1. Dipartimento di Chimica IFM, Università di Torino and NIS - Nanostructured Interfaces and Surfaces - Centre of Excellence, Via P. Giuria 7, 10125 Torino, Italy, and Institut für Mathematische Physik, TU Braunschweig Mendelssohnstraβe 3, D-38106 Braunschweig, Germany