Momentum dependence of polarization charges in electron excitation of nuclei in the oxygen region

We have considered the polarization effects for monopole and quadrupole electron excitations of nuclei in the oxygen region. A phenomenological method has been used in order to extrapolate empirical values for the polarization charges, from the real photon limit, to higher momentum transfers. We find in most cases polarization charges that are nearly constant, for momentum transfers up to 2 or 3 fm^?1. Hence we have provided a justification in the oxygen region for the assumption most frequently made when experiment and theory are compared.