Physics Focus: Molecular Probe Uses a Polarization Flip

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

Attosecond-resolved photoionization of chiral molecules

Just like a thread converts the rotation of a bolt into forward or backward translation depending on the rotation direction, chiral molecules eject electrons forward or backward when ionized by circularly polarized light. This effect, called Photoelectron Circular Dichroism, originates from the scattering of the electrons off the chiral potential. The asymmetric scattering electron dynamics induces a delay between the ejected electrons: we found out that the forward electrons were emitted 7 attoseconds before the backward ones.

Read more: http://www.inrs.ca/english/actualites/are-molecules-right-handed-or-left-handed

Article in Chemistry World: https://www.chemistryworld.com/news/chirality-probed-with-attosecond-precision/3008437.article

En savoir plus: http://www2.cnrs.fr/presse/communique/5353.htm?debut=0

The article: S. Beaulieu et al., Science 358, 1288 (2017)

FigureComm

XUV Spectroscopy without XUV pulses

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.

Physical Review A 95, 041401(R) / arXiv:1701.06352

TRAXTREX

The first time-resolved Photoelectron Circular Dichroism

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 !

  arXiv:1611.06226 or Journal of Physical Chemistry Letters 7, 4514 (2016)

 

Attosecond lighthouse of HHG from excited states

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

lighthouse

Universality of Photoelectron Circular Dichroism

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.

Check out our paper and video abstract on New Journal of Physics

GifAnime.sozi.ogv

Chiral High-harmonic generation

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.
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3369.html

FROMAGE

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)

Photoelectron circular dichroism with quasi-circular high-harmonics

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 photons@15.5 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.

Velocity map imaging of ClN3

Azide XN3 are highly energetic compounds relevant for click chemistry.  We have investigated the ultrafast photodissociation of ClN3 in two energy range, one leading to an azide radical in its linear form and another one leading to its cyclic form. Both the energy balance and the momentum one are recorded at the femtosecond scale. This first experimental investigation of azide compounds in the femtosecond domain shows that the emission angle of the fragments relative to the pump polarisation changes during the dissociation time drastically for the cyclic radical compared to the linear one.