Molar absorptivity

The extinction coefficient (from the Latin extinctio, extinction ) is related to a path length measure of the attenuation of electromagnetic waves through a medium, cf absorbance ( optical ). The weakening is done by scattering and absorption. If the amount of scattering can be neglected, one also speaks of the absorption coefficient.

The absorbance is proportional to the concentration of a substance in a solvent.

Chemistry

In the chemical industry is the extinction coefficient of ( ε, epsilon), more specifically the molar, decadic extinction coefficient (synonym: molar absorption coefficient), a measure of how much electromagnetic radiation is a particular substance in a molar concentration at a pass length of 1 cm and at a certain wavelength absorbed. This term is often used in UV / Vis spectroscopy and photometry. Its value is obtained by the equation

Derived from a fundamental equation of the photometry, the Lambert -Beer law:

E is the absorbance, i.e. the reduction of the intensity of the measured in the photometer light ( to be exact, the absorbance is defined as the decadic logarithm of the ratio of the output intensity I0 and the measured after the sample intensity I, which as a may be referred sample permeability ). The extinction E is a dimensionless quantity.

• ε is the molar decadic extinction coefficient
• C is the molar concentration of the solution in the sample cell
• D is the layer thickness of the sample cell (usually 1 cm)

The common unit of the extinction coefficient is L · mol -1 · cm -1. It is dependent on the wavelength and the temperature of which is measured. Dyes in aqueous solution have its maximum absorption in the visible region (VIS), usually in the extinction coefficient on the order of 105 L · mol -1 · cm -1.

Optics

In this area is referred to (also) the imaginary part of the complex refractive index of the extinction coefficient of term. It is a dimensionless quantity for the weakening power of a medium. The larger, the more the incident electromagnetic wave is absorbed by the material (eg, light ) ( absorbed). Here, the extinction coefficient is strongly dependent on the chemical and crystallographic structure of the material and thus of the physical quantities, such as the wavelength of the radiation, the temperature, etc. (see also: permittivity).

The extinction coefficient is the real part of the complex refractive index and the absorption index (Greek: kappa) linked.

The effect of the imaginary part of the refractive index can be illustrated planar electromagnetic waves derived as follows:

So is negative, then the amplitude of the wave decreases exponentially. The amplitude in the depth of penetration. The intensity of the penetrating shaft applies to the depth of the absorbing medium:

Thus, the extinction coefficient also causes an exponential decay of the light intensity (when ). After the introduction of the absorption coefficient is obtained:

It is sometimes called the extinction coefficient ( see, for example ).