Evidence of phonon-mediated coupling between charge transfer and ligand field excitons in Sr{sub 2}CuO{sub 2}Cl{sub 2}

We present a comparative experimental and theoretical investigation of the two-dimensional charge transfer gap in the strongly correlated material Sr{sub 2}CuO{sub 2}Cl{sub 2}. We observe an Urbach behavior in the absorption profile over a surprisingly wide range of energies and temperatures. We present a model that accounts for phonon scattering to infinite order and that allows us to explain the data accurately by assuming coupling of the charge transfer gap exciton to lower-energy electronic excitations.