G. Meister

Temperature dependence of Shockley-type surface energy bands on Cu(111), Ag(111) and Au(111)

Abstract Using angle-resolved photoemission at very high resolution we have measured thermally induced energy shifts of the Shockley surface states observed around the center \ gG of the surface Brillouin zones on the noble metal (111) surfaces. Based on calculations using the one-dimensional multiple reflection model we demonstrate that the observed shifts can be quantitatively traced back to the temperature-dependent shift of the relevant bulk band gaps which support these surface states. We have addressed care to a precise investigation of the \ gG state on Ag(111), since conflicting results had been reported earlier. Its initial state energy at \ gG is given by E 0 ( T ) = −(75 ± 5) meV + (0.17 meV/K) T in the temperature range up to about 600 K.

High-resolution photoemission study of the surface states near \̄gG on Cu(111) and Ag(111)

We present high-resolution angle-resolved photoemission results concerning the already well-known surface states at the center of the surface Brillouin zone on Cu(111) and Ag(111). Attention is focused on the influence of energy- and angle-resolution, sample temperature, and the proximity of the Fermi edge on the photoelectron line shape. We also discuss, to which extent photohole-lifetimes for Cu and Ag may be inferred from photoemission line-widths.

Oxygen-induced bilayer formation from silver monolayers on W(110)

Abstract We have investigated the room-temperature interaction of molecular oxygen with ultrathin silver layers deposited epitaxially on W(110), using angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and high-resolution electron energy-loss spectroscopy (HREELS). We present experimental evidence that monolayer Ag films are converted into double-layer Ag islands by O 2 after exposures of 2 × 10 −5 Torr·s. This rearrangement opens Ag-free regions of W(110) which are covered by ordered atomic oxygen layers. In contrast, Ag bilayers are not affected by oxygen at all. Obviously, the Ag monolayer on W(110) opens a very effective reaction channel for the rapid dissociation of O 2 and a subsequent site exchange between Ag atoms and atomic oxygen.

Vibrational modes of atomic oxygen on W(110)

Abstract The initial stages of dissociative oxygen adsorption at room temperature on W(110) have been studied by high-resolution electron energy-loss spectroscopy (HREELS) for coverages up to saturation. We resolve the energies h ω of the stretching mode and the wagging mode associated with the different oxygen overlayers characterized as 2D-gas ( h ω = 66 meV , 46 meV ), (2 × 1) − 0.5 ML ( h ω = 72 meV , 48 meV ), (2 × 2) − 0.75 ML ( h ω = 79 meV , 50 meV ), and (1 × 1) − 1.0 ML ( h ω = 82 meV , 53 meV ), respectively. Our data are consistent with the quasi-threefold hollow position at all coverages.

Quantization of electron states in ultrathin xenon layers

Abstract Physisorbed Xe was grown at temperatures T ⩽ 55 K in a layer-by-layer mode on Ag(111) and Cu(100). High-resolution normal-emission photoelectron spectra from the Xe 5p valence levels show clear evidence for a quantization of the electron states, depending on the layer thickness. Their energies and their numbers can be explained within a very simple potential well model. We have also determined values for the electron mean free path in solid xenon from the attenuation of core level photoemission intensities.

Surface anharmonicity: temperature dependence of dipole active vibrations on Ag(110) and Ag(110)(2 × 1)O

Abstract The temperature dependent energy shifts of dipole active surface vibrations on the Ag(110) and the Ag(110)(2 × 1)O surfaces have been studied by high resolution electron energy loss spectroscopy (HREELS). For Ag(110) the energy of the surface phonon resonance around 13 meV decreases by 3.4 meV and the intrinsic width increases from 1.1 to 3.4 meV in the temperature range between 160 and 550 K. For Ag(110)(2 × 1)O the energy of the oxygen stretching vibration around 41 meV decreases by 0.9 meV and that of the surface resonance by 0.8 meV. The measured effects are related to the anharmonicity of the potentials between the vibrating atoms. Our results indicate that on the Ag(110) clean surface the anharmonicity is larger by about a factor of two as compared to the bulk, while bulk-like anharmonicity is observed after completion of the (2 × 1)O reconstruction. These values are in agreement with those observed on Cu(110) by Baddorf and Plummer [Phys. Rev. Lett. 66 (1991) 2770] and on Cu(110)(2 × 1)O by Durr and Baddorf [J. Electron Spectrosc. Relat. Phenom. 64/65 (1993) 691].

Influence of electron-phonon interactions on angle-resolved photoelectron spectra from metals

Abstract The influence of temperature on angle-resolved valence-band photoemission spectra from metals is not well understood. Despite considerable progress in recent years, both experimentally and theoretically, there is no profound understanding of how the electron-phonon coupling enters the experimental data. We present an improved description of temperature effects as revealed in peak intensities and line-widths. The basic constituent of this model is the incorporation of emission and absorption of phonons by the photoelectron. It has been applied to recent high-resolution data from bulk transitions in copper and a promising agreement could be obtained.

Upper limits for d-hole inverse lifetimes in copper

Abstract From high-resolution angle-resolved photoemission spectra we have derived upper limits for the d -hole lifetime-width Г h in copper. For bulk band direct transitions with initial state energies E i between -3.6eV and the Fermi energy E F , we find Г h -3 eV -1 ( E i - E F ) 2 . We discuss implications of this result for the extraction of quasi-particle lifetimes for photoemission spectra.

Diffusion of Ag on Cu(110) and Cu(111) studied by spatially resolved UV-photoemission

We have studied diffusion by mass transport from ultrathin silver islands deposited epitaxially on Cu(110) and Cu(111) at room temperature. Lateral intensity profiles were obtained with a photoelectron spectrometer which has a spatial resolution in the micron range. Analysis of these profiles as a function of annealing temperature and annealing time allows numbers for diffusion constants and activation energies to be derived.

Atomic chemisorption of chlorine on Ag(110) studied by high-resolution electron energy loss spectroscopy

We have studied atomic chemisorption at room temperature of chlorine on Ag(110) using high-resolution electron energy loss spectroscopy (HREELS), supplemented by XPS and LEED. The ClAg vibration energy (around 25 meV) and the line-width of this loss peak show well resolved variations with both chlorine coverage and substrate temperature T. The observed shift with T is related to the anharmonicity of the potential. Based on the Morse potential we derive an anharmonicity parameter xa = 6.2 × 10−2 for the (2 × 1)Cl-overlayer. This indicates that the anharmonicity is enhanced by about a factor of two as compared to the bulk. In contrast, we find xa < 0.2 × 10−2 for c(4 × 2)Cl. By comparison to other data we conclude that the (2 × 1)Cl-phase is a simple overlayer, with no significant reconstruction of the topmost substrate layer.