The polarization of laser light scattered by gases

Abstract

Depolarization occurs when a light is scattered by a gas illuminated by a strong beam of light perpendicular to the line of sight of the observer. Rayleigh's theory describes the mean depolarization ratio of the rotational Raman lines taken together with the strong unshifted line if the polarizabilities refer to the nonrotating vibrational ground state of the molecule. This chapter describes the measurements of the polarization of light scattered from the beam of helium–neon gas laser at low pressures. The intensity, polarization, and the parallelism of the beam permit high accuracy. It provides new values for the depolarization ratios of 24 simple species. It is found that quantum corrections to the classical formula arise from: the effects of frequency change because of rotational Raman scattering, changes in the molecular polarizability with rotational state because of centrifugal distortion, approximations inherent in the polarizability scattering formula, and vibrational Raman scattering. The chapter also gives an experiment conducted to study the polarization of laser light. The results show that the background light—the signal obtained from the evacuated cell—is partly polarized in the vertical plane, and its intensity and the degree of polarization depend on the cleanness of the Brewster angle windows and the configuration of the detector.