Laser reaction control in hot sodium vapor.

Whether in the kitchen, laboratory, or industrial plant, chemistry typically involves mixing together the right proportions of the necessary ingredients, then heating the mixture to get the desired products. The trouble with heating, however, is that the energy needed to break old bonds and form new ones goes into the molecules as random motion. The chemist has little control over where the energy goes, and much of it ends up being wasted.

Lasers offer a means of delivering energy directly to individual molecules. Now, a team of chemists has shown that it's possible to increase the yield of one product while decreasing that of another simply by adjusting the wavelength of the laser light ba thing the molecules in a high-temperature vapor.

Paul Brumer of the University of Toronto, Moshe Shapiro of the Weizmann Institute of Science in Rehovot, Israel, and their coworkers describe the results in a manuscript accepted for publication in Physical Review Letters. "It's a significant step forward , especially because they can [control the reaction] in a harsh environment," says Robert J. Gordon of the University of Illinois at Chicago.

To direct the course of chemical reactions and influence the distribution of reaction products, the chemists exploit subtle quantum effects resulting from the interaction of light and matter (SN: 4/20/91, p. 245).

The researchers use two lasers that boost electrons in a molecule to higher energy states, resulting in two different matter waves in the molecules. Like overlapping water waves, these excitations interfere, either reinforcing or canceling each other. Cha nging the wavelength of one of the lasers can then modify the interference pattern to favor reinforcement or cancellation. Such adjustments can alter the course of the chemical reaction.

"This is a new procedure for achieving quantum interference," Brumer says. Previously, researchers had focused on adjusting the phase relationship (the relative position of the crests and troughs) of two laser beams so that they create quantum interferenc e. With the new technique, this interference effect is generated by the molecule.

"The lasers one needs don't have to be high-quality," Brumer says. "They can jitter or jump about in phase, and one still maintains this quantum interference effect."

Brumer, Shapiro, and their coworkers tested their scheme by illuminating sodium vapor at 370#161#C to 410#161#C with ordinary, pulsed dye lasers. They induced the dissociation of molecules made up of two sodium atoms. By changing the wavelength of one of the lasers appropriately, they could control the proportion of atoms that ended up in a particular excited atomic state.

"Except for one experimental result, no one else has managed to simultaneously enhance the production of one product and deplete the production of another," Brumer says.

The one exception was reported last year by Gordon and his collaborators (SN: 5/27/95, p. 328). That group relied on maintaining certain phase relationships between two laser beams to control whether a molecule of hydrogen iodide broke up or merely ejecte d an electron.

More recently, they did the same experiment with the iodide of deuterium, a heavy version of hydrogen. Deuterium iodide behaves chemically like hydrogen iodide.

"We get different results for deuterium iodide," Gordon says. "Even something as simple as changing a hydrogen atom to a deuterium atom can make a big difference in controllability."

Although chemists are still far from controlling reactions involving large molecules or pinpointing specific bonds within a molecule, these results represent encouraging steps toward those goals.

"People said at the beginning that this would never work," Brumer comments.

"What we have now are some beautiful demonstrations that one can in fact use quantum interference phenomena to control chemical reactions."