During ionization in a strong laser field, an electron is released from a molecule. So far, physicists believed that this is the weakest bound electron in the molecule. A research group has now demonstrated in an experiment that even more strongly bound electrons are released by the ionization in a strong laser field. This new understanding not only advances attosecond research, but also brings researchers closer to the goal of controlling chemical processes. The researchers report on these findings in the journal Science on 16 March 2012.

In one molecule, the electrons move in different orbitals, with no more than two electrons in each orbital. The orbitals have different energy levels. For the highest occupied orbital, the least amount of energy is needed to extract an electron. Therefore, it is obvious that during ionization an electron separates from the highest occupied orbit. However, theoreticians have long had doubts about this thesis because many observations could not be explained well. Although there was evidence in experiments that electrons were released from a lower orbit, so many effects were superimposed that the clear proof was missing. Prof. Marc Vrakking, Director at the Max-Born-Institut (MBI) in Berlin, explains: "We have now delivered this proof with our experiment."

Fig. 1 On the left is a molecule with its orbitals in red and blue (peanut-shaped). The strong laser field, here indicated by a flash, deforms the orbitals, they expand in the image predominantly to the right (red is shown as a grid). Finally, electrons are released from the orbitals. The blue and red (lattice-shaped) areas represent the respective possible locations of the electrons. On the far right are the ions that remain after ionization. The blue ions belong to the blue electrons and are stable, the corresponding red ions are decayed.

The group of researchers from the Canadian National Research Council (NRC), the AMOLF (Amsterdam) and the MBI has ionized a molecule in a powerful laser field in their experiment. The scientists then measured not only the energy of the released electron, but also the molecular ion in parallel. If an electron of the highest occupied orbital is missing, the ion is stable and does not change so fast. However, if an electron of lower orbitals is missing, more energy must have been put into the molecule beforehand. Thus, the molecule is now in an excited and thus unstable state, it is easier to divide. "With the electrons, we could measure electrons with different energies besides those that came from the highest orbit - here it was possible that they came from a lower orbit," says Vrakking. "We then had the proof, when we could see at the same time that the ion had decayed."

The results enable a new understanding in attosecond research, which is based on ionization in strong laser fields. But also for chemical processes completely new possibilities open up. If not only electrons ionize from the highest occupied orbital but also from lower orbitals, then, according to the laws of quantum mechanics, one leaves behind a molecule in which the electrons move very fast, up to an electron current. This affects how the molecule reacts chemically. A reaction can thus be faster - from the femtosecond scale to the attosecond scale. However, the electron flow within a molecule may also cause it to prefer certain chemical reactions. This could be a paradigm shift for the ability of molecules to undergo chemical reactions: they could then be caused by the movement of electrons and not by those of the atomic nuclei. Scientists could directly influence chemical processes.

The measurements were carried out in Canada, the MBI physicists now want to repeat and continue in Berlin.

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