Energy dependence of dispersive effects in sup 12 C

Our investigation of the energy dependence of dispersive effects in elastic electron scattering from {sup 12}C has been extended to a beam energy of 690 MeV. The cross section observed in the minimum of the form factor is larger than that predicted using a static charge density obtained by fitting lower-energy data with the customary phase-shift analysis. This filling-in'' of the minimum at 690 MeV is even larger than that observed previously at 430 MeV. This indicates an increase in the relative strength of higher-order processes in the minimum with an increase of beam energy; the presence of dispersive effects is the most likely explanation for this effect.