A look inside of molecules

To not only observe but really understand a chemical reaction, scientists need to know the behavior of electrons within molecules. Until now, it has not been technically possible to observe electrons because they move unimaginably fast. Now this has been achieved by a group of European researchers using attosecond laser pulses.

An attosecond is a billionth of a billionth of a second. In an attosecond, light travels less than a millionth of a millimeter - that's just the way from one end of a smaller molecule to another. By comparison, light can circle our globe eight times in one second. And that's exactly why the physicists make the immense effort to produce such short laser flashes: they can "photograph" the movement of the electrons within a molecule like in a photo series.

In the European research team Prof. Marc Vrakking, director at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin, worked together with groups and others. from Milan, Amsterdam, Lund (Sweden), Garching, Lyon and Madrid. The physicists first studied the hydrogen molecule (H2) - it is the simplest molecule with two protons and two electrons. The researchers wanted to find out exactly how the ionization takes place in a hydrogen molecule, where an electron is removed from the molecule, and how the remaining electron rearranges itself within the molecule. Marc Vrakking explains: "In our experiment we were able to show for the first time that with an attosecond laser we are actually able to observe the movement of electrons in the molecule. Our experiment can be imagined as follows: First, we irradiated a hydrogen molecule with an attosecond laser pulse. As a result, an electron is released from the molecule - the molecule is ionized. At the same time, we split the molecule into two parts with an infrared laser beam, as with a tiny pair of scissors. Now we have looked at how the charge is distributed over the two fragments - because one electron is missing, one part is now neutral and one part is positively charged. So we knew where the remaining electron was, namely in the neutral part. "

Since the 1980s, scientists have been studying molecules and atoms with the help of femtosecond lasers - a femtosecond is a millionth of a billionth of a second, which is a thousand times slower than an attosecond. This allows movements of atoms and molecules to be tracked, but hardly that of electrons. In 2001 researchers succeeded for the first time to produce a laser flash with a length of 250 attoseconds. First, the technical development of the attosecond laser was in the foreground, as well as their targeted control and measurement. Only gradually are scientists starting to use them for scientific questions.

Although the experiments of the European team with attosecond lasers brought the expected results, there was a surprise for the scientists: to better interpret their measurements, they included a group of theorists from the University of Madrid in the project. The work of the Spaniards brought completely new insights. Dr. Madrid-based Felipe Morales, now working at the MBI, reports: "We have nearly reached our limits with our computing capabilities, spending one and a half million computer computation hours." These calculations showed that the complexity of the problem is far greater than previously thought , For example, it has been shown that the electron removed from the molecule by the attosecond pulse also plays a very important role. Vrakking puts it this way: "We did not solve the problem, as we first thought, we just opened one door. But that makes the whole project even more important and interesting."

Original publication

Electron localization following attosecond molecular photoionization

G. Sansone, F. Kelkensberg, J. F. Pérez-Torres, F. Norales, M. F. Kling, W. Siu, O. Ghafur, P. Johnsson, M. Swoboda, E. Benedetti, F. Ferrari, F. Lépine, J. L. Sanz-Vivario, S. Zherebtsov, I. Znakovskaya, A. L'Huillier, M. Yu. Ivanov, M. Nisoli, F. Martin, M. J. J. Vrakking

Nature 465 (2010) 763-766