The detailed understanding of matter, its phase transitions and its interaction with radiation could be only reached, after its microscopic structure determined by the kind of atoms or molecules as basic constituents of matter had been investigated. This knowledge allowed the controlled optimization of characteristic properties of matter.Atomicphysicsthereforerepresentsnotonlyanareaofimportantfundamental research, but has furthermore many applications which have essentially formed our present technical world. The understanding of materials and their use in daily life, has major impact of our culture and our attitude towards nature and our environment. This textbook is aimed as an introduction to the microscopic world of atoms, - lecules and photons. Itillustrates howourknowledge about themicroscopic structure of matter and radiation came about and which crucial experiments forced an ext- sion and re?nement of existing classical theories, culminating in the development of quantum theory, which is now accepted as the basic theory of atomic and molecular physics. The book therefore starts with a short historical review about the role of - periments for correcting erroneous ideas and proving the existence of atoms and molecules. The close interaction between experiments and theory has been one of the th th reasons for the rapid development of atomic physics in the 19 and 20 centuries.
