4.3.1. Molecular Absorption Attenuation

Molecular absorption attenuation results from an interaction between the radiation and atoms and molecules of the medium (N ₂, O ₂, H ₂, H ₂O, CO ₂, O ₃, Ar, etc.). The absorption coefficient depends on the type of gas molecules and on their concentration. Spectral variations in the absorption coefficient determine the absorption spectrum. The nature of this spectrum is due to the variations of possible energy levels inside the gas mass essentially generated by electronic transitions, vibrations of the atoms and rotations of the molecules. An increase in the pressure or temperature tends to widen the spectral absorption lines by excitation of new possible energy levels and by the Doppler Effect. Molecular absorption is a selective phenomenon which results in the spectral transmission of the atmosphere presenting transparent zones, called atmospheric transmission windows, and opaque zones, called atmospheric blocking windows.

The global transmission windows in the optical range are as following:

The gaseous molecules have quantified energy levels proper to each species. They...