Acknowledgments

First of all I would like to take this opportunity to thank my parents Wilfried and Regina Wuttke for their constant love and support. Furthermore, I want to thank my deceased grandfather Walter Habel for his love and great support, too. I wish to express my appreciation to Dr. Harald Dorn for supervising my thesis work. During the last year he was always patient and available to answer my questions. His concern for my progress far exceeded what any student could expect from a supervisor. Prof. Jan Plefka permitted me to graduate in his group and served as the second reviewer of my thesis. I also wish to thank him supporting my work. Thanks also to Dr. George Jorjadze for helpful conversations and comments. The whole group offered a congenial atmosphere for work. Additionally, I would like to express my gratefulness to colleagues of mine and especially to Markus Hihn and Christian Schön for their moral support, friendship and inspiring conversations about various topics. Thank you to Tina Pietsch for proof reading. Finally, I want to express my appreciation to my dear friend Kati Pietsch for her love, patience and support. Contents