Atomic layer structrure analysis by super-sonic molecular beam techniques.

在这个项目中，我们建立了一个新的超声波分子束机器，它能够研究化学反应的动力学以及分析各种表面的结构。利用氦原子散射（HAS）技术研究了甲烷和乙烷分子在Pt（111）表面上的解离吸附。结果表明，甲烷和乙烷等具有足够高能量的烷烃分子在Pt（111）表面上通过中间体解离吸附，在表面温度高于600 K时，中间体分解为碳和氢，中间体通过甲基分解为乙撑基。重氮化反应过程占主导地位的直接已知分解成碎片时，他们与金属表面碰撞。在这种碰撞过程中，碰撞分子通过在固体中产生声子（直接非弹性碰撞通道）以及通过激发其自身分子的内部模式（内部模式激发通道）来耗散其动能。在甲烷分子的动能范围为300 - 700 meV的情况下，我们对Pt（111）表面反射分子的飞行时间（TOP）测量没有显示出内模激发的迹象。结果表明，直接非弹性碰撞可以用经典的硬立方体模型定性地解释。在LiF（100）这样的粗糙表面上，Washboard模型更适合于解释碰撞。

In this project we have built a new super-sonic molecular beam machine which is capable of studying the dynamics of chemical reactions as well as analyzing the structure of various surfaces. The energy dispersion of 6% and the angular resolution of less than 0.50 of our machine enable us to determine the energy - momentum dispersion relation of the surface phonon generated at the LiF(100) surface.Helium Atom Scattering (HAS) technique is used in studying dissociative adsorption of methane and ethane molecules at a Pt(111) surface. It has been revealed that such alkane molecules as methane and ethane with high enough energy dissociatively adsorb on the Pt(111) surface via intermediates which decompose into carbon and hydrogen at the surface temperature above 600K.The intermediate is found to be ethyldine moiety via methyl moiety.Molecules with such high kinetic energy that Eley-Redeal reaction process is dominant directly are known to decompose into fragments when they collide with metal surfaces. During such collisions, impinging molecules dissipate their kinetic energy by creating phonon s in the solid (direct inelastic collision channel) as well as by exciting internal modes of their own molecules (internal mode excitation channel). In the case of methane molecules with kinetic energy ranging from 300 to 700 meV, our time of flight (TOP) measurements of reflected molecules from a Pt(111) surface have shown no sign of internal mode excitation.Based on TOF and angular intensity distribution measurements of reflected methane molecules from such a flat surface as Pt(111), it is found that direct inelastic collision can be qualitatively explained by the classical Hard Cube Model. It is also found that Washboard model is more appropriate to explain the collision at rough a rough surface such as LiF(100).