Auger-Electron-Probe Time-Resolved Surface XAFS Technique and Its Application to Study of Surface Dynamics

我们发展了一种快速的表面x射线吸收精细结构(XAFS)技术，它用能量分散的同步辐射x射线照射固体衬底，并以空间分辨的方式立即收集从x射线吸收表面物种发出的俄歇电子。在这个项目中，我们的目标是进一步发展这项技术，并将其应用于固体基质上的化学反应的机理研究。目前的改进使我们能够每隔大约1个S同时追踪表面和气相物种的变化。如果将这项技术应用于可重复的过程，我们可以通过使用探测器的时间门来检测亚单层表面物种，时间分辨率为10ms。此外，我们将这一技术扩展到基于波动器的时间分辨x射线光电子能谱，基于这些技术实现了对表面反应动力学的实时监测，取得了以下三大研究成果：(1)在Rh上NO还原反应条件下，发现了以NO二聚体为反应中间体的反应途径；(2)阐明了在Pd上CO氧化反应条件下，表面氧的相变与反应活性的关系；(3)NO诱导的寿命小于毫秒量级的瞬时高密度吸附状态的检测脉冲分子束表面物种快速观测技术的发展及其在表面动态过程中的应用，为研究以前的静态表面技术未能探测到的新的动态过程开辟了一条新的途径。

We have developed a fast surface x-ray absorption fine structure(XAFS)technique where solid substrates are irradiated by energy-dispersed synchrotron-radiation x rays and auger electrons emitted from x-ray absorbing surface species are collected at once with a spatially resolved way. In this project we aimed further development of this technique and its application to mechanistic studies on chemical reactions on solid substrates. The present improvement enabled us to trace changes in both surface and gas-phase species simultaneously every approximately 1 s. If this technique is applied to repeatable processes, we can detect submonolayer surface species with a time resolution of 10 ms by using a time gate for the detector. Furthermore we extended this technique to the undulator-based time-resolved x-ray photoelectron spectroscopy, which enabled us to observe dynamic processes of submonolayer surface species with a time resolution of 2 ms.Real-time monitoring of surface reaction kinetics based on these techniques provided following three major achievements in research that have never been achieved by previous methods.(1) Finding of a novel reaction pathway involving a NO dimer as a reaction intermediate under the reaction condition of NO reduction on Rh(2) Elucidation of cirrelation between phase transition of surface oxygen and reactivity under the reaction condition of CO oxidation on Pd(3) Detection of a transient high-density adsorption state with a life time of less than millisecond order induced by NO pulsed molecular beamThe present development of the fast observation technique for surface species and its application to surface dynamic processes open a way to study novel dynamic processes that have not been detected with previous static surface techniques.