Editing Stimulated emission (section)

== Overview ==
[[Electron]]s and their interactions with [[electromagnetic field]]s are important in our understanding of [[chemistry]] and [[physics]].
In the [[Classical electromagnetism|classical view]], the energy of an electron orbiting an atomic nucleus is larger for orbits further from the [[atomic nucleus|nucleus]] of an [[atom]].  However, quantum mechanical effects force electrons to take on discrete positions in [[Atomic orbital|orbitals]].  Thus, electrons are found in specific energy levels of an atom, two of which are shown below:

[[File:Stimulated Emission.svg|center|550px]]

When an electron absorbs energy either from [[light]] (photons) or [[heat]] ([[phonon]]s), it receives that incident quantum of energy. But transitions are only allowed between discrete energy levels such as the two shown above.
This leads to [[emission line]]s and [[Spectral line|absorption line]]s.

When an electron is [[Excited state|excited]] from a lower to a higher energy level, it is unlikely for it to stay that way forever.
An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. When such an electron decays without external influence, emitting a photon, that is called "[[spontaneous emission]]". The phase and direction associated with the photon that is emitted is random. A material with many atoms in such an excited state may thus result in [[radiation]] which has a narrow spectrum (centered around one [[wavelength]] of light), but the individual photons would have no common phase relationship and would also emanate in random directions. This is the mechanism of [[fluorescence]] and [[thermal emission]].

An external electromagnetic field at a frequency associated with a transition can affect the quantum mechanical state of the atom without being absorbed. As the electron in the atom makes a transition between two stationary states (neither of which shows a dipole field), it enters a transition state which does have a dipole field, and which acts like a small electric [[dipole]], and this dipole oscillates at a characteristic frequency. In response to the external electric field at this frequency, the probability of the electron entering this transition state is greatly increased. Thus, the rate of transitions between two stationary states is increased beyond that of spontaneous emission. A transition from the higher to a lower energy state produces an additional photon with the same phase and direction as the incident photon; this is the process of stimulated emission.