《固体的表面与界面》期末考试作业姓名：刘继琼学号：200907120002 翻译章节：第三章翻译页码：32-42Chapter 3Electron spectroscopy电子能谱3.1 Introduction3.1 引言If we want to learn something about a system, a general experimental approach is a scattering technique: we shoot some particles in a well-prepared state on the target and look at particles coming out of the target (which do not have to be the same). In surface science the most basic questions we want to solve with this approach are for example: Is the surface clean? Which elements are on the surface? And in which chemical compound? What is the exact geometric structure of the surface?如果我们想要了解一个系统，通常的实验方法是采用散射技术：我们向靶发射一些处于特定状态的粒子，观察它们从靶出射的情况（各个粒子出射情况不同），在表面科学中我们想通过这种方法解决的最基本的问题有：表面是否清洁？哪种元素处在表面？以及它们的化学组成？表面的精确的几何结构是什么样的？The most common particles to scatter from surfaces are electrons, ions, atoms and photons both as probe and response particles. An important issue is the surface sensitivity of an experiment. In general, it is high if we choose particles which have a small mean free path in the solid because this means that the detected particles must originate near the surface. The opposite is true, for example, when the scattering of light by the surface is investigated(reflectivity and change of polarization). The photons will penetrate relatively deeply into the crystal. The amount of photons scattering at or near the surface be very small. Hence, light scattering is not a good tool to study surfaces. In some cases we can increase the surface sensitivity by choosing an experimental set-up where we use a very grazing angle of incidence or emission. In this way the particles travel a long way close to the surface, even if their mean free path is relatively long.从表面上散射出来的最常见的粒子是电子，离子，原子和声子，它们即作为探测粒子又作为反应粒子。
一个重要的因素是实验的表面灵敏度。
通常，如果当我们选择的微粒在固体中有一个小的平均自由程时表面灵敏度很高，因为这就意味着探测粒子必须在表面附近产生。
反之亦然，例如，研究表面光的散射（折射率和偏振变化）。
光子将相对较深的深入固体。
在表面及其表面附近散射的光子数量就会很少。
因此，光子散射不适宜用来研究表面性能。
有时候，我们可以选择合适的入射角和出射角来提高表面灵敏度。
这样，及时粒子的平均自由程很长，它们也会在表面附近运动很长的一段距离。
surface sensitivity-表面灵敏度mean free path-平均自由程Very many surface science techniques are based on electrons as a probe. Electrons have very useful properties: they are, at certain energies, very surface sensitive. Electrons in this energy range carry also enough momentum to explore the whole surface Brilloin zone of a material(in contrast to light), they also carry a spin and they are easy to generate and to handle. The extensive use of electrons in surface sciences justifies a lecture explaining the physics of electron-solid in some more detail. Along with this, we will start to learn about some electron-based analytical techniques.很多表面科学技术都是基于电子作为探针进行探测的。
电子有非常有用的性质:他们在一定的能量范围内有很高的表面敏感度。
这些能量范围内的电子有足够的动能来探测材料的整个布里渊区表面（不同于光子），它们伴随自旋并且容易产生和操控。
电子在表面科学中的广泛的运用更详细的证明了一篇报告中所解释的电子固体。
同时，我们将开始学习一些基于电子的分析技术。
Brilloin zone-布里渊区A technique which is of particular interest in this lecture is Electron Energy Loss Spectroscopy (EELS) where a beam of monochromatic electrons is scattered from the surface. A sketch of this experiment is given in Fig.3.1在这个报告中特别提到的一种技术是电子能量损失能谱法，该技术采用的是从表面散射出的一束单色电子，实验示意图如图3.1所示Fig3.1: An EELS experiment. The momentum transfer parallel to the surface is determined by the electron energy and the scattering geometry.图3.1：一个能量损失能谱法的实验。
平行于表面传递的能量有电子能量和几何外形共同决定。
3.2 Why electrons: The mean free path3.2 为什么用电子：平均自由程One of the main reasons to use electrons in surface science is the mean free path of electrons in matter. This mean free path is determined by collisions:用电子作为表面科学的研究的一个主要原因是电子的平均自由程。
这个平均自由程可有电子间的相互碰撞计算出来：,)(）（ττλm kE v E kin kin == (3.1)Where v is the velocity and τ is the collision time. In the Drude model τ is the mean time between two scattering events. In a quasiparticle-picture τ is given by the imaginary part of the self-energy, i.e. by the life time of the quasi-particle. We are interested in energies of electrons between a few eV and many hundred eV. The mean free part of the electrons in this regime is plotted in Fig.3.2. The dashed curve shows a calculation of the mean free path independent of the material and the points are measured data from many elemental solids. The data points scatter more or less around the calculation. The curve is therefore often called a universal curve . The reason for this universality is that the inelastic scattering of electrons in this energy range is mostly involving excitations of conduction electrons, which have more or less the same density in all elements. Note that at lower energies other scattering mechanisms will be important, like the scattering with phonons.其中，v 是电子的速度，τ 是碰撞时间。