Frequency dependence of the Kerr constant

Abstract

The theory of the Kerr effect is developed for wavelengths close to those corresponding to absorption bands. Unlike the transparent regions, where orientation of anisotropic molecules is the major contributor to the Kerr constant, in absorption bands distortion of the molecular structure by the strong electric field is apparently the dominant effect. For molecules with dipole moments in either the ground or the relevant excited states, the sign of the rate of change of the Kerr constant with frequency is determined by the direction of the transition moment with respect to molecular axes, and its magnitude is proportional to the square of the difference between the dipole moments of the ground and excited states. Anomalous dispersion should also occur at frequencies corresponding to forbidden transitions from the ground state, provided there is a third state with dipole transition moments with the ground and excited states. The quantum theory of the Kerr constant of gases is developed and applied to the vibration-rotation spectral region of diatomic and symmetric top molecules. A brief discussion of the potentialities of the effect is presented; since orientation is unimportant, aqueous solutions of electrolytes could presumably be studied after freezing to reduce their conductivity. An experimental method of measuring the frequency dependence of the Kerr constant, using a double beam spectrometer, is proposed.