Electron stimulated desorption of CO molecules from chemisorbed CO layers on Ru(001): a comparison of the distributions of translational and internal energies

The translational and internal energies of CO desorbed by electron impact from well-ordered × layers on Ru(001) are investigated with time-of-flight techniques and resonance enhanced multiphoton ionization (REMPI) spectroscopy. We discriminate four desorption channels with different translational and internal energy distributions. Translational and vibrational energies are positively, crrelated; rotational and vibrational–translational energies are negatively correlated. The microscopic origins of these correlations are discussed.     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

Photoemission study of the Li/Ge(1 1 1)-3 × 1 reconstruction

In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)-3 × 1 reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1 reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the and directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the direction. In the direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)-3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge 3d core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge 3d bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)-3 × 1 case, no double bond between Ge atoms in the top layer was found.     

Angle-dependent ultraviolet photoelectron spectroscopy of sputter cleaned polycrystalline noble metals

Polycrystalline noble metal films are commonly used in practical applications across a variety of different fields. However, the surface electronic structure of the noble metals has primarily only been studied on single crystal substrates. In addition, sputter cleaned polycrystalline noble metal films are commonly used substrates in ultraviolet photoelectron spectroscopy (UPS) studies, but have yet to be systematically studied in terms of their photoemission anisotropy. The angle-dependence of the valence band spectra of sputter cleaned polycrystalline Au, Ag and Cu were studied using angle-resolved UPS. It is found that the photoemission is anisotropic with respect to photoelectron take-off angle. The results for Ag and Cu are in good agreement with previous reports of surface d-band narrowing in polycrystalline noble metal films. However, significant anisotropies in the d-band, s-band and Fermi edge of sputter cleaned Au are observed, which cannot be attributed to surface d-band narrowing alone. The unusual results for Au are attributed to drastic changes in the film morphology near the surface as a result of sputter cleaning.     

Impact of interface geometric structure on organic–metal interface energetics and subsequent films electronic structure

We present the electronic structure of various pentacene thin films grown on Au(1 1 1), Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. A systematic variation of the metal surface such as the substrate metal and its surface symmetry allows a comprehensive discussion on the correlation between the electronic structure and the interface geometric structure. In the monolayer regime, we observed the evidence of the formation of the organic–metal interface state depending on the metal surface, i.e., the interface geometric structure. This evidence is explained by the different organic–metal and intermolecular interactions, which originate from the hybridization of the molecular orbitals with the metal wavefunction. These interface geometric and electronic phenomena can be a seed for the subsequent film growth and resultant films electronic structure.     

Surface photochemistry: from hot reactions to hot materials

The field of surface photochemistry has been rather fruitful for two major reasons: its contribution to the understanding of surface dynamics and its importance in the fabrication of surface materials. On the fundamental side, instead of addressing electronic transitions, it is my attempt here to illustrate the importance of surface photochemistry in understanding dynamics of the ground electronic states, those which are generally associated with thermal reactions. In the world of materials fabrication, I would like to point out the unique advantages of surface photochemistry over gas-phase photochemistry or thermal chemistry in the stringent control of ultra-thin films and ultra-small dimensions, i.e. nanotechnology. Both aspects of surface photochemistry are demonstrated using selected examples from my laboratory.     

Photoexcited processes at metal and alloy surfaces: electronic structure and adsorption site

Photoexcited processes of NO and CO at photon energies ranging from 2.3 to 6.4 eV are investigated on Pt(111), Ni(111) and Pt(111)---Ge surface alloys by reflection-absorption infrared spectroscopy and resonance-enhanced multiphoton ionization. The branching between three competitive processes of desorption, recapture and dissociation upon laser irradiation is dramatically changed on the three surfaces. On Pt(111), NO is either photodesorbed or photodissociated depending on the coverage, while NO is exclusively photodissociated on Ni(111). UV-photon irradiation of NO on Pt(111)---Ge, on the other hand, induces only desorption of NO. Desorption of CO bound at the on-top site of Pt(111) is induced by laser irradiation. The electronic mechanism for photodesorption and competitive branching is discussed in terms of the electronic structure of the substrate and the adsorbate.     

On the choice of a regulator for the extraction of depth profiles from ARXPS data obtained on plasma-treated polystyrene

Regularized oxygen concentration depth profiles were extracted from ARXPS data obtained on a plasma-oxidized polystyrene sample, using a variety of regulators and over a wide range of values for the regularization parameter. The plausibility of the profiles obtained was assessed with respect to strong and weak criteria based on a priori considerations. It was found that the most plausible results were obtained with regulators based upon the slopes or the curvatures exhibited in the profile.     

Correct application of Fresnel’s equations for intensity analysis of angle-resolved photoemission data

The electromagnetic field relevant for the excitation process in angle-resolved photoemission is studied. We show that Fresnel’s equations together with the known bulk dielectric constants can be used to calculate the complex vector potential at the metal surface. A model is developed which accounts correctly for the special experimental geometry with focused light. It is used to calculate the variation of photoemission intensity with changing light incidence angle and polarization. Experimental data for the photoemission intensity as a function of light incidence angle are presented for direct transitions out of bulk, surface and adsorbate states at a Cu(1 1 0) surface. The comparison to our model shows that the application of copper bulk optical constants is justified even when electronic states are localized to the topmost atomic layer.     

Angular distributions of desorbing N2 in thermal N2O decomposition on Rh(1 0 0)

The angular distribution of desorbing N₂ was studied in the decomposition of N₂O(a) on Rh(1 0 0) at 60-140 K by means of angle-resolved temperature-programmed desorption. N₂ desorption shows two peaks at around 80 K and 110 K. At low N₂O coverage, the former collimates far from the surface normal toward the [0 0 1] direction, whereas at high coverage, the desorption sharply collimates along the surface normal. The adsorption form of N₂O and its dissociation were also examined by DFT-GGA calculations. Dissociating N₂O is proposed to be lying along the [0 0 1] direction at low coverage and to change to an upright form bonding through the terminal oxygen at high coverage.