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Chemist in the driver's seat.

Chemist in the driver's seat

MAny chemical reactions occur extremely rapidly. Chemical bonds brek and form in a matter of femtoseconds; atoms shift positions in mere picoseconds. In such cases, what happens during the first tiny fraction of a second often determines how quickly and readily a particular chemicl reaction proceeds. That sensitivity also provides an opening that chemists can exploit for manipulating reactions by directly intervening in the initial stages. The recent development of sophisticated equipment for generating strings of closely spaced laser pulses -- each pulse only a few femtoseconds long -- now makes such manipulation on a submicroscopic scale conceivable.

"By using a proper sequence of short light pulses, the experimentalist can get in there and alter what happens -- can control rather than just watch," says chemist Graham R. Fleming of the University of Chicago.

Quantum mechanics makes this kind of control possible. In studying reaction rates, chemists generally picture the atoms and electrons involved in these processes as particles. They tend to ignore the quantum-mechanically determined interference effects possible when waves associated with particles such as electrons add together as they meet peak-to-peak or cancel each other as they meet peak-to-trough. Although complicated effects undoubtedly occur, chemists usually assume that interactions between these electron waves and waves associated with nearby molecules would smooth out any peaks and troughs into tiny, random ripples before anything of chemical interest happens.

However, some reactions occur so fast that one can't ignore quantum-mechanical effects, Fleming says. In such cases, molecular vibrations and other motions have too little time to wash out wave effects. Indeed, computer simulations show that electron waves can produce an orderly interference pattern that persists through the first moments of a chemical reaction. These models predict that such a pattern would have a substantial influence on how rapidly the reaction proceeds.

To demonstrate this wave effect in the laboratory, Fleming and his collaborators developed a special laser system for generating pairs of femtosecond pulses of visible light so that successive pulses are either in phase (two peaks) or out of phase (one peak and one trough). They studied the effect of these pairs of pulses on electrons in iodine molecules by measuring the amount of light given off by the pulse-excited molecules.

The researchers discovered they could control how much the iodine gas fluoresced by changing the phase relationship between successive pulses. They got less light when the two pulses entered the gas out of phase an d more light when the pulses were in phase, confirming that quantum interference had occurred. In other words, the first pulse would excite electrons in the iodine molecules, and the second pulse, depending on its phase, would either cancel or augment the effect of the first.

"We've demonstrated the simplest kind of control of molecular dynamics," Fleming says. "It remains to be seen whether this technique can be applied to systems of more general interest."

Fleming suggests that quantum effects may play a key role in photosynthesis, explaining why the first step -- the transfer of an electron -- actually occurs much more rapidly and efficiently than predicted by calculations based on conventional theory. By including quantum effects in their calculations, chemists could probably come closer to predicting the correct rate, Fleming says. Someday, researchers may even understand the process well enough to use light pulses to interrupt or accelerate electron transfer, thereby influencing the rate of photosynthesis.
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Title Annotation:using laser pulses to manipulate chemical reactions
Author:Peterson, Ivars
Publication:Science News
Date:Mar 2, 1991
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