Temperature-dependence of the Kerr effect of chloromethanes

Measurements of the Kerr effect (electric birefringence) are reported for the four chloromethanes (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) in the vapour phase over a range of temperature (260-370 K). For CCl₄, the second hyperpolarisability γ^K is found to be (1.23 ± 0.04) × 10^-60 C⁴ m⁴ J^-3, in reasonable agreement with other nonlinear optical measurements. For CH₂Cl₂, a polarisability anisotropy parameter (α₃₃-α) is determined to be (-0.68 ± 0.08) × 10^-40 C² m ² J^-1, demonstrating the inadequacy of the bond additivity approximation. The first hyperpolarisabilities β^K of CH ₃Cl, CH₂Cl₂ and CHCl₃ are relatively small and therefore poorly determined, indicating that hyperpolarisability contributions to the Kerr effect of dipolar molecules may in general be less significant than has previously been suggested.