Note: A helical velocity selector for continuous molecular beams

Abstract

We report on a modern realization of the classic helical velocity selector for gas phase particle beams. The device operates stably under high vacuum conditions at rotational frequencies limited only by commercial dc motor capabilities. Tuning the rotational frequency allows selective scanning over a broad velocity band. The width of the selected velocity distributions at full-width-half-maximum is as narrow as a few percent of the selected mean velocity and independent of the rotational speed of the selector. The selector generates low vibrational noise amplitudes comparable to mechanically damped state-of-the-art turbo-molecular pumps and is therefore compatible with vibration sensitive experiments like molecule interferometry. © 2010 American Institute of Physics. ͓doi:10.1063/1.3499254͔Manipulation and control of electrically neutral particles, such as atoms and molecules in the gas phase is an active field of research. 1 The general aim is to increase the ability to control the motion of particles as they propagate in gas phase beams by means of deceleration, 2-5 trapping, 6 and cooling. 7,8 Today, cold atoms and small molecules are the best controlled physical systems and therefore are the ideal playground to study physics 9 and chemistry 10,11 at the most fundamental level. 12 For more complex systems-starting with molecules of more than about ten atoms-this level of control has not yet been achieved. Velocity selection is one of the first steps to increase control over the motion of large molecules and clusters. For instance, molecule interferometry is challenging today's gas phase manipulation technologies and will gain from improved beam monochromaticity: by reaching maximal interference visibility, 13 by higher precision in interferometric deflection for molecule metrology, 14 as well as by interferometric particle sorting. 15 Cooling and trapping of molecules will benefit from improved molecule velocity selection. 12 Plainly, our device can be used to select slow particles from a wide thermal velocity distribution 16 as proposed in one of the very early approaches to produce cold atoms. 17 As a further example, velocity selection can be used to separate buffer gas cooled molecules thermalized with their lighter coolant atoms propagating at much higher velocities. Furthermore, atom, molecule and cluster lithography techniques 18-20 will benefit from velocity pre-selected particles.We here have picked up the old idea to mechanically separate particles of different velocities, for instance generated by a thermal beam source with a very broad velocity distribution and base our design on earlier helical velocity selectors for neutrons, 21,22 atoms, 23 and the related slotted disk selectors for molecules. 24,25 The main idea is to make use of the difference in propagation time particles need for their passage through a spatial confinement. Such a confinement can be realized by a sequence slits arranged by rotating slotted disks at well defined positions and with well defined phase relations. It turns out ...

Pulsed rotating supersonic source for merged molecular beams

“…9 and 10 include curves for four sets of velocity spreads. These range from (a) utopian 1%, achievable for Stark decelerated beams 17 , but at great cost in intensity; to (b) 5%, attainable with a compact velocity selector 55 with acceptable cost in intensity; to (c) 10%, typical for supersonic beams; to (d) 10% + 20%, similar to our current experiment.…”

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

“…Adapted from Patterson et al [67] In addition to guiding or focusing to increase useful flux, electric or magnetic fields can be used to select particular velocity ranges from the output of the molecular beam, which is useful for collision studies and trap loading experiments (see Section III C), as will be discussed. 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.…”

Nonlinear dynamics of dipoles in microwave electric field of a nanocoaxial tubular reactor

“…Another possible application, which can be useful in precise spectroscopic measurements and collisional studies, is the development of dipole traps and angular velocity selectors in analogy to the well-known translational-velocity separators [67],…”