The terahertz Kerr effect (TKE) of polar molecular vapors is reported. The birefringence signal of fluoroform appears with opposite polarity compared to acetonitrile and water. This behavior is a hallmark of the opposite sign of a new molecular polarizability anisotropy Δα_TKE=α_zz-(α_xx+α_yy)/2$\mathrm{\Delta }{\mathit{\alpha }}_{\mathbf{TKE}}={\mathit{\alpha }}_{\mathit{zz}}-({\mathit{\alpha }}_{\mathit{xx}}+{\mathit{\alpha }}_{\mathit{yy}})/2$, with α_zz${\mathit{\alpha }}_{\mathit{zz}}$ being the polarizability along the permanent dipole moment. As the excitation of the rotational states orients the permanent dipoles along the terahertz electric field, the orientation is translated into an optical birefringence proportional to Δα_TKE$\mathrm{\Delta }{\mathit{\alpha }}_{\mathbf{TKE}}$. Thus, the sign of Δα_TKE$\mathrm{\Delta }{\mathit{\alpha }}_{\mathbf{TKE}}$ is imprinted onto the TKE signal, providing novel insights into the polarizability tensor of water.

报告了极性分子蒸气的太赫兹克尔效应（TKE）。与乙腈和水相比，氟仿的双折射信号极性相反。此行为是一个新的分子极化率各向异性的符号相反的一个标志Δ α _TKE = α _ZZ - （α _XX + α _YY）/ 2$\mathrm{\Delta }{\mathit{\alpha }}_{\mathbf{TKE}}={\mathit{\alpha }}_{\mathit{Z\; Z}}--（{\mathit{\alpha }}_{\mathit{xx}}+{\mathit{\alpha }}_{\mathit{y}}）/\mathrm{2个}$与α _ZZ${\mathit{\alpha }}_{\mathit{Z\; Z}}$是沿着永久偶极矩的极化率。作为旋转状态的激励取向沿太赫波的电场的永久偶极，取向被转换成一个光学双折射正比于Δ α _TKE$\mathrm{\Delta }{\mathit{\alpha }}_{\mathbf{TKE}}$。因此，符号Δ α _TKE$\mathrm{\Delta }{\mathit{\alpha }}_{\mathbf{TKE}}$ 刻在TKE信号上，为水的极化率张量提供了新颖的见解。