Band Spectra:

Band spectra arise from gaseous radicals or small molecules and consist of a series of closely spaced lines which arise from the radiative relaxation of the excited molecule to any of the available vibrational levels in the ground state Figure illustrates the origin of band spectrum of a molecule. You might remember here that the molecule within the vibrational ground state of the excited electronic state might relax to any of the five vibrational stages of the ground state display here. Accordingly the emission spectrum would consist of five closely spaced lines within the example provided here. In complex molecules, the emission band consists of several more lines which are much more closely spaced and appear as a band.

Figure: Emission band spectrum of a molecule