Possibility of optical evidence for charge-transfer excitations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We present a three-level model that addresses the possibility that charge-transfer fluctuations are responsible for the anomalously large intensity of the 155 ${\mathrm{cm}}^{\mathrm{-}1}$ Ba mode observed by Genzel et al. The model also addresses the accompanying Raman signature and the c-axis lattice constant variations with oxygen content.