Yasutaka Takata

Vibrational and anharmonic properties of the surface S-Ni bonds in c(2 × 2)S/Ni(100) and c(2 × 2)S/Ni(110) from the SEXAFS point of view

Abstract Temperature- and angular-dependent S K-edge SEXAFS (surface-extended X-ray absorption fine structure) spectra of c(2 × 2)S/Ni(100) and c(2 × 2)S/Ni(110), where S locates at the hollow site of the Ni surfaces, were measured and analyzed in order to investigate the static surface structure including surface relaxation, anisotropic mean square relative displacements and surface local thermal expansion. By analyzing the S-Ni coordinations for the first and second layer Ni, we observed noticeable thermal vibration and expansion of the surface S-Ni bonds. Significant differences between the two systems are also found in the vertical S-Ni bonds; in the case of c(2 × 2)S/Ni(100) the vibration between S and the second layer Ni has a much lower frequency and larger anharmonicity than that for the first layer Ni because there is no chemical bond with the second layer, while in c(2 × 2)S/Ni(110) the vibration between S and the second layer Ni exhibits higher frequency and smaller anharmonicity possibly due to a shorter bond length. Although it is commonly believed from diffraction studies that the vertical motion is much larger and more anharmonic than the lateral one, the present study suggests that in terms of a relative motion or a local pair potential surface anisotropic vibration is strongly correlated to the bond distance rather than to the bond direction.

Structural and electronic properties of molecularly adsorbed CS2 on Cu(111) studied by X-ray absorption and photoelectron spectroscopies

Abstract The surface structure and the electronic properties of submonolayer carbon disulphide CS 2 molecularly adsorbed on Cu(111) at 92 K were investigated by means of S K-edge surface extended X-ray absorption fine structure (SEXAFS), near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) measurements. The CS 2 molecule is found by NEXAFS to lie nearly flat and linearly on the surface with a tilt angle of the molecular axis of 8 ± 5°. NEXAFS and XPS suggest that the adsorbed species is in a state close to CS 2− 2 . SEXAFS has clarified the adsorption geometry of the CS 2 molecule, where one S atom locates at the three-fold hollow site and the other S is at the bridge site with a slightly longer S-Cu distance. The determined surface structure is discussed comparing to other molecular adsorbate systems.

Local Structures of Carbon Thin Films Synthesized by the Hot Filament Chemical Vapor Deposition Method X-Ray-Absorption Near-Edge Structure and Raman Spectroscopic Studies

Various carbon thin films synthesized by the hot filament chemical vapor deposition (CVD) method with various methane concentrations in hydrogen (1.0-12.5 vol.%) and temperatures of Si substrate (770-1000°C) were characterized by X-ray-absorption near-edge structure (XANES) and Raman spectroscopies as well as by scanning electron microscopy (SEM) and X-ray diffraction. Based on these results, the structural change of carbon thin films is deduced as a function of methane concentration and substrate temperature. Comparison between the Raman and XANES spectra demonstrates the usefulness of XANES spectroscopy for the characterization of carbon thin films and the complementary character between these spectroscopies.

Surface Structure of (√7×√7) R19.1°S/Cu(111) Studied by Surface EXAFS and Back-Reflection X-Ray Standing-Wave Method

Surface local structure of (√7×√7)R19.1° S/Cu(111) was studied by polarization-dependent surface EXAFS and Back Reflection X-ray Standing-Wave (BRXSW) method. A surface structure model is proposed, which satisfies both surface EXAFS and BRXSW results. It consists of three adsorption sites of sulfur atoms; 3-fold hollow site on the top copper layer, embedded site in the top layer, and 3-fold hollow site on the second copper layer. This model is consistent with the recent STM observation.

Spectroscopy Studies of Temperature-Induced Valence Transition on EuNi2(Si1-xGex)2 around Eu 3d–4f, 4d–4f and Ni 2p–3d Excitation Regions

The resonant photoemission spectra and X-ray absorption spectra of temperature-induced valence transition material EuNi 2 (Si 1- x Ge x ) 2 around Eu 3 d –4 f , 4 d –4 f and Ni 2 p –3 d resonant excitation regions have been measured. From the comparison between the resonant photoemission spectra around the 3 d –4 f (bulk sensitive) and 4 d –4 f (surface sensitive) excitation regions, the Eu divalent component originating from the surface is separated from the bulk one. The divalent and the trivalent features of 4 f electrons are obviously distinguished depending on the excitation conditions. It was confirmed that the intensity ratio of the 4 f electronic structures between the Eu divalent and trivalent ions changes as a function of the temperature. The mean valence values estimated from the Eu M -edge X-ray absorption spectra and from the photoemission spectra were rather smaller than those obtained previously from the L -edge X-ray absorption spectra. The reason of the discrepancy is discussed. The spect...

Strong metal-to-ligand charge transfer bands in Ni 2p photoabsorption of K2Ni(CN)4·H2O

Abstract Polarized Ni 2p photoabsorption spectra were measured for a single crystal of planar (D 4h ) Ni II complex with low-spin 3d 8 configuration, K 2 Ni(CN) 4 ·H 2 O, in comparison with powder spectra of CaNi(CN) 4 and BaNi(CN) 4 . Remarkably strong satellite bands are found besides the lowest Ni 2p–3d * atomic lines. Two features in the satellite show different polarization dependencies, and are assigned to the metal-to-ligand charge transfer (MLCT) transitions to out-of-plane e g π * and in-plane b 2g π * ligand orbitals, respectively, in good agreement with ab initio molecular orbital predictions. The low-lying ligand π * orbital is essential in the metal 2p photoabsorption of 3d transition metal compounds with strong π backbonding.

Adsorption of Thiophene on Ni(100) Studied by S K-edge XANES

Adsorption behavior of thiophene (C4H4S) molecule on clean and oxygen-precovered Ni(100) surface at ~90K has been studied by polarization dependent S K-edge XANES. S1s→π* and σ*(S-C) transitions were symmetrically resolved in the polarizaton-dependent XANES spectra of the molecule with a specific orientation on O-precovered Ni(100), which could not be separated and assigned unambiguously in the spectrum of gaseous molecule. From the polarization dependence of those peaks, it is revealed that the C4H4S molecule does not decompose and lies nearly parallel to the surface on both clean and Oprecovered Ni(100) surface. Comparing the present results with those of thiophenol and carbon disulfide, interaction between adsorbed molecules and substrate metal is discussed.

Sample current maximum at the critical angle of x‐ray total reflection

The x‐ray photoemitted electron intensity and sample current of a GaAs wafer are measured at 2 and 3 keV excitation photon energies while varying the glancing angle of the incident x rays. It is found that the sample current and the x‐ray photoemitted electron intensity have similar behavior with respect to the glancing angle. The sample current as well as the photoemission intensity have a maximum at the critical angle of the x‐ray total reflection. This is the first report of the observation of the existence of a sample current maximum at the critical angle of x‐ray total reflection.

XANES and Raman Spectroscopic Studies of Diamond Films Synthesized by Hot Filament CVD

Diamond films synthesized by hot filament CVD were studied by SEM, X-ray diffraction, and Raman and XANES spectroscopies. All of these methods have confirmed that the films prepared at low methane concentrations (1.0 vol.% and 4.0 vol.% in H2) consist of diamond crystals. In contrast to these, the deposit at a high methane concentration (7.5 vol.%) does not provide an X-ray diffraction pattern or a Raman signal characteristic of diamond, indicating the absence of diamond crystals with long-range order. Nevertheless, the XANES spectrum revealed that this specimen is composed of small diamond clusters.

Strong metal-to-ligand charge transfer bands observed in Ni K- and L-edge XANES of planar Ni complexes

r Polarized Ni K-edge and L-edge XANES spectra were measured for some planar Ni complexes with low-spin 3d ~ ~-I configuration, KzNi(CN)a.H20, Ni(Hdmg)2 (Hdmg: 2,3~.1 butanedione dioximato) and [(n-f4Hg)nN]2[Ni(mnt)2] (mnt: 1,2~] dicyanovinylene-1,2-dithiolato). In all the spectra, strong satellite b a n d s a r e f o u n d i n a d d i t i o n t o t h e l o w e s t N i l s , 2p---~3d*atomJc ~] lines. These are mostly assigned to the metal-to-ligand charge transfer (MLCT) transitions to unoccupied states with dominant c ligand rt* components, in good agreement with ab initio .c molecular orbital predictions. The low-lying ligand n* orbital is essential in interpreting the metal K-edge and L-edge XANES of 3d transition metal compounds.