Agradecimentos: I would like to thank very Prof. Ulrik Gelius for having accepted me as a PHD student in his laboratory and for his friendship and guidance. I want also to thank Doc. Svante Svensson who have been a friendly supervisor. Prof. Hans Agren is acknowledged for his stimulating guidance. I am to thank Doc. Leif Karlsson for his always polite and smart answers to an endless number of question. On the Brazilian side I would like to express my great thanks to Nestor who motived me to come to Uppsala and to change to experimental physics (muito obrigado). Amary is also acknowledged. CNPq is acknowledged for financial support. I would like to thank Mike who was a true friend during the work and outside working hours. Gareth is acknowledged for his assistance in the computer and for his friendship. The friends from surface science group : Olle, Sandell, Helena, Bosse, Peter and Erik are thanked for the warm and relaxing atmosphere in the laboratory. All other members of the Fysik I are acknowledged. Mario is thanked for the computer aid. Hans Olliver is acknowledged for his very good computer assistance with the "ladies" Alma and Alice. Many thanks to Jan-Olof Forsell for his assistance with the machine The electronic engineers Hans Rydaker, Patrik Fargerudd and Per Nicoloasen are also thanked. Lars Bolkegard is acknowledged for his help in the workshop. I would like to express my acknowledgement to Prof. Seppo Aksela and Prof. Helena Aksela, for my participation in the H2S project. It was a great pleasure to work with them. M. Y. Adam is also acknowledged for the discussions from which I have learned a lot. Many thanks to Bo Sjögren for stimulating cooperation. Last but not least I would like to thank my wife Lurdinha. Her dedication and love have made possible this work to can trough Ver menos