Introduction

The principle motivation of natural sciences is the desire to learn about the nature of matter and about its interaction with forces, with the main aibeing to understand fundamentally where we all come from and what it is that keeps nature going. In this century, scientists have not only set off to investigate the macroscopic world of the universe as a whole, they have also succeeded in penetrating the microscopic world to learn more about the structure of molecules, of atoms and their nuclei, and they even break up nuclei to find the most elementary building blocks of matter. A major breakthrough for a detailed understanding of the microworld occurred in the twenties when the concepts of quantum mechanics were introduced. They mark the beginning of the victorious evolution of spectroscopic techniques which are today the most widespread tools to investigate the microworld Thereby, matter is subjected to electromagnetic radiation spanning a range of more than ten orders of magnitude in wavelength between X-rays and radio waves. The response of matter to these waves is often very specific, so that it allows detailed insight into its structure and dynamics.