1. FOM Institute for Atomic and Molecular Physics (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, Department of Condensed Matter and Interfaces, Debye Institute, Utrecht University, Princetonplein 5, 3508 TA Utrecht, The Netherlands, and Institute of Chemistry and the Center for Nanoscience and Nanotechnology, the Hebrew University of Jerusalem, Jerusalem 91904, Israel