VIBRATIONAL CONTRIBUTIONS TO 3RD-ORDER NONLINEAR OPTICAL SUSCEPTIBILITIES

Including vibrational levels in the perturbation theory sum for the third-order nonlinear optical susceptibility gamma-introduces terms which have a pure vibrational frequency in the denominator. These terms can be rewritten as a sum over Raman amplitudes. Model calculations on a diatomic system with two electronic states show that these terms are negligible, about 3% of the value obtained ignoring vibrations. In polyacetylene, the Herzberg-Teller contribution to the Raman amplitude is much larger than in the diatomic case. Within the tight binding and harmonic oscillator approximations, the sum over Raman amplitudes is 10% of the value for gamma-obtained ignoring vibrations.