TABLE OF CONTENTS The optical potential plays a vital role in any discussion of direct reactions, because it was learned at an early stage that the scattering in the average field acting between target and projectile distorts the relative motion before and after the direct reaction. Although the optical potential was introduced originally as a phenomenological potential (which is complex to account for the partial absorption out of the elastic channel), its theoretical structure, as described in Chapter 8, was later to be appreciated. Moreover, it was even learned (see Chapter 13) that in those nuclei where there are very strong transitions, their contribution to optical potential can be very specific and identifiable (Glendenning et al., 1968; Glendenning, 1969a). In this chapter we discuss recent work aimed at calculating those components in the optical potential that arise from a particular inelastic mode. The possible advantages lie in the fact that, for the most part, the optical potential receives small contributions from an enormous number of channels at typical collision energies. Because of the exponential growth of the density of nuclear levels with energy, the potential, for the most part, is dominated by the multitude of levels at high excitation. We expect that the higher excitations of neighboring nuclei are basically similar; that the spectra differ principally in the low-energy region. The conclusion is that if the optical potentials of neighboring nuclei differ very much from each other, they do so because of strong collective transitions that are different in the...
