Time-resolved chiral X-Ray photoelectron spectroscopy with transiently enhanced atomic site-selectivity: a Free Electron Laser investigation of electronically excited fenchone enantiomers

Abstract

Chiral molecules are widespread in nature, playing a fundamental role in bio-chemical processes and in the origin of life itself. The observation of dynamics in chiral molecules is crucial for the understanding and control of the chiral activity of photo-excited states. One of the most promising techniques for the study of photo-excited chiral systems is time-resolved photoelectron circular dichroism (TR-PECD), which offers an intense and sensitive probe for vibronic and geometric molecular structure as well as electronic structures, and their evolution on a femtosecond timescale.However, the non-local character of the PECD effect, which is imprinted during the electron scattering off the molecule, makes the interpretation of TR-PECD experiments challenging. In this respect, core-photoionization is known to allow site-and chemical-sensitivity to photelectron spectroscopy. Here we demonstrate that TR-PECD utilising core-level photoemission enables probing the chiral electronic structure and its relaxation dynamics with atomic site sensitivity. Following UV pumped excitation to a 3s Rydberg state, fenchone enantiomers (C 10 H 16 O) were probed on a femtosecond scale using circularly polarized soft X-ray light pulses provided by the free-electron laser FERMI. C 1s binding energy shifts caused by the redistribution of valence electron density in this 3s-valence-Rydberg excitation allowed us to measure transient PECD chiral responses with an enhanced C-atom site-selectivity compared to that achievable in the ground state molecule.These results represent the first chemical-specific and site-specific, enantio-sensitive observations on the electronic structure of a photo-excited chiral molecule and pave the way towards chiral femtochemistry probed by core-level photoemission.I.

Progress and Perspectives of Spectroscopic Studies at Carbon K-edge Using Novel Soft X-ray Pulsed Sources

“…Experimental data confirmed that a soft X-ray FEL source is very effective in timeresolved PECD for studying dissociating chiral cations [34] and for time-resolved photoelectron spectroscopy (XPS) applied to site-specific analysis suitable to distinguish the contributions of different atoms of the same chemical element simultaneously probed [35]. Following these promising results, Faccialà et al investigated at the C K-edge the chirality of fenchone, a prototypical organic molecule, using both time-resolved XPS and PECD [36]. The experiment was carried out at the FERMI-FEL, currently the only facility that delivers high-temporal coherent and circular polarized pulses in this energy domain.…”

Progress and Perspectives of Spectroscopic Studies on Carbon K-Edge Using Novel Soft X-ray Pulsed Sources

“…Experimental data confirmed that a soft X-ray FEL source is very effective in timeresolved PECD for studying dissociating chiral cations [34] and for time-resolved photoelectron spectroscopy (XPS) applied to site-specific analysis suitable for distinguishing between the contributions of different atoms of the same chemical element that are simultaneously probed [35]. Following these promising results, Faccialà et al investigated at the C K-edge the chirality of fenchone, a prototypical organic molecule, using both time-resolved XPS and PECD [36]. The experiment was carried out at the FERMI-FEL, currently the only facility that delivers high-temporal coherent and circular polarized pulses in this energy domain.…”

Ultrafast Polarization-Tunable Monochromatic Extreme Ultraviolet Source at High-Repetition-Rate

“…For this reason, many synchrotron beamlines around the world are dedicated to static XUV circular dichroism experiments in atoms, molecules, and solids in their ground state [85,86,87,88,89]. One of the burgeoning goals in the community is to extend such XUV dichroism experiments to the ultrafast regime, where dynamical out-of-equilibrium properties of matter can be investigated [90,42,91,92,43,93]. The ultrashort duration of XUV pulses generated using high-order harmonic generation makes this approach very appealing for pushing this goal forward.…”