Excited helium under high pressures in the bulk and in nanobubbles

Abstract

We systematically investigate the effects of intense pressures on the excitation energies of helium trapped in bubbles in order to deepen our understanding of the fundamental physics of atoms in extreme conditions. The excitation energy of a confined helium atom is known to differ from that of a free atom being greater in both the bulk liquid or solid or a bubble confined in a metallic matrix state. We compare calculations for the energy shift with both laboratory experiments for bulk systems and results derived from scanning transmission electron microscope (STEM) studies of helium nanobubbles embedded in different matrices. We find excellent agreement between our calculations and the latest extensive measurements in the bulk. However, we find significant discrepancies when we compare with results deduced using the ‘standard’ approach for analysing STEM data. Here, we show the scattering matrix element determining the intensity of this excitation in a STEM experiment is significantly affected by the same environmental factors that shift the excitation energy. Consequently, there is a serious theoretical inconsistency in the way the STEM results are calculated, in that the ‘standard’ approach depends on a supposedly known scattering cross section, whereas we show here that this cross section is itself dependent on the environment. Correcting for this inconsistency does not, in itself, improve agreement.