Extinction coefficient

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The extinction coefficient (from latin extinctio , extinction ' ) is a measure of the attenuation ( extinction ) of electromagnetic waves through a medium , based on the path length through the medium and to the mole concentration of the substance in the solvent . The weakening occurs through scattering and absorption ; if the part of the scattering can be neglected, one speaks of the absorption coefficient .

The extinction coefficient is often used in UV / VIS spectroscopy or photometry .

chemistry

In chemistry, the extinction coefficient (epsilon), more precisely the molar, decadic extinction coefficient ( synonym : molar absorption coefficient ), is a measure of how much electromagnetic radiation a special substance in molar concentration (1  mol / l ) with a passage length of 1 cm and absorbed at a certain wavelength :

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

Denote therein

  • E is the dimensionless absorbance , i.e. H. the reduction in the intensity of the light measured in the photometer (the extinction is more precisely defined as the decadic logarithm of the ratio of the initial intensity to the intensity measured behind the sample, which can also be referred to as the sample transmittance).
  • c the molar concentration of the solution in the measuring cuvette
  • d the path length of the measuring cell (usually 1 cm).

The usual unit of the extinction coefficient is L · mol −1 · cm −1 . It depends on the wavelength, the temperature, often on the pH value and, in the case of many dyes, on the solvent used. It is usually specified for a certain wavelength and at the absorption maximum in relation to the other parameters. Dyes in aqueous solution have extinction coefficients of up to 10 5  L · mol −1 · cm −1  = 10 4  mol −1 · m 2 in their absorption maximum in the visible spectral range (VIS) .

optics

Optical extinction coefficients (green curve) of water between 3 nm and 300 m
Optical extinction coefficients (green curve) of water between 100 nm (UV) and 6400 nm (IR)

In this area, the term extinction coefficient (also ) denotes the imaginary part of the complex refractive index . It is a dimensionless quantity for the weakening capacity of a medium: the larger, the stronger the incident electromagnetic wave (e.g. light) is absorbed by the material. The extinction coefficient depends heavily on the chemical and crystallographic structure of the material and thus on physical parameters such as the wavelength of the radiation, the temperature , etc. (see also: permittivity ).

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

The effect of the imaginary part of the refractive index can be derived from the example of plane electromagnetic waves:

The amplitude in the depth of penetration is . So if it is positive, the amplitude of the wave decreases exponentially .

For the intensity of the penetrating wave, the following applies to the penetration depth of the absorbing medium:

The extinction coefficient therefore causes an exponential decrease in the light intensity .

After introducing the absorption coefficient we get:

Sometimes it is also called the extinction coefficient (see e.g.).

literature

  • Klaus Lüders, Robert Otto Pohl: Pohl's introduction to physics: Volume 2: Electricity and optics . Springer, 2010, ISBN 978-3-642-01627-1 .

Web links

Individual evidence

  1. Klaus Lüders, Robert Otto Pohl: Pohl's introduction to physics: Volume 2: Electricity and optics . Springer, 2010, ISBN 978-3-642-01627-1 , pp. 353 f .
  2. a b Wolfgang Zinth, Ursula Zinth: Optics . 2nd Edition. Oldenbourg Wissenschaftsverlag, 2009, ISBN 978-3-486-58801-9 , p. 22-23 .