The influence of open steps on the surface properties is shown by investigating the interaction of molecular ethene with Cu(410). We find a surprisingly low-temperature, site-selective chemistry at the strongly under-coordinated step sites. Ethene bonds either in a π-bonded or in a di-σ-bonded state or undergoes complete dehydrogenation. All pathways involve the low-coordination sites at the step, since the first species is partially stabilized with respect to low-Miller-index surfaces, while the other two are observed only on Cu(410). When annealing the surface, dehydrogenation and transformation into the di-σ-bonded moiety proceed, both processes being favored by faster heating rates. The so-generated carbon (presumably C2 admolecules) decorates the step edges, thereby blocking the active sites for subsequent dissociation and permitting only π-bonding of ethene. The dipole loss of carbon disappears in high-resolution electron energy loss spectroscopy when annealing to room temperature, indicating that carbon moves to more coplanar or even to subsurface sites where it still influences the surface chemistry. The surface reactivity is recovered when heating the crystal to 900 K since C dissolves then deep enough into the bulk.

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The molecular vibrations of ethene adsorbed on roughened Cu(111) surfaces have been investigated with high resolution electron energy loss spectroscopy and density-functional-theory calculations. The roughness was introduced by sputtering or evaporation of copper, respectively, on the cooled surface. We found stabilization of the ethene layer compared to ethene adsorbed on pristine Cu(111). Furthermore, two new vibrational features observed on the rough surface can be assigned to frustrated translations and rotations of the ethene molecule on surface defects and are indicative of a different binding on the rough surface.

As recently established (J. Am. Chem. Soc. 2008, 130, 12552), ethene adsorption on Cu(410) occurs both molecularly and dissociatively, the latter resulting in carbon contamination of the Surface. Here we report on the coverage-dependent dynamics of C(2)H(4) adsorption on clean and carbon-contaminated Cu(410). For the bare surface, the initial sticking probability has a very weak dependence on kinetic energy and is almost independent of angle of incidence. Molecular adsorption is in both cases precursor-mediated and nonactivated. Ethene dissociation takes place during adsorption as well as upon annealing. Both paths proceed via a molecular precursor. The former is translationally activated, while the latter depends strongly on the heating rate. The presence of preadsorbed carbon, resulting from previous uptakes, affects both the sticking probability and the attained saturation coverage. The latter quantity is shown to be a sensitive probe of carbon precoverage. A scheme of the complicated potential energy surface of this system is derived and discussed

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Defects and morphological changes in nanothin Cu films on polycrystalline Mo analyzed by thermal helium desorption spectrometry

Defects of ultrathin Cu films (3-200 Å) deposited on Mo(1 1 0) at room temperature by e-beam evaporation in ultra-high vacuum are investigated using thermal helium desorption spectrometry. The samples are analysed with both 75 and 1000 eV He + implantation. Cu films transform into islands on annealing and the temperature of this transformation is strongly thickness dependent. Helium release from defects close to the surface of the Cu films (∼450 K), from monovacancies in as-deposited Cu film (50-200 Å), and from defects close to the interface (800-1050 K) are identified. Annealing of monovacancies is mainly responsible for reduction in helium trapping in a 20 Å Cu film prior to film islanding. There is an indication of retrapping of helium released from the first 5 Å layer of the film in the defects located in the overlayers (5-95 Å). Helium in the Cu films survives until the desorption temperature of the film (1200-1300 K).

Thermal helium desorption spectrometry ͑THDS͒ has been used for the investigation of defects and thermal stability of thin Cu films ͑5 -200 Å͒ deposited on a polycrystalline Mo substrate in ultrahigh vacuum. These films are metastable at room temperature. On heating, the films transform into islands, giving rise to a relatively broad peak in the helium desorption spectra. The temperature of this island formation is dependent on film thickness, being 417 K for 10 Å and 1100 K for a 200 Å film. The activation energy for island formation was found to be 0.3± 0.1 eV for 75 Å film. Grain boundaries have a strong effect on island formation. The defect concentration in the as-deposited films is ϳ5 ϫ 10 −4 , for films thicker than 50 Å and more for thinner films. Helium release from monovacancies was identified in the case of a 200 Å film. Helium release was also seen during sublimation of the Cu film ͑ϳ1350 K͒. Overlayer experiments were used to identify helium trapped close to the film surface. An increase of the substrate temperature during deposition resulted in a film that had already formed islands. Argon-ion assistance ͑250 eV͒ during film deposition with an ion/atom ratio of ϳ0.1 resulted in a significant enhancement of helium trapping in the films. The argon concentration in the films was found to be 10 −3 . The temperature of island formation was increased due to argon-ion assistance. The helium and argon desorption spectra are found to be similar, which is due to most of the helium becoming trapped in the defects created by the argon beam. The role of the Mo surface in affecting the defects at the film-substrate interface is investigated. The effect of variation of helium fluence and helium implantation energy is also considered. The present THDS results of Cu/poly-Mo are compared to those of Cu/ Mo͑100͒ and Cu/ Mo͑100͒ reported earlier.