Dynamics of neutral molecules stored in a ring

A decelerated beam of neutral ammonia molecules is injected into an electrostatic storage ring. Electrostatic lenses are used to map the emittance of the decelerator onto the acceptance of the ring. The tangential velocity spread of the package of molecules in the ring is set to less than $1\mathrm{m}∕\mathrm{s}$. The package of molecules can be observed for more than 50 distinct round trips, corresponding to $40\mathrm{m}$ in circular orbit and almost $0.5\mathrm{s}$ storage time, sufficiently long for a first investigation of its transversal motion in the ring.

Figure 1

(Color online) (a) Scheme of the experimental setup. The injection beamline consists of a pulsed valve, a Stark decelerator, hexapoles, and a buncher. (b) The molecules are trapped in a two-dimensional potential well. The center of the ring is to the left. Equipotential lines are shown at $0.02{\text{cm}}^{-1}$ intervals. Molecules are detected in the ring using a uv-laser based ionization detection scheme. The inset shows the vertical position of the potential well along the equilibrium radius for three values of the (ac modulation) voltage on the inner ring electrodes.

Figure 2

The solid line in the lower graph shows the ammonia density at the loading position in the ring as a function of storage time. The dashed line is a $1∕t$ plot, used to guide the eye. In the inset, a measurement of the ammonia density after 49–51 round trips is shown, together with a multipeak Gaussian fit, from which a longitudinal temperature of $300\pm 100\mu \mathrm{K}$ is deduced for the package of molecules in the ring. The upper graph shows the time-integrated signal after each round trip.

Figure 3

(Color online) The upper graph shows the density of ammonia molecules in the ring after 10 round trips as a function of the start time of the ac voltage modulation. The modulation signal is a two-period sine wave with a period of $t=2.56\text{ms}$. The starting point of this modulation signal is scanned stepwise from $t_{start}$ to $t_{end}$. A Fourier transformation of the data is shown in the lower graph (solid curve) together with the frequency spectrum of the two-period sine wave (dashed curve).