Atmospheric Absorption Atmospheric Absorption

The energy structure of atoms and molecules is composed of discrete energy levels. Energy levels are said to be quantised. Photons of electromagnetic radiation also have discrete energy, which is proportional to frequency via Plancks constant, h. Thus E = hf where f is the frequency.

A photon with a frequency such that E1 - E0 = hf0 (where E1 and E0 correspond to a possible transition within the molecule) will be absorbed (as illustrated below) subject to probability whereas no other frequency will interact with the molecule; at least for this energy transition. As a result some energy will be removed from the photon field at exactly the frequency f0 and thus a spectral line will be observed at this frequency.

Transitions can be electronic, vibrational or rotational. The energy levels involved for the different types of transition are generally well separated so that Eel > Evib > Erot. Rotational features in the spectrum typically occur in the far infrared. Vibrational features occur in the mid and near infrared whereas electronic transitions are mainly located in the UV and visible regions. For a given molecule, the contribution of the different possible transitions combined with different broadening effects results in complex absorption features as a function of wavelength. As an example, the graph below shows the absorption cross section of Ozone in the Higgins Bands (UV) for two different temperatures.

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The actual shape of the aborption features and the wavelength regions where aborptions occur are different for each molecule. The following picture shows the main regions where the differents target molecules interact with the solar radiation within the SCIAMACHY spectral range.

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Image Courtesy of IFE/Bremen

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