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When the nuclei catch up with the electrons

In the AttoCOLTRIMS device, the three-dimensional movement of the electrons (blue elliptical arrow) and ions (H2 + and H +, gray arrows) can be detected by coincidence. The combination of an extreme ultraviolet pulse (XUV, blue) and a long and fairly weak infrared pulse (IR, red) in a pump-probe configuration provides the basis for studying the attosecond dynamics of the H2 molecule. Credit: Ultrafast Laser Physics group, ETH Zurich

In an attosecond study of the H2 molecule, ETH Zurich physicists discovered that for light atomic nuclei, as they are contained in most organic and biological molecules, the correlation between electronic and nuclear movements can not be ignored .

The main objective in attosecond science is to understand the dynamics of quantum mechanical systems in their natural time scale. Molecules are among the most interesting systems to investigate, which have a high degree of complexity, particularly when compared to atomic systems. The few attosecond experiments performed with molecules to date have provided valuable information about electron dynamics. In these studies, it is assumed that the dynamics of the nuclei around which electrons evolve are "frozen", since nuclei are much heavier than electrons and, therefore, move more slowly. However, even in the attosecond time regime, the approximation that the electronic and nuclear movement are decoupled from each other is often not justified. In particular, in molecules composed of light atomic species, nuclear motion can be as fast as electron dynamics, resulting in a strong coupling between the two.

A team led by Dr. Laura Cattaneo and Prof. Ursula Keller in the Department of Physics at ETH Zurich has now studied the smaller and lighter molecules, H2, and has explored what happens when nuclear and nuclear movements occur. electronic devices on a comparable time scale. As reported in an article published today in Nature Physics found that in molecules, ionization delays can depend significantly on the kinetic energy of photoelectron and nuclei. (The ionization delays are the time between the absorption of a photon and the emission of an electron during photoionization). This finding extends the concept of ionization delays introduced for atomic systems. The variations of the ionization delays with the nuclear kinetic energy can be as great as the variations with the electronic kinetic energy. This implies that as long as the light atoms are involved in the molecular ionization process, the packet of outgoing electron waves can not unravel from the packet of nuclear waves.

These measurements in the attosecond time scale are based on an experimental approach developed previously in the Keller group. In the so-called AttoCOLTRIMS apparatus (see figure), the attosecond metrology is combined with the COLTRIMS image technique, in which the correlated properties of the fragments of a molecular reaction can be recorded. This experimental capacity was combined with the almost exact ab initio theory, carried out by collaborators of the Autonomous University of Madrid (Spain), to describe the electronic and nuclear movements, as well as the coupling between them.

The importance of this work goes well beyond the simple H2 molecule. Hydrogen atoms are present in most organic and biologically relevant molecules. Understanding the effects and contributions of the coupled electron and nuclear dynamics present in such systems should, therefore, provide a fundamental knowledge that will be important in various fields of research.

Explore more:
A milestone in petahertz electronics

More information:
L. Cattaneo et al, Attosecond coupled electron and nuclear dynamics in H2 dissociative ionization, Nature Physics (2018). DOI: 10.1038 / s41567-018-0103-2

Journal reference:
Physical nature

Provided by:
ETH Zurich

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