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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.


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