ic mechanism it is not known whether the 9. D. L. Novikov, V A. Gubanov. A J. Freeman, Physica C 191, 399 (1 992). in the matrix 10. S. Chakravarty, M. P. Gelfand, S. Kivelson, Scisponding to hopping on (13C, ,12C, , ) , , ence 254, 970 (1 991) v.2 --Versus I t 6 , a n d -' i~6 i clusters wa give rise 11. G. Baskaran and E ' Tossatti. Curr. Sci. 61, 33 (1991). to significant variations in Tc' Or whether 12. J. Bardeen, L N. Cooper, J. R. Schrieffer, Phys. these differences are averaged in the mean-Rev. 108. 1175 (1957). field model used to estimate T,. Calcula-13. P. Morel and P. W. Anderson, rbid. 125. 1263tions for the electronic model are needed to address this essential issue. Our experimental isotope effect data ohtained on a series of specifically substituted Rb3C6, superconductors provide an unambiguous value for the isotope shift in completely substituted fullerene superconductors and elucidate a unique difference between intermolecular and intramolecular isotopic substitution. A theoretical understanding of these experimental results may lead to a single viable mechanism for superconductivity in the fullerenes.

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