i n s t ru m e n t a l s
EM enters the fourth dimension B
mens recorded by the UEM y redesigning the transmission matched the performance of the electron microscope (TEM) and TEM in terms of spatial resoluintegrating an ultrafast laser system, tion, sharpness, and image quality. Ahmed Zewail and colleagues at the The researchers “have demonstrated California Institute of Technology that they can make this modification (Caltech) have taken electron microsto TEM, and it doesn’t degrade the copy to the fourth dimension: time quality of the image,” Ross says. (Proc. Natl. Acad. Sci. U.S.A. 2005, “What they haven’t shown yet is 102, 7069–7073). Akin to strobosomething new that can be learned scopic photography, the 4-D ultraUEM micrograph of a rat intestinal cell at two magnificaby using the pulsing electron source, fast TEM (UEM) captures femtosec- tions. Left scale bar = 1 µm; right scale bar = 100 nm. because the specimens examined in ond flashes of complex structures. (Adapted with permission. Copyright 2005 National Acadthis study were static, not dynamic.” Zewail has coined the term “fem- emy of Sciences, U.S.A.) Researchers plan to use the 4-D tochemistry” to describe his efforts low the path of the condenser, the objec- UEM to study dynamic processes by to study certain chemical reactions and taking measurements of biological and tive, and the projector lenses. The weak transient stages at the ultrafast timescales femtosecond pulses of electrons are read- chemical structures in space and time. at which they actually occur. FemtoZewail is able to draw on his femtoily focused by the optical system. “The chemistry is at the heart of Zewail’s chemistry work involving ultrafast elecUEM is versatile [and easy] to [switch] modification of the TEM from conventron diffraction and spectroscopy and between the UEM mode and the TEM tional to ultrafast. Wayne E. King at the configure it for the UEM. For example, mode,” Zewail explains. Lawrence Livermore National Laboratoin the UEM the time axis is defined by With a conventional EM, the microsry says, “It is our belief that this really the distance steps in the electron pathcopist shines a beam of electrons onto is an exciting new research area that is way. In addition, when the UEM is the specimen. The resulting electron migoing to open up a wide variety of phecombined with cryofixation techniques, crograph is captured either as a still phonomena to be explored in chemistry, bitograph or as a video. “If the event you’re cellular components could be frozen in ology, and material science.” real time, according to Zewail. looking for is extremely fast, you won’t To capture images at femtosecond The molecular EM movie is inching pick it up, because it will have happened resolution, Zewail resolved theoretical closer to reality with the UEM. The high in between the video frames,” explains and technical hurdles, which included time resolution of the UEM will allow Frances M. Ross at IBM Research. overcoming electron–electron repulsion scientists to follow the progress of proThe UEM, however, works like a and limiting the effects of broadening. cesses that were too fast to be captured strobe camera, which uses a series of Thus, his novel redesign of the TEM with conventional methods. “Of great flashes of light to make a movie. Ross generates electrons in a regime below significance will be the direct observation says, “In theory, you could collect a sethe space–charge limit. of transformations in materials, such as ries of these images with very high time The researchers connected the UEM to a very fast high-beam Ti:sapphire laser resolution, so that the time between im- the conversion of solids from one phase to another. Second is the observation ages could be very short. [This] would oscillator. In contrast to a conventional of chemical reactions with microscopy. allow you to pick up an event that you electron source, which produces a conThird is the charting of reactivity in biomight miss if you were using a conventinuous beam of electrons, the UEM delogical cells,” Zewail explains. livers electrons in small pulses. The speci- tional EM.” King believes that these time-resolved Zewail’s group studied calibration men dose is a few electrons per square instruments will be adopted widely by angstrom, a level that limits the radiation specimens of gold, polycrystalline aluscientists. “We are going to start to see minum, and amorphous carbon, in damage of biological specimens. The addition to rat epithelial cells. The snap- major advances in the coming years.” a pulsed design places the source right at —Mary Quirk shots of inorganic and biological specithe cathode surface, so the electrons fol336 A
A N A LY T I C A L C H E M I S T R Y / S E P T E M B E R 1 , 2 0 0 5
© 2005 AMERICAN CHEMICAL SOCIETY
