The realm of chemical reactions is a complex one, involving multiple dynamic processes that impact both the electrons and the nucleus of the atoms involved. One of the most intriguing challenges in the field of chemistry is the detection and analysis of radiation-less relaxation processes known as conical intersections. These processes are crucial for understanding various biological and chemical functions but are notoriously difficult to detect experimentally. The main hurdle lies in simultaneously tracking the nuclear and electronic motions, which occur at comparable ultrafast timescales, making it hard to disentangle their dynamics.
In a recent breakthrough published in Nature Photonics, researchers at ICFO, led by Dr. Stefano Severino and Dr. Maurizio Reduzzi, alongside their colleagues, introduced a powerful tool based on attosecond core-level spectroscopy to investigate molecular dynamics in real-time. This innovative method, with support from theory experts Dr. Karl Michael Ziems and Prof. Stefanie Gräfe, has the capability to overcome the challenges associated with tracking molecular dynamics.
The researchers chose to benchmark their method by studying the gas-phase furan molecule, composed of carbon, hydrogen, and one oxygen atom arranged in a pentagonal geometry. Furan’s cyclic structure makes it an ideal candidate for studying heterocyclic organic rings, which are essential components of everyday products like fuels, pharmaceuticals, and agrochemicals. By time-resolving the entire ring-opening dynamics of furan, the team successfully monitored the fission of the bond between carbon and oxygen, leading to the disruption of its cyclic structure.
Their experimental technique involved using a pump pulse to excite the furan molecule, followed by an attosecond probe pulse to observe the induced changes. By analyzing the changes in the absorption spectrum, the researchers were able to locate the three conical intersections that furan traverses during the ring-opening process. The appearance and disappearance of absorption features, along with their oscillatory behavior, provided clear signatures of the electronic state transitions in furan.
Additionally, the researchers observed a quantum superposition between the initial and final electronic states, resulting in quantum beats during the passage through the first conical intersection. This unique phenomenon, only explainable through quantum theory, was previously difficult to identify in experiments.
The detection of the second conical intersection posed further challenges, as the final electronic state does not interact with photons, making it optically dark. However, the researchers’ spectroscopic tool proved effective in capturing this transition as well as the subsequent ring-opening event, evidenced by changes in the absorption spectrum.
The successful demonstration of real-time molecular dynamics analysis using attosecond core-level spectroscopy opens up avenues for studying a wide range of molecules and their dynamic processes. Beyond furan, this technique can be applied to unravel the complexities of other relevant functions, such as the photoprotection mechanism of DNA bases.
Furthermore, the manipulation of molecular reactions and energy relaxation dynamics presents promising applications for future research. By providing detailed insights into electronic and nuclear coherences, quantum beats, optically dark states, and symmetry changes, this groundbreaking analytical methodology offers a comprehensive understanding of complex molecular processes in their native ultrafast timescale.