A photochemical reaction in its becoming has been observed with unprecedented detail at the Free Electron Laser FERMI in Trieste.
The result of the experiment published in Nature Communications paves the way for investigations that can shed new light on photochemical processes.
“Shooting the movie” of a photochemical reaction, interpreting its hidden details with the help of a computer: this is what has been done, thanks to the extraordinary capabilities of the FERMI free electron laser source in Trieste, by a research team composed of the Universities of Uppsala and Gothenburg in Sweden, the Institut Ruđer Bošković of Zagreb, the Elettra-FERMI Laboratory, the University of Trieste and the Laboratory of Physical Chemistry, Matter and Radiation in Paris. The study was published in Nature Communications.
The researchers wanted to capture the details of a chemical reaction promoted by the absorption of light (photochemical process), to understand how the excitation generated by a light beam induces changes on a target molecule. The first steps in a photochemical process involve changes in the electronic and geometric structure of a molecule over extremely short times measured in femtoseconds (1 fs = 10-15 s), which had so far hindered the accurate reconstruction of the entire sequence of the reaction.
The combination of intensity, energy resolution and very short pulse duration of the FERMI seeded free-electron laser source can now for the first time provide exceptionally detailed information on photoexcitation-deexcitation and fragmentation processes of isolated molecules in pump-probe experiments on the 50-femtosecond time scale.
Photoelectron spectroscopy with high resolution in energy and time, combined with accurate electronic structure and molecular dynamics calculations, has allowed to visualize in its entirety the temporal evolution of the prototype system chosen for the experiment: acetylacetone—a stable molecule used in environmental and medical applications.
“Besides revealing the dynamics of the reaction—explains Maria Novella Piancastelli of the University of Uppsala, principal investigator—a strong point of the experiment lies in the general applicability of the method, which leads us to consider it as the best way to investigate fundamental photochemical processes such as photosynthesis, photovoltaic energy production and vision. The stairway that goes from simple to complex molecules, and from the understanding of phenomena to practical applications is of course a long one, and we are specifically interested in its ‘first step’.
Figure: A pictorial representation of the potential energy surfaces involved in the relaxation mechanism of acetylacetone: the ground state S0 (darker blue), two singlet S2 (ππ*) (light blue) and S1 (nπ*) (orange), and two triplet T2 (nπ*) (light green) and T1 (ππ*) (green) states. This approach based on high-resolution valence spectra backed by high-level calculations is the ultimate way to shed light on fundamental, basic photo processes such as photosynthesis, photovoltaic energy production, and vision.