A. Cavalleri^*, C. W. Siders^†, C. Rose-Petruck^‡, R. Jimenez^§, Cs. Tóth^∥, J. A. Squier, C. P. J. Barty^¶, and K. R. Wilson^**

• The University of California San Diego, 9500 Gilman Drive, La Jolla, California, 92093-0339

K. Sokolowski-Tinten, M. Horn von Hoegen, and D. von der Linde

• Institut für Laser- und Plasmaphysik, Universität Essen, D-45117 Essen, Germany

• ^*Present address: Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA. Email: acavalleir@lbl.gov
• ^†Present address: The School of Optics/CREOL, The University of Central Florida, Orlando, Florida.
• ^‡Present address: Department of Chemistry, Brown University, Providence, Rhode Island.
• ^§Present address: The Scripps Research Institute, La Jolla, California.
• ^∥Present address: The Center for Beam Physics, Lawrence Berkeley National Laboratory, Berkeley, California.
• ^¶Present address: Lawrence Livermore National Laboratory, Livermore California.
• ^**Deceased.