Stig Andersson

Vibrational excitations and structure of H2, D2 and HD absorbed on Ni(100)

Abstract High-resolution electron energy loss spectra of H2, D2 and HD chemisorbed at 200 K on the clean Ni(100) surface reveal vibrational excitations significant for dissociative chemisorption. Comparing the experimental vibrational excitation energy with those derived from an existing model calculation of hydrogen chemisorption at high symmetry sites of a Ni(100) surface identifies the adsorption site as the four-fold hollow.

Vibrational excitations and structure of CO chemisorbed on Cu(100)

Abstract The local adsorption site for CO chemisorbed on Cu(100) at 80 K has been investigated by vibrational spectroscopy at different structural situations as defined by low-energy electron diffraction. The high-resolution electron energy loss spectrum for the c(2 × 2) CO structure (coverage 0.5) is compatible with a terminal Cuue5f8CO bond. The spectrum shows two vibrational loss lines at 43 and 260 meV due to excitations of the Cuue5f8C and Cue5f8O stretcning vibrations respectively. The Cue5f8O stretchning vibration is excited via dipole interaction for primary electron energies 1–4 eV. An effective dynamic charge of 0.64 e is deduced in good agreement with infrared reflection-absorption data. The preceeding CO lattice gas is also characterized by the terminal bond configuration. The spectrum for the CO compression structure at coverage 0.57 is tentatively interpreted in terms of an alternative site (terminal and bridge) absorption model proposed by Pritchard. A brief comparision with the Ni(100)-CO system is made.

Surface vibrations of oxygen and sulphur in the p(2 × 2) and c(2 × 2) structures on Ni(100)

Abstract Vibrational excitations of oxygen and sulphur chemisorbed on Ni(100) in the consecutive p(2 × 2) and c(2 × 2) structures have been investigated by high-resolution electron energy loss spectroscopy (EELS). Vibrational losses due to localized modes are observed at 53 and 39 meV for the p(2 × 2)O and c(2 × 2)O structures respectively and at 46 and 44 meV for the corresponding p(2 × 2)S and c(2 × 2)S structures. The large difference in Vibrational excitation energies for the two oxygen structures as compared to those for sulphur is attributed to differences in the shape of the chemisorption potential well.

Energy dependence of vibrational excitation of adsorbed CO and OH

Abstract To assess the excitation mechanism of vibrationa modes of adsorbates by electron we have measured loss spectra for CO on Ni (100) and OH on NiO (111) as a function of primary electron energy. We find that the dipole interaction describes the data for the CO system but fails completely for the the OH system. We find the loss intensity in OH to be more than an order of magnitude larger than dipole estimates at low energies. This is accompanied by a strong overtone and intrinsic NiO phonon satellite excitation.

Surface vibrational excitations of O in the p(2 × 2)O and c(2 × 2)O structures on Ni(100)

Abstract Surface vibrational excitations of O chemisorbed on the clean Ni(100) surface have been investigated by high-resolution electron energy loss spectroscopy (EELS). The observed vibrational losses in the Ni(100) p(2 × 2)O and Ni(100) c(2 × 2)O surface structures are 53.0 ± 0.5 and 39.5 ± 0.5 meV respectively. The unexpectedly large change in the vibrational excitation energy is attributed to a low potential barrier for oxygen dissolution into the Ni substrate.

Multiple coincidences in surface structure determinations

Abstract We show that when the adsorbate is a weak scatterer of electrons, agreement of calculated LEED spectra with experiment is subject to approximate multiple coincidences in the vertical adsorbate-substrate spacing, with a periodicity of about 0.7 A. Scrutiny of low energy spectra and the appearance of a compression or expansion of the theoretical energy scale with respect to experiment provide means to distinguish between the coincidences.

Infrared spectroscopy of physisorbed and chemisorbed O2 on Pt(111).

The adsorption of O2 on the Pt(111) surface, with particular emphasis on the influence of substrate temperature, has been studied by infrared reflection absorption spectroscopy (IRAS). In the temperature range 30-90 K the IRAS spectra reveal three different molecular adsorption states. A physisorbed state appears below 40 K while chemisorbed peroxo- and superoxo-like states are observed in the whole temperature range, the characteristic vibrational frequencies are at full coverages of 16O2, 1543 cm(-1) and around 700 cm(-1) and 870 cm(-1), respectively. Flash heating from 30 K to 45 K reveal that the physisorbed state acts as a precursor to the superoxo chemisorption. Theoretical calculations suggest that peroxo molecules may occupy both fcc and hcp threefold sites on the Pt(111) surface. However, within the high resolution of the IRAS measurements we only observe one peroxo state in the temperature range 45-90 K, assigned to occupy the fcc site. The peroxo adsorption probability is significantly lower at 45 K than at 90 K, presumably due to reduced thermal activation from the physisorbed precursor state. A longer lifetime in this precursor state at the low temperature results in formation of larger superoxo islands already at low oxygen coverage.

Monolayer K-films on Ni(100) — I. Structure and work function

Abstract Adsorption of K on Ni(100) at room temperature has been investigated experimentally concerning two-dimensional periodicity and work function change. It is found that the K atoms spread uniformly over the Ni surface and that one well-ordered structure of hexagonal symmetry is formed at 4.9 A K nearest neighbour separation. In the most densely packed K-layer this separation was observed to be 4.3 A, i.e. smaller than the bulk separation 4.62 A. The change in work function at low densities was found to correlate qualitatively with the variation of the effective charge on the adsorbed K atoms.

Benchmarking van der Waals density functionals with experimental data: potential-energy curves for H2 molecules on Cu(111), (100) and (110) surfaces.

Detailed physisorption data from experiment for the H(2) molecule on low-index Cu surfaces challenge theory. Recently, density functional theory (DFT) has been developed to account for nonlocal correlation effects, including van der Waals (dispersion) forces. We show that the functional vdW-DF2 gives a potential-energy curve, potential-well energy levels and difference in lateral corrugation promisingly close to the results obtained by resonant elastic backscattering-diffraction experiments. The backscattering barrier is sensitive to the choice of exchange functional approximation. Further, the DFT-D3 and TS-vdW corrections to traditional DFT formulations are also benchmarked, and deviations are analyzed.

Low-energy electron diffraction study of sodium cloride (100) surface

Abstract The (100) surface of a NaCl crystal produced by cleavage in air has been studied by low-energy electron diffraction in the electron energy range of 25–400 eV. The diffraction pattern from the crystal surface was in good agreement with that expected from an ideal NaCl (100) surface in the substrate. A surface expansion of 2.5% is found as measured from the intensity variation of the 00 spot as a function of the electron energy. This expansion is in agreement with the theoretically prediction of surface distorsion by Benson, Freeman and Dempsey 1 ). Deficiency Kikuchi lines were observed in the electron energy range of 150–400 eV. These observations give evidence for multiple scattering.