Pulsed rotating supersonic source for merged molecular beams

Abstract

We describe a pulsed rotating supersonic beam source, evolved from an ancestral device [M. Gupta and D. Herschbach, J. Phys. Chem. A 105, 1626 (2001)]. The beam emerges from a nozzle near the tip of a hollow rotor which can be spun at high-speed to shift the molecular velocity distribution downward or upward over a wide range. Here we consider mostly the slowing mode. Introducing a pulsed gas inlet system, cryocooling, and a shutter gate eliminated the main handicap of the original device, in which continuous gas flow imposed high background pressure. The new version provides intense pulses, of duration 0.1-0.6 ms (depending on rotor speed) and containing ~10 12 molecules at lab speeds as low as 35 m/s and ~ 10 15 molecules at 400 m/s. Beams of any molecule available as a gas can be slowed (or speeded); e.g., we have produced slow and fast beams of rare gases, O 2 , Cl 2 , NO 2 , NH 3 , and SF 6 .For collision experiments, the ability to scan the beam speed by merely adjusting the rotor is especially advantageous when using two merged beams. By closely matching the beam speeds, very low relative collision energies can be attained without making either beam very slow.

A chopper system for shortening the duration of pulsed supersonic beams seeded with NO or Br2 down to 13 μs

“…Slow, velocity-controlled molecular beams have become an attractive tool in the study of molecular dynamics, the measurement of fundamental constants and in conducting high resolution spectroscopy [1][2][3][4][5]. Many advances have been made to produce slow beams such as using a high-pressure seeded expansion [6], pre-cooling from a buffer gas source [7,8], or by passing the gas through a cryo-cooled valve nozzle [9,10] or a roomtemperature nozzle spinning in a counter-rotating configuration [11][12][13]. Molecules in seeded supersonic beams can also be decelerated by undergoing inelastic collisions in a crossed beam set up to remove kinetic energy [14], or by using the interaction of the electric or magnetic dipole moments of the molecules with external optical dipole, electric, or magnetic fields [15][16][17][18][19].…”

Merged-beams for slow molecular collision experiments

“…The merged-beam experiments implemented in our laboratory aim both to test the capabilities of ordinary, basic apparatus [10] and to explore in the cold regime H atom reactions that have been well studied in the warm domain. We note some indicative aspects.…”

“…We have come across only three previous, very brief suggestions [12][13][14]. Our treatment accompanies experiments now underway at Texas A&M University [10].…”

“…With the aim of widening the chemical scope, much effort has been devoted to developing means to slow and cool preexisting molecules. (Compilations are given in [7][8][9][10].) For chemical reactions, however, as yet it has not proved feasible to obtain sufficient yields at very low collision energies, using either trapped reactants or crossed molecular beams.…”

Single-Source, Collinear Merged-Beam Experiment for the Study of Reactive Neutral-Neutral Collisions

“…A different merged-beam approach, which relies on the use of a counter-rotating nozzle in combination with a second beam source, has been proposed by Herschbach and co-workers. 22,23 Besides that, it has also been possible to study low-energy ion-molecule collisions in a merged-beam setup using the Rydberg-deceleration technique. 24 Merged-beam experiments were also performed in the group of H. L. Bethlem using a storage ring that was tangentially crossed by a second supersonic beam.…”