Evaluation of second-order hyperpolarizabilities for systems with low-energy double transitions by the three-level model

Abstract

Second-order hyperpolarizabilities have been calculated by the three-level model for systems having low-energy double transitions. In the three-level model, the second excited state, in addition to the first excited and ground states, is explicitly taken into account. The second-order hyperpolarizabilities calculated by the three-level model gave excellent agreement with the observed values determined by the dc induced second harmonic generation technique, for 4-aminostilbene, p-nitroaniline, and 2-methyl-4-nitroaniline.