RESONANT 3RD-HARMONIC GENERATION IN ALL-TRANS BETA-CAROTENE - THE VIBRONIC ORIGINS OF THE 3RD-ORDER NONLINEAR SUSCEPTIBILITY IN THE VISIBLE REGION

The dispersion of the modulus of the third-order nonlinear susceptibility, \chi(3) zzzz(-3-omega;omega,omega,omega)\, of an all-trans beta-carotene doped polystyrene thin film has been measured by third-harmonic generation throughout the 1 1B(u) absorption band of the beta-carotene. The observed dispersion is interpreted in terms of a three-electronic state model including explicit regard to six Franck-Condon active normal modes in the resonant electronic state. Both the sum-over-states method and, equivalently, the time domain approach are used. A fit of the three-state model to the data shows that the large nonlinear susceptibility of beta-carotene in this wavelength range can be explained by strong dipole coupling between the strong one-photon allowed 1 1B(u) state and a nearby two-photon allowed n 1A(g) state. The electric dipole transition moment between the 1 1B(u) excited state and the n 1A(g) excited state, located at about 22 200 cm-1, was determined to be 31 D.