S. Lehwald

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Abstract Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm −1 for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.

Structure and adsorbate-adsorbate interactions of the compressed Ni(110)-(2 × 1)CO structure

Abstract Electron energy loss spectroscopy (EELS) with a resolution down to 15 cm −1 was used to study the vibrational modes of the Ni(110)-(2 × 1)CO surface. Besides the CO-stretch and the CO-metal stretch vibration we measured a CO-rotational mode, and a “frustrated translation” mode. The observation of only these CO modes over the entire Brillouin zone favours an (bent) on-top adsorption site for the CO molecule. For the CO stretch vibration we observed a Davydov splitting. The dispersion of the CO-stretch vibration can be described by a simple model of electrostatic dipole-dipole interactions and is sensitive to a structural parameter of CO adsorption, namely the lateral displacement from the on-top position along [001]. The dispersion of the “frustrated translation” mode is caused by strong direct CO-CO interactions.

Bending of crystalline plates under the influence of surface stress — a finite element analysis

Abstract Changes in the surface stress Δ τ (s) at the solid–vacuum and the solid–liquid interface are typically measured via the bending of a (thin) crystalline plate. The bending is converted into Δ τ (s) with the help of elasticity theory. Although the conversion is trivial for the unconstrained bending of an elastic isotropic substrate, it becomes less straightforward for single crystals. In this paper we firstly consider the analytical solution for the free two-dimensional bending of a cubic substrate under the influence of a homogeneous, but anisotropic surface stress. Secondly, the bending is calculated numerically for the experimentally mostly realized case of a sample with (100) or (111) surfaces clamped along one edge. In principle, separate numerical calculations are required for each individual material and surface orientation. It is shown that a parameterized form of the analytical solution for the unconstrained bending can still be used in most cases. The appropriate parameters are calculated for various Poisson ratios and crystal anisotropies.

Surface phonon dispersion and adsorbate induced reconstruction of Ni(100)

Abstract The Ni(100) surface reconstructs upon adsorption of half a monolayer of nitrogen. This observation is at variance with a previous report. The reconstructed phase consists of a c(2 × 2) overlayer of nitrogen with a clockwise and counterclockwise rotation of the nickel surface atoms around two nearest neighbor nitrogen atoms, respectively. The same p4g(2 × 2) reconstruction was also observed with carbon overlayers. Recent total energy calculations and lattice dynamic analysis have shown that this reconstruction is driven by internal stresses caused by a repulsive interaction between the first layer nickel atoms. We have measured the phonon dispersion branches of the p4g(2 × 2) nitrogen covered surface along the [110] direction. Two adsorbate induced phonon branches above the bulk phonon branch of nickel are found. The branches are similar to those found for the carbon covered surface and indicate that nitrogen sits close to the surface in the fourfold hollow site. In addition to the adsorbate associated modes surface resonances inside the bulk band and the Rayleigh phonon have been measured. The various modes are identified by comparing experimental results to lattice dynamical calculations.

Adsorption of CO on the unreconstructed and reconstructed Ir(100) surface

Abstract The adsorption of CO on both the (1 × 1) and the (5 × 1) reconstructed Ir(100) surface at room temperature has been investigated using electron energy loss spectroscopy (EELS) and LEED. CO is adsorbed on both surfaces at all coverages in on-top sites. All four vibrational modes of the adsorbate have been detected. Upon adsorption of CO on the (5 × 1) surface the reconstruction is locally lifted giving rise to a (1 × 1) LEED pattern. Adsorption of CO on the (1 × 1) surface leads to a c(2 × 2) structure. The vibrational frequencies of the CO-molecules on both surfaces differ only slightly. At saturation the iridium-CO and the C-O stretching frequencies are 485 and 2075 cm −1 on the (5 × 1) and 497 and 2068 cm −1 on the (1 × 1) surface, respectively. The frequency of the rotational mode of the CO molecule is found to be 425 cm −1 and the frustrated translation at 53 cm −1 , both showing no dispersion along ḡGM direction. The C-O stretching vibration shows dispersion due to dipole-dipole interaction, also when the overlayer is not ordered.

Symmetry and structure of the reconstructed Ni(110)-(2 × 1)O surface

Abstract We have measured the dispersion of oxygen and nickel vibrational modes of the reconstructed Ni(110)-(2 × 1)O surface along the high symmetry directions using momentum-resolved electron energy loss spectroscopy (EELS). Observation of the frequency shifts upon substitution of O 16 by O 18 discriminates adsorbate from substrate phonons. With the scattering plane along [ 1 10] two oxygen modes are observed, which are also dipole active. Their frequency at the \ gG point (at 120 K) is 239 and 382 cm −1 respectively. With the scattering plane aligned along [001] an additional third oxygen mode at about 800 cm −1 is observed off-specular only. According to EELS selection rules the symmetry of the surface is thus C s with the mirror plane oriented along [ 1 10]. This is in agreement with the “saw-tooth” reconstruction model proposed for this surface, however also with a “modified missing-row” model with the oxygen atoms moved out of the long-bridge site along either the [ 1 10] or the [1 1 0] direction.

Electron energy loss spectroscopy: recent advances in technology and application

Recent technological advances in electron energy loss spectroscopy (EELS) are reviewed. The results are demonstrated with high-resolution spectra of localized adsorbate vibrations and shear horizontally polarized surface phonons.

Recent advances in electron energy loss spectroscopy of surface vibrations

Recent advances in electron energy loss spectroscopy are reviewed. After a short discussion of the principal limitations of energy loss spectrometers, the first results obtained with a recently designed spectrometer with angular aberration correction, are presented.

Surface phonons of the clean and H-covered W(110) surface measured with EELS

Abstract The surface phonons of the clean W (110) surface were investigated by EELS. At the Γ -point a dipole active surface resonance at about 190 cm−1 has been resolved and confirmed by slab calculations using central forces up to third neighbors. The Rayleigh wave (RW) has been observed throughout the whole surface Brillouin zone along the Γ H , Γ N , and Γ S direction. The frequencies are 128, 116 and 130 cm−1 at H, N , and S , respectively. The dispersion curves in all three directions agree with the data obtained by He-scattering where such data are available. A second mode within the bulk band has been detected by both techniques and is interpreted as the longitudinal polarized surface phonon of the first layer. For the hydrogen covered surface the RW dispersion curves along Γ H of the (2 × 1) and (2 × 2) superstructure show a slightly higher frequency than for the clean surface around H. On the nearly H-saturated (1 × 1) surface a dip in the dispersion curve of the RW was found at a wave vector of 0.9 A−1 along Γ H : the frequency drops from 110 cm−1 on the clean surface down to 83 cm−1. A second feature detected with He-scattering, showing an even stronger anomaly at the same wave vector, could not be observed in our measurements.

Substrate surface phonons in the cases of saturated (1×1)H/Mo(110) and p(2×2)O/Mo(110): a critical comparison

Abstract We investigated the dispersion of substrate surface phonons of p(2×2)O/Mo(110) with electron energy loss spectroscopy (EELS). As a result, we obtained no significant change compared to the clean surface. This fact is surprising because the shape of the Fermi surface should favour Fermi surface nesting and hence an anomalous softening of the frequencies of the surface phonon modes. Such a softening had been observed for the hydrogen-saturated Mo(110) surface.