We describe how to separate all surface, interface, and bulk tensor co
mponents in optical second-harmonic generation experiments on isotropi
c thin films. Using in situ thickness scans and group theoretical argu
ments concerning the bulk tensor components, we demonstrate that the s
econd-harmonic resonance of C60 at 2HBARomega=3.6 eV is of bulk charac
ter, and, quite remarkably, of magnetic dipole induced origin.