The theory that describes the emission and absorption of electromagnetic radiation. Since it is a quantum theory the processes of emission and absorption are described in terms of the creation and disappearance of photons, with photons being created in emission processes and disappearing in absorption. Absorption occurs when electromagnetic radiation that impinges on a quantum mechanical system, such as an atom or molecules, induces a transition from the ground state of the system to an excited state by virtue of a photon of the radiation being absorbed. Similarly, emission of a photon occurs when an atom returns to the ground state from an excited state, i.e. spontaneous emission occurs. It is also possible for an atom in an excited state to emit a photon by induced emission, a process that is used in lasers.
Subjects: Chemistry — Physics.