Attosecond core-level spectroscopy reveals real-time molecular dynamics

A team of European researchers has developed an attosecond core-level spectroscopy technique that can track the many-body molecular dynamics on its natural ultrafast timescale. Their work was benchmarked with furan, showing the power of their tool by successfully retrieving the entire history evolution of the dynamics and relaxation processes of a heterocyclic organic ring.

Chemical reactions are complex mechanisms. Many different dynamical processes are involved, affecting both the electrons and the nucleus of the present atoms. Very often the strongly coupled electron and nuclear dynamics induce radiation-less relaxation processes known as conical intersections. Such dynamics, which are at the basis of many biological and chemical relevant functions, are extremely difficult to experimentally detect.

The problem arises when researchers attempt to simultaneously trace the nuclear and electronic motion because disentangling their dynamics is difficult, and they occur at comparable ultrafast timescales. That’s why, in the past few years, physicists and chemists have turned capturing the real-time evolution of molecular dynamics into one of their most burning challenges.

However, in a recent Nature Photonics publication, ICFO researchers Dr. Stefano Severino, Dr. Maurizio Reduzzi, Dr. Adam Summers, Hung-Wei Sun, Ying-Hao Chien led by the ICREA Prof. at ICFO Jens Biegert, together with theory support by Dr. Karl Michael Ziems and Prof. Stefanie Gräfe from the Friedrich-Schiller-Universität Jena, have presented a powerful tool based on attosecond core-level spectroscopy to investigate molecular dynamics in real-time, which is capable to overcome the aforementioned challenges.

They have benchmarked their method tracing the evolution of gas-phase furan, an organic molecule made of carbon, hydrogen and one oxygen arranged in a pentagonal geometry. Its cyclic structure gives this kind of species the name of chemical “ring”. The choice was not arbitrary, as furan is the prototypical system for the study of heterocyclic organic rings, the essential constituents of many different day-to-day products such as fuels, pharmaceuticals or agrochemicals. Knowing their dynamics and relaxation processes is thus of huge importance.

Life history of furan unlocked

The team successfully time-resolved the details of the entire ring-opening dynamics of furan, specifically the fission of the bond between one carbon and the oxygen, which breaks its cyclic structure. To achieve this, they tracked the so-called conical intersections (CI), ultrafast gateways between different energy states that furan undergoes in its evolution towards ring-opening.

In their experiment, they first excited the furan molecule with a light beam (the pump pulse). Then, they used an attosecond and much weaker pulse (the probe) to monitor the pump-induced changes in the sample. After the initial photoexcitation, they located the three expected conical intersections in time by analyzing the changes in the absorption spectrum as a function of the delay between pump and probe. The appearance and disappearance of absorption features, as well as their oscillatory behavior, provided signatures of the changes in the electronic state of furan.

Quantum Insights into Furan Dynamics: Detection and Analysis of Ring-Opening Phenomenon

Additionally, they observed that the passage through the first CI transition generates a quantum superposition between the initial and final electronic states, manifesting as quantum beats. This ultrafast phenomenon, explainable only using quantum theory, posed significant challenges in previous experiments. The detection of the second CI was even more challenging, as the final electronic state neither emits nor absorbs photons (it is an optically dark state), making its detection through conventional methods extremely demanding. However, in this case, their platform performed the task as effectively as before.

Following this, the team’s equipment successfully detected the anticipated ring-opening. The transition of the molecule from a closed to an open ring geometry implies a symmetry breaking that leaves an imprint in the absorption spectrum. The researchers’ spectroscopic tool, highly sensitive to nuclear structure, revealed the ring-opening through the emergence of new absorption peaks.

Finally, the molecule underwent relaxation into the ground state (the lowest molecular orbital available) through the third conical intersection, whose transition was accurately time-resolved.

The success of attosecond core-level absorption spectroscopy

All in all, Biegert and his group have proposed and successfully reported on a new analytical methodology to unveil the complex and intricated process that is molecular ring opening in its native ultrafast timescale. The combined high temporal resolution and coherent energy spectrum of their cutting-edge technique allowed them not only to track the transitions of furan across conical intersections, but also to identify electronic and nuclear coherences, quantum beats, optically dark states and symmetry changes, providing an extremely detailed picture of the whole relaxation process.

It is important to highlight that the power of attosecond core-level spectroscopy is not limited to this particular molecule, but consists in a general tool designed to be employed with other species too. Therefore, this new mechanism can bring to light the complex dynamics of relevant functions, such as the photoprotection mechanism of the DNA basis. Furthermore, the researchers identify the manipulation of efficient molecular reaction and energy relaxation dynamics as some of the most promising applications for their work.

Reference article

S. Severino, K. M. Ziems, M. Reduzzi, A. Summers, H.-W. Sun, Y.-H. Chien, S. Gräfe & J. Biegert, (2024) Attosecond core-level absorption spectroscopy reveals the electronic and nuclear dynamics of molecular ring opening,  Nature Photonics, https://www.nature.com/articles/s41566-024-01436-9.