1. Department of Chemistry, University of California, Irvine, California 92697-2025, United States
2. Max Planck Institute for the Structure and Dynamics of Matter, Center for Free Electron Laser Science, Luruper Chaussee 149, Hamburg 22761, Germany
3. Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, United States
4. Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada