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  • Laser Technique Directs Photochemical Reactions

Photonics Spectra
Nov 2006
Manipulating chemical reactions allows researchers to obtain desired outcomes. While chemical reagents typically control such manipulations, a team from the National Research Council’s Steacie Institute for Molecular Sciences in Ottawa and Queen’s University in Kingston, Ontario, Canada, has developed a laser-based method to influence photochemical reactions.

The technique, known as dynamic Stark control, modifies potential energy barriers that ultimately control the direction that chemical reactions follow. Importantly, it is the electric field of the laser pulse and not the absorption of light that operates. To demonstrate this technique, the team applied it to nonadiabatic processes, specifically to photodissociation of IBr, in which charge rearrangements occur along a path where potential energy surfaces intersect.

As reported in the Oct. 13 issue of Science, the investigators initiated the reaction by applying a 100-fs pulse at 520 nm and achieved dynamic Stark control by applying a focused time-delayed 150-fs nonresonant pulse at 1.7 μm. If the time-delayed pulse occurs simultaneously with the initiation pulse, the channel that favors ground-state bromine atoms is enhanced, whereas applying the time-delay pulse as IBr traverses the intersection of potential energy surfaces increases the production of bromine atoms in the excited state. The kinetic energy distributions of neutral ground-state iodine atoms allow complete determination of the ratio of the atoms in the excited state to those in the ground state.

The infrared frequencies are small compared with electronic transition frequencies, thus avoiding absorption of light. The molecule, therefore, is restored to its free-field form after each pulse has passed. Eventually, the method may be applied to more complex chemical systems and may assist in controlling quantum phenomena.

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