Found a new paper on this subject: PRL 89, 093004
Here is the abstract:
A neutral polar molecule experiences a force in an inhomogeneous electric
field. This electric field can be designed such that a beam of polar molecules
is exposed to a harmonic potential in the forward direction. In this potential
the longitudinal phase-space distribution of the ensemble of molecules is
rotated uniformly. This property is used to longitudinally focus a pulsed
beam of ammonia molecules and to produce a beam
