Laser Pulse Controls Chemical Reactions By 'Freezing' Energy Flow

November 03, 1998

CHAMPAIGN, Ill. -- For more than 30 years, chemists have attempted to control chemical reactions by using the pulse of a laser, often with discouraging results. Recent experiments at the University of Illinois indicate scientists may at last be close to achieving their elusive goal.

"We have developed a technique called static coherent control that uses quantum interference to manipulate reactions in polyatomic molecules at the molecular level," said Martin Gruebele, a U. of I. professor of chemistry and a researcher at the university's Beckman Institute for Advanced Science and Technology. "Our goal is to control chemical reactivity by 'freezing' the intramolecular energy redistribution in place with a specially tuned laser pulse."

Vibrational energy flow plays an important role in the reactivity of molecules, Gruebele said. "In the past, researchers have tried to use a fast laser pulse to place energy in a particular bond to perform selective reactions. But the energy redistribution occurred much too quickly for a simple laser pulse to be effective, leading some researchers to conclude that such control was impossible to achieve."

Within the past decade, however, experiments and theories have hinted that intramolecular vibrational energy redistribution occurs more slowly than the rapid exponential decay predicted by standard reaction rate theories. Recent experimental data gathered by Gruebele and his colleagues, as well as full numerical quantum calculations performed by the group, indicate that intramolecular energy flow in several large organic molecules (fluorene, cyclohexylaniline and anthracene) actually follows a much slower power-law decay, making precise control of reactions possible.

"Although the energy redistribution still occurs fairly quickly, an important key is that it flows through multiple paths in the molecule," Gruebele said. "If you send in a laser pulse that causes some of these paths to cancel, you can keep the energy where you put it -- long enough to control a reaction."

Because the vibrational modes of molecules are quantum mechanical objects, they have wave functions that can interfere with one another. A properly tuned laser pulse can adjust the phase of the molecule in such a way that the energy flow slows down, Gruebele said.

With the objective of stemming vibrational dephasing for a time long enough to perform selective chemistry, Gruebele and his colleagues studied the intramolecular vibrational energy redistribution of the molecule thiophosgene using dispersed fluorescence experiments. They also performed quantum dynamics calculations. They then developed a numerical model to determine the optimal laser pulse for intramolecular vibrational energy redistribution control experiments.

"Using a pulse shaped by a 64-element phase and amplitude modulator, the laser freezes the feature in place for 100 times longer than normal," Gruebele said. "By turning off the intramolecular vibrational energy redistribution, our technique of static coherent control can provide a simple and robust route to polyatomic molecular control."

University of Illinois at Urbana-Champaign

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