Toshiaki Ohta

Polarized XANES studies of oriented polyethylene and fluorinated polyethylenes

Polarization dependent XANES spectra were obtained by use of synchrotron radiation for oriented polyethylene and fluorinated polyethylenes: (1) elongated polyethylene (PE) film, (2) elongated poly(vinylidene fluoride) (PVDF) film, (3) poly(tetrafluoroethylene) (PTFE) film, (4) oriented evaporated film of hexatriacontane. CH3(CH2)34CH3 as a model compound of PE, (5) oriented evaporated film of perfluoroeicosane, CF3(CF2)18CF3 as a model compound of PTFF. Pronounced polarization dependence was observed for each compound, which allows unambiguous assignments of the XANES spectrum. Fluorination effects on the XANES spectra was also discussed

Electronic structure of poly(tetrafluoroethylene) studied by UPS, VUV absorption, and band calculations

The electronic structure of poly(tetrafluoroethylene) (PTFE) was studied by UPS, VUV absorption, and ab-initio MO calculations. The UPS spectra give a photoemission threshold energy of 10.6eV, with deeper valence band features consistent with the reported XPS and the oligomer vapour UPS spectra. The UPS spectra are also consistent with the density of states derived from the calculated band structure, which indicates that the uppermost part of the valence band is formed from the C and F 2p orbitals with C-C bonding and C-F antibonding combinations. The VUV absorption spectrum shows an intense peak at 7.7eV, which most probably corresponds to the valence excitation from the top of the valence band to the bottom of the conduction band. With these data, the structure of the occupied and vacant states are deduced.

X-ray absorption spectra of poly-p-phenylenes and polyacenes: localization of π orbitals

Carbon K-edge X-ray absorption near edge structure (XANES) spectra of evaporated films of poly-p-phenylenes and polyacenes were measured by use of synchrotron radiation. Only one dominant resonance associated with a Cls-to-π* transition was observed for each poly-p-phenylene, although there should exist several π* unoccupied orbitals which are energetically well separated from each other. These features were characterized by means of semi-empirical molecular orbital (MO) calculations based on the equivalent core approximation, which leads to a satisfactory agreement between the observed and calculated spectra. The calculations show that the π* orbitals, which are delocalized in the ground state, become localized by the creation of a Cls core hole. Spectral features of polyacenes, which are somewhat different from those of poly-p-phenylenes in their π* regions, indicate a lesser degree of π* localization.

Compact fluorescence x‐ray detector for surface EXAFS and x‐ray standing wave measurements

The design and performance of a compact, ultrahigh vacuum (UHV) compatible x‐ray detector is described which can be applied to measurements of surface EXAFS spectra and x‐ray standing wave (SW) profiles. The detector is a type of gas flow proportional counter. Its compactness allows us to save space around the sample. A 25‐μm‐thick beryllium window separates UHV (<1×10−10 mbar) environment from atmospheric pressure and provides good efficiency to detect low energy x rays. Surface EXAFS spectra of less than 0.1 monolayer chlorine on a Ni substrate are successfully measured with high quality. Its high signal‐to‐background ratio allows us to measure SW profiles, which could hardly be obtained with any electron yield detection technique.

Performance of the cooling system for the soft x‐ray double‐crystal monochromator at the Photon Factory

A crystal cooling system was designed and tested successfully for the soft x‐ray double‐crystal monochromator in an ultrahigh‐vacuum environment at the Photon Factory. The temperature rise at the surface of the InSb first crystal is reduced to less than 45 °C at an average stored current of 200 mA. With the help of a control software, the output beam can be fixed in a position for a wide energy range. In this paper, the heating effect on the double‐crystal monochromator, and the design and performance of a water cooling system are described.

Thiophenol adsorption on Ni(100) studied by S K-edge SEXAFS and XANES

Thiophenol (C6H5SH) adsorption on Ni(100) was studied by S K-edge SEXAFS and XANES spectroscopies. It was found that thiophenol physisorbed multilayers which were deposited at ∼ 130 K desorb at ∼ 180 K, leaving a chemisorbed monolayer species which strongly interacts with the Ni substrate. In this stage, the adsorbed species was identified as phenylthiolate C6H5S, and S locates at the 4-fold hollow site with SNi and SC distances of 2.25 ± 0.02 and 1.84 ± 0.05 A, respectively. From the polarization dependence of the XANES spectra, it was revealed that the axis of the SC bond is nearly normal to the surface. With temperature rise, the SC bond is gradually dissociated and atomic S is left on the 4-fold hollow site.

Surface structure of Cl/Ni(111) determined by surface EXAFS spectroscopy and soft X-ray standing-wave method

Abstract The surface structure of ( 3 × 3 )R30°Cl/Ni(111) has been examined by the combination of polarization-dependent surface EXAFS spectroscopy and the soft X-ray standing wave (SW) method. The surface EXAFS analysis revealed that chlorine atoms locate at the three-fold hollow site with the ClNi bond distance of (2.33 ± 0.02) A, corresponding to the spacing between Cl and surface Ni layers of (1.83 ± 0.02) A. The SW measurement allowed the determination of the displacement of Cl; it is located 1.82 A above the Ni(111) lattice plane. Consequently, surface relaxation is not accompanied by the adsorption of 1 3 of a monolayer (ML) of Cl atoms. The magnitudes of surface relaxation induced by S and Cl adsorption on (100), (110) and (111) surfaces are compared and discussed.

Site-selective adsorption of Cl atoms on Ni(111) studied by back-reflection x-ray standing-wave method

Abstract The surface structure of ( 3 × 3 ) R 30° Cl/Ni(111) as investigated using the back-reflection X-ray standing-wave (SW) method. The SW profiles of the Cl atom were measured for 111 and 11 1 reflections by monitoring the ClK fluorescence yield with a Si(Li) solid state detector. The triangulation of the Cl atom position relative to the (111) and (11 1 ) lattice planes allowed us to distinguish the two inequivalent three-fold hollow sites on this surface. The results indicate that the Cl atoms preferably adsorb on the fcc sites which are directly above the atoms in the third substrate layer.

Surface EXAFS study of (√7 × √7) R19.1° S/Cu(111)

The surface local structure of (√7 × √7)R19.1° S/Cu(111) was studied by the polarization-dependent surface EXAFS method. The results clearly indicate the existence of two or more types of copper coordinations around sulfur atoms and they are incompatible with the previous models proposed by LEED experiments. In order to explain the surface EXAFS results, a new model is proposed, where the Cu(111) surface layer is drastically reconstructed with both lateral and vertical movements, and the sulfur atoms are located at three types of 3-fold hollow sites of the reconstructed surface.

Ru L edge x‐ray absorption studies of the electronic structure of Ru3(CO)12 adsorption and the formation of Ru–Cu bimetallics on Cu(111)

Ru L2,3 edge XANES (x‐ray absorption near edge structure) together with AES (Auger electron spectroscopy) and LEED (low energy electron diffraction) studies of Ru3(CO)12 adsorbed on Cu(111) reveal the following: (a) Ru3(CO)12 adsorbs molecularly on Cu(111) at low temperature (≲−50°). (b) Upon electron bombardment, Ru3(CO)12 dissociates to form bare Ru aggregates which exhibit very broad Ru L2,3 white lines and featureless absorption in the XANES spectrum indicating that the Ru atoms are not well ordered on the Cu(111) surface. (c) Annealing of the bare Ru aggregates on Cu(111) at 450 °C results in the formation of Cu/Ru bimetallic clusters in which the Ru atoms are in the core and exhibit bulk‐like Ru L2,3 XANES while the surface is covered with Cu atoms. Analysis of the Ru L3 white line intensity indicates that charge redistribution in the Ru d band occurs upon the transformation from the bare Ru aggregates to the bimetallic Cu/Ru clusters on Cu(111).