Grand Séminaire d'Institut

Vendredi 20 Octobre 2023 à 11h00.

Photo-Electron Circular Dichroism (PECD): from static measurements to time-resolved, site-specific explorations

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Chirality is ubiquitous in nature, especially in the biosphere, and the so-called chiral recognition process is fundamental for metabolism and drug action. Within a bottom/up approach of molecular complexity, the study of isolated, substrate- and solvent-free chiral species is crucial, but classical chiroptical probes such as Circular Dichroism (CD) in absorption are poorly-adapted to dilute matter because of their very weak associated asymmetries.

At the opposite, two decades ago was introduced a new chiroptical effect called Photoelectron Circular Dichroism (PECD), allowed in the electric dipole approximation, leading to very intense (up to 40 %) forward/backward asymmetries, with respect to the photon axis, in the angular distribution of photoelectrons produced by circularly-polarized light ionization of gas phase pure enantiomers. PECD happens to be a universal, orbital-specific, photon energy dependent chiroptical effect and is a subtle probe of the molecular potential being very sensitive to static molecular structures such as conformers, isomers, clusters, as well as to vibrational motion, much more so than other observables in photoionization such as the cross section (Photoelectron Spectrum-PES) or the usual (achiral) b asymmetry parameter (for a review see [1]). Therefore, PECD studies have both a fundamental and an analytical interest. This last aspect is probably the driving force for the recent extension of PECD studies, beyond the pioneering synchrotron radiation works, towards the laser community.

After an introduction to PECD, several recent results regarding static one VUV-photon valence-shell PECD will be presented, including the specific sensitivity of PECD to conformations [2] as well as a first demonstration of induced-PECD onto an achiral chromophore within a molecular complex [3].

We will then move towards Time-Resolved PECD (TR-PECD), performed with optical lasers to probe intra-molecular relaxation dynamics on femtosecond timescales [4], as well as with the FERMI FEL enabling the chemical-specific, site-specific, and enantiosensitive observation of the electronic structure of a transiently photoexcited chiral molecule via core-shell TR-PECD [5].


[1] R. Hadidi, D. Bozanic, G. Garcia, and L. Nahon, Advances in Physics: X 3, 1477530 (2018).
[2] J. Dupont, V. Lepere, A. Zehnacker, S. Hartweg, G. A. Garcia, and L. Nahon, J. Phys. Chem. Lett. 13, 2313 (2022).
[3] E. Rouquet, M. Roy-Chowdhury, G. A. Garcia, L. Nahon, J. Dupont, V. Lepere, K. Le Barbu-Debus, and A. Zehnacker, Nat. Commun. (accepted).
[4] A. Comby et al., J. Phys. Chem. Lett. 7, 4514 (2016).
[5] D. Faccialà et al., Physical Review X 13, 011044 (2023).

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