B. Hellsing

Kinetics of the hydrogen-oxygen reaction on platinum

Abstract A kinetic model has been constructed primarily to describe the rates of water and hydroxyl desorption during the hydrogen-oxygen reaction on Pt at high temperatures, ∼ 1000 K, and pressures in the range 1–1000 m Torr. The calculated results are compared with experimental observations. The model is based on dissociative sticking of H 2 and O 2 and hydrogen addition to oxygen to form OH. For water formation the two alternative routes OH + H → H 2 O and OH + OH → H 2 O + O are considered. OH decomposition, OH → O + H, is found to be an important reaction step. Using available literature data and results from the model calculations, an enthalpy diagram for the reaction is constructed. It is concluded that a unique enthalpy diagram for the H 2 + O 2 reaction on Pt is still lacking, even at nearly zero coverage. Adsorbate-adsorbate interactions are expected to modify the enthalpy diagram at high coverages. The kinetic equations for various reaction steps have been formulated assuming random distribution of adsorbed species on a uniform surface. At fixed temperature, both routes for H 2 O formation mentioned above can give a reasonably good quantitative description of the OH and H 2 O desorption rates as functions of gas mixture and pressure in the regimes where experimental data are available. However, a closer analysis shows that the relative importance of the two water formation routes could depend sensitively on temperature and gas-phase H 2 /O 2 ratio. High temperature and hydrogen excess favors the OH + H → H 2 O route, while oxygen excess and low temperatures may favor the OH disproportionation reaction. The model has also been used to predict the reaction kinetics at high pressures up to 105 Torr. The latter results may be useful as guides to high-pressure experiments and in calculations of catalytic combustor performance.

Electronic Damping of Atomic and Molecular Vibrations at Metal Surfaces

First-principles calculations of the damping rate of vibrational and translational modes of single hydrogen atoms and molecules chemisorbed on metal surfaces are presented. The decay into electron-hole pair excitations is considered. The metal is described in the so-called jellium model, and the calculations are based on the Kohn-Sham density functional formalism extended to the quasi-static regime. For the actual evaluation of the damping an embedding scheme is used, which through explicit tests is shown to be appropriate. In particular in the homogeneous limit, a comparison is made with an exact phase-shift formula. The local electron density is found to be a key parameter for the damping rate, in particular in situations with no dramatic electron structure at the Fermi level. Adsorbates that induce states around the Fermi level should exhibit an enhanced damping rate. A direct relation between the friction coefficient of an adparticle and the vibrational damping rate is derived. The calculated rate values imply that the electronic mechanism is able to accommodate typical thermal energies of hydrogen atoms impinging on metal surfaces. From this result and comparisons with observed lifetime broadenings of vibrational spectral lines it is concluded that electron-hole pairs provide an important channel for energy transfer at metal surfaces.

Role of Bulk and Surface Phonons in the Decay of Metal Surface States

We present a comprehensive theoretical investigation of the electron-phonon contribution to the lifetime broadening of the surface states on Cu(111) and Ag(111), in comparison with high-resolution photoemission results. The calculations, including electron and phonon states of the bulk and the surface, resolve the relative importance of the Rayleigh mode, being dominant for the lifetime at small hole binding energies. Including the electron-electron interaction, the theoretical results are in excellent agreement with the measured binding energy and temperature dependent lifetime broadening.

Electron-phonon coupling at metal surfaces

Chemical reactions at metal surfaces are influenced by inherent dissipative processes which involve energy transfer between the conduction electrons and the nuclear motion. We shall discuss how it is possible to model this electron-phonon coupling in order to estimate its importance. A relevant quantity for this investigation is the lifetime of surface-localized electron states. A surface state, quantum well state or surface image state is located in a surface-projected bandgap and becomes relatively sharp in energy. This makes a comparison between calculations and experimental data most attractive, with a possibility of resolving the origin of the lifetime broadening of electron states. To achieve more than an order of magnitude estimate we point out the importance of taking into account the phonon spectrum, electron surface state wavefunctions and screening of the electron-ion potential.

Kinetic model and experimental results for H2O and OH production rates on Pt

Kinetic model calculations are compared with experimental data for the catalytic water, and hydroxyl production in the H 2 +O 2 reaction on Pt. The system is studied at intermediate pressure, 0.1 Torr, high temperature, 1100 K, and for various H 2 /O 2 ratios. A Langmuir-Hinshelwood type reaction, with sequential addition of adsorbed atomic hydrogen to O and OH, respectively, is emphasized. The H 2 +O 2 mixture for maximum water production rate is determined by the hydrogen and oxygen sticking coefficients at zero coverage. The hydroxyl production depends relatively strongly on the difference between the activation energies for OH and H 2 O formation.

Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite

We present an experimental and theoretical investigation of K atom desorption from the basal plane of graphite at 83 K induced by low energy photons (3–6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and work function measurements. At monolayer coverage (5.2×1014 atoms cm−2), the dependence of the cross section on photon energy has a threshold at ℏω≈3.0 eV and rises up to a maximum of 1.8±0.4×10−20 cm2 at 4.8 eV. The coverage dependence of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photo-active and close-packed photo-neutral, respectively. The observed photodesorption is a single-photon, nonthermal event, consistent with a substrate-mediated mechanism. The desorption results from attachment of optically excited hot electrons to the empty 4s state of ionized potassium. The theory predicts in this case a Gaussian line shape of...

Theory of surface phonons at metal surfaces: recent advances

Recent studies of the surface dynamics of Al(001) and Cu(111) based on density functional perturbation theory have substantiated the existence of subsurface optical phonon resonances of all three polarizations, thus confirming early predictions of the embedded-atom method. The hybridization of the shear-vertical optical resonance with the longitudinal acoustic phonon branch accounts for the ubiquitous anomalous acoustic resonance as an intrinsic feature of metal surfaces. The DFPT calculation of the phonon-induced surface charge density oscillations shows that helium atom scattering spectroscopy (HAS) can indeed probe subsurface resonances. This opens new perspectives to HAS for the measurement of subsurface phonon dispersion curves in thin films, as proved by recent HAS studies on Pb and Fe ultrathin films on copper. After discussing these recent advances, this paper briefly reviews other important trends of surface dynamics expressed in recent years.

Direct mechanism for photoinduced desorption and dissociation of O2 on Pt(111)

Abstract A theoretical investigation of the cross section for photoinduced direct excitation of O 2 chemisorbed on Pt(111) is presented. In particular, we focus on the excitations that promote desorption and dissociation of the O 2 molecule. Excitation cross sections for sand p-polarized light are calculated as a function of photon energy and angle of incidence. We find reasonable agreement with experiments regarding, (i) the onset for the desorption and dissociation yields as a function of photon energy and, (ii) the angular dependencies of the cross sections for unpolarized light. However, recent polarized light experiments on the similar system O 2 /Pd(111) show qualitatively different angular dependencies. Taking quenching into account, the estimated cross sections for desorption and dissociation are 2 to 4 orders of magnitude less than experimentally measured.

Orbital selectivity in Hund's metals: The iron chalcogenides

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion U and Hund’s rule coupling J . The evolution of the quasiparticle weight Z as a function of the interaction terms reveals a clear crossover at U � 2.5 eV. In the weak coupling limit Z decreases for all correlated d orbitals as a function of U and beyond the crossover coupling they become weakly dependent on U while strongly dependent on J . A marked orbital dependence of the Z’s emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of U . This two-stage reduction of the quasiparticle coherence due to the combined effect of Hubbard U and the Hund’s J suggests that the iron-based superconductors can be referred to as Hund’s correlated metals.

Metallic quantum dots

In a quantum dot, the electrons are confined in all three dimensions to a length scale of the order of the electron Fermi wavelength. Due to the confinement, quantum effects will dominate over the bulk properties and the energy spectrum becomes discrete, similar to that of an atom. In this review, we present a short introduction to electron confinement in nanosize structures and properties related to quantum size effects. Furthermore, we present a model for calculation of the electronic structure of adsorbed quantum dots, where we have focused on the system of Na on Cu(111). Our results are compared to experimental results from scanning tunnelling microscopy and photoemission spectroscopy. In addition, we present a study of CO adsorption on a small Na quantum dot. The resulting charge transfer turns out to depend critically on the size of the quantum dot, and the results are discussed in terms of electron structure and symmetry of relevant electron orbitals.