1. Institute of Chemical Physics, University of Latvia, Raina Blvd. 19, LV-1586 Riga, Latvia
2. Institute of Atomic Physics and Spectroscopy, University of Latvia, Raina Blvd. 19, LV-1586 Riga, Latvia
3. Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Göteborg, Sweden
4. Department of Solid State Physics, Lund University, Lund, Sweden
5. Department of Chemistry and the Tyndall National Institute, University College Cork, Cork, Ireland