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Explanation of dichroism from Field Guide to Spectroscopy

Excerpt from Field Guide to Spectroscopy

The concept of dichroism (literally, "two-colored") comes from mineralogy. Certain minerals, like tourmaline, have different colors when viewed along different crystal axes.

In spectroscopy, dichroism occurs when a material absorbs left circular polarized light in different amounts than right circular polarized light. This is the basis of dichroism spectroscopy.

There are at least three major areas of circular dichroism (CD) spectroscopy:

  • Analysis of chiral molecules. Although there is yet no relationship between the absolute chirality and the preference for polarization type, CD spectroscopy can be used to track the chirality in a sample.

  • The interaction between a vibrational mode and the electronic state of a molecule can cause vibrational circular dichroism, wherein the vibrational modes of a molecule preferentially absorbs one circular polarization of light over another.

  • Magnetic circular dichroism occurs because the selection rules for right circular polarized light are different than for left circular polarized light. In the presence of a magnetic field, previously degenerate absorptions now differ in energy and there is a difference in the absorptions between the two polarizations.

Circular dichroism spectra are typically displayed as difference spectra-the absorption of one polarization minus the absorption of the other polarization.  

D. W. Ball, Field Guide to Spectroscopy, SPIE Press, Bellingham, WA (2006).

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