Direct observation of electronic motion in complex molecules

Direct observation of electronic motion in complex molecules

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  • On 4 February 2022
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Scientists from the Max Planck Institute Stuttgart, Universidad Autónoma de Madrid (UAM) and the IMDEA Nanoscience Institute have realized the first observation of electronic motion in real time and space. This advance represents the first step for the direct study of electronic dynamics in complex molecular systems without having to resort to complex image reconstruction processes that are only accessible for simple molecular systems.

Chemical reactions are the consequence of the movement of electrons in molecules. Therefore, monitoring this movement, which occurs on the time scale of attoseconds (10-18 seconds), is one of the keys, perhaps the most important one, to understand and eventually control these reactions. Experiments and computational modeling carried out in the last two decades using attosecond science techniques have shown that it is possible to generate and follow the evolution of this movement in real time. However, this has been done in an indirect way, since such evolution is obtained from certain (often elusive) characteristics that are observed in photoelectron, photoion, absorption or emission spectra, which has limited its applicability to small molecules. In other words, the motion pictures of electrons that have been made to date do not result from direct observation of electronic motion, but from a reconstruction based on complicated computational algorithms that are not always available and are only applicable to simple systems.

Figura 1. Simplified scheme of the experimental device

On the other hand, it has been well known since the 1980s that scanning tunneling microscopy (STM) allows for direct observation of electron density, without the need for any type of reconstruction. However, this technology is not capable of providing dynamic information on this ultra-fast time scale by itself. Consequently, the ultimate goal, the direct observation of electrons in action, both in real time and in real space, has so far remained inaccessible. In the work that has just been published in the journal Nature Photonics [1], scientists from the Max Planck Institute in Stuttgart, the Universidad Autónoma de Madrid and IMDEA-Nanoscience have been able to combine STM and attosecond technologies to observe, for the first time, the movement of the electron density in the perylenetetracarboxylic dianhydride (PTCDA) molecule, directly and without the need to use any reconstruction procedure, with a spatial resolution of angstroms (10-10 meters) and a temporal resolution of attoseconds. To do this, in the experiment carried out in Stuttgart, a combination of two laser pulses with a duration of less than 6 femtoseconds was used, with a controlled delay of one with respect to the other, which were sent to an ultravacuum chamber equipped with an STM in which the PTCDA molecules had previously been deposited on a gold surface (see Figure 1). The variation of the time delay between the two pulses thus provided the images of the electron density at different times, generating the sequence of frames that allows for a direct visualization of the movement of the electrons in the PTCDA molecules on the attosecond scale (see Figure 2).

The unequivocal proof that the movement observed corresponds effectively to that followed by the electrons in the molecules and not in the gold substrate or the STM tip was obtained from elaborate computational calculations carried out on the Mare Nostrum and Centro de Computación Científica supercomputers of the Spanish Supercomputing Network and UAM.

Authors: Michele Pisarra and Fernando Martín (Universidad Autonoma de Madrid and IMDEA-Nanoscience)

[1] Garg, M., Martin-Jimenez, A., Pisarra, M., Luo, Y., Martín, F., Kern, K. 2021.Real-space sub-femtosecond imaging of quantum electronic coherences in molecules. Nature Photonics https://doi.org/10.1038/s41566-021-00929-1

More info: https://www.nature.com/articles/s41566-021-00929-1
Contact: Fernando Martín, fernando.martin@uam.es.

Acknowledgements: The research leading to this work has been carried out within the framework of the COST Action CA18222 (AttoChem), funded by the European Cooperation in Science and Technology (www.cost.eu). It has been partially funded by MCIN/ AEI /10.13039/501100011033 (grant ref. PID2019-105458RB-I00) and the Comunidad de Madrid (project Y2018/NMT-5028, FULMATEN-CM, co-funded at 50% by the European Social Fund of the Community of Madrid).

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