measured with high precision, both in the cross sections and the small energy intervals, thus allowing meaningful fusion barrier distributions to be extracted. In this representation it is clearly seen that the excitation functions are not smooth and featureless; each is unique and is shown to depend on the details of the structure of the interacting nuclei. The effects of excitation of the collective single phonon states in ' Sm are evident.For the ' 0 projectile, the role of additional coupling to neutron stripping channels with positive Q values can be seen. As expected, the barrier distributions associated with ' Srn and ' W are dominated by deformation effects. However, the data appear to display sensitivity to additional couplings, even though they involve relatively weak inelastic and transfer channels.PACS number(s): 25.70.Jj

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Pusion excitation functions for Sm + 0 and 0 have been measured to high precision.The extracted fusion barrier distributions show a double-peaked structure which is interpreted in terms of coupling to inelastic excitations of the target. The effect of the neutron stripping channel is evident in the reaction with O. These barrier distributions show clearly the signatures of specific inelastic and transfer channels.

High precision measurements of the fusion excitation functions for the reactions 40 Ca 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 coupledchannels 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.

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The role of higher order coupling of surface vibrations to the relative motion in heavyion fusion reactions at near-barrier energies is investigated. The coupled channels equations are solved to all orders, and also in the linear and the quadratic coupling approximations. Taking 64 Ni + 92,96 Zr reactions as examples, it is shown that all order couplings lead to considerably improved agreement with the experimentally measured fusion cross sections and average angular momenta of the compound nucleus for such heavy nearly symmetric systems. The importance of higher order coupling is also examined for asymmetric systems like 16 O + 112 Cd, 144 Sm, for which previous calculations of the fusion cross section seemed to indicate that the linear coupling approximation was adequate. It is shown that the shape of the barrier distributions and the energy dependence of the average angular momentum can change significantly when the higher order couplings are included, even for systems where measured fusion cross sections may seem to be well reproduced by the linear coupling approximation.PACS number(s): 25.70.Jj, 24.10.Eq,