Electric Birefringence in Molecular Hydrogen

The theory of the Kerr effect (electric birefringence) of a dilute gas of diatomic molecules is developed, account being taken of the quantization of rotational energy. The theory leads to corrections to the classical formula for the molar Kerr constant which involve the ratio of the rotational constant hcB0 to the thermal energy kT. These corrections are appreciable at normal temperatures for only a few hydrides; they are particularly important for molecular hydrogen. The Kerr effect has been observed in gaseous H2 from -30 to +60°C and from 25 to 85 atm, a 6328 Å laser light source and phase-sensitive detector being used. The temperature dependence of the Kerr constant is in good agreement with the quantum theory. Analysis of the results yields a value for the hyperpolarizability γ of H2 of (0∙28 ± 0∙03) x 10-36 e. s. u. The Kerr constant of D2 is also reported. The observed pressure dependence of the molar Kerr constant of H2 is compared with theoretical results based on a calculation of the polarizability anisotropy of a pair of molecules.