DISPERSION OF 3RD-HARMONIC-GENERATION OPTICAL SUSCEPTIBILITY IN C-70 THIN-FILMS

We report wave dispersed optical third harmonic generation (THG) studi ed in the fundamental wavelength range of 0.8-1.9 mu m in thin films o f C-70. The THG spectrum shows a broad resonance enhancement in cubic susceptibility chi((3)), with a maximum value of chi((3))(-3 omega; om ega, omega, omega) = (0.9 +/- 0.1) x 10(-10) esu located at 1.42 mu m, and a dramatic decrease at higher energies. We interpret the data in terms of a three-level model consisting of a totally symmetric ground state, along with two excited states: a one-photon state and a two-pho ton state. We predict the presence of a low-lying dipole-forbidden ele ctronic level in close proximity to a dipole-allowed level.