon damping factor. The use of a more realistic smooth free-carrier profile, obtained by the self-consistent solution of the Schrödinger and Poisson equations, leads to plasmon excitations with lower frequencies. Besides Ohmic damping, the calculations show that Landau damping should be incorporated in order to obtain a better fit, particularly at intermediate frequencies.     

Angular distribution patterns of photoelectrons from orbitals of CO adsorbed on Ni(100), Pt(111) and Pt(110)

The synchrotron radiation from BESSY has been used to measure the photoemission from CO orbitals adsorbed as ordered overlayers on Ni(100) c(2 × 2), Pt(111) c(4 × 2) and Pt(110) (2 × 1)p2mg. Angular distribution patterns of photoelectrons from CO orbitals were recorded with a display-type analyzer. The data were compared with differential photoionization cross sections calculated for free and oriented molecules. The results demonstrate the upright orientation of CO on Ni(100) and Pt(111), while CO on Pt(110) shows a marked difference which can be explained by assuming that the CO molecules are tilted in the [001] directions of Pt(110), yielding a (2 × 1)p2mg superstructure observed in LEED. The tilt angle is estimated to about 20°. The structure model is supported by the shape resonances of the 4σ (5σ) orbitals of CO/Pt(110) as compared to CO/Pt(111).     

Fe-contacts on InAs(100) and InP(100) characterised by conversion electron Mössbauer spectroscopy

We have grown 4 nm thin films of ⁵⁷Fe on InAs(100) and InP(100) surfaces by use of MBE and studied the samples by ⁵⁷Fe conversion electron Mössbauer spectroscopy. In the case of InAs, the Mössbauer spectrum showed a sextet due to α-Fe and a further magnetically split component with slightly smaller hyperfine field, which is attributed to interface components. This result indicates that there is a relatively sharp interface between Fe and InAs. On the contrary, the spectrum of the InP sample showed a sextet with very broad lines and a smaller average hyperfine field. This suggests a strong chemical reaction between iron and the substrate, which results in the formation of a poorly crystalline phase.     

Low frequency surface resonance modes in electron energy loss spectroscopy

Electron energy loss spectroscopy has proved a powerful probe of vibrational modes of a wide variety of adsorbed species. Here the primary focus has been on modes with frequency well above the maximum phonon frequency of the substrate. Examples are internal vibration modes of adsorbed molecules, possibly shifted significantly in frequency from their gas phase analogues, and high frequency vibrations of an adsorbed molecule or atom against the substrate. Recent experiments explore features in the energy loss spectrum with frequency below the maximum phonon frequency of the substrate, for ordered overlayers of atoms adsorbed on low index metal surfaces. We shall summarize our theoretical studies of such spectra for several adsorbate/substrate combinations, with emphasis oh the physical origin of the features which appear in the calculations. We obtain a good account of the existing data, within the framework of a rather simple lattice dynamical model, and the calculations show that the features which appear are quite sensitive to the symmetry of the adsorption sits, and other details of the surface geometry. We shall illustrate this with several specific examples.     

On the nonlinear dynamics in the oxidation of CO on Pt(100) and Pt(110) surfaces: Forced oscillations

The effects of low-amplitude periodic modulation of oxygen pressure on the catalytic oxidation on Pt(100) and Pt(110) are investigated with the aid of a set of kinetic equations derived by Cox, Ertl and Imbihl. In the response of the system, subharmonic and superharmonic entrainment bands are found in addition to the fundamental entrainment band. Furthermore, quasiperiodic oscillations are obtained between the entrainment bands.     

Mechanism of an adsorbate-induced surface phase transformation: CO on Pt(100)

The mechanism of the (5 × 20)→(1 × 1) transition of Pt(100) during adsorption of CO has been investigated using a fast Video-LEED technique. Analysis of the coverage-dependence of the intensities of several LEED spots on different surfaces leads to a straightforward model. The phase transition occurs by a nucleation-trapping mechanism of the adsorbed CO. The driving force is the difference in stabilities of CO adsorbed on the reconstructed and unreconstructed Pt surfaces.