The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

Wavevector-resolved electron-energy-loss spectroscopy on the dangling-bond states of Si(111)(2 × 1)

Low-energy-loss spectra of the clean cleaved Si(111)(2 × 1) surface with wavevector resolution are discussed. Parallel to the γ-J line in the surface Brillouin zone large disperson in the dangling-bond bands is found near the energy gap at the J-K′ line. This is in contradiction to the formerly accepted buckling model and favours the recently suggested π-bonded chain model for the (2 × 1) reconstruction.     

High resolution electron-energy-loss spectroscopy of vibrational states of absorbates on (111) tungsten

Electron-energy-loss spectra are reported for adsorbed species on a (111) tungsten surface following exposure to gaseous H₂, D₂ and C₂H₂ at 300 K. Vibrational frequencies are resolved which are characteristic of the chemical and surface structural nature of the adsorbates. Dissociative chemisorption occurs in all cases at low coverage (?1 L exposure). At high coverages, acetylene shows complex coverage-dependent chemisorption behaviour. A transition from dissociative to molecular chemisorption occurs in the range 2 to 4 L exposure with corresponding spectral and work function changes. Fundamental, overtone and combination frequencies are resolved, which serve to identify a single surface molecular complex at high coverage. A comparison of the results with the vibrational excitation energies of free molecules indicates a high-coverage surface molecular complex in which significant rehybridization of the C atom has occurred.     

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.     

Mn/PbTe(111)界面行为的光电子能谱研究

利用X光电子能谱(XPS)对Mn在PbTe(111)表面上沉积生长的界面性质进行了研究.研究表明Mn的沉积使衬底发生了原子尺度上的突变及金属/半导体界面的形成.从X光电子能谱的芯态能级峰来看,随着Mn膜的沉积Pb 4f峰的低结合能端出现了金属Pb的特征新峰,而Te 3d峰的高结合能端却出现了MnTe特征新峰.且随着Mn膜厚度的增加这些新峰变得越来越明显,当Mn膜厚度超过7 ML(monolayer)(即超过Pb,Te的探测深度)时,衬底信号峰完全消失,只剩下金属Pb和MnTe的芯态能级峰.Mn膜厚度继续增 关键词： PbTe半导体 界面形成 光电子能谱 偏析     

Samarium valence changes and reactive interdiffusion at the Si(111)-Sm interface

Synchrotron radiation photoemission studies show that the Si(111) 2×1-Sm interface has a complex morphology involving sequential formation stages with different Sm valence states. For metal coverages up to 2–3 Å, Sm atoms appear only in a divalent state. At higher coverages (up to 10–15 Å) the trivalent Sm configuration dominates and large chemical shifts of Si 2p and Sm 4f and 5p levels are observed. The Sm valence change is related to the onset of the reactive interdiffusion of Sm and Si.