Linear dichroism

Linear dichroism (LD) or diattenuation describes the property of a material whose transmittance depends on the orientation of linearly polarized light incident upon it. As a technique, it is primarily used to study the functionality and structure of molecules.

LD can be defined as the difference between absorption of light polarized parallel and polarized perpendicular to an orientation axis. LD measurements are based on the interaction between matter and light and thus are a form of electromagnetic spectroscopy.

This effect has been applied across the EM spectrum, where different wavelengths of light can probe a host of chemical systems. The predominant use of LD currently is in the study of bio-macromolecules (e.g. DNA) as well as synthetic polymers.

LD uses linearly polarized light, which is light that has been polarized in one direction only. This produces a wave, the electric field vector, which oscillates in only one plane, giving rise to a classic sinusoidal wave shape as the light travels through space. By using light parallel and perpendicular to the orientation direction it is possible to measure how much more energy is absorbed in one dimension of the molecule relative to the other, providing information to the experimentalist.

As light interacts with the molecule being investigated, should the molecule start absorbing the light then electron density inside the molecule will be shifted as the electron becomes photoexcited. This movement of charge is known as an electronic transition, the direction of which is called the electric transition polarisation. It is this property for which LD is a measurement.

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