We have measured the resistive supercondeucting transition temperature in ${\mathrm{C}}_{60}$ single crystals intercalated with isotopically pure ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ and ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ and with natural abundance rubidium. We obtain a rubidium isotope effect exponent of ${\mathrm{\alpha }}_{\mathrm{Rb}}$=-0.028±0.036, a result which implies that the Rb-${\mathrm{C}}_{60}$ optic phonons play at most a minor role in the pairing mechanism of ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$.

• Received 14 February 1994

DOI:https://doi.org/10.1103/PhysRevLett.72.3706