The fog that was not

Proc. Natl. Acad. Sci. U.S.A.

2005

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306

226

Electron microscopy is arguably the most powerful tool for spatial imaging of structures. As such, 2D and 3D microscopies provide static structures with subnanometer and increasingly with ång-strom-scale spatial resolution. Here we report the development of 4D ultrafast electron microscopy, whose capability imparts another dimension to imaging in general and to dynamics in particular. We demonstrate its versatility by recording images and diffraction patterns of crystalline and amorphous materials and images of biological cells. The electron packets, which were generated with femtosecond laser pulses, have a de Broglie wavelength of 0.0335 Å at 120 keV and have as low as one electron per pulse. With such few particles, doses of few electrons per square ångstrom, and ultrafast temporal duration, the long sought after but hitherto unrealized quest for ultrafast electron microscopy has been realized. Ultrafast electron microscopy should have an impact on all areas of microscopy, including biological imaging.femtoscience ͉ structural dynamics ͉ ultrafast electron crystallography ͉ ultrafast electron diffraction Electron diffraction has similarly enabled structural determination, with the membrane protein structure of bacteriorhodopsin (5) being determined by using electron crystallography͞ microscopy (6). All these methods provide the equilibrium structure in crystals or the average structure in solution.The motion of atoms in molecular structures occurs on the femtosecond time scale, and it is now possible to observe such coherent atomic motions in systems of various complexities, from the very small (two atoms) to the very large (e.g., proteins) (7). The mapping in time of dynamical trajectories unravels key features of the forces of motion and the associated effective and reduced energy landscape of structural dynamics. The complete structural determination at different times, however, requires the integration of space and time resolutions in 4D characterization of the structural change (8).Transmission electron microscopy (TEM) with its wideranging arsenal of tools has long been a powerful method in many areas of research (9-20), allowing for subnanometer spatial resolution but lacking ultrashort time resolution. Optical microscopy, using fluorescent probes, e.g., green fluorescent proteins (21,22), has provided the means to visualize dynamic events occurring in vitro and within cells. However, despite possessing the requisite temporal resolution, optical methods are limited in their spatial resolution to the wavelengths used, typically 200-800 nm. As pointed out by Mellman and Warren (23), the ultimate techniques would be those that have the spatial resolution of electron microscopy and the time resolution of optical methods. Many of the most important mechanistic questions can be answered if only we had ''molecular video electron microscopy'' (21). As mentioned above, the atomic length scale can be studied with x-ray and electron diffraction, but for biological and nanoscopic materials with characteristic leng...

Section: Discussionmentioning

confidence: 99%

Four-dimensional ultrafast electron microscopy

Lobastov

Srinivasan

Proc. Natl. Acad. Sci. U.S.A.

2005

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306

226

“…First, there should be no concern about the uncertainty principle in limiting the information to be gained from femtosecond-to-picosecond time resolution, as all structures are prepared coherently (2 and references therein; 19,20). Second, for any dynamical process, the change with time is continuous, and although some global events may occur at longer times-so-called "relevant timescales"-these events are triggered by changes at early times.…”

Section: Zewailmentioning

confidence: 99%

4d Ultrafast Electron Diffraction, Crystallography, and Microscopy

2006

Annu. Rev. Phys. Chem.

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497

341

■ Abstract In this review, we highlight the progress made in the development of 4D ultrafast electron diffraction (UED), crystallography (UEC), and microscopy (UEM) with a focus on concepts, methodologies, and prototypical applications. The joint atomic-scale resolutions in space and time, and sensitivity reached, make it possible to determine complex transient structures and assemblies in different phases. These applications include studies of isolated chemical reactions (molecular beams), interfaces, surfaces and nanocrystals, self-assembly, and 2D crystalline fatty-acid bilayers. In 4D UEM, we are now able, using timed, single-electron packets, to image nano-to-micro scale structures of materials and biological cells. Future applications of these methods are foreseen across areas of physics, chemistry, and biology.

“…3 and 4). However, because of the difference in time scales, femtoseconds for nuclear motions and attoseconds for electron motions, consideration of the energy uncertainty must be addressed (5,6). For chemical and biological structures, the femtosecond resolution is ideal, because on this time scale the energy uncertainty is only a small fraction of the intrinsic energy and the average photon energy can be made in resonance with molecular or material transitions, from the infrared to the UV.…”

mentioning

confidence: 99%

Attosecond electron pulses for 4D diffraction and microscopy

Baum

Proc. Natl. Acad. Sci. U.S.A.

2007

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175

151

In this contribution, we consider the advancement of ultrafast electron diffraction and microscopy to cover the attosecond time domain. The concept is centered on the compression of femtosecond electron packets to trains of 15-attosecond pulses by the use of the ponderomotive force in synthesized gratings of optical fields. Such attosecond electron pulses are significantly shorter than those achievable with extreme UV light sources near 25 nm (Ϸ50 eV) and have the potential for applications in the visualization of ultrafast electron dynamics, especially of atomic structures, clusters of atoms, and some materials.attosecond imaging and diffraction ͉ ponderomotive compression S tructural dynamics of complex systems with many atoms, such as those of chemical reactions and phase transitions, involve intermediates and transition states on a multidimensional energy landscape (see refs. 1 and 2). Likewise, when more than one electron is involved, electron dynamics are expected to define a landscape, because electrons are correlated by Coulomb forces that may result in sequential or concerted interactions (see refs. 3 and 4). However, because of the difference in time scales, femtoseconds for nuclear motions and attoseconds for electron motions, consideration of the energy uncertainty must be addressed (5, 6). For chemical and biological structures, the femtosecond resolution is ideal, because on this time scale the energy uncertainty is only a small fraction of the intrinsic energy and the average photon energy can be made in resonance with molecular or material transitions, from the infrared to the UV. In addition, spatial localization of the nuclear wave packet is nearly complete (7).Because electron dynamics are on the time scale of attoseconds, the applications become that of atoms near or above ionization or in environments such as clusters or special materials. The pioneering work (see ref. 8) in the generation of attosecond pulses at wavelengths of 15-40 nm (30-100 eV) and bandwidths of 10-30 eV (9-14) has shown applications so far in the spectroscopy of Auger processes (15), tunneling in atoms (16), and reconstruction of diatom charge distributions (17). The dynamics resolved is that of a few femtoseconds (18)(19)(20).Here, we focus on the possibility of generating near-15-attosecond free electron pulses and their utilization to extend the domain of electron microscopy and diffraction to the subfemtosecond and attosecond scales. This direct visualization of structures in space and in time has been termed 4D imaging (21-23) and provides the necessary picometer spatial and ultrafast temporal resolution (21,24). From the uncertainty relation, we derive the general requisites for reaching the attosecond time scale with electrons. A compression methodology for electron packets in synthesized optical fields of intensity gratings is discussed, and the reported numerical simulations show the feasibility of such approach under realistic experimental conditions. Attosecond Pulse Generation and CharacteristicsThe generati...