Adiabatic quantum tunneling in heavy-ion sub-barrier fusion

High precision measurements of the fusion excitation functions for the reactions ^{40}Ca + ^{194}Pt, ^{192}Os clearly demonstrate that projectile excitation significantly modifies the potential barrier distribution. In sharp contrast, fusion of ^{16}O + ^{144}Sm appears to show no influence of the projectile excitation on the shape of the barrier distribution. These apparently conflicting conclusions are reconciled in this work, using realistic coupled--channels calculations, which show that high energy states produce an adiabatic potential renormalisation. This result indicates that adiabatic effects restrict, in a natural way, the states which influence the shape of a fusion barrier distribution. The analysis of barrier distributions thus offers a criterion for the relevance of the `counter term' prescription in the Caldeira-Leggett approach.