Whereas single-atom gases have spectra with widely spaced lines, the spectra of molecules consist of groups of many lines lying close together. Molecular spectra are called band spectra because, when observed with spectrographs which have low resolution, luminous bands ostruf unresolved lines appear. Grating spectrographs with high dispersion are able to resolve the fine structure of bands which distinguishes bands from continuous spectra.

  Band spectra, although usually measured by infra-red absorption, are also present in emission sources. In particular the bands of diatomic molecules give rise to background spectra, especially in the visible and UV range of the spectrum. Bands in the far infra-red are clearly composed of line sequences and can be associated with the various energy states of a molecule rotating about the principal axes of inertia. In the near infra-red, band spectra are more complicated. Here, in addition to rotation, linear or distance vibrations of the atoms in the molecule may occur. In the visible and ultra-violet regions conditions are even more complex. Here energy levels of the electron arrangements are added so that the band spectra are based on a triple structure. This is due to the three energy components which produce the large number of lines:

 -the potential energy of the electron arrangement(electron energy)

 - the oscillation energy of the atoms relative to each other

 - the kinetic energy from rotation about an axis passing through the center of gravity

  With this sub-division of energy it has been possible to interpret most band spectra while finding links with the type of bond. Molecular spectroscope is a physical aid for structure analysis, particularly in organic chemistry.

  Molecular or band spectra are only of interest because they give undesirable interference effects in the spectral analysis discussed here.