Surface physics has experienced in the last decades an explosive expansion caused by the development and/or substantial improvement of surface sensitive techniques and UHV apparatuses. It has grown into a mature field of research, with data of the highest accuracy and reproducibility. Surface physics is of great importance for technological applications like field effect devices, molecular beam epitaxy, chemisorption and catalysis, corrosion, surface hardening etc. The present is restricted to the so-called "clean" surfaces, i.e. to surfaces atomically clean and well characterized, leaving the more complex field of the contaminated surfaces and overlayers to a later occasion.weiterlesen