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Accueil > Thèmes de recherche > Plasmas denses et chauds et WDM

Projet CASTORS : CArbon phaSe Transition prObed by Raman Spectroscopy

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Projet CASTORS : CArbon phaSe Transition prObed by Raman Spectroscopy

The CASTORS project, funded by the french research agency ANR, aims to gnaw through challenging problems of contemporary science in the emerging field of both warm dense matter and non-equilibrium physics. This project will explore the ability of time-resolved Raman spectroscopy (T-RRS) to probe dynamics and characterize structural properties of materials in these new regimes. The project will focus on carbon based materials. Carbon presents the particularity to form 3 types of bonds (sp3, sp2 and sp) which directly relates to a specific allotrope (diamond, graphite for the most common bulk forms). In addition to the intrinsic interest of studying carbon, this material is a simple and prototypical covalent material. This type of material is of great interest for other fundamental and applied field of physics.

Raman spectroscopy has been extensively used, as a static tool, to investigate the different structures of carbon allotropes. We contemplate extending these studies to understand processes in ultra-fast non-equilibrium phase transition triggered by optical or x-ray lasers and to characterize the elusive structure of liquid carbon induced by isochoric heating by x-ray laser and induced by laser shock. These two specific problems have to be understood as a step forward in the understanding of a more general topic : the extreme states of matter. This project will take place in the PETRUX group of the CELIA which is involved in WDM and High Energy Density physics. Furthermore, this project involves different teams renowned for their expertise in all the fields required for the success of this project ranging from sample preparation up to state-of-art simulations based on new theoretical developments.

Figure 1 :
Left : Raman spectrum from single CVD diamond crystal.
Right : Raman spectrum from pyrolytic graphite (black curve) and HOPG, highly ordered pyrolytic graphite (red curve). The inset shows the D peak which relates to defect in graphite structure. (Raman measurements by J. Chalupsky, IOP Prague)

Hence the scientific program is divided in three “tasks”, based on three different class of experiments performed at different facilities in collaboration with other teams. Each experiment will be supported by theoretical investigations made by the three theory groups involved in this program. They have developed simulation codes based on different approaches. Two approaches simulate the evolution of the atomic structure. One is based on tight-binding molecular dynamics ; the other one is using an ab-initio approach. The third code will be used to model the absorption of intense x-ray pulses, needed for the task 2. The output of this last simulations will be used as an input for the 2 other code. Comparing the results of these two approaches will allow testing model, and also circumvent their domain of validity. Finally, a special emphasize will be given on involving and train young scientists in this new field of ultra-fast physics. The main experimental tasks we foresee will be organized as follow :
1. The first task will address solid-to-solid, i.e. amorphous t crystal (crystallization) and diamond t graphite (graphitization), phase transition dynamics. It will be performed at the CELIA laboratory. We will take advantage of the on-site fs laser to develop the instrumentation and build-up an expertise in T-RRS. This is also mandatory to “scale” and design the 2 other experiments. As we will use the on-site laser, and thanks to its extremely well characterized and stable performance, an extensive parametric study will be performed. This will allow an in-depth understanding of the processes.
2. The second task will be devoted to phase transition induced by x-ray Free Electron Laser pulse. The idea is to performed similar experiments than in task 2 but using an XFEL pulse as pump. The use of an x-ray pulse will allow testing influence of below and above K-edge excitation on the dynamics of the process. The liquid state will also be investigated, as liquid carbon is more likely to be observed using XFEL pulses. In fact, due to large penetration depth of x-ray (around 1 µm at hv = 270 eV), a large volume of material can be heated, favoring liquefaction. The liquid phase can then be “created” and maintain during a time long enough to be observable. This type of experiment will explore only LDL, in a static mode as described in the following section.
3. The 3rd task will be exclusively dedicated to study the liquid structure of carbon. This is the most difficult task from both the experimental and theoretical point of view. Using shock compression should allow accessing both LDL and HDL. It requires the use of high-energy laser facility (most probably the LULI 2000 facility in France) delivering few shots per day, crippling signal averaging. Only shock compression technic is able to bring sample above the 500 GPa up to 1000 GPa pressure region, where the LDL / HDL border is supposed to lie. These experiments will be the most challenging of this program, and will benefit from collaboration established with the LULI team as well as the CEA one.

Teams involved :
-  M. Störmer, Helmotz Zentrum Geestach - Germany : amorphous carbon target design
-  J. Chalupský - Academy of Sciences of the Czech Republic, - Czech Republic : FEL
-  A. Ravasio, E. Brambrick, A. Benuzzi-Mounaix. Laboratoire d’Utilisation des Lasers Intenses, Ecole Polytechnique - France : Shock experiments
-  S. Brygoo, CEA/DIF - France : Shock experiments
Theory :
-  H.O. Jeschke, University of Frankfurt - Germany : TBMD simulations
-  V. Recoules, CEA/DIF - France : QMD simulations
- O. Peyrusse, CELIA - France

Contact :
Jérôme Gaudin
tel : + 33 (0)540006494

Recent related publications of the group :

-  “Photon energy dependence of graphitization threshold for diamond irradiated with intense XUV FEL pulse” J. Gaudin et al. Phys. Rev. B 88, 060101(R) (2013)
-  “Solid-to-solid phase transition in amorphous carbon induced by intense femtosecond x-ray pulses.” J. Gaudin et al. Phys Rev B 86, 024103 (2012)
- “Picosecond dynamics of laser-induced strains in graphite” M. Harb et al.
Phys. Rev. B 84, 045435 (2011)