24 Janvier – Thesis defense - Sandra Beauvarlet

14 h Amphi A - Building A29 (University of Bordeaux | Talence campus)

Ultrafast electron dynamics in chiral molecules probed by a variety of polarization schemes at the attosecond and femtosecond timescale.

Molecular chirality is an important property in many fields such as biology, chemistry, pharmacology. Chiral molecules refer to the molecules that are not superimposable to their mirror image and exist in two forms called enantiomers which are very difficult to distinguish. However, being able to distinguish them is of critical importance since the interaction of enantiomers with other chiral objects can be extremely different : one of the enantiomers can be a medicine whereas the other one can be a poison (e.g. la thalidomide). These major differences come from chemical reactions ocurring between chiral molecules, interaction which we find interesting to study and understand. To do so, we use ultrashort laser pulses with controlled polarization as a probe for the investigation of electron dynamics in chiral light-chiral matter interaction at femtosecond (fs = 10-15s) and attosecond (as = 10-18s) timescales. This interaction produces asymmetries in the angular distribution of photoelectrons along the laser’s propagation direction. This asymmetry in the photoelectron yield, called photoelectron circular dichroism (PECD), is a signature of the molecular structure and is exactly opposite when the enantiomer or the light helicity is reversed.
In this thesis, we investigate molecular chirality playing with two key parameters : the electric field polarization and its intensity which defines the ionization regime. In resonant multiphoton ionization regime, the photon energy is scanned to reveal the influence of different intermediate states in the chiral response. The ellipticity control allows the selection of a subset of chiral molecules and highlights electronic dynamics and couplings that are not visible in circular polarization.
The ellipticity control is then used in a strong field ionization regime where electron dynamics  are driven by the laser field dynamic. In this regime, the ellipticity allows to find a compromise between electron rescattering and chirality of the electric field. In this work we show the first trace of 2D and 3D chiral asymmetries of about 10 % coming from rescattered electrons ( ~ 35 eV). The elliptical dichroism of rescattered photoelectrons is observed and it varies in the different chiral species studied.
In the remaining parts of the thesis, the polarization becomes more engineered and structured by combining two-color fields linearly and orthogonaly polarized. This field has a zero cycle-averaged chirality but variable instantaneous chirality on a time scale smaller than the optical cycle (< 1.7 fs). In strong field regime, we show that photoelectron dichroism is sensitive to sub-fs dynamics of the electric field. This new polarization and sub-cycle interferometry scheme is then combined with attoclock measurements and calculations to study the contributions to the chiral response of tunneling and electron scattering processes occurring in tunnel ionization. These new observables demonstrate that a chiral sensitive phase and amplitude are imprinted when the electron tunnels through the potential barrier.
Finally, we are interested in a new form of 3D light polarization engineered for high order harmonic generation purposes in chiral molecules. Following a recent theoretical work predicting a very strong sensitivity to chirality in this scheme, we present the preliminary designs for an experiment to come.

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