Temporal And Spatial Characteristics Of Laser-induced Plasma On Organic Materials And Quantitative Analysis Of The Contained Inorganic Elements

This PhD work was devoted to the understanding of the laser-induced plasma on organic materials and the application of laser-induced breakdown spectroscopy (LIBS) to quantitative analysis of these materials. It contributes to deepen our knowledge on the physical mechanisms involved in laser-matter interaction, plasma generation, evolution and expansion of the plasma into the ambient gas, with emphasis on plasmas induced on organic targets. It also intends to improve the performance of LIBS for the analysis of organic materials. The specificity for organic targets fits the current focus of the international community working on LIBS, to improve the control of the plasma induced on this kind of material which has a distinguished optical prosperity with respect to that of metals, better known for laser ablation. It addresses also the growing need to apply the LIBS technique to organic materials for different applications in the environmental, food, or biomedical domains.The works in this thesis were therefore presented in this thesis document according to the following organization.After the General Introduction which introduces the scientific and technological contexts, Chapter Ⅰ recalls the basic theoretical elements necessary to understand the phenomenon of plasma generation by laser ablation, and its evolution in the background gas. Ablation of organic material is emphasized. Procedures and techniques of diagnostics of laser-induced plasma were then presented with a focus on the transient and inhomogeneous nature of the expanding plasma. Chapter Ⅱ focuses on the generation and the evolution of the plasma induced on the skin of a potato, a typical sample of agricultural product. The characteristics of plasma induced on a soft and wet organic target, such as a fresh potato, was something unknown when the thesis work started. These characteristics provide the necessary basis for the quantitative analysis of the trace and ultra-trace metallic elements in these samples. Following this characterization, semi-quantitative analytical results were extracted from LIBS spectra corresponding to potato skin. Chapter Ⅲ is presented in the continuity of Chapter Ⅱ for the application of LIBS to the quantitative analysis of organic materials. A comparative study on the analytical results with LIBS and ICP-AES for milk powders allows an assessment of the performances of quantitative analysis by LIBS for organic materials, and a validation of the CF-LIBS procedure that we have developed. Different from Chapters Ⅱ and Ⅲ where attention was paid to trace metal elements, Chapter Ⅳ studies the behavior of the major elements that make up the matrix of organic material, which are4known organic elements:H, C, O, N. During the decomposition of organic material by laser ablation, these elements can be found in the form of molecular fragments, or recombined into molecular species. We then study in this Chapter the evolution of these species as a function of the laser ablation parameters, the laser wavelength in particular. The thesis document ends with a general conclusion and outlooks.