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arXiv:1707.06291 · Journal of Modern Optics

A buffer gas beam source for short, intense and slow molecular pulses

Stefan Truppe, M. Hambach, Sarah M. Skoff, N. E. Bulleid, J. S. Bumby, Richard Hendricks, E. A. Hinds, B. E. Sauer, Michael R. Tarbutt

Published Nov 5, 2017• 68 citations
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Abstract

Experiments with cold molecules usually begin with a molecular source. We describe the construction and characteristics of a cryogenic buffer gas source of CaF molecules. The source emits pulses with a typical duration of 240 μs, a mean speed of about 150 m/s, and a flux of 5×1010 molecules per steradian per pulse in a single rotational state.

DOI
10.1080/09500340.2017.1384516

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Experiments with cold molecules usually begin with a molecular source. We describe the construction and characteristics of a cryogenic buffer gas source of CaF molecules. The source emits pulses with a typical duration of 240 μs, a mean speed of about 150 m/s, and a flux of 5×1010 molecules per steradian per pulse in a single rotational state.
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We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2 × 10(11) molecules/sr/pulse in the X(2)Σ(+)(v = 0, N(rot) = 0) state, with 140(m/s) forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.
Molecular magneto-optical traps (MOTs) are loaded from cryogenic buffer gas beam (CBGB) sources, which produce a large flux of cold molecules. However, only a small fraction of this flux is slowed and captured. Here, we investigate and optimize one of these CBGB sources through a combination of experiment and simulation to maximize the number of molecules loaded into a MOT of CaF molecules. We experimentally determine the range of molecule velocities from the source that are successfully slowed and captured in the MOT, and perform a systematic study of buffer gas source geometries to identify designs that maximize MOT loading. Our observations agree well with Monte Carlo simulations, which provide physical insight into the dynamics within the source and explain how source geometry influences the subset of molecules that are ultimately trapped. These results provide practical design guidelines for optimizing cryogenic buffer gas beam sources used in laser cooling experiments.
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We demonstrate and characterize a high-flux beam source for cold, slow atoms or molecules. The desired species is vaporized using laser ablation, then cooled by thermalization in a cryogenic cell of buffer gas. The beam is formed by particles exiting a hole in the buffer gas cell. We characterize the properties of the beam (flux, forward velocity, temperature) for both an atom (Na) and a molecule (PbO) under varying buffer gas density, and discuss conditions for optimizing these beam parameters. Our source compares favorably to existing techniques of beam formation, for a variety of applications.