Electrostatic Extraction of Cold Molecules from a Cryogenic Reservoir

We present a method which delivers a continuous, high-density beam of slow and internally cold polar molecules. In our source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electrostatic quadrupole guide. For ${\mathrm{ND}}_3$ the source produces fluxes up to $(7{\pm }_4^7)\times {10}^{10}\mathrm{\text{molecules}}/\mathrm{s}$ with peak densities up to $(1.0{\pm }_{0.6}^{1.0})\times {10}^9\mathrm{\text{molecules}}/{\mathrm{cm}}^3$. For ${\mathrm{H}}_2\mathrm{CO}$ the population of rovibrational states is monitored by depletion spectroscopy, resulting in single-state populations up to $(82\pm 10)\%$.

Figure 1

Schematic view of the experiment. Warm molecules are cooled by collisions with a cold helium gas. Behind the cell an electric quadrupole extracts slow molecules in low-field-seeking states out of the cryogenic environment into the detection chamber, where they are detected by a QMS. Depletion measurements are performed by counterpropagating an UV laser beam through the guide.

Figure 2

(a) Recording cycle to measure guided molecules and the depletion signal. The data are background subtracted and normalized to the average count rate without laser light. (b) QMS signal (normalized to the maximal value) as function of $n_{\mathrm{He}}$. For ${\mathrm{ND}}_3$, the highest signal is obtained with $n_{\mathrm{He}}=1.8\times {10}^{-14}{\mathrm{cm}}^{-3}$. The distribution looks similar for ${\mathrm{H}}_2\mathrm{CO}$. The dashed curve is a guide to the eye.

Figure 3

Depletion as function of $T_{\mathrm{cell}}$ for different states. The dashed lines are guides to the eye.

Figure 4

Depletion as function of $n_{\mathrm{He}}$ for different states. The large statistical error bars at high $n_{\mathrm{He}}$ are caused by the relatively low count rates at these settings. The lines are guides to the eye.