Printed in the United States of America A MICROSCOPIC MODEL FOR THE SECOND-HARMONIC GENERATION FROM

Abstract We discuss the microscopic origin of the Second-Harmonic Generation (SHG) resonance at w=1.81 eV, based on spectroscopic and thickness dependent SHG measurements on C 60 thin films. We assign the three-level diagram responsible for the observed SHG resonance, and show it to be of magnetic-dipole-induced origin. Furthermore we explain the absence of almost any surface contributions, and the narrow line width of the involved HOMO-LUMO (1 1Ag®11T1g) excitation at 1.81 eV