The sign of the polarizability anisotropy of polar molecules is obtained faithfully from terahertz Kerr effect

Optically heterodyned detected terahertz Kerr effect of gases of polar molecules is reported. Strikingly, the birefringence signal from fluoroform is found to have opposite polarity compared to water and acetonitrile. This behavior is a hallmark of the opposite sign of the polarizability anisotropy of these molecules. Resonant excitation of the rotational degrees of freedom of the molecules aligns their permanent dipoles along the terahertz electric field. This alignment is translated into an optical birefringence through the polarizability anisotropy of each molecule. Therefore, the resulting net signal scales with the polarizability anisotropy, whose sign is imprinted faithfully onto the transient birefringence signal.