- 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.
Extreme ultraviolet (XUV) photons carry enough energy to produce highly excited states of matter. Tracking the relaxation of these states, on the picosecond to attosecond timescale, is one of the main challenges of ultrafast science. However, XUV photons are very fragile objects, destroyed by transmission in a few hundred nanometers of most solids. Their use in sophisticated optical experiments is thus a challenging tasks. Here we solve this issue by replacing the absorption of a single XUV photon by the simultaneous absorption of five visible photons, carrying each five times less energy. The exciting light being in the visible domain, it can be manipulated in many ways, for instance by temporal or spatial shaping. The highly excited system relaxes by emitting XUV light whose direct detection is straigthforward. We have thus transformed an challenging transient XUV absorption experiment into a versatile, background-free transient XUV emission experiment.
Unravelling the main initial dynamics responsible for chiral recognition could provide a valuable insight about many biological processes. However this challenging task requires a sensitive enantiospecic probe to investigate molecular dynamics on their natural femtosecond timescale. Here we show that, in the gas phase, the ultrafast relaxation dynamics of photoexcited chiral molecules can be tracked by recording Time-Resolved PhotoElectron Circular Dichroism (TR-PECD) resulting from the photoionisation by a circularly polarized probe pulse. A large forward/backward asymmetry along the probe propagation axis is observed in the photoelectron angular distribution. Its evolution with pump-probe delay reveals ultrafast dynamics. We show for the first time that PECD, which originates from the electron scattering in the chiral molecular potential, appears as a new sensitive observable for ultrafast molecular dynamics in chiral systems. Promising fun in perpective !
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.