Surface reaction recorded in real time

9:51pm, April 2, 2001
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Chemical bonds can form or break incredibly quickly. In the past decade or so, scientists have visualized such ultrafast reactions with laser pulses briefer than a trillionth of a second (SN: 11/13/99, p. 310).

That technique has worked in gases or liquids, but not on surfaces. Now a team of physicists reports real-time viewing of a reaction of molecules on a surface. These reactions lie at the heart of important technologies such as catalytic converters.

The problem with surfaces is that they contain a sea of energy-hungry electrons that can cut short a chemical reaction. A reaction can get started when a laser pulse excites an electron so that it moves into a molecule adhered to the surface. However, the electron's energy usually dissipates among other surface electrons before such a reaction can run its course.

A group led by Henry Kapteyn and Margaret Murnane at JILA, a part of the National Institute of Standards and Technology located at the University of Colorado in Boulder, used a laser pulse to excite a crowd of electrons near a platinum surface. In snapshots taken with pulses of low-energy X rays, the team saw signs that oxygen molecules on the surface had rotated. This is an indication that the molecules were reacting with the laser-energized electrons, the JILA team contends.

Not everyone thinks this is a glimpse of charge transfer. Hrvoje Petek of the University of Pittsburgh says that the transfer of electrons to oxygen molecules may be hard to distinguish from simple heating as a cause for their rotation.

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