of polyatomic systems, including condensed materials, large molecules, etc. Each spectral line corresponds to the difference in two energy levels of an atom. In molecules, these levels can split. When the number of atoms is large, one gets a continuum of energy levels, the so-called spectral bands. They are often labeled in the same way as the monatomic lines.

The bands may overlap. In general, the energy spectrum can be given by a density function, describing the number of energy levels of the quantum