Toward laser control of chemical reactions.

Toward laser control of chemical reactions

Chemists usually rely on relatively crude methods -- such as adjusting the temperature or the concentrations of ingredients--to obtain the products they want. Now, researchers have demonstrated a much more subtle and direct means of controlling a chemical process. They use a pair of laser beams to alter the ionization rate of hydrogen chloride molecules in a predictable way, in effect telling individual molecules exactly what to do.

"What we're doing is so subtle that unless you look for it, you would never see it," says team leader Robert J. Gordon, a chemist with the University of Illinois at Chicago. The researchers manipulate the ionization rate simply by varying the phase relationship between two laser beams -- in other words, by shifting the relative positions of the crests and troughs in two light waves.

This important step brings scientists closer to the long-standing goal of directly controlling the yield and distribution of products in a given chemical reaction. Gordon and his co-workers describe their work in a paper scheduled for the June 1 JOURNAL OF CHEMICAL PHYSICS.

The researchers apply a strategy first proposed by theorists Paul Brumer of the University of Toronto in Ontario and Moshe Shapiro of the Weizmann Institute of Science in Rehovot, Israel. The idea is to offer two alternative pathways for exciting a hydrogen chloride molecule to a specific energy level. One path involves excitation of the molecule by a single 112-nanometer photon of ultraviolet light. The alternative route requires three 336-nanometer photons to achieve the same result. In the final step, an excited molecule absorbs an additional 336-nanometer photon, which forces the ejection of an electron to produce ionized hydrogen chloride.

The Illinois team first generates two laser beams -- one having exactly three times the wavelength of the other. "The crucial thing is that because the 112-nanometer beam is manufactured from the definite phase relationship between the two beams," Gordon says. "If we just took two lasers off the shelf and fired them, there would be absolutely no relation between the phases of the two beams."

The two beams travel together, passing through a chamber containing the gas pressure, the researchers can delay one laser beam relative to the other, altering the phase relationship between the two beams in a predictable, reproducible way.

When they focus the beams on a stream of hydrogen chloride molecules, they find that the rate at which the molecules become ionized depends on the phase relationship between the laser beams. According to quantum mechanics, this effect results from constructive and destructive interference between the two different ways of exciting hydrogen chloride molecules.

This method contrasts with technically much more complicated schemes for achieving control with extremely short, intense pulses of laser light (SN: 3/2/91, p.142). "The key theoretical contribution we made was to show that control has nothing to do with the time scale," Brumer says. "Such [short] pulses are totally unnecessary."

"We've demonstrated that the concept of quantum-mechanical control really works," Gordon adds. "It's not a little effect."

In principle, the same method used to control the ionization of hydrogen chloride molecules should also work for manipulating competing processes that generate different chemical products. "Our experiment is still only partway toward the ultimate goal," Gordon says. "We are now trying other molecules. We want to get two different sets of products and be able to control one product's [yield] at the expense of the other."