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Extreme Ultraviolet Circular Time-Resolved Spectroscopy

Periodic Reporting for period 3 - EXCITERS (Extreme Ultraviolet Circular Time-Resolved Spectroscopy)

Reporting period: 2019-09-01 to 2021-02-28

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. 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 aims at developing vectorial attosecond spectroscopy using elliptical strong fields and circular attosecond pulses, and to apply it for the investigation of chiral molecules. To achieve this, the project will (1) establish a new type of highly sensitive chiroptical spectroscopy using high-order harmonic generation by elliptical laser fields; (2) create and characterize sources of circular attosecond pulses; (3) use trains of circularly polarized attosecond pulses to probe the dynamics of photoionization of chiral molecules and (4) deploy ultrafast dynamical measurements to address the link between nuclear geometry and electronic chirality. The developments from this project will set a landmark in the field of chiral recognition. They will also completely change the way ellipticity is considered in attosecond science and have an impact far beyond the study of chiral compounds, opening new perspectives for the resolution of the fastest dynamics occurring in polyatomic molecules and solid state physics.
Tracking the photoionization dynamics of chiral molecules requires extreme temporal resolution because these dynamics occur on an attosecond timescale. We thus initially thought that this could only be possible using attosecond extreme ultraviolet pulses. However we found a way to measure the dynamics with femtosecond laser pulses using photoelectron interferometry to reach a one-attosecond resolution. This has enabled us to discover a tiny asymmetry in the time it takes for an electron to leave a chiral molecule when ionized by circularly polarized light (only 7 attoseconds).
Since photoionization is an attosecond process, it should be sensitive to the instantaneous shape of the electric field. To verify this, 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.
In parallel to the investigation of the chiral photoionization process, we studied the influence of chirality in the creation of bound electronic wavepackets. We have discovered that in chiral molecules, the wavepackets produced by circularly polarized light showed 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.
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 (6.6e14 photons/s at 18 eV, corresponding to an average power of 1.9 mW). This source is being coupled to a new coincidence imaging electron-ion spectrometer, which has been built within the project.
We have also used the high-repetition rate laser directly to probe molecular chirality, and 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.
The work performed so far has led us to discover several new chiroptical phenomena – chiral photoionization delays, photoexcitation circular dichroism, photoelectron elliptical dichroism… This constitutes an important fundamental breakthrough in chiral light-matter interaction. From a technological point of view, we now have a very bright source of extreme ultraviolet pulses and a coincidence electron ion imaging spectrometer, which puts us in a very good position to probe complex ultrafast dynamics occurring in chiral molecules, and to address the fundamental questions related to the birth and death of chirality, the transfers from nuclear to electronic degrees of freedom, or the mechanisms responsible for chiral recognition. The results obtained in this first period are very encouraging: we now have a bright source of extreme ultraviolet pulses, and a good expertise in chiral photoionization dynamics. We will combine these two to perform measurements using a new coincidence electron-ion imaging spectrometer. This will enable us to probe complex ultrafast dynamics occurring in chiral molecules, and to address the fundamental questions related to the birth and death of chirality, the transfers from nuclear to electronic degrees of freedom, or the mechanisms responsible for chiral recognition.