The dynamics of desorption induced by atomic hydrogen: HD/Cu(111)

Recent observations of a direct reaction between adsorbates and hydrogen atoms incident from the gas phase are interpreted in terms of an Eley-Rideal reaction. A detailed comparison of the experimental data for the HD/Cu(111) system with quantum mechanical model calculations corroborates such an interpretation. The peculiar isotope effect observed can be understood from the different dynamical implications of appropriately rescaled potential energy surfaces. The width of the measured time-of-flight spectrum is explained from the overlapping contributions of the populated vibrational levels. The angular distributions are rationalized by contributions both from ‘indirect’ events, where the incident atoms make several bounces in the surface well prior to reaction, and ‘direct’ reactive events.     

The dynamics of desorption induced by atomic hydrogen: HD/Cu(111)

Recent observations of a direct reaction between adsorbates and hydrogen atoms incident from the gas phase are interpreted in terms of an Eley-Rideal reaction. A detailed comparison of the experimental data for the HD/Cu(111) system with quantum mechanical model calculations corroborates such an interpretation. The peculiar isotope effect observed can be understood from the different dynamical implications of appropriately rescaled potential energy surfaces. The width of the measured time-of-flight spectrum is explained from the overlapping contributions of the populated vibrational levels. The angular distributions are rationalized by contributions both from ‘indirect’ events, where the incident atoms make several bounces in the surface well prior to reaction, and ‘direct’ reactive events.     

Adsorption and desorption of hydrogen on bare and Xe-covered Cu(111)

Using polarization-modulated ellipsometry to monitor adsorbate coverage in-situ, we studied the activated adsorption of filament-heated molecular hydrogen on Cu(111) and subsequent isothermal desorption of hydrogen adatoms. The adsorption is characterized by a zeroth-order kinetic with a constant sticking probability of S₀=0.0062 up to θ=0.25, followed by a Langmuir kinetic until the saturation coverage θ_s=0.5 is reached. The desorption follows a second-order kinetic with an activation energy of 0.63 eV and a pre-exponential factor of 1×10⁹ /s. A pre-adsorbed monolayer of Xe atoms on Cu(111), with a desorption activation energy of 0.25 eV and a pre-exponential factor of 8×10¹⁴ /s, efficiently blocks the subsequent adsorption of hot molecular hydrogen, making physisorbedXe useful as templates for spatial patterning of hydrogen adatom density on Cu(111). PACS 68.43.Jk; 78.68.+m; 81.15.-z; 82.40.Np     

Oxidation of Cu(111): two new oxygen induced reconstructions

The chemisorption of oxygen on Cu(111) has been studied by scanning tunneling microscopy. Two hitherto unknown well ordered oxygen induced reconstructions with extremely large unit cells, 29 and 44 times the 1 × 1 surface unit, are presented. The reconstructions emerge as coincidence lattices between the 1 × 1 surface lattice and a hexagonal substructure, which is associated with the (111) plane of Cu₂O.     

XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111)

We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ～0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The $\text{I}\text{4}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ core electron binding energy in both cases decreases by ～0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas.     

Adsorption and desorption of hydrogen and ethene on clean Si (111) faces

Workfunction, surface conductivity, field effect mobility and desorbed species have been measured after adsorption of hydrogen and ethene on heated clean silicon (111) faces. Whereas the hydrogen studies mostly show the same drastic changes as with cleaved faces, ethene does not change the electronic parameters except the electron affinity due to the dipole moment of the adlayer. During that treatment desorption of ethene and hydrogen due to decomposition is observed. The results enable a study of hydrogenation of ethene for simultaneous adsorption.     

Sorption,chemisorption and desorption of hydrogen by neodymium overlayers and Nd/Cu ultra thin alloy films on Cu(100)

The various modes of hydrogen uptake exhibited by Nd overlayers and Nd/Cu ultra thin alloy films on Cu(100) have been investigated by LEED, UPS, XPS and thermal desorption measurements. Ultra thin Nd overlayers exhibit very low sticking probabilities for H₂ (~10⁻⁴) — far lower than the values characteristic of thick Nd films. This behaviour is associated with the unusual surface structure adopted by the rare earth when present as a very thin film. Codeposition of Nd and H₂ leads to the formation of sorbed hydrogen and is accompanied by valence charge transfer from Nd to H. The kinetics of H₂ desorption from alloy films and Nd overlayers are markedly different: this feature provides a sensitive test for the overlayer → alloy transformation.     

Atomic Diffusion in Cu/Si （111） and Cu/SiO2/Si （111） Systems by Neutral Cluster Beam Deposition

The Cu films are deposited on two kinds of p-type Si （111） substrates by ionized duster beam （ICB） technique. The interface reaction and atomic diffusion of Cu/Si （111） and Cu/SiO2/Si （111） systems are studied at different annealing temperatures by x-ray diffraction （XRD） and Rutherford backscattering spectrometry （RBS）. Some significant results are obtained： For the Cu/Si （111） samples prepared by neutral dusters, the interdiffusion of Cu and Si atoms occurs when annealed at 230℃. The diffusion coefficients of the samples annealed at 230℃ and 500℃ are 8.5 ×10^-15 cm^2.s^-1 and 3.0 ×10^-14 cm^2.s^-1, respectively. The formation of the copper-silicide phase is observed by XRD, and its intensity becomes stronger with the increase of annealing temperature. For the Cu/SiO2//Si （111） samples prepared by neutral dusters, the interdiffusion of Cu and Si atoms occurs and copper silicides are formed when annealed at 450℃. The diffusion coefficients of Cu in Si are calculated to be 6.0 ×10^-16 cm^2.s^-1 at 450℃, due to the fact that the existence of the SiO2 layer suppresses the interdiffusion of Cu and Si.     

Substrate mediated long-range oscillatory interaction between adatoms: Cu /Cu(111)

A quantitative study of the long-range interaction between single copper adatoms on Cu(111) mediated by the electrons in the two-dimensional surface-state band is presented. The interaction potential was determined by evaluating the distance distribution of two adatoms from a series of scanning tunneling microscopy images taken at temperatures of 9-21 K. The long-range interaction is oscillatory with a period of half the Fermi wavelength and decays for larger distances d as 1/d(2). Five potential minima were identified for separations of up to 70 A. The interaction significantly changes the growth of Cu/Cu(111) at low temperatures.     

The Tamm surface state on Cu(111) and Ag(111)

It is shown, that the well-known d-electron Tamm surface-state emission observed in photoelectron spectra from Cu(111) at the M̄ point in the surface Brillouin zone, is indeed due to such a surface state and not a bulk band transition as recently suggested L. Wallden, Solid State Commun. 59, 205 (1986). A similar surface state on Ag(111) is reported.     

Surface lattice dynamics and inelastic He scattering in Cu(111)

Summary We present a calculation of the Cu(111) surface dynamics in the framework of the multipole model. The electronic degrees of freedom include dipole and quadrupole deformabilities of the conduction electron density, the multipole expansion points being located at the midpoints between nearest-neighbour ions. The model accounts for the anomalous longitudinal resonance by an increase of dipolar deformability at the surface. Moreover the model explains in a straight-forward way the intense He scattering from the longitudinal resonance via the dipolar and quadrupolar modulations of the surface electron density. The surface dipolar contribution also explains the intense electron scattering from the optical surface resonance localized on the second layer.     

Long-lived excited states at surfaces: Cs/Cu(111) and Cs/Cu(100) systems

One-electron and multielectron contributions to the decay of transient states in the Cs/Cu(111) and (100) systems are studied by a joined wave-packet propagation and many-body metal response approach. The long lifetime of these states is due to the Cu L and X band gaps which reduce the electron tunneling between Cs and Cu. In the (111) case, the decay is mainly by inelastic e-e interaction, whereas in the (100) case, electron tunneling is dominating. This accounts very well for the experimental findings [Bauer et al., Phys. Rev. B 55, 10 040 (1997) and Ogawa et al., Phys. Rev. Lett. 82, 1931 (1999)].     

The interaction of hydrogen and carbon monoxide with oxygen on Cu(111)-Fe Surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(111)-Fe surfaces containing different amounts of iron has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and if, at larger iron deposits, γ-Fe₂O₃ is present, γ-Fe₂O₃ can be reduced to Fe₃O₄. The maximum reaction rate is proportional to the square of the total copper surface. With hydrogen all oxygen can be removed. The reduction proceeds via a number of different stages. This is explained by the subsequent occurrence of γ-Fe₂O₃, Fe₃O₄ Fe_0.95O and Fe.     

Electron beam induced desorption and dissociation of CO chemisorbed on Ir(111)

Election beam induced perturbations of CO chemisorbed on Ir(111) have been measured using LEED and AES. The total interaction cross-section for electron-stimulated desorption and dissociation is found to be 0.8 to 1.7 × 10^?17 cm² near $\text{1}\text{3}$monolayer coverage at a beam energy of 86 eV. This total cross-section is estimated to be 1 × 10^?17 cm² when defined with respect to the primary flux of a 2.5 keV beam. Electron-stimulated dissociation is found to occur at 1–2% of the rate of stimulated desorption.     

Magnetic properties of (111) Cu/Fe multilayers

The magnetic properties of (111) Cu/Fe multilayers grown on Cu underlayers of several thickness (2000-500-50 Å) on cleaved mica have been investigated by Mössbauer spectroscopy and SQUID magnetometry. The analysis of experimental results suggests that 2.5 monolayers of ψ-Fe interfaced with Cu evolves partially from paramagnetic to ferromagnetic states as the Cu underlayer thickness is reduced to 50 Å and partially from weak to strong antiferromagnetism.