1. National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland, Department of Physics, Trinity College Dublin, Dublin 2, Ireland, Facultes Universitaires Notre-Dame de la Paix, Laboratoire de RMN, 61 Rue de Bruxelles, B-5000 Namur, Belgium, and Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, H1525 Budapest, P.O. Box 49, Hungary