Optical model with angular-momentum-dependent absorption, exchange of two nucleons, and the scattering ofO16onO18near the Coulomb barrier

The properties of the optical model with angular-momentum-dependent absorption are studied mainly using the scattering of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ on ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ for energies near the Coulomb barrier. The results have been compared to two-nucleon transfer predictions. The $L$-dependent form factor has been taken as $f\left(L\right)={\{1+\exp [\frac{(L-L_C)}{\Delta L_C}\left]\right\}}^{-1\mathrm{}}$. The following have been found: (1) There exists an approximate continuous ambiguity concerning the parameters $L_C$ and $\Delta L_C$ which implies a simplification with respect to practical calculations. (2) The $L$ cutoff in the imaginary potential produces an $L$ staggering of the reflection coefficients similar to the one by the exchange model. (3) Both models yield a different energy dependence of the $L$ staggering, since the oscillations of the reflection coefficients are, in the $L$-dependent optical model, connected with size resonances; in the case of exchange, however, with the symmetry of the reaction and the bound-state wave function of the exchanged particle.