https://doi.org/10.1351/goldbook.08586
Raman intensity with the electric vector of the scattered electromagnetic radiation perpendicular to that of the incident radiation divided by the Raman intensity with electric vectors parallel.
Note: In normal Raman scattering with linearly polarized exciting radiation, \(\rho \le 0.75\) for totally symmetric vibrations and \(\rho = 0.75\) for all other vibrations.