2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/28190We use a local model to study the formation and the structure of the low energy charge transfer excitations in the insulating Cu-O$_2$ plane. The elementary excitation is a bound exciton of spin singlet, consisting of a Cu$^+$ and a neighboring spin singlet of Cu-O holes. The exciton can move through the lattice freely without disturbing the antiferromagnetic spin background, in contrast to the single hole motion. There are four eigen-modes of excitons with different symmetry. The p-wave-like exciton has a large dispersion width. The s-wave-like exciton mixes with p-state at finite momentum, and its dipole transition intensity is strongly anisotropic. The model is in excellent agreement with the electron energy loss spectra in the insulating Sr$_2$CuO$_2$Cl$_2$.5 pages, 5 figures and 1 table. Submitted to Phys. Rev. BStrongly Correlated Electronstext