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Published February 3, 2004 | Published
Journal Article Open

Ultrafast electron crystallography: Transient structures of molecules, surfaces, and phase transitions


The static structure of macromolecular assemblies can be mapped out with atomic-scale resolution by using electron diffraction and microscopy of crystals. For transient nonequilibrium structures, which are critical to the understanding of dynamics and mechanisms, both spatial and temporal resolutions are required; the shortest scales of length (0.1–1 nm) and time (10^–13 to 10^–12 s) represent the quantum limit, the nonstatistical regime of rates. Here, we report the development of ultrafast electron crystallography for direct determination of structures with submonolayer sensitivity. In these experiments, we use crystalline silicon as a template for different adsorbates: hydrogen, chlorine, and trifluoroiodomethane. We observe the coherent restructuring of the surface layers with subangstrom displacement of atoms after the ultrafast heat impulse. This nonequilibrium dynamics, which is monitored in steps of 2 ps (total change <=10 ps), contrasts that of the nanometer substrate. The effect of adsorbates and the phase transition at higher fluences were also studied through the evolution of streaks of interferences, Bragg spots (and their rocking curves), and rings in the diffraction patterns. We compare these results with kinematical theory and those of x-ray diffraction developed to study bulk behaviors. The sensitivity achieved here, with the 6 orders of magnitude larger cross section than x-ray diffraction, and with the capabilities of combined spatial ({approx}0.01 Å) and temporal (300–600 fs) resolutions, promise diverse applications for this ultrafast electron crystallography tabletop methodology.

Additional Information

© 2004 National Academy of Sciences. Contributed by Ahmed H. Zewail, December 15, 2003. We thank Professor N. Lewis and Ms. L. Webb for functionalizing the silicon surface. This work was supported by the National Science Foundation. Some support was also provided by the Air Force Office of Scientific Research. F.V. acknowledges partial financial support from the Swiss National Science Foundation, and S.C. was supported by a Millikan fellowship at the California Institute of Technology.

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