Measurements of the frequency dependence of the microscopic second-order nonlinear electronic susceptibility are reported for the organic system $p$-nitroaniline. The results are suitably described in terms of a theoretical microscopic mechanism involving highly charge-correlated electron states dominating the second-order nonlinear optical response of the system as presented in earlier theoretical calculations.

• Received 11 November 1982

DOI:https://doi.org/10.1103/PhysRevLett.50.350