Optimized cell geometry for buffer-gas-cooled molecular-beam sources

We have designed, constructed, and commissioned a cryogenic helium buffer-gas source for producing a cryogenically cooled molecular beam and evaluated the effect of different cell geometries on the intensity of the produced molecular beam, using ammonia as a test molecule. Planar and conical entrance and exit geometries are tested. We observe a threefold enhancement in the ${\mathrm{NH}}_3$ signal for a cell with planar entrance and conical-exit geometry, compared to that for a typically used “boxlike” geometry with planar entrance and exit. These observations are rationalized by flow field simulations for the different buffer-gas cell geometries. The full thermalization of molecules with the helium buffer gas is confirmed through rotationally resolved resonance-enhanced multiphoton ionization spectra yielding a rotational temperature of 5 K.

Figure 1

(a) Schematic of the cryogenic buffer-gas-cell setup. A two-stage cryocooler holds a copper cell, cooled to 3.4 K base temperature. The cell has a small volume at the front where cold helium (orange dots) is introduced, which then flows radially into the main thermalization cell. Warm molecules (red dots) are injected from a heated stainless steel capillary, carried by the radial helium flow, and eventually thermalize to the buffer-gas temperature (blue dots) via collisions. The produced molecular beam is detected in a time-of-flight mass spectrometer; see text for details. (b) The buffer-gas cell has detachable entrance and exit end caps, which can either be planar, with a ${180}^{\circ }$ opening angle, or conical, with a ${106}^{\circ }$ opening angle, resulting in four distinct geometries.

Figure 2

Measured ammonia-ion signal (solid circles), from SFI of ammonia, as a function of helium flow rate for the four geometries of the cryogenic cell; the signal decreases from PC over CC and PP, to CP. Lines are third-order polynomial fits to guide the eye. All geometries show an initial linear increase, until a maximum is reached around $12{\text{ml}}_{\text{n}}\text{/}\min$ helium flow. The cell geometry significantly affects measured signal intensity, with a planar-conical cell providing the most intense molecular beam.

Figure 3

Simulations of the helium flow field for four different geometries of our cryogenic cell. Depicted are streamlines with the helium flowing from left to right. For clarity, only one half of the cylindrically symmetric $r,z$ plane is shown.

Figure 4

REMPI spectrum of ${\mathrm{NH}}_3$ measured using a planar-conical cell at room temperature (black, upper line) and cooled to 3.8 K (red, lower line), together with corresponding pgopher simulations at 4, 5, and 6 K rotational temperature. In the inset, the different simulations can be distinguished on the right peak, which is strongest for 6 K, intermediate for 5 K, and weakest for 4 K. The 5 K simulation reproduces the experimental data very well, indicating full thermalization of the molecules with the buffer gas.