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  • Laser Controls Surface Reactions

Photonics Spectra
Feb 2002
Richard Gaughan

The ability to manage the location and timing of chemical reactions is key to advanced industrial and biochemical processes. A new laser technique promises unprecedented control over surface reactions.

Although it is relatively easy to create the necessary conditions for a chemical reaction to occur, it is difficult to control the balance of various parameters on a microscopic level. For example, oxygen is adsorbed on a catalytic surface covered by carbon monoxide only when the partial pressures of the reactants and the temperature of the surface reach particular, interdependent values. Where the reaction occurs, the balance is so delicate that local, random instabilities dominate the surface chemistry.

Researchers from Fritz Haber Institute in Berlin and from Princeton University in New Jersey have used a continuous-wave argon-ion laser from Spectra-Physics Inc. to control the adsorption of oxygen onto platinum. They first prepared the single-crystal surface by immersing it in a CO atmosphere. Then they introduced O2.

Because the temperature was just below that required to displace the CO, no O2 could be adsorbed onto the platinum. The researchers then directed the 80-µm focal spot of the 7-W laser to the surface. After 5 ms of illumination, the temperature at the spot rose by 2 to 3 K, above the threshold to initiate adsorption.

A stationary spot created an elliptical pattern of oxygen adsorption, and a moving spot created a V-shaped wake pattern. Using ellipsomicroscopy for surface imaging -- imaging the polarization state of light that was reflected from the surface at an oblique angle -- the researchers were able to monitor the unique adsorption features introduced at the surface.

After demonstrating open-loop adsorption control, they investigated closed-loop control, using a CCD camera operating at 30 Hz. They arbitrarily selected four regions on an image as feedback locations and another four as laser targets. The intensity at the feedback areas, a function of molecular species that were adsorbed at those locations, determined the location and dwell time of the laser spot.

Before active control, they observed a periodic fluctuation in surface adsorption. When active control was initiated, however, the structure of the adsorption was disturbed.

Directions of manipulation

Harm Hinrich Rotermund, leader of the surface-reaction imaging group at Haber Institute, said that such investigations into the manipulation of surface adsorption provide insight into other chemical reactions as well. And the technique should enable not only the control of a reaction on a very localized scale, but also the generation of arbitrarily shaped reaction structures that can be key steps in chemical processes.

"We are moving in many directions: further investigations of specific surface interactions, refinement of the active control to produce specific patterns and also applications of our technique as a step for a more complex reaction," Rotermund said.

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