For a fiber, these directions would be with the polarization of the light parallel and perpendicular to the fiber axis. Let the fiber be oriented along the z axis of a Cartesian coordinate systems. The light beam enters the fiber along the x axis. In the parallel experiment, the polarization is in the xz plane. In the perpendicular, the polarization is in the xy plane.
The objectives are to determine differences in the intensity of the absorption, and perhaps also the frequency of maximum absorption, in the two spectra. If the sample is isotropic, no such differences are expected. But if the sample is anisotropic, and if this anisotropy produces a preferred orientation of the transition moments of the chromophores responsible for absorption, differences in the two spectra may occur.
An example is provided by using infrared linear dichroism to study an oriented sample of poly(L-alanine), prepared so that the chain axis is aligned with the z axis of the experiment. Linear infrared dichroism in the region of the Amide I band probes the orientation of the C=O bond in the amide units. The transition moment can be taken to be parallel with the C=O bond because the Amide I band is predominantly due to stretching of this bond.
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