Calculation of Error in Depolarization Ratio Measurement Due to Finite Collection Angle in Laser Raman Spectroscopy

Abstract

We calculate the error introduced in the measurement of the depolarization ratio (ρ) of strongly polarized lines due to the finite angle of collection employed in a laser Raman spectrometer. The mathematical formulation takes explicit account of both the collection lens and the polarization analyzer, placed to select the polarization of the scattered beam. It is shown that for the commonly employed geometry where the analyzer is rotated by 90° to select the polarized and depolarized intensities, keeping the polarization of the incident beam normal to the mean scattering plane, the relative error (Δρ/ρ) does not exceed 8%, even for a collection cone of half angle as large as 25° for ρ as small as 0.01. The present work also shows that the results in the literature which suggest that the relative error in the depolarization ratio is independent of ρ are incorrect.