Stokesregel

Stokesregel is a principle in quantum mechanics that describes the allowed transitions between energy levels in atoms. It states that when an electron in an atom transitions from a higher energy state to a lower energy state, it can only emit or absorb a photon with a specific energy corresponding to the difference between those two energy levels. This means that the angular momentum of the electron must change by one unit (or zero in some specific cases) during the transition.

Specifically, Stokes' rule applies to the selection rules governing spectroscopic transitions. For electric dipole transitions, which are the most common, the change in the azimuthal quantum number (l) must be plus or minus one (Δl = ±1). The principal quantum number (n) can change by any integer, and the magnetic quantum number (m) can change by zero or ±1 (Δm = 0, ±1). However, Stokes' rule focuses on the requirement that the electron's orbital angular momentum changes in a specific way.

Violations of Stokes' rule can occur in certain circumstances, such as in the presence of strong magnetic fields or when considering forbidden transitions. These forbidden transitions are less probable but can still be observed under specific conditions. The concept is fundamental to understanding atomic spectra and the interaction of light with matter. It helps predict which spectral lines will be observed for a given atom or molecule.