Spin-resolved photoemission by circularly polarized light from Au on Ag(111): experiment and theory

The electronic structure of 1 ML and 2 ML Au on Ag(111) has been studied experimentally and theoretically by spin-, angle- and energy-resolved photoemission in the highly symmetrical set-up of normally incident circularly polarized VUV radiation and normal electron emission. The measured spectra agree rather well with their relativistically calculated counterparts and can be consistently interpreted with the aid of spin- and layer-resolved densities of states. The spin polarization of spectral peaks directly reflects the double group symmetry type of the the corresponding initial states. For 1 ML (2 ML) Au on Ag(111) there are two (four) Au-derived states, which are confined to the adlayers (“film states”). For 2 ML Au, the “film-state” photoemission intensity depends strongly on the photon energy, which is understood as an interference effect. Since the agreement with the experimental photoemission spectra is improved by calculations assuming an additional Ag layer on top, and since the Au-derived experimental features are substantially broadened, an intermixing of Au and Ag is very likely.     

Spin-resolved photoemission from Ag(111): Theory and experiment

Using circularly polarized synchrotron radiation between 14 and 24 eV, spin-resolved normal photoemission spectra have been measured from an unreconstructed Ag(111) surface. Corresponding spectra were calculated by means of a fully relativistic one-step theory of photoemission together with the bulk band structures for real and complex potentials, using two different local approximations for the exchange-correlation potential. Experiment and theory employing anX potential agree well with regard to existence and positions of peaks. Relative peak heights match except for an observed enhancement at photon energies, at which two or more direct interband transitions may resonate.     

Spin-resolved photoemission study of in situ grown epitaxial Fe layers on W(110)

Spin- and angle-resolved photoemission spectroscopy of in situ grown epitaxial Fe layers on W(110) shows bulk-like behavior for more than two atomic Fe layers. For about ten and more atomic layers of Fe we find a spin polarization to be about -100% near the Fermi energy and +80 % between 1 eV and 3 eV binding energy. For the bilayer of Fe drastic changes in the spin-resolved spectra and a 20 % enhancement of the spin polarization compared to the bulk value are observed. The monolayer of Fe is ferromagnetically ordered with a spin polarization reduced by about 50%. A switching of the easy magnetization axis from [001] to [11̄0] is observed in the spin polarization with decreasing Fe layer thickness near d = (65±5) Å.     

Coverage dependence of neopentane trapping dynamics on Pt(111)

Adsorption probabilities for neopentane on Pt(111) were measured directly using supersonic molecular-beam techniques at coverages ranging from zero to monolayer saturation, incident translational energies between 18 and 110 kJ mol⁻¹ and incident angles between 0° and 60° at a surface temperature of 105 K. The adsorption probability was found to increase with coverage up to near monolayer saturation at all incident translational energies and incident angles. The coverage dependence of the adsorption probability predicted by a modified Kisliuk model with enhanced trapping into the second layer exhibits good quantitative agreement with the experimental values. The angular dependence of the adsorption probability decreases with increasing coverage, suggesting that the effective corrugation of the gas–surface interaction potential increases with the adsorbate coverage. The initial adsorption probability into the second layer onto the covered surface decreases from 0.95 to 0.75 with increasing energy over the energy range studied, and exhibits total energy scaling. A comparison with second-layer trapping data of simpler molecules onto covered Pt(111) indicates that the structural complexity of adsorbed neopentane molecules facilitates collisional energy transfer during adsorption.     

Thickness dependence of the spin- and angle-resolved photoemission of ultrathin,epitaxial Ni(111)/W(110) layers

The thickness dependence of the magnetic band structure of ultrathin, epitaxial Ni(111)/W(110) layers has been studied by spin and angle-resolved photoemission spectroscopy. The changes of the spin-resolved photoemission intensities upon reducing the layer thickness depend strongly on the wavevector along the -L line of the Brillouin zone. The measured exchange splitting atk 1/3(-L) andk 1/2(-L) is found to be independent of the layer thickness for layers consisting of 3 or more atomic layers, while decreases rapidly with the layer thickness atk2/3(-L). This behavior is very similar to the temperature dependence of the spin-resolved photoemission spectra of bulk Ni(111) at the samek-points.     

Temperature dependence of Pt(111) surface relaxation

A careful investigation of the Pt(111) surface has been carried out, using the MeV heliumscattering technique. The anomalously large surface relaxation effect reported previously has been identified as an experimental artifact resulting from an unexpectedly large surface damage effect at low temperature. Optimum conditions have been established for minimizing the main experimental sources of error: background subtraction, radiation damage, and deviations from the Rutherford scattering law. Using these optimum conditions, a series of scattering measurements has been made over the temperature range 40–300 K. At all temperatures, we observe a significant anisotropy in the 〈111〉 angular scans, indicating an outward relaxation of the Pt(111) surface plane. By comparing this observed anisotropy with a set of Monte Carlo simulations, a value of 0.03 ? 0.01 Å (i.e. 1.3 ? 0.4%) is obtained for the surface relaxation. The temperature dependence of the surface peak also indicates that the enhanced vibrational amplitude of the surface atoms is not nearly as large as had been derived previously from high-temperature LEED studies.     

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.     

Core level photoemission study of Au deposited on Pt(111) in the submonolayer range

A photoemission study, using a synchrotron source, on the deposition of submonolayer amounts of Au on Pt(111) has been performed. The Au 4f emission shifts continuously to higher binding energies for increasing Au coverages. A good fit to the experimental spectra has been obtained by only one Doniac-Sunjic function. We discuss the initial versus final state effects in photoemission experiments on small supported particles.     

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.     

AES,photoemission and work function study of the deposition of Cs on (100) and (111)B GaAs epitaxial layers

Work function, photoemission and AES measurements have been made on (100) and (111)B epitaxial GaAs layers as a function of caesium coverage. It has been shown that the photoemission maximum and work function minimum occur at a coverage of $\text{2}\text{3}$ of a monolayer. The reduction of the work function takes place in distinct stages with changes in the rate of fall occurring at $\text{1}\text{8}$ and $\text{1}\text{2}$ of a monolayer for the (111)B face and $\text{1}\text{8}$, $\text{1}\text{4}$, $\text{1}\text{3}$ and $\text{1}\text{2}$ of a monolayer for the (100) face. This indicates the presence of specific adsorption sites which have not been observed in previous work on these faces.     

Observation of high-energy Pt(111) surface resonances excited by laser-generated XUV radiation

The surface electronic structure of platinum is investigated by photoemission using extreme ultraviolet light. High-order harmonics of amplified femtosecond Ti:sapphire radiation in the range h=20 to 50eV are employed. In the normal emission spectra of Pt(111), three well-distinguished maxima at fixed kinetic energies of E_kin=10,12.5 and 19eV are observed. These structures, which are far above the Fermi energy, show a strong dependence upon adsorption of CO. This observation and layer-resolved photoemission calculations indicate the surface character of the involved unoccupied platinum states. These empty states are populated by the primary photo-excited electrons, which decay into the unoccupied states due to inelastic scattering. The pronounced emission of the electrons originates mainly from these surface resonances. PACS 79.60.-i; 42.65.Ky; 71.20.-b; 79.20.Hx; 79.60.Dp     

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     

Ultraviolet photoemission studies of acetylene and ethylene adsorptions on the Pt(111) surface: Correlations with LEED studies

The chemisorption of acetylene and ethylene on platinum (111) surfaces for T ≥ 300 K has been studied with ultraviolet photoelectron spectroscopy (UPS) at 21.2 eV. An activated metastable-stable acetylene transition observed recently in low-energy electron diffraction (LEED) intensity-energy profiles has been seen with the UPS spectra. The upperlying electronic levels of the metastable acetylene state are related to a shifted gas-phase acetylene spectrum. The stable acetylene state appears to involve a stronger molecule-surface interaction and probable rehybridization, consistent with the LEED analysis showing the molecule to be situated in a triangular position at covalent Pt-C distances. Ethylene is founf to dehydrogenate at room temperature to the stable acetylene species on Pt(111) surfaces.     

Strain analysis of epitaxial ultrathin films on Pt(111)

X-ray photoelectron diffraction (XPD) measurements have demonstrated that Ni ultrathin films on Pt(111) exhibit a single domain fcc stacking pseudomorphic to the substrate, with a consequent trigonal distortion of the Ni unit cell in order to accommodate the in-plane expansion of the Ni lattice parameter of about 11% with respect to its bulk value. We show that the amount of interlayer contraction and the strain energy resulting form the trigonal distortion are very well predicted by a strain analysis in the framework of simple linear elasticity theory. Strain analysis also allows to discuss in some detail the controversial case of Co growth on the same substrate. Finally, we discuss the dependence of the strain energy of the overlayer on the substrate crystal orientation and its effects on chemisorption experiments performed on heteroepitaxial monolayers (HML).     

A photoelectron spectroscopy study of ultra-thin epitaxial alumina layers grown on Cu(111) surface

Thin epitaxial alumina layers were grown on the Cu(111) surface using simultaneous aluminum deposition and oxygen exposure. Different substrate temperatures during the deposition resulted in layers with different thicknesses, growth rates, crystallinity and epitaxy. Low energy electron diffraction measurements confirmed the epitaxial growth for substrate temperatures above 870 K. The Al 2p doublet was studied by means of photoelectron spectroscopy in order to determine the alumina termination at the metal-oxide interface. A strong dependence on the preparation temperature was found and both aluminum and oxygen terminated interfaces were created.