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Consider molecule of interest be dissolved in a solvent (and this molecule has absorption in the visible region). We are using a broadband visible light source say tungsten lamp. This light passed through a polarizer and then split into two beams, one passing through sample cell, and other through reference cell having solvent.

Would there be difference in the electronic absorption spectra when measured keeping the polarizer parallel or perpendicular with respect to the incident polarization ? Also, no external electric field is present and we have randomly oriented molecules.

(Consider the transmission of optics and detector are corrected for this polarization difference)

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    $\begingroup$ Only if there is a good reason for the solute to have a preferred orientation in the liquid (external electric field, say). $\endgroup$ – Jon Custer May 25 '16 at 14:51
  • $\begingroup$ @Jon - Thanks for the point about electric field. $\endgroup$ – ankit7540 May 25 '16 at 14:53
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With the experimental setup you describe, you would not observe any intentional polarization dependence, because the light passes only through a beam splitter, not a polarizer.

There can be unintentional polarization due to the nature of optical materials and manufacturing, caused by

  • the diffraction grating,
  • the polarization contribution of mirror reflections, and
  • the diffraction grating slits.

With that out of the way, UV/Vis absorption is polarization dependent. In theory, polarization can occur in any spectroscopic technique where the polarization of the incident EM radiation can be controlled. To quote the Wikipedia article,

In general, any kind of anisotropy in the media results in some sort of change in polarization. Such an anisotropy can be either inherent to the media (e.g., in the case of a crystal substance), or imposed externally (e.g., in the presence of magnetic field in plasma).

Assuming that no other external fields are applied other than your probe beam and your bulk phase has no anisotropy, any differential absorption of polarized light must come from the molecules themselves, namely those that are "optically active". Linear dichroism and particularly circular dichroism are used to describe the differential absorption of linearly and circularly polarized light, respectively. Electronic (UV/Vis) CD is widely applied in the determination of protein secondary structure and the absolute configuration of stereocenters in chiral molecules. An ECD spectrum is usually presented as a difference spectrum between the left- and right-polarized spectra, which themselves are no different from a typical UV/Vis spectrum.

The parent dichroism article is a good starting point for learning about these spectroscopies.

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  • $\begingroup$ Thinking about the first sentence, since in the setup I describe I do keep a polarizer to get linearly polarized excitation light. Do you mean that the beam splitter may induce polarization aberration for the two split beams ? $\endgroup$ – ankit7540 Jan 22 '17 at 15:31
  • $\begingroup$ I'm not sure why I thought that you didn't have a polarizer present. Are you sure you don't have a monochromator instead of a polarizer? I didn't intend to say that the beam splitter would cause an aberration, but it might be possible. I will ask my 2D spectroscopy friend. I'm thinking a real answer to your question with your setup is "yes", with an explanation of orientation dependence (like here). $\endgroup$ – pentavalentcarbon Jan 22 '17 at 16:14

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