Evaluation method for Raman depolarization measurements including geometrical effects and polarization aberrations

Abstract

In this article, we address the notoriously difficult problem to quantitatively link measured Raman depolarization values to theoretical polarizability tensor quantities, since quantum calculations do not incorporate experimental parameters. For this, we introduce a numerical model to calculate, for realistic experimental configurations, effective Raman line strength functions, f, which find their way into depolarization ratios, r. The model is based on interlinked integrations over the angles in the light collection path and a finite Raman source volume along the excitation laser beam. The model deals also with the conditional aperture parameters, associated with more than one optical component in the light collection path. Finally, we also can take into account polarization aberrations introduced by the sample cell windows. The procedure was fully tested for Raman depolarization spectra of selected hydrogen isotopologues. Distinct aspects affecting Raman depolarization data were validated, namely: (1) excitation polarization impurities; (2) extended Raman excitation volumes; (3) Raman light collection over finite solid angles; and (4) polarization aberrations introduced by optics in the light collection path. The correction of the experimental measurement data for the aforementioned effects resulted in depolarization ratios for the Q 1 (J ") Raman lines of H 2 and T 2 , which mostly differed by less than 5% from those obtained by quantum-calculations.

Accurate depolarization ratio measurements for all diatomic hydrogen isotopologues

“…The solid angle is defined explicitly to obtain correct definitions of integration bounding curves that intrinsically account for the correct transmission of off‐centre scattered light through multiple apertures such as sample cell windows, collection lenses, etc. Polarization aberrations by stress‐induced birefringence in the cell windows can reduce the polarization cleanness . In our case, that means the incident laser light is no longer perfectly polarized after passage through a laser window.…”

“…This is obtained via a numerical integration over the angles θ , φ , and the position along the laser beam axis, z . In Schlösser et al ,. this model is described in detail.…”

“…In the simplification of the correction model, only the polarization aberration effect in the incident laser beam is considered . While the polarization aberrations introduced by the Raman collection optics are estimated to be nearly negligible, and are therefore omitted in our treatment, this notion is not entirely correct.…”

“…The correction procedure model depends on the accuracy of the input parameters . These are the distance between the limiting diaphragm to the scattering centre, and its aperture opening radius.…”

“…In brief, our methodology allows one to take into account aspects normally affecting Raman depolarization measurements, namely (1) excitation polarization impurities; (2) extended Raman excitation volumes; and (3) Raman light collection over finite solid angles. These corrected depolarization ratios can then be directly compared with expressions that are based on theoretical polarizability tensor elements …”