1. Applied Physics, GAP, University of Geneva, 22 Ch. de Pinchat, 1211 Geneva 4,
Switzerland
2. Institute of Applied Physics, University of Bern, Sidlerstr 5, 3012 Bern,
Switzerland
3. Department of Physics, Institute for Quantum Electronics, ETH-Zurich, 8093 Zurich,
Switzerland
4. Department of Physics, University of Zurich, Winterthurerstr 190, 8057 Zurich,
Switzerland
5. Physical Chemistry Laboratory, ETHZ,
Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland