HYPERPOLARIZABILITY OF INTERACTING ATOMS

The effects of molecular collisions on the hyperpolarizabilities of spherical atoms are considered. Closed-form expressions are given for the second hyperpolarizability of a pair of atoms in the presence of dipolar coupling for five cases, namely, a static electric field, third-harmonic generation, electric-field-induced second-harmonic generation, the Kerr effect, and four-wave mixing. The longest range contribution to the mean hyperpolarizability varies as R(-6); it is less than one-third of the magnitude of the effect deduced by Donley and Shelton, but agrees with that obtained by Hunt. For a dissimilar pair of atoms, a collision-induced first hyperpolarizability proportional to the inverse fourth power of the separation is predicted.