6.1.1.1 Definitions and historical layout

This chapter provides an introduction of elastic scattering and diffraction of electrons and positrons. Of the various techniques devoted to the study of crystalline surfaces, those dealing with the scattering of charged particles are the most widely used. In particular, the elastic scattering of electrons and positrons have been accepted as one of the principal tools for obtaining crystallographic data, i.e. for finding out the exact positions occupied by atoms in the first atomic layers. The chapter also focuses on various techniques like low-energy electron diffraction or elastic low-energy electron diffraction (LEED or ELEED), very low-energy electron diffraction (VLEED), medium-energy electron diffraction (MEED), reflection high-energy electron diffraction (RHEED), low-energy positron diffraction (LEPD), polarized and spin polarized LEED (PLEED and SPLEED) for interaction of charged particles with surfaces. They are all based on similar mechanisms of interactions of the particle with the solid, but their specific use covers different, and often complementary, aspects of the physics of surfaces. With the development of the UHV technology, able to keep surfaces clean for a long enough period to allow experimental observations, diffraction techniques emerged as the most promising means to study the solid surfaces and the gas-solid interaction.