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An absorption band is a range of wavelengths (or, equivalently, frequencies) in the electromagnetic spectrum which are able to excite a particular transition in a substance. See absorption spectrum. Since energetic transitions can take place in both directions, many absorption bands can also act as an emission band.
According to quantum theory, atoms and molecules can only hold certain defined quantities of energy, or exist is specific states. Therefore, in order for a substance to change its energy it must do so in a series of "steps" conforming to the allowed states that it may exist in. Each of these steps corresponds to a particular energy, which may be represented as a wavelength of light (or a spectral line). Thus, when light of one of these specific wavelengths interacts with a molecule, it can absorb it while other wavelengths pass. This is the origin of the absorption spectrum.
However, not all molecules are exactly the same, even those of the same substance. They will be moving differently, vibrating and rotating differently, and have different neighbors. For this reason each transiton happens at a slightly different wavelength in different molecules. Thus, rather than an infinitely narrow absorbance line in the spectrum, a band is observed - a range within which a particular transition may take place under the right conditions.
Absorption band characteristics
A wide variety of band shapes exist, and the analysis of the band shape can be used to determine information about the transition that casues it. All the same, in many cases it is convenient to assume that a spectral band is an easily modeled shape such as a Gaussian or Lorentzian.
There are many mechanisms by which line broadening into bands can occur, including:
Types of absorption band
Electronic transitions mainly take place at energies corresponding to the UV and visible part of the spectrum. The main factors that cause broadening of the spectral line into an absorption band are the distributions of vibrational and rotational energies of the molecules in the sample (and also those of their excited states). In gas phase spectroscopy, the fine structure afforded by these factors can be discerned, but in solution-state spectroscopy, the differences in molecular microenvironments further broaden the structure to give smooth bands. Electronic transition bands of molecules may be from tens to several hundred nanometers in breadth.
Vibrational transitions take place in the infrared part of the spectrum, at wavelengths of around 1-30 micrometres.
Rotational transitions also take place in the infrared, but a lower energies than vibrational transitions.
Absorption bands of interest to the atmospheric physicist
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Absorption_band". A list of authors is available in Wikipedia.|