An absorption band is a range of wavelengths, frequencies, or energies (a series of very closely spaced absorption lines) in the electromagnetic spectrum resulting from absorption by molecules that is characteristic of a particular transition from the initial to the final state in a substance.
According to quantum mechanics, atoms and molecules can only hold certain defined amounts of energy or exist in certain states. When electromagnetic radiation is absorbed by an atom or molecule, the energy of the radiation changes the state of the atom or molecule from an initial state to a final state. The number of states in a given energy range is discrete for gaseous or dilute systems, with discrete energy levels.
Condensed systems, such as liquids or solids, have a continuous density of states distribution and often have continuous energy bands. For a substance to change its energy, it must do so in a series of “steps” by absorbing a photon. This absorption process can move a particle, such as an electron, from an occupied state to an empty or unoccupied state. It can also move an entire vibrating or rotating system, such as a molecule, from one vibrational or rotational state to another, or it can create a quasiparticle, such as a phonon or plasmon in a solid.