Photoinduced Dynamics

Studying chemical reactions in an aggregate is an elegant way to investigate the effects of a reaction medium on the dynamics of a chemical reaction. The « Cluster Isolated Chemical Reaction » (CICR) technique is particularly interesting in this field due to its flexibility: this technique allows for the selection of a known number of reactive atoms or molecules and their interaction with a medium, the aggregate, whose structure and temperature are known. The aggregate plays two successive roles: first, it serves to collisionally capture the reactants, and then it mimics the solvent. The motivations for isolating reactions on van der Waals (or helium) aggregates are of several types:

Using the flexibility of the CICR technique to create a study object that is difficult to obtain otherwise, while ensuring spectroscopic information about it.
Determining the exact stoichiometry of a reaction.
Studying how the aggregate disrupts the reaction occurring within it. Two effects exist: i) mechanical by hindering movements, ii) electronic by altering the relative heights of the different potential surfaces involved in the studied chemical reactions, or even modifying the surfaces themselves. The induced dipole/permanent dipole interaction indeed stabilizes the potential surfaces or the regions of potential surfaces corresponding to a charge transfer compared to those for which there is no charge transfer.

The time delay of a photoelectron emitted angularly resolved in the molecular reference frame carries the signature of the interaction of the electron ejected from a molecular orbital by the absorption of an XUV photon with the electrostatic potential representing the electronic structure of the molecule. The determination of this observable is generally the objective of complex two-photon (XUV – IR) experiments, temporally resolved on the attosecond scale. We have demonstrated that such photoionization delays can be determined by measuring the molecular emission diagrams of photoelectrons spectrally resolved in the XUV domain using synchrotron radiation (SOLEIL). The reaction illustrating these experimental and theoretical results involves the interference between direct ionization and resonant ionization due to a shape resonance, described by a multichannel Fano model (see figure). The resonant state, responsible for the modulation of the cross-section, increases the ionization delay by retaining the electron before ionization and modifies the emission diagram.

The ionization time delay can also be measured using a time-resolved approach, employing the RABBIT method which involves measuring the time difference taken by an electron escaping from an ion, for different ionization energies. In collaboration with LIDYL, ILM, and LCPMR, and by introducing an angular approach to the measurement, we were able to reconstruct the film of the ionization process in helium.

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The study of photoionization processes in ionic species is essential for the characterization and modeling of many plasmas, whether they are laboratory-based (laser plasmas, tokamaks, etc.) or of astrophysical interest (stars, nebulae, etc.) and planetary (ionospheres). From a fundamental perspective, understanding photon-matter interactions in complex atomic systems poses a real challenge for theoretical models aimed at describing the various competing interactions within the atomic electronic structure. The photoionization process allows, among other things, for tracking the evolution of the relative intensity of electronic correlations in isonuclear series as a function of the variation in the intensity of the central potential.

Experiments on the photoionization of ionic species began in the 1980s using synchrotron radiation provided by the ACO ring and continued at Super ACO, and then on the Pléiades beamline at Soleil, where the current MAIA experimental setup has a permanent installation. Only one setup, located in Germany, competes with it.

Among the numerous results obtained, we can mention the study of the collapse of the 4f orbital in iodine, xenon, or barium depending on the initial charge state of the ion, the study of isonuclear series of ions of astrophysical interest (C, N, O, Fe), or the study of heavy systems whose modeling requires the use of UTA. These studies have extended to more complex systems such as hydrides.

Collaborations:
E.T. Kennedy, J.P. Mosnier  (Dublin College University)
C. Blancard (CEA Bruyères-le-Châtel)
B. Mc Laughlin (Harvard Smithsonian Center for Astrophysics)
T. Gorczyca (Western Michigan University)
S. Cargniato (LCPMR, Paris)

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