Establishment of the soft X-ray energy dispersive XAFS method and its applications to surface chemistry

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The purpose of this project is to improve the energy-dispersive NEXAFS method which we have developed both in energy resolution and measuring time and to apply this method to surface chemistry, surface magnetism and surface dynamics.The activities during the project are as follows ;(1)CO oxidation reaction on Pt(111). The dispersive NEXAFS method was used to trace the change of the O and CO coverages during the CO exposure on O pre-covered Pt(111). Detailed reaction rate analysis and Monte Carlo simulations have given two new findings. One is that there is another reaction path of CO oxidation as well as that proposed with the previous STM measurement Second is that CO adsorption has an effect to control the oxygen diffusion and the total reaction cannot be explained by a simple LangmuirHirishelwood reaction.(2)Water formation reaction on Pt(111). The catalytic water formation reaction was investigated by the energy dispersive NEXAFS. An oxygen covered Pt(111) surface with the (2x2) st … More ructure was exposed to gaseous hydrogen (5.0x10x9 Torr) at constant surface temperatures (120-140 K). O K-edge NEXAFS spectra were measured during the reaction with a time interval of 35 s. Quantitative analyses of the spectra provided the coverage changes of the adsorbed species (O,OH, and H_2O). The reaction is composed of three steps, which are characterized by an induction period (I), fast increase in coverage of OH and H_2O with consuming O (II), and slow conversion of OH to H_2O after the complete consumption of O (III). It was also found that the maximum OH coverage becomes smaller at a higher temperature. The kinetic Monte Carlo simulation has reproduced the three characteristic reaction steps ; in the first step OH domains are created through 2D aggregation of H_2O (I), after the nucleation process the second step sets in where the OH domains propagate by the autocatalytic cycle until they contact with each other (II), and finally the merged OH domains convert to H_2O (III). The Reaction Diffusion method was also applied to this system. It explained the reaction behavior in a wide surface area.(3)NO+N reaction on Ph(111) : By applying the dispersive NEXAFS, method, we succeeded in observing a reaction intermediate, N_2O. We also found an interesting reaction behavior : the reaction rate increases by decreasing the substrate temperature in the region of 100-370 K.(4)The adsorption structure of methylthiolate (CH_3S) self-assembled on Au(111), a long-standing controversial issue, has been unambiguously determined by scanned-energy and scanned-angle S 2p photoelectiMn difrraction. The methylthiolate molecules are found to occupy atop sites with a S-Au distance of 2.42 °A. The angular distribution of the S 2p photoelectrons due to forward scattering reveals that the S-C bond is inclined by approximately 50± from the surface normal towards both the [^-211] and [^-12^-1] (nearest-neighbor thiolate) directions.(5)A depth-resolved technique in the x-ray magnetic circular dichroism (XMCD) has been devebped and applied to Fe/Cu(100) and Fe/Ni/Cu(100) in order to observe the magnetic depth profile directly. It was confirmed that the surface two layers of the 7 ML Fe/Cu(100) are ferromagnetically coupled, while the inner layers are in the spin density wave (SDW) state at 130 K. The technique enables to extract the XMCD spectra from the surface ferromagnetic (FM) and inner SDW regfons separately, indicating that the FM/SDW interface has an antiparallel magnetic coupling, and that the SDW region has the bulk-like feature. For Fe/Ni/Cu(100), we have observed magnetically live surface layers and some thickness-dependent magnetic coupling between the Fe surface and Ni film. Less