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Extreme Ultraviolet Circular Time-Resolved Spectroscopy

Periodic Reporting for period 4 - EXCITERS (Extreme Ultraviolet Circular Time-Resolved Spectroscopy)

Reporting period: 2021-03-01 to 2021-12-31

Chiral molecules exist as two forms, so-called enantiomers, which have essentially the same physical and chemical properties and can only be distinguished via their interaction with a chiral system, such as circularly polarized light. Many biological processes are chiral-sensitive and unravelling the dynamical aspects of chirality is of prime importance for chemistry, biology and pharmacology – as for instance a molecule can be a medicine while its mirror image is a poison. Studying the ultrafast electron dynamics of chiral processes requires characterization techniques at the attosecond (10 −18 s) timescale. Molecular attosecond spectroscopy has the potential to resolve the couplings between electronic and nuclear degrees of freedom in such chiral chemical processes. There are, however, two major challenges: the generation of chiral attosecond light pulse, and the development of highly sensitive chiral discrimination techniques for time-resolved spectroscopy in the gas phase. This research project aimed at developing vectorial attosecond spectroscopy using elliptical strong fields and circular attosecond pulses, and to apply it for the investigation of chiral molecules.
The EXCITERS project has led to technical and conceptual advances that have enabled us to perform the first time-resolved measurements of chiral molecular dynamics in the gas phase, opening the field of chiral femtochemistry, but also to measure and control the electron dynamics in chiral molecules on the attosecond timescale using strong laser fields. We have introduced new schemes to resolve the influence of molecular chirality on fundamental processes such as quantum tunneling, as well as to demonstrate the importance of the instantaneous rotation of the electric field in chiral light-matter interaction. The ensemble of spectroscopic techniques introduced within the project can now be deployed to investigate a broad variety of ultrafast processes in chiral molecules.
Tracking ultrafast chiral dynamics requires two key ingredients: a source of chiral light pulses, and a light-matter interaction scheme producing chiro-sensitive signals. These two elements have been developed in parallel during the project, resulting in a variety of schemes addressing complementary aspects of molecular chirality.
One of the pillars of the project is the development of a new circularly polarized attosecond light source in the extreme ultraviolet range, based on very high repetition rate lasers. We have purchased a new laser system and developed a new beamline to produce extremely bright extreme ultraviolet pulses, reaching record photon fluxes. We have compared different strategies to produce circularly polarized radiation. This source has been coupled to a new coincidence imaging electron-ion spectrometer, which has been built within the project, and is now used for chiral photoionization experiments.
We initially thought that tracking attosecond chiral dynamics could only be possible using attosecond extreme ultraviolet pulses. However we found several ways of measuring such dynamics with strong femtosecond laser pulses: the temporal resolution is in that case not given by the pulse duration but by the oscillation period of the electric field. We have demonstrated that significant chiral responses could be measured in this strong field regime. We have then used photoelectron interferometry to measure a tiny difference between the time it takes for an electron to leave a chiral molecule or its mirror image, when ionized by circularly polarized light (only 7 attoseconds). Next we have combined multiple electric fields to sculpt a light field whose rotation direction switches every few hundreds of attoseconds. While this field carries zero net chirality, we have demonstrated that the ionized electrons could be deeply affected by its instantaneous chirality, stressing the ultrashort character of the photoionization process. This scheme has enabled us to discover that quantum tunneling through a rotating potential barrier was sensitive to the chirality of the barrier. Last, we have used elliptically polarized strong laser fields to drive ionized electrons to re-collide with their parent ion and measured laser-induced chiral rescattering.
In parallel to the investigation of the attosecond chiral photoionization process, we have tracked the ultrafast relaxation of photoexcited chiral molecules by chiral femtochemistry. We have discovered that in chiral molecules, bound wavepackets produced by circularly polarized light show a very strong asymmetry, leading to a new type of chiral observable we called photoexcitation circular dichroism. We showed that this observable was an extremely sensitive probe of ultrafast electronic chiral dynamics.
Last, we have performed extensive studies of the resonant multiphoton ionization of chiral molecules, benefiting from the high-repetition rate of the laser. We have discovered a new way to measure enantiomeric excesses in chemical samples with unprecedented speed and accuracy using photoelectron elliptical dichroism. We have conducted fundamental investigations of this processes, and demonstrated its applicative potential.
At the beginning of the project, the photoionization of chiral molecules was mostly investigated using synchrotron radiation and a few experiments using femtosecond lasers had just been carried out in the multiphoton regime. The strong field ionization regime was not really considered as a good way to probe chirality, because the influence of the potential is generally weak in this regime. We have not only shown that a significant chiroptical response could be measured in this regime, but also that it offered a unique framework to perform chiral attosecond spectroscopy. Using multiple laser pulses and controlling the polarization state of the light led us to discover several new chiroptical phenomena – chiral photoionization delays, photoexcitation circular dichroism, photoelectron elliptical dichroism, chiral rescattering, sub-cycle gated photoelectron dichroism... This constitutes an important fundamental breakthrough in chiral light-matter interaction. We have shown that chirality plays a role at all steps of the ionization process: it imprints a phase and amplitude modulation in tunneling and electron scattering, and has a dramatic influence in Coulomb focusing and rescattering.
We have investigated the sensitivity of these new chiroptical observables to molecular structure by comparing the response of different systems. We have also carried out the first time-resolved femtosecond chiral dynamics measurements in the gas phase, demonstrating that chiral photoionization possessed a unique structural sensitivity. We are currently in the process of extending these investigations to more complex dynamics.