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Molecular dynamics on the femtosecond timescale: a close look at photochemical reactions

The photodynamics of a prototypical system, acetylacetone, have been revealed in unprecedented detail using the FERMI seeded free-electron laser source, paving the way for deeper investigations of photochemical processes.
The first steps in photochemical processes involve changes in electronic and geometric structure on extremely short timescales. Thanks to an experimental setup unique in the world and the support of advanced molecular dynamics calculations, a team of researchers from the Universities of Uppsala and Gothenburg, Sweden, the Institut Ruđer Bošković, Zagreb, Croatia, the Elettra laboratory, the University of Trieste, Italy, and the Laboratory of Chemical Physics, Matter and Radiation, Paris, France, has shed light on the very fast dynamics of acetylacetone after photoabsorption.
The combination of intensity, energy resolution and very short pulse duration of the FERMI seeded free-electron laser source recently build at the Elettra laboratory, can now provide exceptionally detailed information on photoexcitation-deexcitation and fragmentation processes of isolated molecules in pump-probe experiments on the 50-femtosecond (1 fs = 10-15 s) time scale.
Acetylacetone is a stable molecule used as solvent or as chelating agent with potential environmental and medical applications. The chemical formula and possible fragmentation pathways of acetylacetone are shown in Figure 1.
When it absorbs a photon which promotes one valence electron to an empty molecular orbital, this systems undergoes a series of fast rearrangements, implying very rapid changes of electronic and geometric structure, on a timescale of tens of femtoseconds.

Figure 1 Chemical formulae for the two tautomeric forms of acetylacetone, and possible fragmentation pathways.

In the experiment, the molecule has been excited with an optical laser of 261 nm (1 nm = 10-9 m) wavelength (the so-called pump), and the evolution of the system has been followed by ionizing the photoexcited species created that way by a 19.23 eV photon beam from the FERMI free-electron laser (the so-called probe) with varying time delays (from zero to 2,000 fs) between the pump and the probe. In this way, several photoexcited species have been characterized by valence photoelectron spectroscopy, ion spectroscopy and molecular dynamics calculations.
In more detail, a clear picture of the evolution of the system is reached, showing that the photoexcitation from the S0 (ground state) to the S2 (ππ*) (bright) state is followed by a conical intersection connecting with the S1 (nπ*) (dark) state, and then the T1 (ππ*) state is reached through ultrafast S1 (nπ*)/T2 (nπ*) crossing which is immediately followed by internal conversion to T1 (ππ*). A minor pathway leading back to the ground state is also identified. Observed fragmentation yielding CHx species is related to the onset of the T1 (ππ*) state formation.
A schematic view of the photoexcitation-relaxation mechanism and of the potential surfaces of all ground and excited states is shown in Figure 2.

Figure 2. 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.



This research was conducted by the following research team:

R.J. Squibb1, M. Sapunar2, A. Ponzi2, R. Richter3, A. Kivimäki4, O. Plekan3, P. Finetti3, N. Sisourat5, V. Zhaunerchyk1, T. Marchenko5, L.Journel5, R. Guillemin5, R. Cucini3, M. Coreno3,6, C. Grazioli3,6, M. Di Fraia3,6, C. Callegari3,6, K.C. Prince3,7, P. Decleva4,8, M. Simon5, J.H.D. Eland1,9, N. Došlić2, R. Feifel1 and M.N. Piancastelli5,10

1Department of Physics, University of Gothenburg, Gothenburg, Sweden.
Institut Ruđer Bošković, Zagreb, Croatia.
3 Elettra - Sincrotrone Trieste SCpA, Trieste, Italy.
4 Consiglio Nazionale delle Ricerche-Istituto Officina dei Materiali, Trieste, Italy.
5 Sorbonne Universités, UPMC Univ Paris 06, Laboratoire de Chimie Physique-Matière et Rayonnement, Paris, France.
6 Consiglio Nazionale delle Ricerche–Istituto di Struttura della Materia, Trieste, Italy.
Molecular Model Discovery Laboratory, Department of Chemistry and Biotechnology, Swinburne University of Technology, Melbourne, Australia.
8 Dipartimento di Scienze Chimiche e Farmaceutiche, Universitá di Trieste, Trieste, Italy.
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford, UK.
10 Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.

Contact persons:

Maria Novella Piancastelli, e-mail: maria-novella.piancastelli@physics.uu.se


R.J. Squibb, M. Sapunar, A. Ponzi, R. Richter, A. Kivimäki, O.Plekan, P. Finetti, N. Sisourat, V. Zhaunerchyk, T. Marchenko, L. Journel, R. Guillemin, R. Cucini, M. Coreno, C. Grazioli, M. Di Fraia, C. Callegari, K.C. Prince, P. Decleva, M. Simon, J.H.D. Eland, N. Došlić, R. Feifel and M.N. Piancastelli, “Acetylacetone photodynamics at a seeded free-electron laser”, Nature Communications 9, 63 (2018). DOI:10.1038/s41467-017-02478-0

Last Updated on Thursday, 25 January 2018 16:46