  Important information on momentum resolved low energy charge response can be
extracted from anomalous properties of bond stretching in plane phonons
observed in inelastic neutron and X-ray scattering in cuprates and some other
perovskites. We discuss a semiphenomenological model based on coupling of
phonons to a single charge mode. The phonon dispersion and linewidth allow to
locate the energy of the charge excitation in the mid infrared part of the
spectrum and to determine some of its characteristics. New experiments on
oxygen isotope substitution could allow to achieve a more detailed description.
Corresponding relations following from the model can be used for the
interpretation of experiments and as test of the model.
