  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 Pd90Pt10 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.
