Temperature Dependence, Orientation Correlation, and Molecular Fields in Second-Harmonic Light Scattering from Liquids and Gases

The properties of second-harmonic light scattering in fluids are derived in terms of the molecular polarizabilities, orientation correlation among molecules, and the molecular field Fα. Measurements of scattering intensity in CCl4 and water, from about 10° to 60°C, are reported. The variations with temperature, and therefore apparently the effects of changes of orientation correlation, are slight. The influence of preferred orientation in CCl4, of the type deduced from x-ray diffraction, is calculated. The third-rank molecular polarizability tensor consists of a fourth-rank tensor contracted with Fα, in addition to the term previously known, which is independent of Fα. With Fα arising from molecular multipole moments, the new term explains some measured depolarizations in liquids, and accounts for the spectral line, but not the background, observed in methane gas. In the case of molecular dipole moments, the new term can cause spectral narrowing.