Excessive vibration excited by the laser pulse. The movement slows down around the reaction transition state, which is observed by the X-ray laser before the carbon dioxide rapidly leaves the surface. The illustration is made by Henrik Öström.
Excessive vibration excited by the laser pulse. The movement slows down around the reaction transition state, which is observed by the X-ray laser before the carbon dioxide rapidly leaves the surface. The illustration is made by Henrik Öström.


Seeing the transition state of a chemical reaction has long been considered impossible, but has now become a reality. This fundamental advance will increase the understanding of how chemical reactions occur. This may in turn lead to better methods for tailoring reactions to store energy, for example through recycling of CO2 and how we make fertilizers for the future food supply.

“It has been my dream for many years to follow chemical reactions on surfaces in real time. This is the result of a long-term systematic work where we, at Stockholm University, have built up experiments with ultrashort optical laser pulses”, says Henrik Öström, senior lecturer in chemical physics at Stockholm University.

The researchers studied one of the most important reactions in an automobile emission catalytic converter; when poisonous carbon monoxide (CO) becomes carbon dioxide (CO2). In the experiment, the researchers had CO and oxygen settle on a surface of ruthenium and started the reaction with an ultra-short pulse from an optical laser, heating up the surface to well over 1,000 degrees Celsius (more than 3,000 degrees Fahrenheit). With the heating the adsorbed atoms and molecules begin to vibrate rapidly and move across the surface of the metal, greatly increasing the likelihood that they will collide with each other and create a bond. The research team was able to follow – in real time using the X-ray laser– how the electronic structure changed when CO collided with oxygen atoms and the two tried to bind to each other.

The traditional image of the transition state is that the molecules pass through too quickly to be observed. Therefore, it was a big surprise that a large percentage of them actually turned out to be there an unexpectedly long time, even though only a small percentage went on to really form carbon dioxide. Most of them fell back to the original CO plus oxygen. Theoretical modelling of molecular motion and what changes that can be observed in various situations played an important role in the interpretation of the experiments.

“It's a bit like rolling marbles up a hill where most only just reach the top and roll back again. Near the top the marbles slow down and that's actually where they spend the most time. We see many attempts to react but only a few manage to go all the way”, says Lars GM Pettersson, professor of theoretical chemical physics at Stockholm University.

The unique X-ray laser is used to create high-intensity X-ray pulses with a wavelength that makes it possible to see atoms and molecules, and with a pulse duration which is short enough to resolve their movements. This technique has made it possible to capture the various stages of chemical reactions in a way that has never before been possible. The X-ray laser facility that has been used is situated at Stanford University, USA.

“This experiment shows the great importance of X-ray laser facilities as they open up for studying in detail in the future how catalysts really work. In the end, a deeper understanding may lead to more efficient catalysts that can be used in chemical energy transformations”, says Anders Nilsson, professor of chemical physics at Stockholm University and Stanford University.

The team also included researchers from the LCLS, SUNCAT at SLAC / Stanford, Helmholtz-Zentrum Berlin, the University of Hamburg, Fritz-Haber Institute of the Max Planck Society, DESY and the University of Liverpool. Funding from the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the DOE Office of Science, the Volkswagen Foundation and the German Research Foundation (DFG).

The recently published work by Henrik Öström et al. can be read here: http://www.sciencemag.org/content/early/2015/02/11/science.1261747

Description of the X-ray laser LCLS.

A short YouTube presentation of the project and the results.