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Femtobiology & Femtochemistry | Theory & Computation | Video

Breaking Boundaries


As with stop-motion photography and flash stroboscopy, in 4D imaging the molecular motion has to be resolved into frames using a sequence of flashes, in our case the probing electron pulses. However, 4D electron imaging demands the marriage of ultrafast probing techniques with those of conventional microscopy and diffraction, as well as the development of concepts describing the simultaneous temporal and spatial resolutions of atomic scale. Since the first reports from this laboratory, the technical and theoretical machinery had to be further developed, and currently at Caltech there are five table-top instruments for studies of gases, condensed matter and biological systems. Examples of the experimental setups for UED, UEC and UEM are displayed below. The conceptual framework of the approach is as follows. Upon the initiation of the structural change by either heating of the sample, or through electronic excitation induced by ultrashort laser pulses, a series of electron pulses is used to probe the specimen with a well-defined time delay. An electron diffraction pattern or a micrograph is then obtained. A series of images recorded at a number of delay times provides a direct insight into the temporal evolution of the structure. Isolated reactions are studied by using collisionless molecular beams (UED), while crystals and surfaces are examined either in the reflection or transmission mode (UEC). For microscopy, the electron beam typically penetrates a nm-scale sample (UEM). Because electrons strongly interact with air, the 4D imaging measurements are usually performed in vacuum.

Central to these approaches is the generation and propagation of ultrafast coherent electron packets in space and time (see abstracts below). Although the spatial resolution can reach the atomic scale, imaging must take into consideration the coherence volume of a single electron and the nature of the contrast function in the pulsed mode. The temporal resolution is typically limited by the probing-pulse width and by the difference in group velocities of electrons and the light used to initiate the dynamical change. With tilted optical pulses we can now reach limits of time resolution down to regimes of fs and, possibly, as. The paradigm shift was the realization that imaging can be achieved using timed and coherent single-electron packets, which are free of space-charge effects. Images develop in about the same time as that of an N-electron pulse, but now the time resolution is under control. Increasing the number of electrons in the pulse can, in principle, result in a single-pulse imaging, especially suited for irreversible processes that result in permanent damage. The ever-ongoing research in the areas of laser and electron optics is expected to provide a solid ground for further experimental developments in our laboratory.

Selected Publications

4D electron microscopy: Imaging in space and time, A. H. Zewail, J. M. Thomas, Imperial College Press, London, 2010.
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4D attosecond imaging with free electrons: Diffraction methods and potential applications, P. Baum, A. H. Zewail, Chem. Phys. 2009, 366, 2-8.
[Web link] [Abstract] [Top of page]

Temporal lenses for attosecond and femtosecond electron pulses, S. A. Hilbert, C. Uiterwaal, B. Barwick, H. Batelaan, A. H. Zewail, Proc. Natl. Acad. Sci. USA 2009, 106, 10558-10563.
[Web link] [Abstract] [Top of page]

Ultrashort electron pulses for diffraction, crystallography and microscopy: Theoretical and experimental resolutions, A. Gahlmann, S. T. Park, A. H. Zewail, Phys. Chem. Chem. Phys. 2008, 10, 2894-2909.
[Web link] [Abstract] [Top of page]

Attosecond electron pulses for 4D diffraction and microscopy, P. Baum, A. H. Zewail, Proc. Natl. Acad. Sci. USA 2007, 104, 18409-18414.
[Web link] [Abstract] [Top of page]

Breaking resolution limits in ultrafast electron diffraction and microscopy, P. Baum, A. H. Zewail, Proc. Natl. Acad. Sci. USA 2006, 103, 16105-16110.
[Web link] [Abstract] [Top of page]

4D ultrafast electron diffraction, crystallography, and microscopy, A. H. Zewail, Annu. Rev. Phys. Chem. 2006, 57, 65-103.
[Web link] [Abstract] [Top of page]

Diffraction, crystallography, and microscopy beyond three dimensions: Structural dynamics in space and time, A. H. Zewail, Phil. Trans. R. Soc. A 2005, 364, 315-329.
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The uncertainty paradox: The fog that was not, A. H. Zewail, Nature 2001, 412, 279.
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Chemistry at the uncertainty limit, A. H. Zewail, Angew. Chem., Int. Ed. Engl. 2001, 40, 4371-4375.
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Femtosecond transition-state dynamics, A. H. Zewail, Faraday Discuss. Chem. Soc. 1991, 91, 207-237.
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