The absorption of e.m. radiation by molecules, in different physical states, can be attributed to specific (quantized) changes in their electronic and/or vibrational and/or rotational energy. Subsequent quantized molecular vibrational energy levels are equidistant so that all vibrational transitions occur, for each molecule, by the emission/absorption of radiation at a specific wavelength. Depending on the specific amount of energy required to modify the status of electrons within the atoms composing the molecules, as well as the one required to modify the molecule's vibrational and rotational energy, different wavelengths can be adsorbed. As in the case of atomic spectra which are fully determined by the electronic energy level structure depending on the atomic number, rotational and vibrational energy levels of molecules depends on specific characteristics (number, masses, distances, inertia momentum, elastic constant, etc.) of the atoms composing the molecule itself which make specific and characteristic for each molecule associated absorption spectra. In the Earth's atmosphere the effect of atomic/molecular absorption is significant at wavelength between 1nm and about 1cm. Considering the optical and microwave spectral ranges used in Earth's remote sensing from space it should be noted that:
a) Visible, Near Infrared and Short wave IR radiation (400-3000 nm) is adsorbed mostly for electronic transitions within atoms. In the SWIR region (after 1000nm) forbidden vibrational absorption lines can be observed (overtones and related combinations).
b) e.m. radiation in the Medium and Thermal IR (up to 100.000 nm) spectral range are mostly adsorbed for operating vibrational energy transitions in H2O, CO2 and O3 molecules
c) e.m. radiation in the Far IR up to the Microwave's spectral range (0,035-1 mm) is mostly adsorbed for operating rotational transitions in water vapur molecules. As, in principle, such electronic, vibrational and rotational transitions can contemporary occur (and usually occur considering the collective effect of the enormous number of molecules that can be present even in a small volume of terrestrial atmosphere) molecular spectra results in a complex composition of absorption lines (bands).