R. Mitsumoto

Core hole effect in NEXAFS spectroscopy of polycyclic aromatic hydrocarbons: Benzene, chrysene, perylene, and coronene

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS), indicating significant core-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which affects both transition energy and intensity. During the course of this analysis, the core-hole effect in benzene was also analyzed. It was found that the magnitude of the core-hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final stat...

SOFT X-RAY ABSORPTION, UV PHOTOEMISSION, AND VUV ABSORPTION SPECTROSCOPIC STUDIES OF FLUORINATED FULLERENES

In this paper, chemical bonding and electronic structures of fluorinated fullerenes: C60Fx and C70Fx have been investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy1 and UV photoemission spectroscopy (UPS), which are useful methods for examining unoccupied and occupied states, respectively. With these results, we could derive an energy diagram for C60Fx. Electronic absorption spectroscopy of C60Fx solution was also measured and compared with theoretical simulations.

Polarized VUV absorption and reflection spectra of oriented films of poly(tetrafluoroethylene) (CF2)n and its model compound

Polarized vacuum-UV absorption and reflection spectra were measured for oriented films of polytetrafluoroethylene (PTFE) (CF2)n and its model compound perfluorotetracosane (PFT) n-C24F50. The lowest energy excitation leading to an intense peak was found to be polarized along the molecular chain, and its energy decreases with the chain length (8.05 eV for PFT and 7.66 eV for PTFE). The polarization and chain-length dependence are discussed with the aid of information by theoretical calculation, NEXAFS, and electron transmission spectroscopy.

Fluorination of fullerene C60 and electrochemical properties of C60Fx

Abstract Fluorinated fullerene polycrystalline powders C 60 F x have been obtained by direct fluorination of C 60 at 100°C. Electrochemical behaviors of C 60 and C 60 F x were investigated by the cyclic voltammetry and galvanostatic discharges on Li/1M LiClO 4 PC/C 60 and C 60 F 4 cells. The electronic structure changes of C 60 upon fluorination were also investigated by XANES and UPS. Three stepwise reversible reduction peaks (2.50, 2.15 and 1.85 V), and an irreversible reduction peak (2.8–3.2 V) corresponding to CF reduction were observed on the cyclic voltammograms. The fluorination of C 60 renders the open circuit voltage (OCV) higher by 1 V (OCV = 4 V vs. Li reference electrode), and the reversible reduction potentials nobler by 0.15 V. These electrode behaviors reflect the changes in the C 60 electronic structure through fluorination.

Polarized XANES studies on the mechanical rubbing effect of poly(tetrafluoroethylene) and its model compound

Abstract The structural change of the evaporated perfluorotetracosane (PFT) n-CF3 (CF2)22CF3 and poly(tetrafluoroethylene) (PTFE) (CF2)n films by the mechanical rubbing process was examined with use of XANES spectroscopy. In the PTFE film the chains were almost parallel to the surface and were uniaxially realigned along the rubbing direction, while in the PFT film the chains after evaporation were oriented normal to the surface, and this orientation was hardly affected by the rubbing process. The chains of the PFT evaporated on the rubbed PTFE film aligned along the rubbing direction.

Surface analysis of CVD carbon using NEXAFS, XPS and TEM

Abstract Chemical and morphological characterization of carbon thin films produced by chemical vapor deposition (CVD) of hydrocarbon gases onto a hard carbon substrate has been performed using various surface analytical techniques. Semiquantitative analyses of the surface concentration of C C double bonds have been conducted using near edge X-ray absorption fine structure (NEXAFS) at the C K edge and core level X-ray photoelectron spectroscopy (XPS). NEXAFS spectra of CVD carbon films are similar to those of well-ordered graphite. Rather surprisingly, however, it has been revealed that the CVD carbon films contain less C C double bonds than that of the hard carbon. Intensities of π-π * shake-up satellite of the C 1s XPS peak have been measured, and the results were consistent with those of NEXAFS. Smaller concentrations of C C double bonds can be attributed to the incomplete dehydrogenation due to the relatively low temperature of the CVD treatment. High resolution transmission electron microscope (TEM) images of CVD carbon showed preferential orientation of graphite-like plane units in the direction parallel to the surface, suggesting the formation of large molecules in gas phase reaction prior to deposition.

Polarized NEXAFS studies on the mechanical rubbing effect of poly(tetrafluoroethylene) oligomer and its model compound

The structural change of the evaporated perfluorotetracosane (PFT) n-CF3(CF2)22CF3 and poly (tetrafluoroethylene) (PTFE) (CF2)n films by the mechanical rubbing process was examined with use of NEXAFS spectroscopy. In the PTFE film, the chains were almost parallel to the surface and were uniaxially realigned along the rubbing direction, while the chains in the PFT film after evaporation were oriented normal to the surface, and the orientation was hardly affected by the rubbing process. The chains of the PFT evaporated on the rubbed PTFE film aligned along the rubbing direction.

NEXAFS studies on the temperature dependence of surface structure of solid and liquid pentacontane (n-CH3(CH2)48CH3)

The surface molecular orientation of pentacontane (n-CH3(CH2)48CH3) film evaporated on Cu substrate was studied with NEXAFS (Near Edge X-ray Absorption Fine Structure) spectroscopy at various temperatures. The surface molecular orientation of the film was found to be changed by raising temperature from room temperature to just below the bulk melting point, and the pre-transitional ordered structure at the surface of the liquid phase was observed even above the bulk melting point.

Crystal Structure of C60F x

Abstract Fluorinated C60, C60F x (20<x<45), in which the fluorine atoms are bound to the carbon atoms, form a face-centered cubic lattice with a 0=17.1 A.

X-ray absorption near-edge structure (XANES) spectroscopy of fullerenes: inner-shell excitonic effects in fullerenes and the XANES spectrum of a higher fullerene C76

Abstract X-ray absorption near-edge structure (XANES) spectra of fullerenes C60, C70 and C76 were measured at the K-edge of carbon. Comparison of XANES and inverse photoemission spectra of C60 and C70 revealed that the core excitonic effects is small, indicating the usefulness of XANES spectroscopy for probing the density of unoccupied states (DOUS) of fullerenes. The variation of the magnitude of the core excitonic effect among various delocalized π-electron systems is discussed in relation to the dimensionality and size of the systems. The observed XANES spectra were compared with theoretically calculated DOUS.