Tailored laser pulses transform chemical reactions

21 Feb 2003

Carefully-shaped femtosecond pulses of light can now enhance the yield of chemical reactions and synthesize novel molecules. Gustav Gerber, one of the pioneers of laser chemistry, talks to Rob van den Berg about his work.

From Opto & Laser Europe March 2003

For hundreds of years chemists have been heating, stirring and pressurizing their reaction mixtures to encourage molecules to undergo specific changes. Unfortunately, such "shake and bake" techniques are not always particularly selective or efficient, especially when it comes to the synthesis of complicated molecules which may require many processing steps.

Recent optical research by chemists and physicists around the world looks set to change that situation. For several years, scientists have been quietly developing ways to directly manipulate the electronic structure of the molecule using laser pulses, forcing individual bonds to break and reattach.

The ultimate aim is to establish optical synthesis techniques that can deliver a higher yield with less unwanted by-products, or produce molecules that are hard to make by other means. The idea is that laser pulses aligned to the exact frequency of a chemical bond can deliver just enough energy to cause selective breakage of bonds.

In initial experiments, unfortunately, this selectivity was lost due to the rapid redistribution of energy within the molecule. As a result, laser excitation became simply an expensive method of heating up the entire molecule. However, the use of femtosecond Ti:sapphire lasers, pulse-shaping devices and sophisticated feedback algorithms now looks set to change the situation.

Coherent control Today's state-of-the-art approach to laser chemistry is called coherent control. It involves making an initial guess at the shape of the optical pulse that will excite the molecules and start the reaction. The pulse is generated and the reaction products that form as a result are detected and analysed by a learning algorithm. It then adjusts the pulse's duration, phase and amplitude to increase the yield of the reaction. After several iterations the best pulse shape for the reaction is found. In a typical experiment, it takes a matter of minutes to test thousands of different laser pulses and find the most suitable pulse parameters.

Gustav Gerber, a professor of experimental physics at the University of Würzburg, Germany, is one of the technique's pioneers. Gerber thinks that the technique offers many possibilities - creating novel stable or metastable molecules, obtaining better image-quality in microscopy and improving optical data transfer, for instance. "This scheme was first applied to increase the fluorescence of a large dye molecule, but already, one year later, we have used it to optimize the outcome of a dissociation reaction," Gerber said.

In the coherent control method, two diffraction gratings and a liquid-crystal multi-channel modulator (LCM) are used to adjust the shape of the optical pulse. The first grating separates the individual wavelengths within the pulse, while the multi-channel modulator gives each wavelength a specific phase delay. Finally, the wavelengths are recombined into a single pulse by the second grating. The process not only reshapes the pulse but also reorders the wavelengths within it so that they strike molecules in a predetermined sequence.

But because the LCM has 128 channels that can each be set at 4000 different levels, an astronomical number of different laser pulse shapes can be generated. To find the optimal shape, an evolutionary algorithm is used. Each pulse is labelled by the setting of the individual modulator channels. This setting works as a kind of genetic code and forms the basis for a process similar to natural selection.

Using a random setting of the individual modulator channels, the chemical reaction in question is initiated, and its products are detected. The reaction is then performed again using slightly different settings. A computer analyses the results and determines which are the most "successful" pulses - that is, which have generated the largest quantity of the desired product. Only the most successful pulses are used to generate offspring, becoming the "parents" for the next generation of pulses. After a surprisingly small number of generations, the optimal pulse shape is reached.

The system acts rather like a very efficient analogue computer, solving the problem depending on the conditions set by the experimenter. "Basically, it is the system itself that determines what it needs at each instant of time," said Gerber."It is as if the molecule effectively solves the Schrödinger equation, and determines at what time it needs what frequencies."

On the experimental side, in the last four years Gerber's group has discovered numerous examples of reactions in which the product yield may be enhanced using the coherent control technique.

"For instance, in the molecule CH2ClBr it is very difficult to break only the C-Cl bond and leave the C-Br bond intact," said Gerber. "Our method proved to be four times better than conventional synthetic chemistry." His group has obtained a similar enhancement in the removal of a carbonyl ligand (CO) from organometallic complexes like Fe(CO)₅.

"In the pharmaceutical industry synthetic chemists use acid groups to protect certain parts of a molecule during reactions," said Gerber. "After that you have to get rid of these acid groups, but unfortunately, other molecular groups often break off at the same time and are lost."

Gerber has succeeded in decreasing this unwanted loss by a factor of five. He was also the first to make the method work in liquids by selectively exciting complex dye molecules that have overlapping absorption spectra. It had been thought that the complex interactions between the excited molecule and surrounding solvent molecules would cause problems, but this has not turned out to be the case.

Theoretical success Researchers now want to take a step further, and find out what it is that determines the optimal laser field (pulse shape). For relatively simple processes, like the ionization of a calcium atom, Gerber succeeded in finding out why the laser pulse came out as it did. But he admitted: "If we want to try to understand what the molecules are telling us, we need the help of theoreticians."

A group from the Free University of Berlin led by Ludger Wöste and Jörn Manz has recently managed to decipher the reaction dynamics underlying the optimal laser field as determined by coherent control. "With ab initio quantum calculations and simulations of wave packet dynamics we were able to decode the optimal femtosecond pulse generated by adaptive learning techniques," explained Wöste. "That was our main goal: to understand exactly what the light pulse does to the molecule. The first part of the pulse is for excitation and the second for ionization."

Gerber also explains that a common criticism of laser chemistry - that it is a technique that will never be suitable for generating macroscopic quantities of a product - is also no longer an issue. His approach is as follows:

"We first determine the optimal laser field with a 1 kHz laser in the gas phase, but then copy this on a different system with a much higher repetition-rate and perform the same experiment in the liquid phase at a much higher density," said Gerber. "In this way we are able to produce the same microgram to milligram quantities in 24 hours as 'classic' synthesis of a complicated substance, which requires many separate steps. And the operating costs of a laser are much lower."

Gerber and his team are now looking into using LCMs to change the polarization state of each laser pulse. In this way they hope to be able to address the 3D properties of molecules and pull a chemical bond in a specific direction. Ultimately, this may enable synthesis of either the left or right-handed form of molecules - a capability that can be crucial for making pharmaceutical compounds, where often only one of the two forms is biologically active.

Ultimately, the pulse-shaping techniques developed for laser chemistry could also have important applications in other fields. In telecommunications, the shape of a light pulse travelling along an optical fibre tends to distort over long distances. The effect, known as dispersion, might be preventable by pre-compensating the light pulses with a pulse shaper developed for laser chemistry.

In biological imaging, multi-photon laser microscopes use femtosecond pulses to generate 3D images of a sample. The images from deeper layers tend to be poor-quality, because the laser light is distorted as it passes through the upper layers.

"With a pre-compensated pulse, we might be able to correct for this, increasing the resolution and contrast ratio of the images," said Gerber. He sees similar benefits in the fields of materials micromachining with lasers and laser surgery.