Time-Resolved Chiral X-Ray Photoelectron Spectroscopy with Transiently Enhanced Atomic Site Selectivity: A Free-Electron Laser Investigation of Electronically Excited Fenchone Enantiomers

Chirality is widespread in nature, playing a fundamental role in biochemical 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 photoexcited states. One of the most promising techniques for the study of photoexcited 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 nonlocal 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 utilizing 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 (${\mathrm{C}}_{10}{\mathrm{H}}_{16}\mathrm{O}$) 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. This chemical-specific, site-specific, and enantiosensitive observation of the electronic structure of a transiently photoexcited chiral molecule is expected to pave the way toward chiral femtochemistry probed by core-level photoemission.

Popular Summary

Chiral molecules cannot be rotated to create their own mirror image. The two mirror-image structures of such molecules, called enantiomers, may have dramatically different properties when interacting with another chiral molecule. As such, chirality plays a fundamental role in biochemical processes and in the origin of life itself. It also has widespread applications in the synthesis of drugs, pheromones, odors, and scents. Understanding the contribution of each specific atom to the overall chirality of complex molecules, particular during chemical reactions, therefore has wide appeal. Here, we demonstrate that it is possible to get new insights into the time-dependent local chirality of a molecule after photoexcitation by using the site specificity of a soft x-ray probe.

In fenchone, a prototypical chiral compound, we use circularly polarized light to distinguish the contribution to the molecular chirality from the different carbon atoms during the dynamics following photoexcitation. In particular, exciting the molecule with an ultrashort laser pulse produces a separation in energy between the contributions of different molecular sites, which can then be observed by photoionization with circularly polarized soft x-ray pulses from a free-electron laser.

This approach offers a sensitive probe for the study of ultrafast dynamics in chiral molecules. We expect that it will advance chiral femtochemistry, allowing researchers to unravel the chiral contribution of each atomic site during an ultrafast reaction.

Figure 1

(a) Experimental setup. The circularly polarized XUV probe (CPL) and the visible pump with linear horizontal (LH) polarization are focused quasicollinearly in the molecular beam of S-(+)-fenchone enantiomers. (a.1) 3D representation of the S-(+)-fenchone, where only carbon (black) and oxygen (red) atoms are shown, while hydrogen atoms are hidden for the sake of clarity. Carbon atoms are labeled from 1 to 10. (b) Simplified energy level diagram of the excitation scheme. (b.1) 3D representation of the $\mathrm{LUMO}+1$ orbital ($3s$ Rydberg state). (b.2) 3D representation of the HOMO orbital. Representations (a.1), (b.1), and (b.2) share the same molecular orientation.

Figure 2

C $K$-edge ionization of ground state S-(+)-fenchone. (a) Experimental PES recorded at 300 eV photon energy and (b) a theoretically predicted spectrum based on the calculated C $1s$ binding energies. These are marked with green and violet sticks, the lengths of which are intended for clarity only. The gray area in (a) represents the $1\sigma$ confidence interval, multiplied by 10 for clarity. The associated experimental PECD results appear in (c), while in (d) the theoretically predicted PECD for S-(+)-fenchone is presented. The gray area in (c) represents the $1\sigma$ confidence interval. In both (c) and (d), the backward asymmetry from the ${\mathrm{C}}_1$ localized emission reaches almost 20%. Theoretical PECD asymmetries for S-(+)-fenchone (solid line) and R-($-$)-fenchone (dashed line) as a function of photon energy are presented for the ${\mathrm{C}}_1$ peak in (e) and the unresolved average of the ${\mathrm{C}}_{2-10}$ peaks is presented in (f). Dots and diamonds in (e) indicate experimental synchrotron data for S-(+)-fenchone and R-($-$)-fenchone, respectively (see Ref. [14]). The experimental $h\nu =300\mathrm{eV}$ results from our current work for the ${\mathrm{C}}_1$ and ${\mathrm{C}}_{2-10}$ peaks are shown with violet and green square dots in (e) and (f), respectively, along with the corresponding $1\sigma$ error bars.

Figure 3

(a) PES at $t_A=-200\mathrm{fs}$ (dashed line) and $t_B=100\mathrm{fs}$ (dotted line), difference $\mathrm{\Delta }{\widehat{\sigma }}_0(t_B)={\widehat{\sigma }}_0(t_B)-{\widehat{\sigma }}_0(t_A)$ of the sliding averaged PES ${\widehat{\sigma }}_0$ (solid line and $1\sigma$ error bars). (d) $\mathrm{\Delta }{\widehat{\sigma }}_0(t)$ for all the measured delays $t$. (a),(d) The energies $E_1$, $E_2$, and $E_3$ are indicated by dashed green, violet and red lines, respectively. (e) ${\widehat{\sigma }}_0(t)$ for $E_1$, $E_2$, and $E_3$ indicated by green, violet, and red dots, respectively, along with the $1\sigma$ error bars. A single exponential of 3.3 ps decay time, corresponding to the measured lifetime of the $3s$ Rydberg state [9] is shown with solid lines. The amplitude and time zero of this exponential are fitted to the experiment and the $1\sigma$ confidence interval of the fit is represented by a gray shaded area. (b),(c) Calculated binding energies for the ground state (b) and excited $3s$ Rydberg state (c), indicated by red (${\mathrm{C}}_1$), blue (${\mathrm{C}}_2$, ${\mathrm{C}}_3$), and orange (from ${\mathrm{C}}_4$ to ${\mathrm{C}}_{10}$) dots and vertical lines, where the vertical offset is intended for clarity only. Theoretical PES in (b) and (c) are shown with dashed and dotted line, respectively.

Figure 4

(a) Experimental PECD (black lines) and PES (gray lines) at $t_A=-200\mathrm{fs}$ (dashed lines) and $t_B=100\mathrm{fs}$ (solid lines), and corresponding $1\sigma$ PECD confidence intervals (shaded gray area), for S-(+)-fenchone. Energy $E_2$ is indicated by a vertical violet dashed line. (b),(c) Calculated PECD [S-(+)-fenchone] for the electrons emitted from the carbon $1s$ site from a ground electronic state (b) and from excited $3s$ Rydberg state (c), indicated by red (${\mathrm{C}}_1$), blue (${\mathrm{C}}_2$, ${\mathrm{C}}_3$), and orange (from ${\mathrm{C}}_4$ to ${\mathrm{C}}_{10}$) lollipops. Total PECD in (b) and (c) shown with dashed and dotted black line, respectively. The theoretical PES is also drawn for reference (gray lines). (d) Measured TR-PECD at $E_2=292.53\mathrm{eV}$, shown with violet dots along with the $1\sigma$ error bars. The static value is reported at $t=-\infty$. The theoretical PECD from the model is shown as a dashed line overlaid to the experimental data.

Figure 5

The $a_{lm}$ coefficients, $l=1–6$, for the calculated S-(+)-fenchone ${\mathrm{C}}_1$ $1s^{-1}$ PAD. See Table 1 and note that this figure is for right circular polarization ($p=-1$).