Tunneling and optical transmission studies have been performed on supe
rconducting samples of Rb3C60. At temperatures much below the supercon
ducting transition temperature T-c, the energy gap is 2 Delta = 5.2 +/
- 0.2 meV, corresponding to 2 Delta/k(B)T(c) = 4.2. The low temperatur
e density of states and the temperature dependence of the optical cond
uctivity resemble BCS behavior, although there is an enhanced ''normal
state'' contribution. The results indicate that this fulleride materi
al is an s-wave superconductor, but the superconductivity cannot be de
scribed in the weak coupling limit.