Dipolwahlregeln

Dipolwahlregeln, often translated as "dipole selection rules," are a set of principles in quantum mechanics that govern the probabilities of transitions between different energy states of an atom or molecule due to the absorption or emission of electromagnetic radiation. These rules arise from the conservation of angular momentum and parity during the interaction of the light with the system.

In essence, dipole selection rules state that transitions are most likely to occur when the change in the angular momentum quantum number ($\Delta l$) is plus or minus one ($\Delta l = \pm 1$) and the change in the magnetic quantum number ($\Delta m$) is zero or plus or minus one ($\Delta m = 0, \pm 1$). Furthermore, transitions are forbidden if the parity of the initial and final states is the same. Parity refers to the symmetry of the wavefunction with respect to inversion through the origin.

These rules are a consequence of the fact that the interaction Hamiltonian, which describes the coupling between the electromagnetic field and the system, is proportional to the electric dipole moment operator. This operator has specific symmetry properties that dictate which transitions are allowed. While these are the electric dipole selection rules, other types of transitions, such as magnetic dipole or electric quadrupole transitions, can also occur, but they are generally much weaker.

Dipole selection rules are crucial for understanding and interpreting atomic and molecular spectra. They explain why certain spectral lines are observed while others are absent, providing valuable information about the electronic structure and properties of matter. They are fundamental to spectroscopy and have wide applications in physics, chemistry, and astronomy.