Headlines 2014

Nov 26, 2014

Determination of radiolytic yield in a spatio-temporal structure of ionization tracks of high energy heavy ion is the big challenge in liquid water for radiation chemistry modeling of high LET (linear energy transfer) particles especially in a context of nuclear plant and in hadrontherapy. OH radical is the most oxidant species evolved in radiation chemistry processes immediately following the ionization of water molecule. It is formed within the femtosecond.

Simultaneous effects of LET and temperature, if we consider them in competition, could then reveal the dynamic structure of radicals around the track. To access to these information, the experimental challenge was designing a specific optical flow cell resistant pressure, temperature and letting the ions reaching the solution, knowing these ions have short range in material. These experiments were performed at GANIL in collaboration with CIRIL.

Mar 26, 2014

Motions of the carbonyl group of the Photoactive Yellow Protein (PYP) chromophore have been investigated, for the first time, by femtosecond time-resolved circular dichroism (tr-CD) spectroscopy, in the near-ultraviolet spectral region. The quantitative analysis of the tr-CD signals shows that, upon excitation, the carbonyl group undergoes a fast (≪0.8 ps) and unidirectional flipping motion in the excited state with an angle of ca. 17−53°. For the subset of proteins that do not enter the photocycle, tr-CD spectroscopy provides strong evidence that the carbonyl group moves back to its initial position, leading to the formation of a nonreactive ground-state intermediate of trans conformation. The initial ground state is then restored within ca. 3 ps.

Jul 22, 2014

Les propriétés des molécules flexibles sont fonction de leur forme et leurs applications intéressent de multiples domaines (médicaments, nouveaux matériaux…). Un enjeu majeur dans ce domaine est de pouvoir contrôler leur repliement pour in fine contrôler leurs propriétés. De nombreuses techniques existent pour caractériser les différentes conformations que peuvent adopter ces molécules flexibles, dont la spectroscopie IR. La présente étude, impliquant une équipe du LIDyL en collaboration avec deux équipes de l’Université Paris-Sud, porte sur une fonction chimique très appréciée dans la synthèse de molécules flexibles de forme contrôlée : la fonction hydrazide (–CO-NH-N<).

Les conformations des systèmes flexibles sont structurées en partie par l’hyperconjugaison (interaction stabilisante des électrons d’une liaison σ avec une orbitale vide adjacente) dont la mise en évidence expérimentale directe restait jusqu’à présent très limitée. Cette étude montre que la spectroscopie IR est extrêmement sensible aux effets d’hyperconjugaison se produisant au cœur des hydrazides, offrant ainsi un puissant diagnostic conformationnel.

En parvenant à caractériser ces effets sur des hydrazides, la spectroscopie IR ouvre ainsi des perspectives non seulement pour la caractérisation des conformations de nombreux systèmes flexibles, mais aussi pour l’étude plus fondamentale du concept d’hyperconjugaison et de sa modélisation.

May 20, 2014

A major challenge for the study of laser-matter interactions at ultrahigh intensity is to find simple methods both to control these interactions, and to characterize them on very small spatial scales (micron) and temporal (femtosecond to attosecond). In this context, I will show how, through a very simple shaping of a femtosecond laser beam of high power, one can generate 'plasmas gratings' resistant to ultraintenses laser pulses, at the surface of initially flat targets (glass plate). Using a diffractive imaging technique called 'ptychography', these transient plasma  gratings can then be used as diffracting objects to reconstruct  spatially, in amplitude and in phase, the harmonic field En (x) generated at the surface of the target. These results have widely exceeded our original goals, and I will put some emphasis on the unexpected path which led us there.

 

Optically Controlled Solid-Density Transient Plasma Gratings, Phys. Rev. Lett. 112, 145008 – Published 11 April 2014, S. Monchocé, S. Kahaly, A. Leblanc, L. Videau, P. Combis, F. Réau, D. Garzella, P. D’Oliveira, Ph. Martin, and F. Quéré, DOI: http://dx.doi.org/10.1103/PhysRevLett.112.145008

 

Contact CEA  : Fabien QUERE

 

May 16, 2014

One of the goals in physical chemistry is to follow reaction paths in details. Many techniques have been developed with studies conducted in cells, effusive or supersonic beams, using laser spectroscopy, mass spectrometry etc. as investigation tools.

 

An original technique called "Cluster Isolated Chemical Reaction" (CICR) has been developed by the group "Dynamique Réactionnelle" of LIDyL/LFP for spectroscopic and reaction dynamics studies [1]. It allows determining the stoichiometry of the observed processes on a nanoreactor consisting of a van der Waals cluster. Recently, it has been successfully applied to helium droplet, which is a quite fascinating medium joining a very low temperature (0.37 K), superfluidity and finite size. The latter allows trapping and isolating a controlled number of molecules and stabilizing complexes which have a very low stability. The study of such systems provides very accurate information on the intramolecular potential energy surfaces (PES), improving the reliability of the associated models.

Mar 26, 2014

Motions of the carbonyl group of the Photoactive Yellow Protein (PYP) chromophore have been investigated, for the first time, by femtosecond time-resolved circular dichroism (tr-CD) spectroscopy, in the near-ultraviolet spectral region. The quantitative analysis of the tr-CD signals shows that, upon excitation, the carbonyl group undergoes a fast (≪0.8 ps) and unidirectional flipping motion in the excited state with an angle of ca. 17−53°. For the subset of proteins that do not enter the photocycle, tr-CD spectroscopy provides strong evidence that the carbonyl group moves back to its initial position, leading to the formation of a nonreactive ground-state intermediate of trans conformation. The initial ground state is then restored within ca. 3 ps.

Jul 02, 2014

Elementary chemical acts are often associated with the crossing of a saddle point: 2 colliding reagents are climbing up a valley of the ground state potential energy surface which describes their interaction; they reach a saddle point and run downhill a valley toward the reaction products. The landscape is more complicated when electronic excitation is present since several potential surfaces are coupled, often via conical intersections (see Fig.1). Potential surfaces are very distorted in their vicinity, allowing ultra-fast exchanges - ~ 100 fs = 10-13 s or less - between electronic energy and deformation of the molecular skeleton. The Reaction Dynamics group at LIDyL/LFP is very active in this field for clarifying of the role of these intersections. It combines femtochemistry pump-probe experiments with sophisticated theoretical approaches.

A recent result on electronically excited 2-hydroxypyridine revealed a surprising dynamics (REF. and Fig.1). The molecule was excited in the S1 state (Fig.1, purple arrow), in the vicinity of a saddle point where the dominant electronic configuration is pp*. The wave packet which is formed moves out of this area along trajectories as those schemed as red and brown spaghettis in Fig.1. Departure from the excitation area takes place in ca 100 fs, mostly via deformations of the pyridine cycle. A slower presumably chaotic evolution follows (~1.3 ps) when the movement of the H-atom bonded to the O-atom couples other deformations of the molecular skeleton. This corresponds to a roaming behavior of the wave packet which spreads in areas where np* is the dominant electronic configuration. Roaming behaviors have been invoked repeatedly to interpret time unresolved experiments. The reference below reports for the first time their direct observation in a time-resolved femtochemistry experiment.

May 16, 2014

One of the goals in physical chemistry is to follow reaction paths in details. Many techniques have been developed with studies conducted in cells, effusive or supersonic beams, using laser spectroscopy, mass spectrometry etc. as investigation tools.

 

An original technique called "Cluster Isolated Chemical Reaction" (CICR) has been developed by the group "Dynamique Réactionnelle" of LIDyL/LFP for spectroscopic and reaction dynamics studies [1]. It allows determining the stoichiometry of the observed processes on a nanoreactor consisting of a van der Waals cluster. Recently, it has been successfully applied to helium droplet, which is a quite fascinating medium joining a very low temperature (0.37 K), superfluidity and finite size. The latter allows trapping and isolating a controlled number of molecules and stabilizing complexes which have a very low stability. The study of such systems provides very accurate information on the intramolecular potential energy surfaces (PES), improving the reliability of the associated models.

Nov 26, 2014

We report on a 400 nm broad-band type I frequency doubling in a non-collinear geometry with pulse front tilted and chirped femtosecond pulses (l0 = 800 nm, FTL pulse duration ~ 45 fs). With moderate power densities (2 to 10 GW/cm2) thus avoiding higher-order nonlinear phenomena, the energy conversion efficiency was up to 65%. Second-harmonic pulses of Fourier transform limited pulse duration shorter than the fundamental wave were generated, exhibiting a good beam quality and no pulse-front tilt. High energy (20 mJ/pulse) was produced in a 40 mm diameter and 6 mm thick LBO crystal. To the best of our knowledge, this is the first demonstration of this optical configuration with sub 100 fs pulses. A good agreement between experimental results and simulations is obtained.



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