Helium at High Pressure

The energy-loss spectrum of 4He at 13.4 GPa.

The electronic structure of crystalline helium has been measured by inelastic X-ray scattering experiments carried out in part at HPCAT, 16-ID-D. The energy-loss spectra reveal a cut-off energy of 23 eV, a sharp exciton, and a continuum of excitations. Spectra were measured over a range of pressures in a diamond anvil cell, and for a range of scattering angles. The experiments identified the energy minimum at the Brillouin zone center and maximum at the zone edge.  Ab-inito calculations of the energy-loss function based on the Bethe-Salpeter and density functional methods enabled direct comparison to measurements and provided a quantitative picture of helium exciton.
* H.K. Mao et al., PRL 105, 186404 (2010).