A NEW MOLECULAR THEORY OF FIELD GRADIENT INDUCED BIREFRINGENCE USED FOR MEASURING ELECTRIC QUADRUPOLE-MOMENTS

Measurement of the birefringence induced in a gas by an applied electric field gradient has been interpreted in terms of an existing molecular theory to yield the electric quadrupole moment of its molecules, whether dipolar or not. A different expression relating the birefringence to the quadrupole moment and other molecular properties is derived in the present paper, using a recent eigenvalue theory of light propagation in matter. Such an expression is required to be independent of molecular origin, even though a quadrupole moment depends in general on the choice of origin. The new result satisfies this requirement only if the primitive quadrupole moment, as opposed to the traceless, is used. In addition, it is shown that the earlier theory is inapplicable to the quadrupole moment experiment, in that it describes an incoherent phenomenon whereas birefringence is a coherent effect.