1. Department of Physics and Astronomy, Laboratory of Surface Modification, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854-8019, USA
2. Charged Particle Optics Group, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, Netherlands
3. Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA