Dispersion effects in elastic electron scattering from Pb208

Dispersion corrections to elastic electron scattering from 208Pb at energies up to 150 MeV are estimated within the second-order Born approximation. All strong transient nuclear excited states with angular momentum up to L = 3 and natural parity are taken into consideration. It is found that at small scattering angles the high-lying L = 1 isovector states provide the dominant contribution to the dispersive change of the differential cross section and of the beam-normal spin asymmetry. At the back-most angles these changes are considerably smaller and are basically due to the isoscalar L = 2 and, to a lesser extent, L = 3 states. In comparison with the findings for 12C, the dispersive cross-section modifications for 208Pb are significantly larger, while the spin-asymmetry changes are much smaller at all angles. The latter originates from the destructive interference of the multipole contributions in 208Pb that switch sign when proceeding from L to L + 1. This mechanism might be the origin of the small beam-normal spin asymmetries recently measured in 208Pb at much larger energies.