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 and COLTRIMS. These research interests 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-LIDYL-CELIA call,  and via LASERLAB Europe.

                   https://erc.europa.eu/sites/default/files/LOGO_ERC-FLAG_EU%20NEGATIF.jpg

One of our main research interests is the investigation of ultrafast dynamics of chiral molecules in the gas phase, on the femtosecond and attosecond timescale. This is the goal of the EXCITERS project - Extreme Ultraviolet Circular Time-Resolved Spectroscopy - funded by the European Reseach Council (ERC) under the European Union's Horizon 2020 research and innovation programme. This project aims at developping innovative tools based on high-repetition rate fiber laser systems, coincidence electron-ion imaging and high harmonic spectroscopy, to study chiral molecules. 

 

 

Highlights

Physics Focus: Molecular Probe Uses a Polarization Flip

Read an article on Physics describing our recent results on shaping the sub-cycle optical chirality of a laser field to probe molecular chirality:

Focus: Molecular Probe Uses a Polarization FlipFocus: Molecular Probe Uses a Polarization Flip

The results are published in Physical Review X:

Controlling Subcycle Optical Chirality in the Photoionization of Chiral Molecules,

S. Rozen, A. Comby, E. Bloch, S. Beauvarlet, D. Descamps, B. Fabre, S. Petit, V. Blanchet, B. Pons, N. Dudovich, and Y. Mairesse, Phys. Rev. X 9, 031004 (2019)

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Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism

The fast and accurate analysis of chiral chemical mixtures is crucial for many applications but remains challenging. Here we use elliptically-polarized femtosecond laser pulses at high repetition rates to photoionize chiral molecules. The 3D photoelectron angular distribution produced provides molecular fingerprints, showing a strong forward-backward asymmetry which depends sensitively on the molecular structure and degree of ellipticity. Continuously scanning the laser ellipticity and analyzing the evolution of the rich, multi-dimensional molecular signatures allows us to observe real-time changes in the chemical and chiral content present with unprecedented speed and accuracy. We measure the enantiomeric excess of a compound with an accuracy of 0.4% in 10 min acquisition time, and follow the evolution of a mixture with an accuracy of 5% with a temporal resolution of 3 s. This method is even able to distinguish isomers, which cannot be easily distinguished by mass-spectrometry.

Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism
A. Comby, E. Bloch, C. M. M. Bond, D. Descamps, J. Miles, S. Petit, S. Rozen, J. B. Greenwood, V. Blanchet & Y. Mairesse, Nature Communications 9, 5212 (2018)

PicturePEELDlite

Decoupling excitation and ionization steps in photoelectron circular dichroism

Multiphoton ionization by circularly polarized laser pulses is a powerful way to probe chiral molecules in the gas phase. The ionization process often involves going through intermediate states, in a resonance-enhanced multiphoton ionization scheme. The importance of these resonances in the photoelectron circular dichroism (PECD) remained unclear. To elucidate it, we decoupled the photoexcitation and photoionization steps, using two laser pulses with independent polarization states. We found that the ionizing photons had by far the greatest influence on the resulting PECD signal. Changing the helicity of the ionization laser pulse leads to a sign change of the PECD, while changing the helicity of the excitation laser pulse only slightly changes the magnitude of the PECD. We even observed PECD using a linearly polarized excitation laser pulse, as long as the ionization laser pulse was circularly polarized.

Scilight article on our work: https://aip.scitation.org/doi/10.1063/1.5060729

Multiphoton photoelectron circular dichroism of limonene with independent polarization state control of the bound-bound and bound-continuum transitions
S. Beaulieu, A. Comby, D. Descamps, S. Petit, F. Légaré, B. Fabre, V. Blanchet, and Y. Mairesse, The Journal of Chemical Physics 149, 134301 (2018); https://doi.org/10.1063/1.5042533

 Decoupling      

 

Extreme Ultraviolet Blast Beat at 10 Millions BPM

Our new laser system, BlastBeat, delivers two synchronized beams of 130 fs pulses at 1030 nm, with 50W average power each and a repetition rate between 166 kHz and 2 MHz. (In hardcore punk (as well as grindcore, deathcore and death and black metal), two synchronized beats played ultrafast are called a blast beat and its speed is quantified in Beats Per Minute, BPM).
We have developped a new high-harmonic generation beamline using this laser, and implemented an original and simple characterization technique to determine the absolute gas density profile in the generating medium. This has allowed us to optimize the generating conditions to reach perfect phase matching, producing a bright XUV source at 10 millions BPM (166 kHz).

Absolute gas density profiling in high-order harmonic generation
A. Comby, S. Beaulieu, E. Constant, D. Descamps, S. Petit, and Y. Mairesse, Optics Express 26, 6001

PXCD - Photoexcitation Circular Dichroism

Chiral effects appear in a wide variety of natural phenomena and are of fundamental importance in science, from particle physics to metamaterials. The standard technique of chiral discrimination—photoabsorption circular dichroism—relies on the magnetic properties of a chiral medium and yields an extremely weak chiral response. We have proposed and demonstrated an orders of magnitude more sensitive type of circular dichroism in neutral molecules: photoexcitation circular dichroism. This technique does not rely on weak magnetic effects, but takes advantage of the coherent helical motion of bound electrons excited by ultrashort circularly polarized light. It results in an ultrafast chiral response and the efficient excitation of a macroscopic chiral density in an initially isotropic ensemble of randomly oriented chiral molecules. We have probed this excitation using linearly polarized laser pulses, without the aid of further chiral interactions. Our time-resolved study of vibronic chiral dynamics opens a way to the efficient initiation, control and monitoring of chiral chemical change in neutral molecules at the level of electrons.

Nature Physics 14, 484 (2018)

PXCD