The Stark effect is the shifting and splitting of atomic or molecular spectral lines when an external electric field is applied.
First observed in 1913 by Johannes Stark, this phenomenon provided important early evidence for the interaction between electromagnetic fields and atomic structure. The effect arises because the electric field perturbs the energy levels of electrons, altering the frequencies of light emitted or absorbed during electronic transitions. As the electric analogue of the Zeeman effect, the Stark effect is analysed using perturbation theory and plays a significant role in spectroscopy and quantum mechanics.
The precise way in which these spectral lines shift, however, depends on both the internal structure of the atom or molecule and the strength of the applied electric field. In practice, two principal regimes are distinguished: the linear Stark effect and the quadratic Stark effect.
The linear Stark effect is defined as the first-order energy correction in perturbation theory and occurs when the energy shift is directly proportional to the strength of the applied electric field. This behaviour typically appears in systems with degenerate energy levels, such as in the hydrogen atom. In contrast, the quadratic Stark effect corresponds to the second-order energy correction and occurs when the energy shift is proportional to the square of the electric field strength. This is the more common situation for atoms and molecules with non-degenerate energy levels.
Details of these two effects will be discussed in the next two articles.