Controlling chemical reactions by laser

Researchers have used ultrashort laser pulses to preferentially remove one of the hydrogen atoms from acetylene C₂H₂ – a symmetric hydrocarbon –showing for the first time that directional control of hydrogen bond breaking in a symmetric molecule is possible.

Publishing their work in Nature Communications, the scientists at the American University of Sharjah, UAE, King Abdulaziz University, Saudi Arabia, and the Ludwig Maximilians University and Max Planck Institute for Quantum Optics, Germany, suggest their achievement could help direct the pathways of chemical reactions.

Chemistry is based on breaking bonds and/or forming new ones. While the choice of reactants and reaction conditions can be controlled, the subsequent trajectory of a reaction generally cannot, because the nuclear and electronic processes that cause such reactions occur on a different timescale where quantum mechanics takes over. 

The scientists used very short laser pulses to tap into these timescales – in the order of a millionth of a billionth of a second – and manipulate the quantum properties of the reactants to make more likely an otherwise random outcome of a reaction. 

“Employing laser as a light-reagent to steer the outcomes of chemical reactions will provide new insights towards understanding the underlying physics and chemistry that govern matter on its most fundamental level,” says Ali Alnaser, a physicist at the American University of Sharjah, and the study’s lead author. It could also lead to the synthesis of new chemical substances that could not have been created in the past.

Erik Lötstedt, a researcher from the University of Tokyo, who was not involved with the study, says that "the paper is remarkable since they show how a short laser pulse can induce asymmetric bond breaking in a polyatomic molecule, compared to most previous studies that employed diatomic molecules.” 

Electrons in a molecule can only occupy certain allowed quantum energy states. During a chemical reaction, the molecule enters an activated reaction intermediate state for a very short period, and the evolution of a reaction depends on the state occupied by electrons in this state. 

Upon interaction with laser pulses, which have specific electric field waveforms, the quantum state occupied by electrons in the reaction intermediate can be altered. This is dependent on the shape of the electric field waveform of the applied laser. For ultrashort laser pulses, with a duration of only a few femtoseconds, a manipulatable property of lasers known as Carrier Envelope Phase (CEP) determines the exact electric field waveform of these pulses. Therefore, adjusting CEP allows the researchers to probe laser-driven dynamics in atoms, molecules and solid targets, explains Alnaser.

The team shone laser pulses with varying CEP at a reaction chamber to study the effect of CEP on the laser-induced ionization and fragmentation of acetylene. “We can measure the dependence of these processes and their yields on the value of the CEP. Knowing this dependence will enable tailoring certain pulses with certain shapes that will optimize a certain reaction pathway over another, hence optimizing a specific reaction outcome,” explains Alnaser. 

The team also managed to detect the trajectory taken by a proton, or any other charged particle, in response to a certain laser polarization. “We can then correlate the preferred left/right ejection of these charged particles with the value of the CEP. Preferential ejecting to the left or right corresponds to selective breaking of the left/right bond inside the acetylene linear molecule in this case,” adds Alnaser.  

Despite impressive advances in the arena of laser-controlled chemical reactions, most techniques developed so far manipulate small molecules, and the controlled synthesis of more complex materials will require a much more sophisticated understanding of laser-matter interaction.

“Quantum control is not so far replacing the traditional chemical methods of synthesis,” says Ahmed Zewail, who won the 1999 Nobel prize in chemistry for his pioneering work in time-resolved spectroscopy and femtochemistry. “However, the intellectual richness of the field is initiating new avenues for future possibilities.” 

 

Lötstedt added that there was a long way to go before this kind of selective bond breaking can be applied to anything beyond unimolecular reactions of small molecules in the gas phase.