selectieregel

Selectieregel, often referred to in German as a selection rule (the standard term in physics is Auswahlregel), is a concept that describes restrictions on which transitions, changes, or selections are allowed in a given process. The form selectieregel is largely a direct calque or bilingual usage and is not the prevalent term in most German technical literature, where Auswahlregel is preferred. Nevertheless, the idea behind a selectieregel is widely used across physics and mathematics.

Selection rules arise from fundamental principles such as conservation laws and symmetry properties of a system. They determine which transitions or processes are allowed and which are forbidden, thereby simplifying predictions and analyses. In quantum mechanics, for example, selection rules constrain electronic transitions in atoms and molecules. Electric-dipole transitions typically require changes in angular momentum and parity, so many transitions are forbidden under the dipole approximation if these conditions are not met.

Beyond spectroscopy, selection rules appear in vibrational spectroscopy (IR activity requires a change in dipole moment; Raman activity involves changes in polarizability), in nuclear and particle physics (gamma transitions follow angular-momentum and parity constraints), and in solid-state physics where symmetry dictates allowable electronic and vibrational modes. Group theory and the Wigner–Eckart theorem provide systematic methods to derive these rules from the symmetry of the system.

See also: selection rule.