1. J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, CZ-18223 Prague 8, Czech Republic, Physics Department, Lancaster University, Lancaster, LA1 4YB, United Kingdom, Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences, Konkoly-Thege M. út 29-33, H-1121 Budapest, Hungary, and Department of Biological Physics, Eötvös Loránd University, Pázmány Péter sétány 1/A H-1117 Budapest, Hungary