Longitudinal Focusing and Cooling of a Molecular Beam

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 with a longitudinal velocity spread of $0.76\mathrm{m}/\mathrm{s}$, corresponding to a temperature of $250\mu \mathrm{K}$.