Continuous Loading of an Electrostatic Trap for Polar Molecules

A continuously operated electrostatic trap for polar molecules is demonstrated. The trap has a volume of $\approx 0.6{\mathrm{cm}}^3$ and holds molecules with a positive Stark shift. With deuterated ammonia from a quadrupole velocity filter, a trap density of $\approx {10}^8{\mathrm{cm}}^{-3}$ is achieved with an average lifetime of 130 ms and a motional temperature of $\approx 300\mathrm{mK}$. The trap offers good starting conditions for high-precision measurements, and can be used as a first stage in cooling schemes for molecules and as a “reaction vessel” in cold chemistry.

Figure 1

(a) Exploded view of the trap with input and output quadrupoles. The quadrupole guide filters slow molecules from the cooled effusive source and guides them into the trap. Here the slowest molecules reside till they find their way out through one of the two quadrupole guides. The trapped gas is analyzed with a mass spectrometer positioned behind the output quadrupole. (b) Electric field distribution in the $xy$ plane for electrode voltages of $\pm 5\mathrm{kV}$ (left) together with the field distribution in the $yz$ plane containing the input quadrupolar guide (right).

Figure 2

(a) The raw detector signal at the output quadrupole as a function of time when the voltage at the input quadrupole is switched. The electrodes carry voltages of $\pm 4.5\mathrm{kV}$. (b) Deconvoluted detector signals as a function of time for different trapping-field configurations. A rapid decay is observed when the output is switched off at $t=1\mathrm{s}$. The trap dynamics is revealed by rapidly switching the voltage on one small ring electrode from 4.5 kV to 0, 2, and 4.5 kV after finishing the filling process in order to create an artificial hole in the trap. When the particular electrode is set to 0 kV a clear signal loss can be observed, whereas the slow decay caused by trapped molecules becomes dominant for higher voltages when the trap hole is closed. The signal trace obtained with output off serves as a reference.

Figure 3

Velocity distribution of trapped ${\mathrm{ND}}_3$ molecules derived from data obtained with electrode voltages of $\pm 4.5\mathrm{kV}$ (squares). The line is a fit to the measured data of the functional form $(2v/{\alpha }^2)\exp [-v^2/{\alpha }^2]$, with the characteristic longitudinal velocity $\alpha =16.6\pm 1\mathrm{m}/\mathrm{s}$. The triangles denote simulation results.