Theoretical Studies of the Kerr Effect II: The Influence of Pressure

A statistical-mechanical theory of the Kerr constant is presented, and expressions are derived which are applicable at any density. For anisotropic molecules without permanent dipole moments, the Kerr constant is expanded, powers of the polarizability and the leading non-vanishing terms retained. To this order of accuracy, the molecular Kerr constant of molecules of this type is unaffected by pressure changes provided the forces acting between the molecule are independent of orientation. Experimental data for some compressed non-polar gases support this conclusion. For orientationally dependent intermolecular forces, the Kerr constant is expanded in powers of the density and the leading term representing a departure from ideal behaviour is discussed in some detail for both polar and non-polar molecules.