Precise fusion excitation functions measured for the reactions Ca-40 Zr-90,Zr-96 yield barrier distributions which are qualitatively very
different, While the lighter system is well described by coupled-chann
els calculations including multi-phonon excitations, this coupling sch
eme fails completely for the heavier system, possibly due to the role
of transfer channels. (C) 1997 Elsevier Science B.V.