Exciton dispersion and electronic excitations in hcp He-4 - art. no. 156402

We present first measurements of the dispersion of excitons in solid helium , taken on a single hcp He-4 crystal along the c axis. In agreement with st udies on helium clusters, the major energy-loss peak can be interpreted as an intermediate molecular-type exciton, as we do not observe Wannier-like e xcitations. The measurements are in the (0 0 2) periodic zone, with the exc iton energy dispersing along the c axis with a minimum at the Gamma point. A calculated conduction band minimum at 31.0 eV above the valence band at G amma is supported by our data at energies above the exciton energy, leading to an exciton binding energy of 8.4 eV.