Continuous Centrifuge Decelerator for Polar Molecules

Producing large samples of slow molecules from thermal-velocity ensembles is a formidable challenge. Here we employ a centrifugal force to produce a continuous molecular beam with a high flux at near-zero velocities. We demonstrate deceleration of three electrically guided molecular species, ${\mathrm{CH}}_3\mathrm{F}$, ${\mathrm{CF}}_3\mathrm{H}$, and ${\mathrm{CF}}_3\mathrm{CCH}$, with input velocities of up to $200{\mathrm{m}\mathrm{s}}^{-1}$ to obtain beams with velocities below $15{\mathrm{m}\mathrm{s}}^{-1}$ and intensities of several ${10}^9{\mathrm{mm}}^{-2}{\mathrm{s}}^{-1}$. The centrifuge decelerator is easy to operate and can, in principle, slow down any guidable particle. It has the potential to become a standard technique for continuous deceleration of molecules.

Figure 1

(a) Centrifuge decelerator, top view. For a description of the molecules’ propagation through the decelerator, see text. $\Omega$ is the centrifuge rotation speed. Color bar labels $L$ and $H$ stand for low and high electric field, respectively. (b) Force vector diagram explaining the origin of the deceleration force (for details, see text). Red curve $A$: spiral-shaped quadrupole guide (“longitudinal” molecular trajectory in the rotating frame) rotating with an angular velocity $\Omega$ about the axis $O$; blue curve $B$: trajectory in the laboratory frame for a given molecule’s input velocity $v_{\mathrm{in}}$ and a centrifuge angular velocity $\Omega$.

Figure 2

Sketch of the output velocity distributions for different centrifuge rotation speeds $\Omega$. The centrifuge deceleration results in a shift to lower velocities and a broadening of the velocity distribution. Red lines $A$ and blue lines $B$ designate the fractions of the velocity distributions cut off by the prefilter and by the exit bend, respectively (see text). Green lines and green shaded areas $C$ show fractions of the velocity distribution exiting from the centrifuge. Orange dotted lines $D$ display molecules with insufficient kinetic energy, which do not reach the exit of the centrifuge, and are reflected back towards the source.

Figure 3

Experimental results for ${\mathrm{CF}}_3\mathrm{H}$ ($m=70\mathrm{u}$). (a) Velocity distributions of the continuous output beam of ${\mathrm{CF}}_3\mathrm{H}$ molecules at a guide voltage of $\pm 5\mathrm{kV}$ for different centrifuge rotation speeds. The range of velocities trappable in a 1-K-deep trap is highlighted in green. (b) Velocity distributions of the continuous output beam of ${\mathrm{CF}}_3\mathrm{H}$ molecules at a guide voltage of $\pm 2\mathrm{kV}$ for different centrifuge rotation speeds. The range of velocities trappable in a 1-K-deep trap is highlighted in green. (c) Nozzle-pressure dependence of the integrated count rate of molecules with longitudinal velocities between 0 and $15{\mathrm{m}\mathrm{s}}^{-1}$ (corresponding to an energy of 1 K) for the centrifuge at rest and for the centrifuge at a rotation speed of 33.3 Hz. The blue asterisk designates the lack of signal at zero pressure. All quoted errors stem from counting statistics of the time-binned time-of-flight signal.

Figure 4

Dependence of the output flux of ${\mathrm{CF}}_3\mathrm{H}$ on the centrifuge rotation phase for different centrifuge rotation speeds. For $2/3$ of the rotation cycle the molecules experience a continuous guiding potential around the periphery of the centrifuge provided by the stationary annular guide encompassing it. With increasing rotation speed, a phase shift is observed, the total count rate rises, and the modulation depth decreases. For further explanations, see text.