1. Institute of Applied Physics; Justus-Liebig-University Giessen; Heinrich-Buff-Ring 16 35392 Giessen Germany
2. Institute of Physical Chemistry; Justus-Liebig-University Giessen; Heinrich-Buff-Ring 58 35392 Giessen Germany
3. Laboratory of Measurement Technology; Saarland University; Campus A5 66123 Saarbrücken Germany
4. Département de Physique; Ecole Polytechnique; Route de Saclay 91128 Palaiseau Cedex France
5. Institute of Theoretical Physics; Freie Universität Berlin; Arnimallee 14 14195 Berlin Germany
6. Department of Chemistry; University of Paderborn; Warburger Straße 100 33098 Paderborn Germany