Melting Line and Structure of Hot dense Fluids probed by X-ray diffraction

Warm dense simple molecular fluids constitute a large fraction of planetary interiors. Their current microscopic understanding is essentially based on ab-initio calculations. Novel states are predicted such as: ionic water and ammonia, metallic oxygen and hydrogen, or polymerized carbon dioxide and nitrogen with possible first-order transition. These remain to be tested by experiment. In this contribution, we will present some developments which have enabled to perform structural measurements on dense simple molecular fluids in the 100 GPa range and few thousand Kelvin. An elaborate sample environment has been implemented to homogeneously laser heat the molecular system. A multichannel collimator is used to filter the overwhelming x-ray diffraction background contribution from the diamond anvils. A careful data analysis is applied to extract the liquid structure factor and the radial distribution function. Observing the liquid diffraction is essential to unambiguously determine the occurrence of melting. Measurements on laser heated nitrogen, gold and xenon will be presented to illustrate this experimental approach.