Solar energy is a renewable, abundant energy and can be considered as one of the most promising energies of the future. Nature, in the course of evolution, has modelled a very efficient system for using this energy: photosynthesis. Photosynthesis converts carbon dioxide and water in the presence of light into sugars and oxygen:

6CO₂ + 6H₂O + hv → C₆H₁₂O₆ + 6O₂

This apparently simple reaction involves two reaction systems in a chain of elementary reactions whose initial mechanism involves a charge separation which is insufficiently understood and especially, why the quantum yield of this step approaches unity?. It is one of the challenges of the 21^st century to understand the different stages of these complex reactions and to draw inspiration from them to transform the solar energy into storable energy. Chlorophyll and its dimers are involved in several stages of this reaction. To understand their roles, it is important to know their vibrational and vibronic structures. This is the aim of our work. We use a new spectroscopic method: the photodetachment of an electron from a cooled dipolar anion. Indeed, if the excess electron of the dipolar anion is weakly bound to the molecule. The molecular structure of the neutral species and the dipolar anion are very similar. The properties of the anion are characterised by measuring the kinetic energy of the electron ejected after photoexcitation. This new spectroscopy
allows access to the vibrational modes of the chlorophyll cycle in the absence of a perturbation and thus to compare these directly with high-level quantum calculations.

We are developing a new experimental setup that combines an electrospray source, a cooled ion trap and a velocity imaging detector, dedicated to negative ions and photoelectron detection. During the course of the internship, the cryostat for cooling the trap will be set up and the following molecules will be studied as examples: Protoporphyrin IX (Heme without iron atom) in its anionic form and anionic Pheophytin (Chlorophyll without Mg atom). The trainee will thus participate in the first studies of the vibrational structure of cooled anions, first on these model molecules and then on the Chlorophylls. This project is carried out in the framework of a collaboration with a team of chemists specialised in Chlorophylls and two teams of theorists.

This internship may be followed by a funded thesis (ANR ELECTROPHYLLE).

Contact : Niloufar SHAFIZADEH