时间相关离子散射谱(TDISS)用于研究气体覆盖度

THE APPLICATION OF TIME-DEVELOPED ION SCATTERING SPECTROSCOPY ON THE MEASUREMENT OF GAS COVERAGES ON SAMPLES IN UHV

本文介绍一种新的在超高真空装置内测量样品上气体覆盖度的方法:时间相关离子散射谱(TDISS).作者使用100 eV的He^+离子入射,测量了不同暴露量的氧、CO、丙烯和氢在Rh(111)表面上的覆盖度.使用前三种气体的实验取得了满意的结果,可以迅速地获得吸附等温线形式的气体暴露量和覆盖度间的关系曲线;氢覆盖度的测量效果不理想,原因在于入射离子对其强溅射作用.文中分析了这一方法的优缺点,并预示了下一步工作的方向,即将此法的应用推广到多晶、薄膜、粉末样品及多组元样品.

A new method, Time-Developed Ion Scattering Spectroscopy ( TDI-SS ) , was utilized to measure the coverage of gases on samples in ultra-high vacuum systems( UHV). As well known coverage data are very important for catalysis and surface science researches, but few means can be reliable enough to give precise values conveniently. Despite ISS has long been used to investigate the interaction between adsorba-tes and substrate with its unique monolayer -detecting ability, the preferential sputter of the incident ions to the adsorbates often prevents its being used as a good tool for coverage measurement.In this work the measured energy of the scattered ious was fixed at that corresponding to Rh to record the ISS signal in a time-based way during gas chemisorption, resulting in symptotic sorption isotherms. From the data it can be found that the TD1SS curves of adsorbates and substrate are mirror-symmetric, so that the signal of the substrate, which is two orders of magnitude stronger than that of the adsorbate, can be used to deal with the sorption process.With a TDISS curve, the height at the beginning represents the amount of surface Rh atoms on clean surface, while during chemisorption the height of the curve represents that of Rh atoms still accessible to incident ions. Therefore the height difference between the ISS signals of clean and covered Rh surfaces was correspondent to the amount of covered Rh atoms, and the coverage was defined as the ratio between this differ- ence and the tSS signal of clean surface.Moreov er in TD1SS the energy of incident ions used can be reduced to 100eV which was considered as not suitable for ISS, so that the penetrating and sputter effects of the ions are well attenuated. Altogeehrt the coverages of four gases, namely oxygen, CO, propene and hydrogen, on a Rh ( 111 ) single crystal were studied, of which the first three gave satisfactory results: within minutes an isotherm was drawn,which tells the relation between coverage and exposure. For hydrogen the measurement was not successful, as the sputter effect of the incident ions, in spite of their being only 100 eV. interfered the surface to be stably equilibrated. Experimentally the incident angle of the ion beam should be small to minimize the influence of the ''shadowing cone' of the adsorbate atoms to bare Rh atoms. It is expected that in the future the application of this method can be extended to the study of polycrystal, thin film and even powdered samples for both mono- and multimetallic catalysts.