Presentation of SAMSON Team (Spectroscopie Atomique and Moléculaire, Structuration de Surfaces et Optique Non-linéaire)
Group leader : BARILLE Régis (PR)
Tenured staff members
Michel Chrysos (PR)
Sylvie Dabos-Seignon (CR HC)
Denis Gindre (PR)
Matthieu Loumaigne (MC)
Florent Rachet (MC)
Bouchta Sahraoui (PR)
Non-tenured staff members
Vincent Colin (PhD student)
Leandro NUNEZ (PhD student)
Henri Piel (PhD student)
Anna Popczyk (PhD student)
Théo Travers (PhD student)
Karolina Waszkowska (PhD student)
Short description of the SAMSON group activities
Our activities are on materials and their structure and dynamics. There are two main research directions in the group: the first one is on molecular interactions in dilute or dense media; the second is on the diagnosis of optical properties and the photoinduced molecular structuring.
The media studied span a vast range of possibilities, going from simple molecular or supermolecular systems to complex organic or inorganic structures, to polymers and to molecular or macromolecular surfaces; the linear or nonlinear properties of all those systems are analyzed by means of photons or toward applications in photonics. Our publications deal with intermolecular interactions in gases, molecular cooperativity in liquids and solids, photon-induced molecular structuring, as well as with the optical diagnostics of complex chemical compounds. New phenomena, such as the self-organization of a surface relief, the photon-induced molecular encryption, or some collective gyration effects in nanoengines tailored to specific needs in monocrystals or polymers, open the door toward future applications in photonics.
Finally, some straightforward links with the environmental physicochemistry are established via the study of collective processes, either in atmospheric and planetary media or in the dense phase.
Molecular interactions in dilute or dense media.
(M. Chrysos, F. Rachet)
Research interests in our group are part of two complementary orientations: on the one hand, atomic scale computer simulations of dense media, such as supercooled liquids and glass and polymers; on the other hand, processes that are induced by interactions between atoms or molecules in dilute media (non-polar gases, etc). The works by our group aim to deepen our understanding of how specific processes operate in materials, or explain observations that have remained unexplained with the current models.
They also aim to develop applications dealing with transport or environmental properties.
Both experiment and theory are parts of the know-how. Specifically, numerical simulations with the dense matter have revealed the appearance of cooperative phenomena, such as the huge increase of the viscosity of a liquid as temperature is decreased, which are particularly pronounced in the supercooled state. Interestingly, the underlying mechanisms of such effects, still unexplained, are by no means linked to any structural change of the medium. They however seem to be universal properties, in the sense that they characterize the supercooled liquids, the glass and the polymers.
We have shown recently that such cooperative effects manifest themselves in soft matter also. This finding suggests the occurrence of an even larger universality than the one quoted above. According to a possible explanation put forward, these surprising effects would be related to a cooperative motion between the molecules. One of our most spectacular recent findings consists in the observation (made numerically) that, in certain media, such as supercooled water, cooperativity is greatly enhanced upon confinement of the medium inside a nano-porous material (see figure). Our researches have applications across the range of medical science, for the preservation of living media or for drugs.
As far as dilute media are concerned, the main focus of our group is the study of molecular interactions in gases. Light scattering spectroscopy is used to probe the interactions. In our experiments all the effort is put into sensitivity. A Raman equipment is used that has been optimized especially for the detection and processing of signals as low as only few photoelectrons per week. To make it easier to grasp the idea, this situation is similar to a photographer’s camera that takes a photo of a single burning candle on the Moon. As for theory, our group develops methods, mostly quantum-mechanical, that are original and task-specific. The purpose of these methods is the study of collision-induced Raman-scattering and infrared-absorption spectra. Such spectra lie with specific processes that bring into play the interactions between gas molecules.
Owing to their strong collective character and to the very dim light signals they generate, these interactions are challenging situations for both detection and theory. These interactions have a major role in many physical phenomena, and especially in the greenhouse effect of some planetary atmospheres. More particularly, their significance in the physics of carbon dioxide and of methane is all but negligible. As has been shown by our recent results, vibrational modes that are infrared-absorption-inactive in those molecules can contribute substantially to the absorption by these gases, owing to the interactions between the gas molecules.
Through our studies, the whole topic of the collision-induced phenomena has enjoyed a renewal of interest in recent years.The role of the collisions in the interception, absorption and scattering of photons, which had thus far only partiallybeen understood and largely underestimated garnered global media attention, and it was highlighted by the international press.(See, for instance, “Physics Today” N° 6, volume 61, pp. 20-21, June 2008; “Pour la Science”, N° 368, June 2008). More recently, our group managed to observe and to successfully analyze collision-induced spectral bands generated by double vibrational transitions.
These bands are fingerprints of two transitions occurring simultaneously in two interacting molecules, and for this reason their Raman signals are far weaker than those generated by standard collision-induced processes. The feebleness of these transitions suggests them as effective benchmarks for detection and theory. Such unstructured bands are suspected of being actors in atmospheric processes. To see these signals, we use gas mixtures rather than pure gases. In so doing, this “double process”, so unlikely, takes place in spectral regions that are totally free from any spectral signature due to the individual molecules. We have already observed and analyzed such transitions within gaseous mixtures of greenhouse molecules and inert ones.
For more information :
M. Chrysos, F. Rachet, N.I. Egorova, A.P. Kouzov, “Intermolecular Raman spectroscopy long-range interactions: the CO2-Ar collision-induced n3 CO2 band“, Phys. Rev. A 75, 012707 (2007).
M. Chrysos, A.P. Kouzov, N.I. Egorova, F. Rachet, “Exact low order classical moments in collision induced bands by linear rotors: CO2-CO2 “, Phys. Rev. Lett. 100, 133007 (2008).
M. Chrysos, S. Dixneuf, F. Rachet “Anisotropic collision-induced Raman scattering by Ne-Ne: Evidence for a nonsmooth spectral wing“, Phys. Rev. A 80, 054701 (2009).
S. Dixneuf, M. Chrysos, F. Rachet, “Isotropic and anisotropic collision-induced Raman scattering by monoatomic gas mixtures: Ne-Ar“, Phys. Rev. A 80, 022703 (2009).
S. Dixneuf, M. Chrysos, F. Rachet, “Anisotropic collision-induced Raman scattering by the Kr:Xe gas mixture“, J. Chem. Phys. 131, 074304 (2009).
I.A. Verzhbitskiy, M. Chrysos, F. Rachet, A. Kouzov, “Evidence for double incoherent Raman scattering in binary gas mixtures: SF6-N2 “, Phys. Rev. A 81, 012702 (2010).
M. Chrysos et I. A. Verzhbitskiy, “Evidence for an isotropic signature in double vibrational collision-induced Raman scattering: A point-polarizable molecule model“, Phys. Rev. A 81, 042705 (2010).
Diagnostics of optical properties and photoinduced molecular structuring
(R. Barillé, B. Sahraoui, S. Dabos-Seignon, D. Gindre)
In this second direction of activities, we develop new processes for optical storage of information in polymers using the properties of assisted disorientation of azobenzenes (see figure). The goal of these techniques is the mapping of the nonlinear susceptibility (c2) of materials with high resolution. We carry out experiments by imaging microscopy of femtosecond second harmonic generation (SHG). This is a new technique, efficient which allows by an optical control of the molecular dimerization ratio to give rise to encoded images at the nanoscale. This process, which thus becomes a means of marking very difficult to detect, is currently under patent by the CNRS entitled ‘reversible recording material support for optical storage.
We realize experiments of atomic force microscopy to characterize the morphology of surfaces in the goal to develop electronic components based on polymer and organic compounds or to develop new materials for nonlinear optics. In parallel we realize we conduct researches with photo switchable molecules like azobenzenes in the goal to induce and understand the mechanisms of surface modification or thin film patterning. A sum-up of this activity has been published in ‘’Pour la Science’’ N° 378, (April 2009). Among allthe different results we have shown the possibility of spontaneous structuration of the surface of an azopolymer thin film with only one laserbeam. We have also created the concept of neuro-photonics with the possibility given by the surfacerelief patterns to be erased and reconfigured. Several of our works concern thediagnostic,functionalization and characterization of specific components for applications in optoelectronics systems.