Spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has been used to investigate as fluidic phase in nanoubbles embedded in a metallic P d90P t10 matrix. Using the 1s → 2p excitation of the He atoms, maps of the He distribution, in particular of its density an pressure in bubbles of different diameter have been realized, thus providing an indication of the involved bubble formation mechanism. However, the short-range Pauli repulsion mechanism between electrons on neighboring atoms seems insufficient to interpret minute variations of the local local measurements performed at the interface between the metal and the He bubble. Simulations relying on the continuum dielectric model have show that these deviations could be interpreted as an interfzce polarization effect on the He atomic transition, which should be accounted for when measuring the densities within the smaller bubbles.Confined fluids in nanosized volumes constitute challenging objects for both basic and technological aspects. The investigation of the structural features and dynamics of nanojets has given rise to spectacular experimental studies and theoretical simulations [1]. Another ideal system is represented by gas confined in nanocavities. It is the case of inert gas atoms coalescing as a fluid or a solid to fill nanocavities in metals, with spherical or faceted morphologies depending of the local pressure. In the case of Xe in Al, an interfacial ordering has been demonstrated by high resolution electron microscopy [2]. These small gas-filled cavities therefore behave as high-pressure cells, providing the boundary conditions for the evaluation of the physical properties of encapsulated gases. A most challenging problem is the evaluation of gas density and pressure in such cavities.Among the possible systems, He nanobubbles in metals have attracted the attention of many researchers, because of their high technological interest in the aging of the mechanical properties of materials involved in nuclear reactors [3]. Measurements averaging the information over large populations of bubbles, the size distribution of which being controlled by TEM, have first been performed by NMR [4] and by a combination of optical absorption and electron energy-loss spectroscopy (EELS) [5]. The first of these studies has revealed a solid-fluid transition at 250K for bubble pressures ranging from 6 to 11 GPa (i.e. He atomic densities from about 100 to 200 nm −3 ). The second study comparing UV absorption spectroscopy on a synchrotron and high energy resolution EELS without spatial resolution on He + implanted Al thin foils, have identified the blue shift of the He 1s → 2p transition (with respect to its value of 21.218 eV for the free atom) as a hint for evaluating the local pressure . Theoretically, Lucas et al. [6] have confirmed that this blue shift of the He K-line should be attributed to the short-range Pauli repulsion between the electrons of neighboring He atoms. Consequently, this effect should increas...

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