1. Contribution from the European Synchrotron Radiation Facility, Grenoble Cedex 38043, BP 220, France, National Creative Research Initiative Center for Time-Resolved Diffraction, Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Republic of Korea, and Laboratoire de Physique, Théorique des Liquides, Université Pierre et Marie Curie, Case Courrier 121, 4 Place Jussieu, Paris Cedex 75252, France