nondipolar

Nondipolar is an adjective used in physics and chemistry to describe phenomena that do not involve the electric dipole moment or dipole-allowed transitions. In the dipole approximation, interactions of light with matter are dominated by the electric dipole term. Nondipolar effects arise when the dipole term is absent or negligible, and higher-order terms in the electromagnetic interaction, such as magnetic dipole (M1) and electric quadrupole (E2), become relevant.

In chemistry, nonpolar molecules are sometimes described as nondipolar because they possess no permanent electric dipole moment. This occurs when molecular geometry and electronegativity symmetry cause bond dipoles to cancel. Nonpolar molecules include N2, O2, CH4, and CCl4. Note that a molecule can be composed of polar bonds yet be overall nonpolar due to symmetry.

In spectroscopy, nondipolar transitions are those that are not electric-dipole (E1) allowed. They include magnetic dipole (M1) and electric quadrupole (E2) transitions and higher-order multipoles (E3, M2, etc.). These transitions are typically weaker and have different selection rules. Nondipolar contributions can be observed in high-resolution spectra, in forbidden lines, and in transitions where dipole intensity is suppressed by symmetry.

Beyond molecules, the concept appears in the multipole expansion of radiation and light–matter interaction, where the full description includes a series of multipole terms; each term has characteristic angular and polarization properties. The term nondipolar emphasizes the presence or significance of non-dipole processes in a given system.