An attosecond look into the Interior of Molecules
For the first time ever, a European research team has managed to use attosecond laser pulses to observe the motion of electrons in molecules. This work is an important step in unraveling the movements of electrons that are at the basis of some elemental chemical reactions. The results are published in Nature on the 10th of June.
To observe and understand chemical reactions, scientists want to know how electrons move within molecules. But the electrons move so incredibly fast that up to now nobody had succeeded in actually watching electrons as they move. A group of European researchers has now achieved this goal using attosecond laser pulses.
An attosecond is a billionth of a billionth of a second. During an attosecond, light covers a distance of less than one millionth of a millimeter –equivalent to the diameter of a DNA molecule. Researchers are making great efforts to create attosecond laser pulses: by using these pulses they can “photograph” the fast movement of electrons within molecules, just as a fast photo camera can freeze the movement of a runner at top speed.
In the European team, researchers from the FOM Institute AMOLF worked together with groups from - among others – Milan (Italy), Lund (Sweden), Garching (Germany), Lyon (France) and Madrid (Spain). The team examined ionization of the hydrogen molecule (H2) – nature’s simplest molecule, with just two protons and two electrons. In this process, one electron is removed from the molecule, and the remaining electron undergoes a rearrangement. Team member Freek Kelkensberg from AMOLF explains, “In our experiment we were able to show for the first time that with the help of an attosecond laser we really have the ability to observe the movement of electrons in molecules. First we irradiated a hydrogen molecule with an attosecond laser pulse. This led to the removal of an electron from the molecule – the molecule was ionized. In addition, we split the molecule into two parts using an infrared laser pulse. This allowed us to examine how the remaining charge distributed itself between the two fragments – since one electron is missing, one fragment will be neutral and the other positively charged. We knew where the remaining electron could be found, namely in the neutral part.”
With femtosecond lasers, the movement of atoms and molecules can be tracked, but not that of electrons. In 2001, researchers were for the first time able to produce a light flash with a length of only 250 attoseconds. This was followed by considerable efforts on the development of attosecond lasers as well as the control and measurement of the pulses. More recently, scientists are beginning to use the attosecond pulses to address problems in natural science, such as the first molecular application that was published now.
Although the European team’s experiments with attosecond lasers produced some of the results they had hoped for, there were also surprises in store for the scientists. In order to improve the interpretation of the measurements, they involved a group of theoreticians from the University of Madrid in the project. The Spanish researchers’ work brought completely new insights. Dr. Felipe Morales from Madrid, reports, “We needed one and a half million hours of computer time to understand the problem and really reached the limits of current state-of-the-art supercomputers.” The calculations showed that the complexity of the problem was far greater than previously thought. For example, it was shown that the electron that is removed from the molecule by the attosecond laser pulse still plays an important role in the subsequent dynamics of the ionized molecule that is left behind. Project leader Marc Vrakking from AMOLF Amsterdam describes it as follows, “We did not – as we originally expected – solve the problem. On the contrary, we merely opened a door. But in fact this makes the result even more important and interesting.”
This work forms the starting point of a research field in which electron dynamics in molecules is studied on an attosecond time scale. In the future, more complex systems will be delved into with improved techniques, with the aim to gain better insight into the role of electron movements in chemical reactions, so that these will be better understood and controlled.
The experimental study was performed as part of a European network financed by the EU in which young researchers are trained. The research is partially financed by FOM and NWO.
More information
For more information you can contact:
Freek Kelkensberg, FOM institute AMOLF, +31 (0)20 75 47 100
Marc Vrakking, FOM institute AMOLF, +31 (0)20 75 47 100
Nature-Paper: Electron localization following attosecond molecular photoionization
DOI: 10.1038/nature09084