Absorption band

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.

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Overview

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

Band shape

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.

Band width

There are many mechanisms by which line broadening into bands can occur, including:

• Vibrational broadening
• Rotational line broadening
• Doppler broadening

Types of absorption band

Electronic transitions

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

Vibrational transitions take place in the infrared part of the spectrum, at wavelengths of around 1-30 micrometres.

Rotational transitions

Rotational transitions also take place in the infrared, but a lower energies than vibrational transitions.

Other transitions

• Absorbance bands in the radiofrequency range are found in NMR spectroscopy.

Absorption bands of interest to the atmospheric physicist

In oxygen:

• the Hopfield bands, very strong, between about 67 and 100 nanometres in the ultraviolet (named after John J. Hopfield);
• a diffuse system between 101.9 and 130 nanometres;
• the Schumann-Runge continuum, very strong, between 135 and 176 nanometres;
• the Schumann-Runge bands between 176 and 192.6 nanometres (named for Victor Schumann and Carl Runge);
• the Herzberg bands between 240 and 260 nanometres (named after Gerhard Herzberg);
• the atmospheric bands between 538 and 771 nanometres in the visible spectrum; and
• a system in the infrared at about 1000 nanometres.

In ozone:

• the Hartley bands between 200 and 300 nanometres in the ultraviolet, with a very intense maximum absorption at 255 nanometres (named after Walter Noel Hartley);
• the Huggins bands, weak absorption between 320 and 360 nanometres (named after Sir William Huggins);
• the Chappuis bands (sometimes misspelled "Chappius"), a weak diffuse system between 375 and 650 nanometres in the visible spectrum (named after J. Chappuis); and
• the Wulf bands in the infrared beyond 700 nm, centered at 4,700, 9,600 and 14,100 nanometres, the latter being the most intense (named after Oliver R. Wulf).

In nitrogen:

• The Lyman-Birge-Hopfield bands, sometimes known as the Birge-Hopfield bands, in the far ultraviolet: 140– 170 nm (named after Theodore Lyman, Raymond T. Birge, and John J. Hopfield)

See also

• Franck-Condon principle
• Spectroscopy
• Spectral line