Static Valence-shell PECD : from Early Works to Recent Studies

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, about 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, on which the photoelectron is scattered off, being very sensitive to static molecular structures such as conformers, isomers, clustering/complexation, 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 a dual fundamental and analytical interest. This last aspect is probably one of the driving forces for the recent extension and blooming of PECD studies, beyond the pioneering synchrotron radiation works, towards the laser (and now FEL) communities.

After a broad introduction to PECD, several recent results regarding static valence-shell PECD will be presented, including striking examples of the specific sensitivity of PECD to conformations [2-4]. Besides, recent results on enantio-pure helicenes demonstrating the sensitivity of PECD to helical chirality will be also shown.

In the conclusion, I will stress the universality of PECD, observed from free molecules to nanoparticles [5], and which may be at play in numerous natural environments including the interstellar medium where it has been suggested as a possible symmetry-breaking process in link with the origin of life’s homochirality [6].

[1] L. Nahon, G.A. Garcia, I. Powis, Valence shell one-photon photoelectron circular dichroism in chiral systems, Journal of Electron Spectroscopy and Related Phenomena, 204 (2015) 322-334.

[2] R. Hadidi, D.K. Božanić, H. Ganjitabar, G.A. Garcia, I. Powis, L. Nahon, Conformer-dependent vacuum ultraviolet photodynamics and chiral asymmetries in pure enantiomers of gas phase proline, Communications Chemistry, 4 (2021) 72.

[3] J. Dupont, V. Lepere, A. Zehnacker, S. Hartweg, G.A. Garcia, L. Nahon, Photoelectron Circular Dichroism as a Signature of Subtle Conformational Changes: The Case of Ring Inversion in 1-Indanol, J Phys Chem Lett, 13 (2022) 2313-2320.

[4] E. Rouquet, J. Dupont, V. Lepere, G.A. Garcia, L. Nahon, A. Zehnacker, Conformer-Selective Photoelectron Circular Dichroism, Angew. Chem. Int. Ed. Engl., 63 (2024) e202401423.

[5] S. Hartweg, G.A. Garcia, D.K. Božanić, L. Nahon, Condensation Effects on Electron Chiral Asymmetries in the Photoionization of Serine: From Free Molecules to Nanoparticles, The Journal of Physical Chemistry Letters, 12 (2021) 2385-2393.

[6] R. Hadidi, D. Bozanic, G. Garcia, L. Nahon, Electron asymmetries in the photoionization of chiral molecules: possible astrophysical implications, Advances in Physics: X, 3 (2018) 1477530.