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 $250\mu \mathrm{K}$.

Figure 1

Scheme of the experimental setup. A pulsed beam of ammonia molecules is decelerated and passes through a buncher. The arrival time distribution of the package of molecules at the laser interaction zone is recorded using a UV-laser based ionization detection scheme.

Figure 2

Expanded view of the end of the decelerator, the buncher, and the detection region. The calculated longitudinal phase-space distribution of the ammonia molecules is given at the exit of the Stark decelerator ($t=0\mathrm{m}\mathrm{s}$), at the entrance ($t=1.743\mathrm{m}\mathrm{s}$) and exit ($t=2.100\mathrm{m}\mathrm{s}$) of the buncher, and in the laser detection region ($t=2.652\mathrm{m}\mathrm{s}$), relative to the position in phase space of the synchronous molecule.

Figure 3

Density of ammonia molecules 33 cm behind the buncher as a function of time (at $t=0\mathrm{m}\mathrm{s}$ the synchronous molecule is at the exit of the decelerator) for different voltages on the buncher, using two bunching stages. In the inset similar measurements recorded at a 4.5 cm distance behind the buncher, using three bunching stages, are shown. The results of trajectory calculations through the entire setup are shown underneath the experimental data.

Figure 4

Calculated longitudinal phase-space distribution at the moment that the buncher is switched on ($t=1.743\mathrm{m}\mathrm{s}$) and off ($t=1.982\mathrm{m}\mathrm{s}$) with voltages of $+3.8$ and $-3.8\mathrm{k}\mathrm{V}$. The projections of the longitudinal velocity distributions on the $v_z$ axis are given for both cases. On the right hand side, the measured (upper trace) and calculated (lower trace) TOF distributions 33 cm behind the buncher are shown.