1. Institute of Physics, The Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8, Czech Republic
2. Microelectronics and Materials Physics Laboratories, University of Oulu, P. O. Box 4500, FI 90014, Finland
3. Department of Chemistry and 4D LABS, Simon Fraser University, 8888 University Drive, Burnaby, B.C., V5A 1S6, Canada