Surface reaction recorded in real time

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.