Comparative study of vibrations in submonolayer structures of potassium on Pt(111)

We present results of a comparative study of the vibrational spectrum and local density of phonon states in ordered p(2 x 2) and (√3 x √3)R30° structures formed by potassium atoms on the Pt(111) surface. The calculations were performed with tight-binding interatomic interaction potentials. It was found that the mode associated with vertical displacements of K adatoms has an energy of about 20 meV in both K structures. The strength and energy of this mode slightly decreases with increasing coverage. This result is in good agreement with available experimental data. As in time-resolved second harmonic generation measurements, we observed low frequency modes for both structures considered, which are caused by the interaction of potassium with the second layer of the substrate.     

Magnetic properties of Co clusters deposited on Pt(111)

Using the fully relativistic version of the Korringa–Kohn–Rostoker method for electronic structure calculations within local spin density functional theory, the magnetic and spectroscopic properties of Co clusters deposited on Pt(111) have been investigated. Of central interest was the role of spin–orbit coupling, since it influences the spontaneous formation and orientation of magnetic moments and gives rise among other things to the occurrence of orbital magnetic moments, magnetic anisotropy energy and magnetic circular dichroism in X-ray absorption. The results have been complemented by calculations of the exchange coupling parameters J_ij used within Monte Carlo simulations on the basis of the extended classical Heisenberg Hamiltonian. This allowed us to simulate the magnetic properties at finite temperatures, which are of central importance for applications. PACS 73.20.-r; 71.15.Rf; 73.22.-f; 87.64.Ni     

Angle resolved photoemission study and calculation of the electronic structure of the Pt(111) surface

Angle resolved photoemission spectra of Pt(111) were measured along the direction of the surface Brillouin zone (SBZ). The electronic structure of a semi-infinite Pt(111) crystal was calculated applying the LMTO-TB approximation to aid interpretation of the spectra. The experimental spectra are well described by the calculated bulk band structure. Both the experiment and calculation reveal a surface state near the Fermi level in the neighborhood of the point of the SBZ.     

Spin-resolved photoemission from epitaxial Au layers on Pt(111): Coverage dependence of the bandstructure and evidence of surface resonances

Au/Pt(111) has been studied by spin-, angle- and energy-resolved photoemission with normal incident circularly polarized synchrotron radiation of BESSY and normal photoelectron emission for different Au coverages. The prepared layers were characterized by LEED and Augerelectron spectroscopy and turned out to grow up two dimensional and epitaxially. In the photoemission experiments the development of the 3-dimensional bandstructure in the -direction could be observed. For a coverage of 2.6 layers the highest occupied spin-orbit split bands are located at about 0.6 eV lower binding energy than the corresponding bands for a 3D-Au crystal and show dispersion which is, however, weaker than in a 3D-Au crystal. A 5 layer Au adsorbate was found to have already the same dispersion and energetic location as a Au(111)-crystal. For thick gold layers, which behave in photoemission like Au(111)-crystals, we find structures that cannot be due to direct transitions into a free electron like final band. The coverage dependence and spin polarization of these structures show that some of them are due to surface resonances, while the origin of one strong peak could not yet be explained conclusively. In addition we find strong hybridization and two avoided crossings in the occupied part of the bandstructure.     

Effect of surface reconstruction on the photoemission cross-section of the Au(111) surface state

The photoemission cross-section of the Shockley surface state of Au(111) is studied over a wide range of photon energies both experimentally and theoretically. The measurements are fully understood based on the theoretical analysis within a one-step ab?initio theory of photoemission. The constant initial state spectrum is shown to be very sensitive to the structure of the topmost atomic layer. A maximum in the constant initial spectrum at 60?eV is identified as a fingerprint of the Au(111) surface reconstruction.     

A new spin effect in photoemission with unpolarized light: Experimental evidence of spin polarized electrons in normal emission from Pt(111) and Au(111)

Unpolarized light ejects spin polarized electrons from Pt(111) and Au(111) even if the electron emission occurs normal to the surface. For off normal incidence of 11.8 eV, 16.9 eV, and 21.2 eV radiation, and for the main peaks in the photoemission spectra, a degree of spin polarization of up to 30% or more is found for the spin polarization component P _y perpendicular to the reaction plane. A crystal rotation about its surface normal does not change P _y . P _y is largest for transitions from bands with symmetry ₆ ³ . All these experimental findings agree with a recent theoretical prediction [1] of a new spin effect by Tamura and Feder.     

Vectorial volume effect in interfacial photoemission from Cu(111) and Au(111) at low photon energies

Pronounced angle-of-incidence and polarization dependences of the photoemission yields from Cu(111) and Au(111) were observed in the photon energy range of 3.4–5.0 eV with the interfacial photoemission-into-electrolyte technique. Strong evidence was obtained that these vectorial features are due to the anisotropic bulk excitation of photoelectrons and restrictive escape condition of parallel k-vector conservation. Calculations, based upon a simple model of anisotropic bulk photoemission, show good agreement with the experimental results.     

Angle-resolved photoemission study of Pd(111) and Co/Pd(111)

Electronic structure of epitaxial Co films on Pd(111) has been investigated as a function of the Co overlayer thickness, by using angle-resolved photoemission spectroscopy (ARPES). It is found that experimental band structures for Pd(111) and work functions for Co/Pd(111) agree reasonably well with the results of band structure calculations. Compared to Pd(111), valence band ARPES spectra of Co/Pd(111) support the existence of the induced magnetic polarization of Pd atoms near the interface, suggesting large hybridization between Co 3d and Pd 4d states near the interface.     

The adsorption of methanol on Pt(111), an IR reflection and UV photoemission study

IR reflection (absorption) spectroscopy (IRS) under grazing incidence and UV photoemission (UPS) with He I and He II radiation were used to study the adsorption of CD₃OD and CH₃OH, respectively, on Pt(111) at temperatures between 90 and 350 K. At 90 K a first chemisorbed layer of methanol consists of molecules being strongly chemisorbed at their oxygen end. On top of this first layer two other phases of methanol are detected. For both the second and third phase the IR spectra indicate hydrogen bonding between the molecules. From UPS a stronger contribution to chemical bonding at the oxygen end of the molecules is derived for the second phase. On top of the second phase amorphous multilayers condense which exhibit a phase transition into the crystalline α-ice modification of methanol upon annealing to 125 K. Thermal desorption proceeds via loss of the α-ice multilayers (135 K), desorption of the second (“amorphous”) methanol phase to a final dissociation of molecules in the first layer into CO and H. About 3% of a monolayer of CO remains on the surface at temperatures above 230 K. No stable intermediate in the decomposition of methanol into CO is detected.     

Island shape selection in Pt(111) submonolayer homoepitaxy with or without CO as an adsorbate

The microscopic selection mechanisms of single-layer island shapes in Pt(111) homoepitaxy with or without minute amounts of CO adsorbate have been investigated theoretically. For clean growth, only triangular islands of a fixed orientation are obtained within a wide range of growth temperatures, with the orientation uniquely determined by a disparity in the rates of atom supply to an island corner site from the two island edges defining the corner. This novel picture is further corroborated by growth predictions in the presence of CO, whose preferential decoration of one type of the island edges reverses the intrinsic rate disparity for atom supply, thereby inverting the island orientation.     

Narrow photoemission peak at the Fermi level in Fe/Cu(111)

We have studied electronic states of Fe adatoms deposited on Cu(111) by photoemission spectroscopy and have found a narrow peak at the Fermi level in the Fe 3d spectral function. The Fe 3d spectral function would be consistent with the Anderson impurity model, which has been widely used for the interpretation of bulk Ce and Yb compounds. This result indicates a strong reduction of hybridization between the Fe 3d state and the conduction-band states and an enhancement of effective Coulomb interaction for the Fe 3d electrons in an Fe adatom on Cu(111).     

Lifetimes of image-potential states on the Pt(111) surface probed by time-resolved two-photon photoemission spectroscopy

We have investigated the population dynamics of image-potential states on the clean Pt(111) surface. The first two image-potential states have been resolved exhibiting lifetimes of 26±7 fs and 62±7 fs. Those lifetimes are in contrast to the (111) surfaces of Ag and Cu, where the n=2 state is degenerate with bulk states leading to lifetimes shorter than 20 fs. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

STM study of Au(111) growth on mica

Gold films were thermally evaporated in vacuum on heated cleaved mica substrates. The substrate temperature was immediately decreased after finishing the growth. Samples prepared at various temperatures from 580 K to 890 K and evaporation rates smaller than 1 nm/s were studied by a scanning tunneling microscope (STM) under ambient conditions. Structure and defects of Au(111) surfaces were investigated and discussed with respect to growth conditions.     

Angle-resolved photoemission study of no chemisorption on Ag(111) surface

An angle-resolved photoemission study using synchrotron radiation has been carried out to investigate the adsorption of NO on Ag(111). At 150 K, NO is adsorbed molecularly on the Ag(111) surface. The NO molecule is found to have an upright orientation from a study using incidence angle dependent measurements and symmetry selection rules. The upright orientation is also confirmed from the study of the shape resonance of the 5σ level. Photon induced dissociation of the chemisorbed NO was observed and is briefly discussed.