Applications of linear Dichroism Spectroscopy

Abstract

Linear dichroism is the phenomenon of anisotropic absorption of light. It is shown by materials containing oriented molecules for which the absorption varies with the direction. The absorption intensity is proportional to the square of the scalar product between the electric field vector of the light and a molecule-characteristic transition moment vector, the absorption being maximum when the light vector is polarized parallel to the transition moment and zero when perpendicular to it. Linear dichroism can therefore provide (1) directions of transition moments when the molecule orientation is known (spectroscopic applications), or (2) information on molecular orientation when the transition moments are known (structural applications). Both applications are useful in several chemical systems, but so far the use of linear dichroism has been confined to a relatively small number of specialized laboratories, not least because of a lack of appropriate commercial instruments. Plane-polarized spectra have long been measured on crystals and other well-oriented materials, but systems with less complete orientation have usually been studied by birefringence which has allowed greater sensitivity. Birefringence and linear dichroism are related by the dispersion equations and therefore in principle contain the same basic information. However, linear dichroism is better suited for practical use since it is related in a very simple way to more-or-less well-separated quantal transitions, while birefringence is a complicated average over all transitions in the molecule.