Polarization-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy: a new probe of molecular symmetry through accurate determination of the Raman depolarization ratio

Abstract

A broad-band multiplex CARS spectroscopic system was constructed for precise polarization-controlled measurements. The directions of polarization of the two incident laser beams and that of the CARS signal beam are controlled precisely by two polarizers and an analyzer set on high-precision prism mounts. Polarized CARS spectra were recorded with high signal-to-noise ratios for a number of different analyzer settings, typically at intervals of 1° of the analyzer axis. The observed changes in band shapes are interpreted by a global simulation analysis in which all the band shapes at different analyzer settings are reproduced using a small number of intrinsic bandshape parameters. The analysis leads to very accurate determination of depolarization ratios, accurate within an error of ±0.002 in the most favorable case. The method may well be called polarization-resolved CARS spectroscopy. Depolarization ratios of two typical organic liquids, cyclohexane and 1,2-dichloroethane, were determined. The two non-totally symmetric bands of cyclohexane gave a value of 0.75 within an experimental uncertainty of 0.002. The two totally symmetric bands of 1,2-dichloroethane, which have been reported to be depolarized despite their symmetry, showed depolarization ratios very close to but meaningfully smaller than 0.75. Copyright © 2000 John Wiley & Sons, Ltd.