Molecules in Motion: Advanced Spectroscopy Captures Molecular Dynamics in Real Time

Researchers have developed a new method that uses attosecond nuclear-level spectroscopy to capture molecular dynamics in real time.

The mechanisms behind chemical reactions are complex, involving many dynamic processes that affect both the electrons and the nuclei of the atoms involved. Often, the dynamics of tightly bound electrons and nuclei give rise to non-radiative relaxation processes known as conical junctions. These dynamics support many important biological and chemical functions, but are extremely difficult to detect experimentally.

The challenge in studying these dynamics stems from the difficulty of tracking nuclear and electronic motion at the same time. Their dynamics are entangled and occur on ultrafast time scales, which has made capturing molecular dynamic evolution in real time a major challenge for both physicists and chemists in recent years.

Now, a team of researchers from ICFO has developed a powerful tool based on attosecond nuclear level spectroscopy to probe molecular dynamics in real time, successfully overcoming this challenge.

The team, led by ICREA Prof. Jens Biegert, includes ICFO researchers Dr. Stefano Severino, Dr. Maurizio Reduzzi, Dr. Adam Summers, Hung-Wei Sun and Ying-Hao Chien, and Drs. Karl Michael Ziems and Prof. Stefanie Gräfe from Friedrich-Schiller-Universität Jena, who provided theoretical support. Their findings were recently published in the journal Photonics of nature.

Study of Gas Phase Furan

The researchers benchmarked their method by tracking the gas-phase evolution of furan, an organic molecule made of carbon, hydrogen and an oxygen arranged in a pentagonal geometry. Its cyclic structure gives this type species the name of the chemical “ring”. The choice was not arbitrary, since furan is the prototype system for the study of heterocyclic organic rings, the essential components of many different everyday products, such as fuels, pharmaceuticals or agrochemicals. Understanding their dynamics and relaxation processes is essential.

Dynamics of Furan Ring Opening

The team was able to time-resolve the details of the entire ring-opening dynamics of the furan, that is, the cleavage of the bond between one carbon and the oxygen, which breaks its cyclic structure. To do this, they had to track the so-called conic intersections (CIs), ultrafast gateways between the different energy states that the furan undertakes in its evolution toward ring-opening.

In their experiment, a light beam (pump pulse) first excited the furan molecule. An attosecond and a much weaker pulse (probe) were then used to monitor the pump-induced changes in the sample. After the initial photoexcitation, the three expected conical junctions were timed 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, give signs of changes in the electronic state of furan.

They could see that going through the first CI transition generates a quantum overlap between the initial and final electronic states, which manifests itself in the form of quantum beats. This ultrafast phenomenon, which can only be explained using quantum theory, was extremely difficult to identify in previous experiments. The second CI was in principle even more challenging to capture, since the final electronic state neither emits nor absorbs photons (it is an optically dark state) and thus its detection through conventional methods is extremely demanding. However, in this case, their platform did the job as before.

After that, the opening of the ring was supposed to happen, and the team’s equipment was again victorious in its discovery. The transition of the molecule from a closed geometry to an open ring implies a symmetry breaking that is embedded in the absorption spectrum. The spectroscopic tool used by the researchers proved to be extremely sensitive to the nuclear structure, and the opening of the ring was shown as the appearance of new absorption peaks.

Finally, the molecule was relaxed to the ground state (the lowest available molecular orbital) through the third conical junction, whose transition was again time-resolved with precision.

Success of Attosecond Core Level Absorption Spectroscopy

Biegert and his group have successfully proposed and demonstrated a new analytical methodology to unravel the complex process of molecular ring opening on its ultrafast time scale. The combined high time resolution and coherent energy spectrum of their state-of-the-art technique allowed them not only to track furan transitions through conical junctions, but also to identify electronic and nuclear coherences, quantum beats, optically dark states, and changes in symmetry, providing an extremely detailed view of the entire relaxation process.

The power of nuclear attosecond level spectroscopy is not limited to this particular molecule, but is a general tool designed to be used with other species as well. Therefore, this new mechanism may shed light on the complex dynamics of relevant functions, such as the photoprotection mechanism of DNA base. Additionally, the researchers identify the manipulation of efficient molecular reactions and energy relaxation dynamics as some of the most promising applications for their work.

Reference: “Attosecond Core Level Absorption Spectroscopy Reveals Electronic and Nuclear Dynamics of Molecular Ring Opening” by S. Severino, KM Ziems, M. Reduzzi, A. Summers, H.-W. Sun, Y.-H. Chien, S. Gräfe and J. Biegert, 29 April 2024, Photonics of nature.
DOI: 10.1038/s41566-024-01436-9