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Molecular beam brightening by shock-wave suppression.

Yair Segev, Natan Bibelnik, Nitzan Akerman, Yuval Shagam, Alon Luski, Michael Karpov, Julia Narevicius, Edvardas Narevicius

Published Mar 1, 2017• 18 citations

Abstract

Supersonic beams are a prevalent source of cold molecules used in the study of chemical reactions, atom interferometry, gas-surface interactions, precision spectroscopy, molecular cooling, and more. The triumph of this method emanates from the high densities produced in relation to other methods; however, beam density remains fundamentally limited by interference with shock waves reflected from collimating surfaces. We show experimentally that this shock interaction can be reduced or even eliminated by cryocooling the interacting surface. An increase of nearly an order of magnitude in beam density was measured at the lowest surface temperature, with no further fundamental limitation reached. Visualization of the shock waves by plasma discharge and reproduction with direct simulation Monte Carlo calculations both indicate that the suppression of the shock structure is partially caused by lowering the momentum flux of reflected particles and significantly enhanced by the adsorption of particles to the surface. We observe that the scaling of beam density with source pressure is recovered, paving the way to order-of-magnitude brighter, cold molecular beams.

DOI: 10.1126/sciadv.1602258

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Supersonic beams are a prevalent source of cold molecules used in the study of chemical reactions, atom interferometry, gas-surface interactions, precision spectroscopy, molecular cooling, and more. The triumph of this method emanates from the high densities produced in relation to other methods; however, beam density remains fundamentally limited by interference with shock waves reflected from collimating surfaces. We show experimentally that this shock interaction can be reduced or even eliminated by cryocooling the interacting surface. An increase of nearly an order of magnitude in beam density was measured at the lowest surface temperature, with no further fundamental limitation reached. Visualization of the shock waves by plasma discharge and reproduction with direct simulation Monte Carlo calculations both indicate that the suppression of the shock structure is partially caused by lowering the momentum flux of reflected particles and significantly enhanced by the adsorption of particles to the surface. We observe that the scaling of beam density with source pressure is recovered, paving the way to order-of-magnitude brighter, cold molecular beams.

S>In the production of intensive molecular beams using nozzles, compression shocks must be avoided as much as possible since they cause a broadening of the velocity distribution and a reduction of the particle current density of the molecular beam. The conditions for the occurrence and the effect of such compression shocks was investigated by measurement of the velocity distribution and the beam intensity for the case when a convergent nozzle is used for the beam production. It was shown that under given conditions a strong compression shock connected with the collision system of the freely expanding gas beam occurs. This compression reduces the beam intensity in single-molecule beams by an order of magnitude and in condensed beams by two orders of magnitude. When this strong compression shock released from the first beam stop is avoided by a suitable selection of the research conditions, weak compression shocks emanating from the peeler can still occur. Possible reduction of this weak compression shock is discussed. (tr-auth)

A supersonic beam of noble gas atoms is a source of unprecedented brightness. A novel short pulse supersonic nozzle is developed with beam intensity that is higher by at least an order of magnitude than other available sources. We show how this beam can be coherently slowed and focused using elastic reflection from single crystals. Simulations show beam fluxes of 10 11 atoms s −1 at velocities of 50 m s −1 and temperatures of less than 20 μK in the longitudinal direction. Possible applications of this slow beam to the study of atom–surface interactions and atom interferometry are discussed.

The field of cold molecules has become an important source of new insight in fundamental chemistry and molecular physics. High-resolution spectroscopy benefits from translationally and internally cold molecules by increased interaction times and reduced spectral congestion. Completely new effects in scattering dynamics become accessible with cold and controlled molecules. Many of these experiments use molecular beams as a starting point for the generation of molecular samples. This review gives an overview of methods to produce beams of cold molecules, starting from supersonic expansions or effusive sources, and provides examples of applications in spectroscopy and molecular dynamics studies.

Abstract Well-collimated, high-intensity beams of neutral atoms have many applications ranging from atom microscopy to atom interferometry to ultracold atomic physics. We experimentally demonstrate a method for brightening a supersonic atomic beam and observe an increase in the phase space density by a factor of at least 20. Our scheme relies upon a permanent magnetic hexapole lens to focus a divergent beam of neutral atoms emitted from a supersonic nozzle and transverse laser cooling as the beam converges downstream from the lens so as to create a dense, narrow, Doppler-collimated atomic beam. In principle, this method can be repeated multiple times in series for further beam brightening.

Bright beams of cold molecules provide valuable quantum sensors for measuring permanent electric dipole moments that violate fundamental symmetries, which enables these tabletop experiments to probe energy scales exceeding those available with current or near-future particle colliders. The precision of such experiments generally scales with the number of molecules that are detected per unit time, typically a few meters downstream of the place where the molecular beam is created. So, improving the precision -and thereby probing new physics at higher and higher energy scales- requires increasing the downstream flux of molecules. This can be achieved by collimating the molecular beam to maintain a high number density. These molecular experiments can borrow techniques for control and manipulation from the extremely well-developed field of atomic physics. One such technique is laser cooling, which is a powerful method to achieve this boost in brightness, whose implementation however remains challenging due to the complex energy level structure of molecules. In this work, we demonstrate and analyze laser cooling of a focused cryogenic beam of BaF molecules. We conclude that with some follow-up steps this method will increase the beam brightness by a factor 100, a crucial milestone towards a high precision measurement of the electron electric dipole moment (eEDM).