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

该项目的目的是改进我们在能量分辨率和测量时间方面开发的能量色散NEXAFS方法，并将该方法应用于表面化学，表面磁性和表面动力学。项目期间的活动如下：(1)CO在Pt（111）上氧化反应。采用分散型NEXAFS法，对O预覆盖Pt（111）在CO暴露过程中O和CO覆盖的变化进行了追踪。详细的反应速率分析和蒙特卡罗模拟给出了两个新的发现。一是除了之前STM测量提出的反应路径外，CO氧化还有另一条反应路径。二是CO吸附有控制氧扩散的作用，整个反应不能用简单的LangmuirHirishelwood反应来解释。(2) Pt（111）上的成水反应。采用能量分散NEXAFS对催化成水反应进行了研究。在恒定的表面温度（120-140 K）下，将具有（2x2） st…More结构的氧覆盖Pt（111）表面暴露于气态氢（5.0 × 10x9 Torr）中。在反应过程中以35s的时间间隔测量O K-edge NEXAFS光谱。光谱的定量分析提供了吸附物质（O，OH和H_2O）的覆盖变化。该反应分为三个步骤，其特点是有一个诱导期(I)，随着O （II）的消耗，OH和H_2O的覆盖率迅速增加，O （III）消耗完后，OH转化为H_2O的速度缓慢。同时发现，温度越高，最大OH覆盖面积越小。动力学蒙特卡罗模拟再现了三个特征反应步骤；在第一步中，OH域通过H_2O的二维聚集(I)产生，在成核过程之后，第二步开始，OH域通过自催化循环传播，直到它们相互接触（II），最后合并的OH域转化为H_2O （III）。反应扩散法也应用于该体系。它解释了大表面积下的反应行为。(3) Ph（111）下NO+N反应：采用分散型NEXAFS方法，成功地观察到了反应中间体N_2O。我们还发现了一个有趣的反应行为：在100-370 K范围内，随着衬底温度的降低，反应速率增加。(4)利用扫描能量和扫描角度s2p光电timn衍射，明确了甲基硫酸盐（CH_3S）自组装在Au（111）上的吸附结构，这是一个长期争议的问题。发现甲基硫酸盐分子占据了S-Au距离为2.42°a的顶部位置。由于正向散射导致的s2p光电子的角分布表明，S- c键向[^-211]和[^-12^-1]（最近邻硫化物）方向倾斜约50±。(5)建立了x射线磁圆二色性（XMCD）深度分辨技术，并将其应用于Fe/Cu（100）和Fe/Ni/Cu(100)，直接观测磁性深度剖面。结果表明，7ml Fe/Cu（100）的表面两层是铁磁耦合的，而内层在130 K时处于自旋密度波（SDW）状态。该技术能够分别从表面铁磁区和内部SDW区提取XMCD光谱，表明FM/SDW界面具有反平行磁耦合，SDW区具有块状特征。对于Fe/Ni/Cu(100)，我们观察到具有磁性的活表面层，并且在Fe表面和Ni薄膜之间存在一些与厚度相关的磁耦合。少

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