We report a resonant inelastic x-ray scattering study of the dispersion relations of charge transfer excitations in insulating La$_2$CuO$_4$. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of $sim 2.2$ eV at the zone center, and a dispersion of $sim 1$ eV. The spectral weight of this mode becomes dramatically smaller around ($pi$, $pi$). The second peak shows a smaller dispersion ($sim 0.5$ eV) with a zone-center energy of $sim 3.9$ eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.
Download