- Publication : vendredi 11 septembre 2009 17:03
HarMoDyn is a research project that aims at measuring ultrafast molecular dynamics using high-order harmonics and strong-field driven attosecond electron wavepackets, as well as fs-REMPI (resonant enhanced multiphoton ionization) coupled to velocity map imaging. These research interest lead us to develop as well theoretical tools to describe the main steps of the undergoing pump-probe experiments.
The HarMoDyn team is located in the laboratory CEntre Lasers Intenses et Applications (CELIA - CNRS UMR 5107) at Universite de Bordeaux , 351 Cours de la Libération, 33405 TALENCE CEDEX, France.
CELIA facilities are accessible through the national LOA-IRAMIS-CELIA call, and via LASERLAB Europe.
In High-order Harmonic Generation (HHG), the electrons that tunnel ionize from a given electronic state generally photorecombine onto the same state. We have demonstrated that during a few-cycle driving laser pulse, some Rydberg states can be populated and open a new channel for HHG : the ionization from excited states and recombination to the ground state. Using the attosecond lighthouse technique, we showed that the high-harmonic emission from Rydberg states is temporally delayed by few-femtosecond compared to the usual non-resonant HHG.
Samuel Beaulieu, Seth Camp, Dominique Descamps, Antoine Comby, Vincent Wanie, Stéphane Petit, François Légaré, Kenneth J. Schafer, Mette B. Gaarde, Fabrice Catoire, Yann Mairesse, Physical Review Letters 117, 203001 (2016), arXiv:1603.07905v2
When circularly polarized light ionizes chiral molecules, more electrons are emitted forward or backward the light propagation axis. This asymetry, which reverses with the light helicity or molecule's handedness, is called Photo-Electron Circular Dichroism. PECD is an extremely sensitive probe of molecular chirality in the gas phase. We have investigated PECD in all ionization regimes, from the single XUV photon absorption to tunnel ionization by mid-infrared lasers, and found out that PECD was a universal effect.
We have recently demonstrated that high-order harmonic generation by elliptical laser fields allowed two enantiomers of a chiral species to be distinguished. The resulting harmonic intensity depends on the handedness of the molecule, enabling one to discriminate two enantiomers, even with laser ellipticities as low as 1%. This effect originates from attosecond chiral hole dynamics. The strong laser electric field ionizes the molecules, leaving a hole in the ion which rotates under the influence of the laser magnetic field in a few hundreds of attoseconds. The rotation of this hole is probed by the recollision of the electrons accelerated by the laser field. The contribution of magnetic dipole transitions is enhanced by the interferometric nature of the process. As a result, the technique has an exceptionally high sensitivity in terms of chiral discrimination, up to two orders of magnitude above usual optical techniques.
FROMAGE is not only cheese, it is now also a technique which enables retrieving the phase and amplitude of the harmonic emission from excited molecules: Frequency Resolved Opto-Molecular Gating. We used this technique in combination with two-color high-harmonic generation to decouple the ionization and recombination steps in the generation process in vibrating N2O4 molecules. This enabled us to reveal a strong modulation of the recombination dipole moment. Physical Review Letters 116, 053002 (2016)
We have recently discovered that elliptical laser pulses focused in a specific molecular medium resulted in the emission of quasi-circular high-order harmonics. Optimizing the different parameters in the experiment enabled us to create a bright, coherent source of femtosecond pulses in the XUV domain (2x106 email@example.com eV at 1 kHz). We used this source to photoionize chiral molecules in the gas phase, and measured a forward/backward asymetry in the angular distribution of the ionized electrons. This asymetry is the signature of photoelectron circular dichroism, a purely dipolar process which is very sensitive to molecular structure.
This experiment constitutes the first XUV photoelectron circular dichroism measurement with a table-top system, all other studies being performed using synchrotron radiation. This works opens the way to femtosecond and attosecond time-resolved studies of chirality.