Yoshinori Kitajima

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.

Surface structures and electronic properties of SO2 adsorbed on Ni(111) and Ni(100) studied by S K-edge X-ray absorption fine structure spectroscopy

Surface structures and electronic properties of submonolayer sulfur dioxide, SO2, molecularly adsorbed on Ni(111) and Ni(100) at ∼ 170 K have been investigated by means of S K-edge surface extended X-ray-absorption fine structure (SEXAFS), near-edge X-ray-absorption fine structure (NEXAFS) and S 1s X-ray photoelectron spectroscopy (XPS) measurements. NEXAFS and SEXAFS have clarified that on both surfaces the SO2 molecule adsorbs with the molecular plane parallel to the surface and the elongated intramolecular SO bond lengths. The S atom directly interacts with substrate Ni, located at the bridge sites on both surfaces. Such adsorption geometries for submonolayer SO2 on metal surfaces have never been reported before. Comparing to other SO2metal systems the determined surface structures are discussed in terms of chemical bonding.

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.

Structural studies of adsorbed alkanethiols on Cu(111) by use of S and C K-edge X-ray absorption fine structures

Surface structures of submonolayer alkanethiol CH3(CH2)n−1SH (n=6,12) adsorbed on Cu(111) at room temperature (0.3 ML) were investigated by means of S K-edge surface extended X-ray absorption fine structure (EXAFS) and S, C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. Polarization dependence of the C K-edge NEXAFS spectra revealed that alkanethiol molecules adsorb with the tilt angle of 12 ± 10° from the surface normal. The S K-edge surface EXAFS determined that the SCu distance is 2.31 ± 0.02 A and that the S atoms locate at a deep three-fold hollow site with significant lateral outward movements of the nearest neighbor Cu atoms.

Adsorption of SO2 on Cu(100) studied by X-ray absorption fine structure spectroscopy and scanning tunneling microscopy

Abstract The room temperature adsorption of SO2 on Cu(100) was studied using S K-edge X-ray absorption fine structure spectroscopy, Auger electron spectroscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy (STM). It is known that the disproportionation reaction 3SO2(a)→S(a) + 2SO3(a) occurs on the Cu(100) surface. The surface coverages of the reaction product adsorbates as well as their respective geometric structures are reported. XAFS modeling calculation results indicate that the atomic S adsorbs in the fourfold hollow site and trigonal pyramidal SO3 adsorbs with the sulfur away from the surface. Thus, the three oxygen atoms are directly bonded to the Cu substrate. STM images show the coexistence of two different phases of the (2 × 2) and c(4 × 6) domains. The (2 × 2) unit cell contains one SO3 species but no S atoms. In contrast, the c(4 × 6) unit cell contains one SO3 and two S atoms. The presence of such domains implies a drastic migration of the reaction products on the surface.

Spectrometer for polarized soft X-ray Raman scattering

An experimental system for polarized soft X-ray Raman scattering spectroscopy has been constructed. The soft X-ray spectrometer is based on the Rowland circle geometry with a holographic spherical grating. Three types of gratings are used to cover the energy range from 18 eV to 1200 eV. According to a ray-trace simulation, the resolution is expected to be 200 meV at 700 eV by using a 10 micro m slit width. The polarized and depolarized soft X-ray Raman scattering spectra can be measured by rotating the soft X-ray spectrometer around the axis of the incident beam. Preliminary measurements of polarized and depolarized spectra were accomplished at undulator beamline BL-2C of the Photon Factory.

Design of a Holographically Recorded Plane Grating with a Varied Line Spacing for a Soft X-ray Grazing-Incidence Monochromator.

A new design concept is presented for a plane grating with a varied line spacing for the Monk-Gillieson mounting monochromator. A light path function including both a spherical mirror and a varied-line-spacing grating is defined to optimize groove parameters. Aspheric wavefront recording optics are utilized to fabricate a grating holographically. Ray-tracing results show that the varied-line-spacing grating eliminates aberrations significantly and affords a high resolving power as a total optical system of a soft X-ray grazing-incidence monochromator. The effects of errors in recording parameters and in the radius of the spherical mirror are described, and possible ways to compensate for these errors are discussed.

Structural and electronic properties of adsorbed C4H4S on Cu(100) and Ni(100) studied by S K-XAFS and S-1s XPS

Abstract Surface structures and electronic properties of submonolayer thiophene C4H4S adsorbed on Cu(100) (0.07 ML, 220 K) and Ni(100) (0.10 ML, 43 K) have been investigated by means of S K-edge surface extended X-ray-absorption fine structure (SEXAFS), near-edge X-ray-absorption fine structure (NEXAFS) and S 1s X-ray photoemission spectroscopy (XPS) measurements. It is revealed that the C4H4S molecules adsorb with the molecular plane parallel to the surface and the S atom locates at the bridge site on both surfaces. The distances of S-substrate atoms, and of the SC bond in C4H4S, as well as the degree of charge transfer from the substrate to the C4H4S π ∗ orbital were determined. These results indicate that C4H4S interacts more strongly with the Ni substrate than with the Cu substrate.

Surface structure of SO2 adsorbed on Ni(110) studied by S K-edge X-ray absorption fine structure spectroscopy

Abstract Surface structure and electronic properties of submonolayer sulfur dioxide SO2 molecularly absorbed on Ni(110) have been investigated by means of S K-edge surface-extended X-ray absorption fine structure (SEXAFS), near-edge X-ray absorption fine structure (NEXAFS) and S 1s X-ray photoemission spectroscopy (XPS) measurements. The SO2 molecule was found by SEXAFS and NEXAFS to adsorb with the molecular plane parallel to the surface and an elongated SO bond distance compared to that of a free molecule. The S atom was revealed to locate on the long- and short-bridge sites with almost equal probabilities. No azimuthal-angle dependence was detected as for the SO bond direction. A significant amount of charge transfer from the substrate to the SO2 π∗ orbital was found by NEXAFS and XPS. Such structural and electronic properties are essentially similar to those of SO2 on Ni(100) and Ni(111).

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.