Electronic Excitations in hcp 4He\mathrm{^4He} at 61.5 MPa and 4.3 K Studied by Inelastic X-Ray Scattering Spectroscopy

The dynamic structure factor S(→q,ω) of electrons in solid 4He was measured with 1.6 eV energy resolution by means of inelastic scattering of synchrotron x rays from a hcp single crystal for →q along the c axis and for q=0.45,0.97,and1.24a.u. The hcp crystal was grown in situ at 61.5 MPa and 4.3 K. A q-independent energy loss peak at 21.9±0.3eV could unambiguously be identified. These signals are the first directly measured electronic excitations in solid helium. They are discussed in terms of excitonic excitations