Ultraviolet dispersion of the donor–acceptor charge transfer contribution to the second order hyperpolarizability

Abstract

We have measured for the first time electric field induced optical mixing (ω+2ω=3ω) in the liquid state. Two molecules DANS (trans‐4‐dimethylamino‐4′‐nitrostilbene) and p‐NA (p‐nitroaniline) possessing large intramolecular charge transfer interactions were used and the second order mixing hyperpolariabilities β (3ω) were determined to be β (3ω) = (−7.2±2.6i) ×10⁻²⁸ and β (3ω) = (+9.6±38i) ×10⁻³⁰ esu for DANS and p‐NA, respectively, with 3ω=3.50 eV. Combining these measurements with the simple two level quantum mechanical model yields the theoretical prediction of β (2ω) = (+38+15i) ×10⁻²⁸ and β (2ω) = (+48+5.3i) ×10⁻³⁰ esu for the second harmonic hyperpolarizability of DANS and p‐NA, respectively. These predictions are not in good agreement with the experimental values ‖β (2ω) ‖ =2.1×10⁻²⁸ esu (obtained using the measurements of Oudar and Chemla) and β (2ω) =+22×10⁻³⁰ esu.