Formation of doubly excited He atoms during scattering of He ions from a clean and oxygen covered Ni(1 1 0) surface

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with energies between 60 eV and 1 keV with a Ni(1 1 0) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage of the target surface with oxygen. For a controlled O₂ adsorption on the Ni(1 1 0) surface we can explain the resulting changes in the electron spectra by the modification of the work function of the target surface. Thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures provides an alternative interpretation of recent work where the local electron spin polarization of a Ni(1 1 0) surface was deduced from changes in the electron spectra as function of target temperature.     

Electron spectroscopy of surfaces by impact of metastable He atoms: CO on Pd(110)

Deexcitation of metastable He1 2¹S (excitation energy E1 = 20.6 eV) or 2³S (E* =19.8 eV) atoms at a clean Pd(110) surface proceeds through a two-stage process (resonance ionization + Auger neutralization, RI + AN). The measured electron energy distribution reflects the self-convolution of the local density of states of the outmost atomic layer. A CO adlayer suppresses the RI step and the spectra are caused by Auger deexcitation (Penning ionization). Comparison with corresponding UPS data allows identification of the valence orbitals of the adsorbate. Emission up to the Fermi level is ascribed to contributions from the 5σ level. The effectively available excitation energy in front of the adlayer is lowered by 0.5 eV. Extensive data on the variation of the intensities from the adsorbate valence levels with angle of incidence as well as of emission are presented and are analyzed in terms of an empirical model.     

Radiative decay of doubly excited states in helium below the He+ (N = 2) ionization threshold

The radiative decay of (1)P(o) doubly excited states in helium has been investigated using a novel apparatus in which metastable atoms and vacuum ultraviolet photons are detected. The intensity ratio of the energetically narrow (sp,2n-)(1)P(o) and (2p,nd)(1)P(o) series to the broader (sp,2n+)(1)P(o) series is strikingly enhanced in comparison with conventional photoabsorption, photoion, or photoelectron measurements using synchrotron radiation. The experimental approach is a new way forward for the study of energetically narrow doubly excited states.     

A UPS and LEED/Auger study of adsorbates on Fe(110)

Ultraviolet photoemission spectroscopy (UPS) and LEED/Auger were used to study adsorbed species of C, N, O, S, CO, NO, and C₂H₂ on Fe(110). The complicated “carbon ring” LEED patterns were shown to be due to atomic carbon and/or nitrogen. Molecular nitrogen does not stick at or above room temperature on Fe(110). The optical excitation probability of the 3p electrons of segregated sulphur is found to have a Cooper minimum aroundhω=40.8 eV. Carbon monoxide chemisorbs molecularly at room temperature and then dissociates slowly. Only dissociative CO adsorption was observed atT=385 K. Acetylene also adsorbs molecularly but does not dissociate at room temperature. By contrast, nitric oxide chemisorption is completely dissociative at room temperature.     

Study of the sputtering of adsorbates by low energy ions (O on Ni)

Sputtering or ion impact desorption of adsorbed layers has been studied by low energy ion scattering and model calculations. For adsorption of a monolayer or less (e.g. oxygen on nickel) the desorption by ions can be readily observed by He⁺ ion scattering from the development of the adsorbate and substrate signal as a function of time. Desorption cross sections for Ne⁺ and He⁺ ions in the energy range from 500 eV to 1600 eV are given. These results are compared with the model calculations and the influence of various parameters can be studied by this comparison. The contribution of different processes to the sputtering mechanism is discussed.     

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).     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

The substitution of Fe2+ ions by Ni2+ ions in green rust one compounds

The oxidation of Fe(OH)₂ in the presence of Cl^– or CO ₃ ^2– ions leads, in the first stage of the reaction, to chloride-containing green rust one (GR1), 3Fe(OH)₂· Fe(OH)₂Cl·nH₂O, or carbonate-containing GR1, 4Fe(OH)₂·Fe₂(OH)₄CO₃·nH₂O, respectively. These GR1 compounds give the ferric oxyhydroxides by further oxidation. If a hydroxide Ni _x Fe_1–x (OH)₂ is initially precipitated, the reaction leads to a nickelous-ferric compound isomorphous to the ferrous-ferric GR1, but stable with respect to the oxidizing action of air. Similarly, the oxidation of a nickelous-ferrous hydroxide, in the presence of excess OH^– ions, leads to a nickelous-ferric GR1, a layered hydroxide with anionic interlayers made of OH^– ions and water molecules. The Mössbauer spectra of these nickelousferric GR1 display two ferric doublets, D₀ withIS=0.34 mm/s andQS=0.45 mm/s and D₁ withIS=0.36 mm/s andQS=0.86 mm/s. The existence of a ferrous-ferric GR1 incorporating OH^– ions, a compound never observed so far, is strongly suspected.     

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.     

Hydrogen desorption from Ni sites on (110) CuNi surfaces

Hydrogen adsorption on Ni-rich (110) CuNi alloy surfaces has been studied by means of thermal desorption spectroscopy. After adsorption near room temperature the hydrogen desorption spectra exhibit a coverage dependence similar to that known from pure (110)Ni. Besides a slightly composition dependent desorption energy the alloy surfaces behave like a (110)Ni surface diluted by practically inert Cu. These results are compared to those reported by Yu Ling and Spicer.     

Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

Ionization of metastable He(1s2s) atoms by alkalated surfaces

We calculate the ionization probability for singlet and triplet metastable He(1s2s) atoms scattered from potassium covered surfaces, on the basis of the resonant tunnelling process. We do not assume the trajectory approximation, but we do assume classical motion on the potential energy curves, with crossing between neutral and ionic curves at a given point calculated from electronic tunnelling probabilities. Energy spectra of the He⁺ ions are also calculated. The results are found to be somewhat sensitive to tunnelling probability as a function of distance from the surface. Results are compared with experimental ionization probabilities measured by Roussel. The agreement is found to be fair if the tunneling rate is parameterized according to the theory of Janev et al. A simple interpretation is given.     

Capture of polarized electrons from Fe(110) into excited terms of fast atoms scattered at grazing incidence

We observe the emission of circularly polarized light after grazing scattering of fast ions at a magnetized Fe(110) surface. Changing the state of magnetization of the target affects the polarization of the emitted light due to capture of spin polarized electrons. Some aspects of atom-surface interaction and investigations of surface magnetism are discussed.     

On the diffraction of He by a Ni(110) surface

The diffraction of low-energy He atoms by a Ni(110) surface is studied using hard and soft wall models for the elastic scattering. The effects of thermal vibrations and inelastic processes on the He intensities are estimated. It is shown that the experimental spectra imply a corrugation height along the [001] direction that is maximally 0.12 au. A value of 0.30 au, calculated recently within a scheme that assumes the repulsive interaction to be proportional to the electron density of unperturbed Ni(110), leads to “double rainbow” diffraction patterns qualitatively different from those observed.     

Inelastic scattering of He atoms from a Cu(110) surface

A He beam has been used to measure inelastic scattering from a Cu(110) surface. The scattering was a result of predominantly single phonon events and both energy gain and loss processes were observed. This was in contrast to Cu(001) where only energy loss was observed. For Cu(110), aligned in the [001] azimuth, partial dispersion relations have been measured which meant that phonons with ΔQ values up to the zone boundary value have not been detected under the present experimental conditions. The elastic incoherent component varied with azimuthal angle for Cu(110).