An electric field shifts and can split the energy eigenvalues of an atom. This is the Stark effect; its hydrogen atom ground-state case is quadratic in a weak field.
The observable in Stark effect spectroscopy is a change in a spectral line, rather than a direct reading of an individual energy eigenvalue. If an upper energy eigenstate emits a photon into a lower energy eigenstate , the Planck constant relates its frequency to the difference:Here denotes the applied electric field component along a chosen axis. Both levels can move, and electric-dipole selection rules for hydrogen decide which pairs produce observable spectral lines. This makes Stark effect spectroscopy a probe of energy eigenstates and their electric dipole moments: the slope is the corresponding dipole component by the Hellmann–Feynman theorem. The comparison with Zeeman splitting is useful: there an applied magnetic field supplies the perturbation.
The Stark effect is the phenomenon in which the energy levels of atoms and molecules are shifted and split in the presence of an external electric field. This effect is named after the German physicist Johann Stark, who discovered it in 1913. When an external electric field is applied, it interacts with the electric dipole moments of the atoms or molecules.
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