We describe how to separate all surface, interface, and bulk tensor components in optical second-harmonic generation experiments on isotropic thin films. Using in situ thickness scans and group theoretical arguments concerning the bulk tensor components, we demonstrate that the second-harmonic resonance of ${\mathrm{C}}_{60}$ at 2ħω=3.6 eV is of bulk character, and, quite remarkably, of magnetic dipole induced origin.

• Received 16 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.3569