Knowledge Bank

When a pulsed ruby laser is brought to focus in a liquid composed of molecules which lack inversion symmetry a small amount of light whose frequency is twice that of the laser is scattered. This scattering is due to orientation fluctuations and its spectral width is governed by orientation relaxation processes. Linewidths have been measured for several liquids (they ranged from $12cm-1$ for $CCl4$ to , the instrument limit) and used to compute (assuming Lorentzian shapes) the relaxation time $rs$. The results are compared to $r1$, the dielectric relaxation time and $r2$, the relaxation time determined from measurements of the wings of the Rayleigh line. In the Debye approximation these times are related by $rn4\pi a3\eta n(n+1)$ where a is the molecular radius and $\eta$ the viscosity. This relation is restricted to non-structured liquids (for which $rn\ge rs$, the structure time) and also may not be used to compare $r1$ and $ra$ when the static molecular dipole moment $\mu \to$ is different from zero. In this case those elements of $\beta \int$*** tensor (which describes the induced dipole moment according to $p\to =\mu \to +\alpha :E\to E\to +\beta E\to E\to E\to$ which transform as $\mu \to$ will relax with $r1$ the remainder with $r3$.