Pendular state

A pendular state is a field-dressed rotational quantum state in which a molecule's orientation is concentrated near a preferred direction, resembling small angular librations of a pendulum. It arises when the orienting or aligning field interaction competes strongly with rotational kinetic energy.

A permanent dipole in a static electric field has an interaction proportional to $-\cos \theta$, which distinguishes opposite directions. An anisotropic induced-polarization interaction often depends on $-{\cos }^2\theta$, aligning an axis without necessarily orienting its two ends. The field strength and rotational constant determine whether a weak-field perturbation treatment is adequate.