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Diffuse Interstellar Bands: A Cosmic Mystery
(Released November 2009)

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  by Pam Graham  

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Spectral Characteristics

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The atomic spectrum is an effect of the quantized orbits of electrons around the atom. In other words, a single mechanism, electronic transition, produces atomic spectra, which are characterized by sharp lines.

Molecular spectra are much more complex than atomic spectra. Molecules, formed when atoms bind together, also exhibit electronic transitions similar to those of an atom. However, in the more complex molecular structure, internuclear distances are quantized in discrete vibrational energy states, with consequent vibrational transitions. Additionally, a molecule has the freedom to rotate in space about various axes, resulting in discrete rotational energy states.

While atomic spectra can undergo a number of weak line broadening processes, the three classes of molecular transitions lead to numerous spectral lines superimposed on each other, closely spaced in wavelength and displaying an easily recognizable banded structure. It was the first detection of these substructures in the profiles of several DIBs that pointed to the molecular nature of DIB carriers.

DIB component chart
The γ6614 diffuse band showing three (possibly four) components. The band shape conforms to P, Q and R rotational branch structure that is common in molecular electronic spectra

Armed with these basics concerning the ISM and spectral analysis, a survey of the main DIB theories that have come and gone, as well as a brief discussion of why tracing the DIB carriers is an important pursuit, becomes more straightforward.

Go To DIB Theories and Importance to Astrophysics

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