1. R. W. Schoenlein, T. E. Glover, P. Balling, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
2. W. P. Leemans, P. Volfbeyn, M. Zolotorev, K.-J. Kim, S. Chattopadhyay, Accelerator and Fusion Research Division, Center for Beam Physics, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. A. H. Chin, Department of Physics, University of California, Berkeley, CA 94720, USA.
4. C. V. Shank, Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Physics, University of California, Berkeley, CA 94720, USA.