1. Applied Physics Department and Center for Ultrafast Optical Sciences, University of Michigan, Ann Arbor, Michigan 48109
2. FEI Company, Hillsboro, Oregon 97124, and Fontana Laboratories, The Ohio State University, Columbus, Ohio 43210
3. Electron Beam Analysis laboratory, University of Michigan, Ann Arbor, Michigan 48109
4. Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109
5. Electron Microscopy Center, Argonne National Laboratory, Argonne, Illinois 60439
6. EECS Department and Center for Ultrafast Optical Sciences, University of Michigan, Ann Arbor, Michigan 48109