Momentum-Dependent Charge Transfer Excitations in Sr2CuO2Cl2 - Angle   Resolved Electron Energy Loss Spectroscopy
  Electron-hole pair excitations in the insulating cuprates $Sr_2CuO_2Cl_2$ were investigated by angle-resolved electron energy loss spectroscopy. The optically allowed and optically forbidden transitions were observed to be strongly anisotropic in $Cu-O_2$ plane. The former show a large energy dispersion $\sim 1.5$eV along [110], and the latter appear at a higher energy position ($\sim 4.5$eV) only along [100], but not along [110]. We interpret these results as transitions involving excitons. A small exciton model is examined to explain both the observed features. 
Y.Y. Wang, F.C. Zhang, V.P. Dravid, K.K. Ng, M.V. Klein, S.E.   Schnatterly and L.L. Miller