Description du sujet de thèse

The dynamics of the formation of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium and in soot is still under debate, partly because there are an extremely large number of possible isomers to their building block, which is benzene C6H6. Indeed, the most stable structure i.e. benzene, well known to chemists, but as soon as energy is absorbed by this molecule, a multitude of metastable molecular structures can be reached and act as an energy reservoir for a chemical reaction that may take place following a collision. On the other hand, in these aromatic molecules, the replacement of two carbon atoms by a boron atom and a nitrogen atom leads to a molecule with an identical number of electrons and a similar structure (we call it CC/BN isosteres). On such compounds, the optical and electronic properties are strongly modified by the appearance of a non-zero dipole moment, which makes these molecules very interesting for applications in organic semiconductor materials. For example, it has been suggested that BN isosteres of polycyclic aromatic hydrocarbons could facilitate singlet fission, a process thought to increase the efficiency of solar cells.
The objective of the thesis is to study the energetics and dynamics of metastable isomers and BN isosteres of benzene in the gas phase by different spectroscopic techniques. We will focus on specific molecules that will be studied in the framework of an international collaboration between ISMO (Université Paris-Saclay, UPsay) and Ingo Fischer's group at the University of Würzburg (UWzg) in Germany [1].
The thesis project focuses mainly on the studies carried out at UPsay: time-resolved spectroscopy at the femto (10-15 s) and picosecond (10-12 s) scales to elucidate photo-induced dynamics in the molecules of interest. The samples, synthesized by UWzg collaborators, will be vaporized by heating (possibly followed by flash thermolysis), and photo-excited by a femtosecond laser pulse in the UV. The relaxation dynamics will be probed by time-resolved photo-ionization spectroscopy using the pump-probe technique. The probe laser pulse will be at 400 nm, 800 nm or 62 nm (20 eV) produced by high-order harmonics generation. Photoionization spectroscopy [2] is an ideal method for probing the ultrafast relaxation dynamics of excited states, because ionization of transient or final states of different energies produces photoelectrons of distinct kinetic energies[3]. In addition, no transitions are prohibited by selection rules. Thus, the complete relaxation dynamics of molecules after excitation in UV can be observed for the first time, on the femtosecond and picosecond time scales.

[1] A. Röder, J. Petersen, K. Issler, I. Fischer, R. Mitrić, and L. Poisson, 'Exploring the Excited-State Dynamics of Hydrocarbon Radicals, Biradicals, and Carbenes Using Time-Resolved Photoelectron Spectroscopy and Field-Induced Surface Hopping Simulations,' The Journal of Physical Chemistry A, vol. 123, pp. 10643-10662, (2019).
[2] I. Fischer and S. T. Pratt, 'Photoelectron spectroscopy in molecular physical chemistry,' Physical Chemistry Chemical Physics, 2022.
[3] N. Kotsina and D. Townsend, 'Improved insights in time-resolved photoelectron imaging,' Physical Chemistry Chemical Physics, vol. 23, pp. 10736-10755, 2021.

Contexte de travail

This experimental study is part of a larger international and interdisciplinary scientific project, called Dynastere, combines organic synthesis (UWzg), molecular spectroscopy (UWzg & UPSay), reaction dynamics (UPsay), laser developments (UPsay), femtosecond XUV laser (UPsay), development of data analysis codes in Python (UPsay) and theoretical chemistry (UWzg).

Contraintes et risques

The thesis will include stays in Germany, the objectives of which will be refined according to the skills and motivations of the candidate.