Low and High Field Seeking Molecules

A polar molecule's classification as low or high field seeking depends only on its rotational state and the strength of the electric field, not on any intrinsic property of the molecule. A static electric dipole aligned with an inhomogeneous field is pulled toward the strongest field, but a dipole aligned antiparallel with the field is drawn in the direction of the weakest field. A ground state polar molecule tends to align with the field and is a high field seeker. But for a tumbling molecule, the rotation looks something like a pendulum rotating "over the top": It turns slowest through the least-favorable configuration and fastest through the most favorable (aligned) direction. So a rotating molecule ends up spending more time with its dipole aligned antiparallel with the field and is a low field seeker.

If the field is strong enough, however, it can enforce more alignment and prevent even a rotationally excited polar molecule from tumbling, which turns it back into a high field seeker. In that case the molecule simply oscillates like a pendulum. For every rotational energy state there is a critical electric field above which the polar molecule will become a high field seeker. This is a classical picture of a quantum mechanical effect, of course, so it leads to some inconsistencies. For example, a rotationally excited molecule that is prevented from rotating by a strong electric field is difficult to picture intuitively.