Absorption Band

Absorption Band

When a photon is absorbed the electromagnetic field of the photon disappears and the state of the system that absorbs the photon changes. Energy, momentum, angular momentum, magnetic dipole moment and electric dipole moment are transported from the photon to the system. Because there are conservation laws, that have to be satisfied, the transition has to meet a series constraints. This results in a series of selection rules. It is not possible to make any transition that lies within the energy or frequency range that is observed.

The strength of an electromagnetic absorption process is mainly determined by two factors. First it is important to realize that transitions that only change the magnetic dipole moment of the system are much weaker than transitions that change the electric dipole moment and that transitions to higher order moments like quadrupole transitions are weaker than dipole transitions. Second, not all transitions have the same transition matrix element, absorption coefficient or oscillator strength.

For some types of bands or spectroscopic disciplines temperature and statistical mechanics plays an important role. For (far) infrared, microwave and radio frequency ranges the temperature dependent occupation numbers of states and the difference between Bose-Einstein statistics and Fermi-Dirac statistics determines the intensity of observed absorptions. For other energy ranges thermal motion effects, like Doppler broadening may determine the linewidth.