Femtosecond Footage of Methane's Private Life

Flickers of laser light can clock the speed of a chemical reaction, timing the knitting and breaking of each molecular bond. Now scientists have rigged this stopwatch to trip a kind of camera, taking stop-action pictures of the molecules as they change shape during the reaction. The technique, described in today's issue of Nature, could allow researchers to better understand how complex biomolecules such as enzymes carry out reactions essential for life.

Physics and chemistry professor Ahmed Zewail and his colleagues at the California Institute of Technology married two previously independent lines of research: femtochemistry, in which pairs of brief laser pulses initiate and monitor a chemical reaction, and electron diffraction, in which a molecule's structure is determined from the scatter of electrons fired at a crystal containing billions of copies of that molecule.

Zewail's team used a laser to fire a pulse of photons into a vacuum chamber filled with a heavy form of methane containing two iodine atoms. The photons began breaking apart some of the methane molecules--essentially starting a reaction stopwatch. A second pulse, fired a fraction of a second later, hit a metal-coated cathode-ray tube, stripping away electrons. The electrons flew into the vacuum chamber, where some of them ricocheted off the dissociating methanes. These stray electrons lit up charge-coupled device cameras arranged at the far end of the chamber. By analyzing the diffraction pattern, the researchers could construct an image of the bombarded molecules.

Varying the length between the initial and follow-on pulses, the researchers were able to create multiple images, akin to a movie, depicting changes in the shape of the methane molecules over time. Because electron diffraction is widely used to image the structures of enzymes and other large biomolecules, the new work holds out hope for seeing the structures of those more complex molecules change during reactions, says Zewail. And although that dream hasn't been realized yet, says Carl Lineberger, a femtosecond chemist at the University of Colorado, Boulder, "it's very exciting to see people are taking steps to seeing electron diffraction in real time."