Filming molecular chiral dynamics from the inside with FEL

Chiral molecules are essential for understanding many aspects of chemistry, biology, and physics. A chiral molecule is non-superimposable with its mirror image and exists in two different forms called enantiomers. The reactivity and biological and pharmacological activity of chiral molecules can vary significantly depending on the configuration of the enantiomers. Understanding this property is crucial for developing technologically innovative and advanced solutions in materials science, pharmaceuticals, and catalysis.

The scientific communities of biology and chemistry have devoted significant efforts to exploring the phenomenon of chirality, yet much remains unknown about the ultrafast dynamics of chiral compounds. Time-Resolved Photo-Electron Circular Dichroism (TR-PECD) has recently emerged as a promising approach for investigating time-dependent chiral dynamics, as it enables researchers to observe the enantio-dependent structural relaxation of a molecule on a femtosecond timescale. The technique involves utilizing an ultrashort circular pulse to ionize a photo-excited molecule from the valence shell, where the transient dichroism of the medium is mapped on the forward-backward asymmetry of the photoelectron emission along the pulse propagation axis. Despite its potential, the non-local character of this approach makes the interpretation of the experimental results challenging.

In this study, we used the circularly polarized radiation produced by a free-electron laser to measure the transient chirality of photo-excited fenchone (C10 H16O, Fig. 1a), by directly photo-ionizing the molecule from its core-level carbon 1s states and by measuring the PECD asymmetry of the emitted photo-electrons in a time-resolved way. In this case the initial orbital is localized, achiral, and chemically specific. It thus serves as an ideal in-situ probe of the chiral electronic potential through which the photoelectron scatters. This provides an enantio-sensitive probe of the transient chirality of the electronic structure of the molecule that is both site- and chemical- specific.

Figure 1 of top-stroy by Faccialà et al., Phys. Rev. X 13, 011044 (2023)

Figure 1 (a): 3D representation of the S-(+)-fenchone molecule, where only carbon (black) and oxygen (red) atoms are shown, hydrogen atoms omitted. (b) Energy-level diagram of the excitation and detection scheme.

We performed the experiment at the Low Density Matter (LDM) beamline of FERMI, the only FEL providing circularly polarized soft X-ray pulses up to the carbon K-edge with high temporal coherence. The excitation scheme of the experiment is depicted in Fig. 1b. A two-photon absorption of the linearly polarized ultrafast UV pump excites the molecule into the diffuse 3s Rydberg state, and the relaxation dynamics is probed in the wake of excitation by photo-ionizing the molecule with circularly polarized FEL probe pulses. We employ the velocity map imaging (VMI) spectrometer of the LDM beamline to measure the PECD of the photo-electron angular distribution.

With this experiment we show how the excitation provided by the pump not only initiates the molecular dynamics, but also causes transient chemical shifts of the carbon 1s sites, allowing to further enhance the site-specificity of the technique. In particular, the combination of a site-selective enantio-sensitive probe and the chemical shift triggered by excitation allowed us to measure the local chirality of the electronic environment around a subset of carbon atoms (carbons 2 and 3 in Fig. 1a) that in the ground state are mixed with the contributions from all other sites.  

The ability to “photograph” the chirality of a molecule from the perspective of individual atoms during an ultrafast process is a significant step forward in the study of chiral molecules, paving the way for advanced Time Resolved - chiral - X-ray Photoelectron Spectroscopy (TR-chiral-XPS). From a wider viewpoint, the understanding of chiral dynamics attainable through this type of experiments will provide novel insights into fundamental questions in photochemistry and biochemistry related to the nature of chirality and chiral recognition.

Adapted from the original under the terms of the Creative Commons Attribution 4.0 International license

This research was conducted by the following research team:

D. Faccialà1, M. Devetta1, S. Beauvarlet2, N. Besley3, F. Calegari4,5, C. Callegari6, D. Catone7, E. Cinquanta1, A. G. Ciriolo1, L. Colaizzi4, M. Coreno6,7, G. Crippa1,8, G. De Ninno6,9, M. Di Fraia6, M. Galli4,8, G. A. Garcia10, Y. Mairesse2, M. Negro1, O. Plekan6, P. Prasannan Geetha1,8, K. C. Prince6, A. Pusala1,8, S. Stagira1,8, S. Turchini7, K. Ueda11, D. You11, N. Zema7, V. Blanchet2, L. Nahon10, I. Powis3 and C. Vozzi1.
1 Istituto di Fotonica e Nanotecnologie-CNR (CNR-IFN), Milano, Italy
2 Université de Bordeaux-CNRS-CEA, CELIA, Talence, France
3 School of Chemistry, University of Nottingham, Nottingham, United Kingdom
4 Center for Free-Electron Laser Science, DESY, Hamburg, Germany
5 Physics Department, Hamburg University, Hamburg, Germany
6 Elettra-Sincrotrone Trieste, Basovizza, Italy
7 Istituto di Struttura della Materia-CNR (ISM-CNR), Roma, Italy
8 Dipartimento di Fisica, Politecnico di Milano, Milano, Italy
9 Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia
10 Synchrotron Soleil, Gif sur Yvette, France
11 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

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D. Faccialà, M. Devetta, S. Beauvarlet, N. Besley, F. Calegari, C. Callegari, D. Catone, E. Cinquanta, A. G. Ciriolo, L. Colaizzi, M. Coreno, G. Crippa, G. De Ninno, M. Di Fraia, M. Galli, G. A. Garcia, Y. Mairesse, M. Negro, O. Plekan, P. Prasannan Geetha, K. C. Prince, A. Pusala, S. Stagira, S. Turchini, K. Ueda, D. You, N. Zema, V. Blanchet, L. Nahon, I. Powis and C. Vozzi, "Time-Resolved Chiral X-Ray Photoelectron Spectroscopy with Transiently Enhanced Atomic Site Selectivity: A Free-Electron Laser Investigation of Electronically Excited Fenchone Enantiomers", Phys. Rev. X 13, 011044 (2023); DOI: 10.1103/PhysRevX.13.011044.

Last Updated on Wednesday, 19 April 2023 16:00