1. Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala, Sweden, Center of Molecular Devices, Physical Chemistry, Royal Institute of Technology, Teknikringen 30, SE-10044 Stockholm, Sweden, Fysikum, AlbaNova University Center, Stockholm University, SE-106 91 Stockholm, Sweden, and Center of Molecular Devices, Organic Chemistry, Royal Institute of Technology, Teknikringen 30, SE-10044 Stockholm, Sweden