Kerr effect in methane and its four fluorinated derivatives

Electric birefringence has been measured at a wavelength of 632.8 nm over a range of temperature and pressure in the gases CH₄, CH₃F, CH₂F₂, CHF₃ and CF₄. The results yield values of the second hyperpolarizability γ for CH₄ and CF₄ and, for CH₃F, CH₂F₂ and CHF₃, the first hyperpolarizability β and the difference (α₃₃-α) between the optical polarizability component in the direction of the molecular dipole and its mean. For CH₂F ₂ (α₃₃-α) is unusually small; the birefringence changes sign as the pressure is increased, the change occurring at 1.7 bar at 244°K. Limitations of a bond additivity model for β are exposed, and the importance of hyperpolarizability contributions to the Kerr constant demonstrated. The density dependence of the birefringence in the five gases is compared with the predictions of simple theoretical models, but agreement is in most cases poor.