dipoleforbidden

Dipoleforbidden refers to a spectroscopic transition between two quantum states of a molecule or atom that is not allowed by the rules of quantum mechanics, specifically the selection rules governing electric dipole transitions. These selection rules dictate that for a transition to be permitted via the absorption or emission of a photon through an electric dipole interaction, a change in the dipole moment of the system must occur. This change is typically associated with a specific change in quantum numbers, such as angular momentum.

Transitions that are dipoleforbidden can still occur, but they are significantly less probable than allowed transitions. These less probable transitions may proceed through other mechanisms, such as magnetic dipole transitions or electric quadrupole transitions, which involve different interaction operators and have different selection rules. In some cases, dipoleforbidden transitions can become weakly allowed due to perturbations that mix different quantum states, breaking the strict symmetry requirements of the selection rules. Observing dipoleforbidden transitions can provide valuable information about the electronic structure and symmetry of the system, and they are often observed in high-resolution spectroscopy, particularly for molecular electronic transitions or vibrational transitions in molecules with specific symmetries. The term "forbidden" is therefore a relative one, indicating a low probability rather than an absolute impossibility.