Co adsorption on potassium promoted Fe(110)

CO adsorption on potassium covered Fe(110) has been studied using UPS, XPS, AES and flash desorption. It was found that CO adsorbs molecularly at room temperature with a larger binding energy than on clean Fe(110). The CO saturation coverage increases and the sticking coefficient decreases with increasing potassium coverage. On heating, the probability of adsorbed CO dissociating increases with the amount of potassium present. The UPS spectra show that the CO 4σ peak is shifted by 0.8 eV to higher binding energies on Fe(110) + K and that at 21.2 eV the peak due to the 1π + 5σ orbitals is split into a double peak. The catalytic relevance of the measurements is discussed with reference to the Fischer-Tropsch synthesis.     

H2O分子在Fe(100), Fe(110), Fe(111)表面吸附的第一性原理研究

采用第一性原理研究了H2O分子在Fe(100),Fe(110),Fe(111)三个高对称晶面上的表面吸附.结果表明,H2O分子在三个晶面上的最稳定结构皆为平行于基底表面的顶位吸附结构.H2O分子与三个晶面相互作用的吸附能及几何结构计算结果表明H2O分子与三个晶面的相互作用程度不同,H2O分子与Fe(111)晶面的相互作用最强,其次是Fe(100),相互作用最弱的是Fe(110)表面,而这与晶面原子的排列密度相关.吸附体系的电子结构计算结果也得出了相似的结论.同时电荷布居分析表明,H2O分子与Fe表面相互作用时,O原子与基底原子之间的电荷交换使基底Fe原子表面带负电,导致表面电位降低,也促使Fe表面更易于发生电化学腐蚀反应.     

The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

Oxygen adsorption on (110) silver

The adsorption of oxygen on a (110)Ag surface is investigated by means of Auger electron spectroscopy, LEED and low energy helium ion scattering (IS). With LEED two ordered structures, i.e. (3×1) and (2×1) were observed at oxygen exposures of 1700 L and 7000 L respectively. The oxygen signal observed by AES and IS increases monotonically with oxygen exposure. The signals can be related to absolute coverage by comparison with Δφ measurements and by the use of the LEED data. With this calibration and with theoretical scattering cross-sections the IS measurements allow the position of the adsorbed oxygen to be estimated. The observation of a strong azimuthal anisotropy of the IS signal, e.g. a large oxygen signal if the plane of scattering is parallel to the [110] direction and a relatively small oxygen signal in the [100] direction, leads to the conclusion that the oxygen is adsorbed in a bridge position between two Ag atoms of the [110] surface channels, its centre being slightly below the centres of the Ag atoms.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

Periodic lattice distortions in epitaxial films of Fe(110) on W(110)

The epitaxial growth of Fe(110) on W(110) at 500 K is analyzed using LEED and AES. Frank-van der Merwe growth is established by AES. According to LEED, pseudomorphism occurs up to θ = 1.64, where every W atom of two topmost W layers is just covered by exactly one Fe atom. For 2?θ?9, characteristic reflection-multiplets are observed, symmetric about the basic Fe(110) reflections, which are interpreted in terms of periodic lattice distortions. The latter are caused by interaction with the misfitting W substrate.     

Angle-resolved photoemission from CO on Fe(110)

We report normal emission, angle-resolved photoemission measurements of the system p (1×2)Co-Fe(110) at several photon energies. The lower bind-energy feature, commonly attributed to emission from both the 1π and 5σ molecular orbitals of CO, shifts from 6.9 to 8.1 eV (relative to the Fermi energy) as the photon energy is increased from 21 to 30 eV. We attribute this dispersion to a large variation in cross section for photoionization of the 1π- and 5σ-derived orbitals and use symmetry selection rules to Identify the 6.9 eV component as the 1π and the 8.1 eV component as the 5σ. This ordering is reversed from the gas phase ordering.     

The adsorption of ammonia on an Fe(110) single-crystal surface studied by high-resolution electron energy-loss spectroscopy (eels)

The EELS spectra of ammonia adsorbed on an Fe(110) surface at 110 K reveal four different adsorption states of molecular ammonia with increasing coverage : chemisorption at “on-top” sites, chemisorption at multi-coordinated sites, adsorption in a second layer, and multilayer condensation.Thermal processing of an ammonia-covered sample to 155 K causes desorption of both the condensed phase and the second layer without any fragmentation of ammonia.Further heating of the sample leads to a much weaker desorption of molecular ammonia up to a temperature of 260 K. EELS spectra recorded after heating to 290 K show only small amounts of atomic nitrogen and hydrogen present on the surface, indicating partial decomposition of ammonia.The formation of species such as NH₂ (ads) or NH (ads) during the thermal-processing experiments could not be observed.     

Hydrogen adsorption on Pd/TiFe (110) surface

Adsorption of hydrogen on the TiFe (110) surface covered by palladium monolayer was investigated using the full potential linearized augmented plane wave method within the local density approximation. Influence of palladium coating to adsorption properties of the TiFe (110) surface as well as difference it from Pd/TiFe (100) are discussed.     

The adsorption entropy of H on W(110)

Recently, Nahm and Gomer have measured the increase in entropy associated with the adsorption of hydrogen on the W(110) surface. We discuss the implications of this data, and address it within the framework of a model introduced recently [Phys. Rev. Lett. 68 (1992) 2846; Surf. Sci. 287/288 (19930 837] to describe hydrogen dynamics on this surface.     

Monte Carlo simulations of hydrogen adsorption on the W(110) and Mo(110) surfaces

Kinetics of low-temperature hydrogen and deuterium adsorption on W(110) and Mo(110) surfaces have been studied by the real-time Monte Carlo simulations. Recently reported qualitative dependence of the adsorption characteristics on variation of the H₂ flux is described in terms of the dynamical equilibrium between incident and desorption fluxes and improved conditions for accommodation for the hydrogen molecules at high incident fluxes. The role of the intrinsic precursor state in hydrogen dissociative adsorption is analyzed.Received: 16 February 2004, Published online: 28 May 2004PACS: 82.65. + r Surface and interface chemistry; heterogeneous catalysis at surfaces - 02.50.Ng Distribution theory and Monte Carlo studies - 82.20.Wt Computational modeling; simulation     

Phonons at the Fe(110) surface

The in-plane density of phonon states of clean Fe(110) surface was measured separately for the first, second, and further atomic monolayers using nuclear inelastic scattering of synchrotron radiation. The results show that atoms of the first layer vibrate with frequencies significantly lower and amplitudes much larger than those in the bulk, and that vibrational spectra along two perpendicular in-surface directions are different. The vibrations of the second layer are already very close to those of the bulk. The good agreement of the experimental results and the first-principles calculations allows for detailed understanding of the observed phenomena.     

Vibrational spectra of ammonia adsorbed on Fe(110)

EELS spectra of ammonia adsorbed on a Fe(110) surface at 120 K reveal three different adsorption states of molecular ammonia. Thermal processing of the ammonia covered Fe(110) surface to 315 K indicates fragmentation of the NH₃ molecules into atomic hydrogen and nitrogen. Formation of an NH₂ intermediate is not observed whereas the existence of NH_ad species cannot be excluded at present.     

Magnetism of ultrathin Fe(110) films

Application of conversion electron Mössbauer spectroscopy (CEMS) to structural and magnetic analysis of ultrathin films and their interfaces is reviewed. Fe(110) films were prepared on W(110) under UHV conditions and analyzed in situ. CEMS provides detailed information on the mode of growth and film structure and on magnetic hyperfine fields, B _hf. Local structure of B _hf across the film is discussed in relation to modifications of magnetic order caused by the finite (including monolayer) film thickness and by the electronic structure of the interface.     

Energetics of Carbon deposition on Fe(100) and Fe(110) surfaces and subsurfaces

Spin-polarized density functional theory calculations have been performed to investigate carbon deposition and carbide formation on Fe(100) and Fe(110) at different carbon coverage. On Fe(100) with increasing carbon coverage, the most stable carbon adsorption configuration changes from surface carbon adsorption to surface carbon diffusion into subsurface, and surface carbon clustering is not favored thermodynamically. However, surface carbon clustering is more favored kinetically than carbon diffusion; and carbon diffusion into subsurface and surface carbon clustering become competitive. On Fe(110) with increasing surface carbon coverage, the most stable adsorption configuration changes from surface carbon adsorption to surface carbon diffusion into subsurface, and this process is favored both kinetically and thermodynamically, and surface carbon clustering is neither favored nor competitive. Surface carbon deposition might form on Fe(100), while carbide formation might be found on Fe(110).     

Phase transitions on centred rectangular lattice gases: A model for the adsorption of H on Fe(110)

A lattice gas model on a centred rectangular lattice with short range pair and triple interactions is investigated. Ground state calculations give a large number of ordered structures. Phase diagrams, correlation functions and critical exponents of several transitions to (2 × 1), (3 × 1) and (4 × 1) structures are obtained from transfer matrix scaling and Monte Carlo methods. Isotropic scaling is verified. Many experimental observations for the adsorption of H on Fe(110) are reproduced by this lattice gas model.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

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.