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Absorption

Genspect Absorption

Genspect models absorption as a probability, a unitless quantity varying between 0 (no absorption) and 1 (complete or 100% absorption).

The Absorption Process

As radiation passes though matter it is attenuated though absorption processes. Absorption is caused by interactions between the incident field and the medium. The medium can be any solid, fluid or gas. Conceptually, this interaction process may be understood from a Quantum mechanical perspective. As photons or energy quanta intersect the matter, each one has a certain probability that it will interact with the substance and be absorbed.

In the simplest example, we consider a single photon incident on a single molecule of matter. The chance that the photon will interact with the molecule is dependent on the particular photon's frequency (or wavelength) and the electromagnetic properties of the medium. For this gas example, the probability of interaction is related to the transition energies required to change the molecule's energy state. Given that the photon is passing though a certain volume, we can relate the probability of interaction as the ratio of a cross-sectional area of the molecule and a cross-sectional area of the volume. The cross-sectional area of the molecule varies with frequency and is sometimes termed the absorption coefficient.

This simple example may be easily generalized to explain absorption behavior. If we now consider a unit of path, the probability of absorption is proportional to the molecular density and the length of the path. In both cases, there are more molecules to interact with. As each interaction may be modeled as an independent event, the probability of absorption is linear with respect to density and path length; this relation is known as the Beer-Lambert law (also called Beer's law or the Beer-Lambert-Bouguer law).

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