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In situ synchrotron radiation photoelectron spectroscopy was performed during the oxidation of the Ge(100)-2 x 1 surface induced by a molecular oxygen beam with various incident energies up to 2.2 eV from the initial to saturation coverage of surface oxides. The saturation coverage of oxy-gen on the clean Ge(100) surface was much lower than one monolayer and the oxidation state of Ge was +2 at most. This indicates that the Ge(100) surface is so inert toward oxidation that com-plete oxidation cannot be achieved with only pure oxygen (O2) gas, which is in strong contrast to Si surfaces. Two types of dissociative adsorption, trapping-mediated and direct dissociation, were confirmed by oxygen uptake measurements depending on the incident energy of O2. The direct ad-sorption process can be activated by increasing the translational energy, resulting in an increased population of Ge2+ and a higher final oxygen coverage. We demonstrated that hyperthermal O2 beams remarkably promote the room-temperature oxidation with novel atomic configurations of ox-ides at the Ge(100) surface. Our findings will contribute to the fundamental understanding of oxygen adsorption processes at 300 K from the initial stages to saturated oxidation. [ABSTRACT FROM AUTHOR]

Highlights: [•] We report the Cu2O formation on Cu3Au(110) with energetic O2 molecular beams. [•] Dissociative adsorption of O2 on Cu3Au(110) was accompanied with Cu segregations. [•] Oxidation-induced Cu segregation is key process to promote the Cu2O formation. [Copyright &y& Elsevier]

The thermal oxidation of Si(0 0 1) surface under 1000 K in the O2 pressure of 1 × 10-4 Pa has been in situ investigated using real-time photoemission spectroscopy with high energy-resolution synchrotron radiation. Using a reaction kinetics model, we found that the oxidation at 1000 K progressed with a two dimensional island growth mechanism involving desorption of SiO molecules. Si2+ species relating to the backbond oxidation at the topmost Si dimers appeared in conjunction with Si1+ species at the initial oxidation stage at 1000 K. We clarified that the topmost Si atoms bonding to two oxygen atoms played an important role as an initial adsorbate. Since Si oxidation states with higher oxidation numbers, such as Si4+ and Si3+, appeared at the early oxidation stages as well, we concluded that SiO2 adsorbates constructed with the Si4+ species were preferentially formed even in the oxide nucleus as well as the two dimensional islands on the Si(0 0 1) surface at 1000 K. [Copyright &y& Elsevier]

We have experimentally studied the dissociative adsorption of O2 on a clean Si(0 0 1)-2 × 1 surface at room temperature from the viewpoint of time resolved photoemission spectroscopy for surface chemical reactions induced by the O2 incident energy. Combining the high-resolution and intense synchrotron radiation with the supersonic molecular beams technique, we have succeeded in the analysis of the time evolution of Si oxidation states (Si1+, Si2+, Si3+ and Si4+) induced by the translational kinetic energy of O2 (EtO2=0.04, 0.6, 2.0 and 3.0 eV). Furthermore, we proposed that the oxidation on a clean Si(0 0 1)-2 × 1 surface progressed in the mechanism of almost consecutive Si oxidation in the low translational kinetic energy region. It was found that the translational kinetic energy of O2 induced the adsorption of oxygen atom at the bridge site between the Si dimer and at the backbonds of the Si dimers and the subsurface. [Copyright &y& Elsevier]

The influence of translational kinetic energy of incident O2 molecules for the passive oxidation of clean Si(0 0 1) surfaces at room temperature has been studied by photoemission spectroscopy using high-resolution synchrotron radiation. The O2 incident energy was controlled up to about 3 eV via supersonic seed molecular beam techniques. Predicted two potential energy barriers for the O2 dissociative chemisorption were indistinct. Photoemission spectra of Si-2p and O-1s as a function of incident energy revealed the incident energy induced oxidation, in which the oxide layers thickness was controlled to the extent of 0.6 nm. [Copyright &y& Elsevier]

Highlights: [•] We report the initial oxidation process of TiAl with a hyperthermal O2 molecular beam. [•] The precursor-mediated adsorption is the dominant initial step of oxidation. [•] Blue colored TiO2 and Al2O3 containing layers was fabricated on TiAl surface. [ABSTRACT FROM AUTHOR]

Abstract: An initial oxidation dynamics of 4H-SiC(0001)-(√3×√3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0001)-(√3×√3)R30° surface was exposed to oxygen molecules with translational energy of 0.5eV at 300K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si–Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation. [Copyright &y& Elsevier]

Dissociative adsorption of hyperthermal O2 molecules on Cu{1 0 0} was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From precise measurements of the O-uptake curves, it was found that the dissociative adsorption of hyperthermal O2 molecules follows a simple Langmuir-type model and the reaction order depends on the incident energy. An O2δ--mediated adsorption mechanism can interpret the incident-energy dependence of the reaction order. These results suggest the possibility of tuning dissociative adsorption via the selection of the incident kinetic energy of the impinging O2 molecules. [Copyright &y& Elsevier]

Oxygen adsorption and subsequent oxide formation on Cu(110) using a hyperthermal oxygen molecular beam (HOMB) were investigated using X-ray photoelectron spectrometry. The O-uptake curves, which were determined from the evolution of the O-1s peaks, indicate that simple Langmuir type kinetics can describe dissociative adsorption of O2with an incident energy (Ei) below 0.5 eV under Θ ≤ 0.5 ML. The reaction order dependence on Eiimplies two competing dissociation mechanisms, trapping-mediated and directly activated adsorption. Oxidation at Θ ≥ 0.5 ML proceeds rather effectively using highly energetic HOMB at Ei≥ 1.0 eV. The azimuthal dependence of the sticking probability during the effective oxidation using HOMB incidence suggests that the added rows, which consist of the Cu−O structure, shade the reactive hollow sites in the trough where oxygen penetrates into the subsurface. The surface Cu2O formed with highly energetic HOMB incidence decomposes with desorbing subsurface oxygen even at room temperature, demonstrating that HOMB can induce a metastable surface structure that cannot be produced in the thermal equilibrium process. [ABSTRACT FROM AUTHOR]

Abstract: Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0001) surface in high coverage region. The Ru(0001) surface pre-covered with oxygen atoms of 0.5monolayer, which corresponds to the p(2×1)-O structure, was dosed to oxygen molecules with translational energy of 0.5eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface. [Copyright &y& Elsevier]