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Pulses put chemists in control

31 Jan 2003

Scientists calculate the optimal pulse shape to enhance the product yield in a chemical reaction.

A research team from France and Germany has calculated the ideal pulse shape needed to maximize the product yield of a chemical reaction. In subsequent experiments, the optimized pulse improved product yield by 60% (Science 299 536).

Chemists and laser scientists have long dreamt of controlling the outcome of a reaction simply by illuminating molecules with a burst of laser light, but until now all experiments have relied on a trial-and-error method based on genetic algorithms.

Now, Leticia Gonzalez and her colleagues at Berlin's Free University and the Louis Pasteur Institute in Strasbourg have used quantum calculations and wave packet dynamics simulations to predict precisely the pulse that is needed.

"This is the first synergetic experimental and theoretical work unraveling the detailed mechanism of an optimal laser pulse controlling a chemical reaction and leading it to the desired product," Gonzalez told Optics.org.

The team illuminated an organometallic molecule, CpMn(CO)3, with pulses from a Ti:sapphire laser. This photo-excitation ionizes the molecule to produce one of two possible products.

After studying the reaction kinetics with pump-probe experiments and making their calculations, the team identified a pulse sequence that according to their simulation should produce one of the products exclusively.

The tailored pulse sequence was found to consist of two dominant subpulses followed by a small third one. This "ideal" pulse was produced by a liquid crystal phase modulator. Each of these lasted about 40 fs and they were seperated by 85 fs, with the initial subpulse having a slight blue-shift.

Although the complex chemical dynamics involved in the reaction make a 100% yield impossible, the team found that they could enhance the yield of the desired product by 60% over the "natural" yield.

In their paper, the researchers conclude that three elements of the optimal control mechanism should be applicable to other molecular systems. First, the target product should be produced by two main pulses. Relatively few photons should be used, to reduce Stark shifts that can lead to competing reaction channels. Finally, they say that it should be possible to suppress undesired reaction channels by producing wavelength-shifted subpulses within the overall pulse sequence.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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