Near-barrier polarisation potentials for 6,7Li + 208Pb

A previous optical model analysis of 6,7Li + 208Pb elastic scattering data at near barrier energies found a marked difference in the energy dependence of the surface strength of the real part of the best fit optical model potentials for the two isotopes. We show that this difference can be accounted for by the difference in strength of the real polarisation potentials produced by the breakup couplings in 6Li and 7Li. We show that this is in turn largely due to the differing breakup thresholds of 6Li and 7Li.