Adsorption active sites -Key factor on the adsorption ability of pyridine molecules on Ag surfaces

Experimental results on the study of pyridine molecules adsorbed on Ag films and Ag foils under atmospheric and ultra-high vacuum environments by the secondharmonic generation technique suggest that micro-scaled active sites existing in the evaporated Ag film play the key role for this kind of adsorption.From Su-Zhou University     

The coefficient of surface stiffness for a pinned interface

We derive nonperturbatively — within the meanfield version of the Landau-Ginzburg-Wilson theory — the expressions for the constrained order parameter, for the coefficient of surface stiffness, and for the effective interface potential of a fluctuating interface in the presence of a flat substrate. The derived expressions display the typical dependence on the distancel between the interface and the substrate; the exponentially decaying contributions are multiplied by polynomials inl. We show that the expressions for the coefficient of surface stiffness and for the effective interface potential have identical form as those proposed recently by Jin and Fisher (Phys. Rev.B 47, 7365 (1993)).Dedicated to Herbert Wagner on the occasion of his 60th birthday     

Electronically stimulated desorption

In the valence region, desorption of neutrals predominates; nevertheless most research has been on ions up to recently. New methods have made the investigation of neutrals easier, and angle as well as energy distributions have been reported; on this basis the salient mechanisms can be deduced both for chemisorbed and physisorbed layers. Desorption of ions and, in particular, of fragment ions, is strongly enhanced following higher excitations. These can most easily be reached via Auger decay of adsorbate core holes. Considerable detail of understanding is obtainable through polarization-dependent measurements with synchrotron radiation. The importance of many-body interactions at surfaces (increased localization by hole-hole interaction; increased delocalization by screening) becomes obvious. The field of stimulated desorption has profitted strongly from the improved understanding derived from the mature state of surface spectroscopies.     

Investigation of self-organizing thiol films by optical second harmonic generation and X-ray photoelectron spectroscopy

The adsorption of self-organizing thiol films on polycrystalline gold substrates was investigated in situ by optical second harmonic generation. Growth of the films could be detected at submonolayer coverage. For a comparison with classical surface analytical methods the thiol coverage was measured ex situ by X-ray photoelectron spectroscopy. We find that formation of ordered thiol films up to monolayer coverage can be described by Langmuir adsorption kinetics.     

An adsorption-desorption study of Cu on Mo(110)

Thermal desorption spectroscopy (TDS), adsorption-desorption equilibrium (ADE), adsorption transient (AT) and desorption transient (DT) measurements are used to study the adsorption and condensation of Cu on Mo(110). Fitting the ADE results with various statistical-mechanical models gives good agreement with the desorption energies derived from the TDS, AT, and DT measurements. At coverages above two monolayers (ML) three-dimensional nucleation is seen in the kinetic measurements.     

Droplet suction on porous media

We study the forced aspiration of small ( mm) and large ( cm) liquid drops, deposited on prewetted porous membranes, and pumped mechanically with a constant current J. Two kinds of membranes are used where the pores are i) disconnected, cylindrical and calibrated or ii) interconnected “sponge-like”. Whatever the size of the drops and the intensity J of the current, two suction regimes are observed versus time: 1) a “locked” regime, when the drop is pinned, with a dynamic contact angle decreasing from advancing () to finite receding () contact angle; 2) an “unlocked” regime, where the contour line recedes with a constant contact angle closed to . In both regimes, the shape of the drop remains quasistatic, during the suction process, i.e. a spherical cap for small drops and a flat “gravity pancake” for large ones. Received 19 January 2000     

Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.     

Angle and speed distributions of hydrogen desorbing thermally from metal surfaces

On the basis of the theory developed in a previous paper (Part I), the angle and speed distributions of D₂ molecules desorbing thermally from the Ni(111) surface are numerically calculated in the present paper (Part II). The dynamical motion of the activated complex of the recombinative desorption process is calculated by using a model potential surface, which involves a few parameters introduced to describe the necessary features of the potential surface at the transition state. Numerical calculations are carried out, and the results show that the present dynamical theory reproduces very well a variety of the characteristic features of the experimental results, e.g., sharply focused angular distribution, mean translational energy lower than 2k _B T at grazing angle, and the non-Maxwellian profile of the TOF distribution.     

Formation of a knudsen layer in electronically induced desorption

For intense desorption fluxes, particles desorbed by electronic transitions (DIET) from a surface into a vacuum may thermalize in the gas cloud forming above the surface. In immediate vicinity to the surface, however, a non-equilibrium layer (the Knudsen layer) exists which separates the recently desorbed, non-thermal particles from the thermalized gas cloud. We investigate by Monte Carlo computer simulation the time it takes to form a Knudsen layer, and its properties. It is found that a Knudsen layer, and thus also a thermalized gas cloud, is formed after around 200 mean free flight times of the desorbing particles, corresponding to a desorption of 20 monolayers. At the end of the Knudsen layer, the gas density will be higher, and the flow velocity and temperature smaller, than literature values indicate for thermal desorption. These data are of fundamental interest for the modeling of gas-kinetic and gas-dynamic effects in DIET.     

Angle and speed distributions of hydrogen desorbing thermally from metal surfaces

A quantum theoretical treatment of the angle and speed distributions of recombinatively desorbing hydrogen from metal surfaces is proposed. The desorption rate is discussed in the framework of the transition state theory. The recombinative reaction process of hydrogen due to thermal activation leads to the formation of an activated complex in the transition state. In the vicinity of a saddle point on a three-dimensional potential energy surface, the translational motion of the activated complex in the direction perpendicular to the metal surface is accompanied by its center-of-mass vibrational motion parallel to the metal surface. In order to carry out the quantum mechanical calculation, the potential surface is replaced by a simplified model potential, which provides a square potential barrier along the surface normal. It is shown that, on leaving the potential barrier, the activated complex is reflected by the boundary of the potential barrier with a certain probability and, at the same time, the center-of-mass modes of vibration with frequencies v ₁ and v ₂ are coupled with the translational motion along the surface normal. Vibrational wave functions in the momentum representation are used to calculate the transmission coefficient, which is incorporated into the conventional rate formula. The angle-dependent speed distributions of desorbing molecules are derived from the rate formula.     

Photoacoustic signal enhancement at visible and X-ray wavelengths

Good agreement between theory and experiment is found for photoacoustic (PA) signal enhancement by volatile liquids adsorbed on solid samples in a microphonic cell illuminated with visible light. When the same solid samples are illuminated with synchrotron X-radiation significantly smaller PA signal enhancements are observed than are predicted by theory because of radiolysis of the liquid vapors. The introduction of the volatile liquids is nevertheless valuable for the X-ray photoacoustic measurements because it raises the signal above the threshold for detectability.     

Transfer of Au Cusp-clusters on Si(111) 7×7 surfaces from a pure Au tip of a scanning tunneling microscope

Received: 16 August 1996/Accepted: 6 March 1997     

The scattering of thermal He atoms at ordered and disordered surfaces

The use of thermal energy atom scattering (TEAS) as a surface investigation tool is reviewed. Specific properties of thermal He scattering, which are particularly advantageous in surface analysis, are discussed; the main causes which hampered so far the broad application of this technique to the investigation of ordered surfaces are analysed. The recent development of TEAS based on the exploitation of the large cross section for He diffuse scattering of adsorbates and vacancies is reviewed. This development extends the capabilities of TEAS to a straightforward investigation of various aspects of disordered surfaces so far hardly accessible to other methods. Examples concerning the lateral distribution of adsorbates and defects, the mutual attraction and repulsion, 2D-phase transitions, mobility and surface diffusion are given.     

Anchoring transitions with polar and non-polar nematic liquid crystals on incompletely oxidized silane substrates

The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles, with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed for this symmetry breaking term. Received: 18 May 1998     

Organic chemistry at interfaces studied by optical second-harmonic and IR-vis sum-frequency generation

Three selected systems are presented which demonstrate the versatility of second-order nonlinear techniques for interface analysis and thin film monitoring. With second-harmonic generation processes at buried interfaces can be monitored in real time as is shown by thiol adsorption on polycrystalline gold from solution. The time dependent second-harmonic signal shows a strong wavelength dependence due to resonances. In situ monitoring of thin-film formation by second-harmonic generation can differentiate between the reaction at the interface and the subsequent film growth. The chemical vapor deposition of polyamic acid is presented as example. Hydrogen adsorption on diamond C(111) is analysed by sum-frequency generation. It is demonstrated that sum-frequency generation has the sensitivity and specificity to trace chemical reactions at surfaces and to infer on the orientation of chemical bonds.     

Filling carbon nanotubes

Received: 2 March 1998     

Interaction of atomic hydrogen with metal surfaces

Received: 23 March 1998/Accepted: 21 August 1998