Continuous guided beams of slow and internally cold polar molecules

Abstract

We describe the combination of buffer-gas cooling with electrostatic velocity filtering to produce a high-flux continuous guided beam of internally cold and slow polar molecules. In a previous paper (L.D. van Buuren et al., arXiv: 0806.2523v1) we presented results on density and state purity for guided beams of ammonia and formaldehyde using an optimized set-up. Here we describe in more detail the technical aspects of the cryogenic source, its operation, and the optimization experiments that we performed to obtain best performance. The versatility of the source is demonstrated by the production of guided beams of different molecular species.

The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules

“…Molecules of a particular forward velocity range can be selected by rotating mechanical objects [86]; however, using fields is much less technically challenging, especially in a cryogenic environment. The first realization of this method [87] (and later realizations with buffer gas cooled beams [5,15,16,67]) was to simply bend one of the guides described above, similar to what is shown in Figure 14. Molecules with high enough kinetic energies in the forward direction will simply pass over the potential barrier, and only the more slow-moving molecules will remain in the guide.…”

“…A purely effusive beam should have a forward velocity which, according to Eq. (47), does not change with source pressure; however, in buffer gas beam sources with good extraction [3,7,9,16] the forward velocity of the molecules indeed changes with source pressure. Slowing cells (Section II C 2) can offer near-effusive velocity distributions, but typically have ∼ 1% extraction efficiency [5,10].…”

“…can be used to change the output of a buffer gas cooled beam to suit a particular application [5,15,16,67,84]. Guiding the species of interest can be useful for a number of reasons.…”

“…It should be noted that regardless of technique, the density of the species is typically (though not in the case of some capillary filling schemes [15,16]) < 1% of the number density of the buffer gas. This fact will be important later on, because it allows us to treat the the gas flow properties as being determined solely by the buffer gas, with the species as a trace component.…”

Cooling and collisions of large gas phase molecules

“…1,5 In related work, a beam of slow (11 m s À1 ), but rotationally and vibrationally warm (300 K), PerfluoroC 60 (mass 4 6000 amu) was produced by filtering slow molecules from a warm sample. 6 Cold samples of large molecular ions are routinely produced and trapped.…”