Molecular interactions in optically induced nonlinearities

This is a review of the more important theoretical and experimental results or intensity-dependent refractive index in relation to self-focusing and nonlinear scattering of light in liquids. The part played within the framework of the molecular-statistical theory of these processes by radial and angular molecular correlations, molecular redistribution, various nonlinear polarizations induced by fluctuations of electric fields of neighboring molecular multipoles, and geometrical shape of the molecules, is discussed. Numerical evaluations of the various contributions are given and compared with the available experimental data. It is found that while investigation of optically induced nonlinearities in gases and rarefied substances yields only values of linear and nonlinear optical polarizabilities of atoms or molecules, work on condensed systems yields information on the values of molecular quadrupoles and octupoles as well as on the functions accounting for molecular correlations of various kinds.