You can't even bat an eyelash in the time it takes Ahmed Zewail to finish an experiment. The Egyptian-born chemical physicist pioneered the use of ultrashort laser pulses to witness the dance of atoms as they knit and break chemical bonds. Today, that new slow-motion view earned Zewail the 1999 Nobel Prize in Chemistry.
Before the late 1980s, few had ever dreamed that it would be possible to see chemistry in action on the molecular level, as it all happens so fast. Some reactions, such as the rusting of a nail, may seem sluggish, but that's because molecules must first overcome an energy barrier, which can be very steep. Once that has happened, however, the making and breaking of bonds occurs in an instant.
To capture that action, Zewail, of the California Institute of Technology in Pasadena, developed what may be described as the world's fastest flash camera, using laser pulses that last a mere 10 to 100 femtoseconds, or quadrillionths of a second. In a tabletop laser, an initial pulse--the flash--provides target molecules the energy they need to surpass the energy barrier. A second pulse fired mere femtoseconds later illuminates the resulting chemical reaction: The light spectra characteristic of the starting materials or byproducts are captured by a nearby detector. By varying the time interval between the two pulses, it is possible to track the chemical reaction from start to finish.
Although Zewail's work initially focused on simple reactions of gaseous molecules, his group and others around the world have since pushed the technology to chronicle chemical changes in liquids and solids as well, thus spawning the new discipline of femtosecond science.
"It was a wonderful choice and a well-deserved award," says Paul Corkum, who heads femtosecond science research at the National Research Council in Ottawa, Canada. Zewail's work opened a new vista by revealing chemistry at its most basic level, Corkum says. And that has led to new insights into everything from how plants capture sunlight in photosynthesis to how we still manage to see at night when the light is faint. "It's intrinsically important to follow chemical reactions as they occur," says Corkum.