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EPJ Techniques and Instrumentation

Merged neutral beams

Andreas Osterwalder

Published Jul 24, 2015• 48 citations
Publisher

Abstract

A detailed description of a merged beam apparatus for the study of low energy molecular scattering is given. This review is intended to guide any scientist who plans to construct a similar experiment, and to provide some inspiration in describing the approach we chose to our goal. In our experiment a supersonic expansion of paramagnetic particles is merged with one of polar molecules. A magnetic and an electric multipole guide are used to bend the two beams onto the same axis. We here describe in detail how the apparatus is designed, characterised, and operated.

DOI
10.1140/epjti/s40485-015-0022-x

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A detailed description of a merged beam apparatus for the study of low energy molecular scattering is given. This review is intended to guide any scientist who plans to construct a similar experiment, and to provide some inspiration in describing the approach we chose to our goal. In our experiment a supersonic expansion of paramagnetic particles is merged with one of polar molecules. A magnetic and an electric multipole guide are used to bend the two beams onto the same axis. We here describe in detail how the apparatus is designed, characterised, and operated.
Within this thesis two experiments will be described. In the first experiment a velocity filter was used to obtain a beam of polar molecules (ND3) with a translational temperature of less than 5K. For the first time the dynamics of individual rotational states inside a velocity filter could be investigated directly. Usually a velocity filter defines only a maximumvelocity that can pass the filter. We successfully implemented two different approaches where molecules below a certain minimal velocity are removed from the beam, turning the velocity filter into a bandpass filter. The second and more interesting experiment is the merged beamexperiment. We developed and constructed the first apparatus which successfullymerged two guided beams, one consisting of polar molecules, the other of paramagnetic particles, into an overlapping parallel movement. With this approach we could lower the temperature at which the reaction of metastable Ne* with ND3, NH3 and CH3F has been investigated by 3 orders of magnitude down to the sub-kelvin regime. Especially the stereochemistry of those reactions revealed an unexpected energy-insensitivity at temperatures below 200K. As our setup is very general the investigation of manymore neutral-neutral reactions will be possible, opening a new route into an temperature regime where only a few reactions have been studied before.
We have developed an experiment for the investigation of neutral molecular collisions in the gas phase at temperatures as low as 100 mK. These low temperatures are obtained by merging two supersonic expansions, using an electric and a magnetic guide, and by matching the velocities of the beams. Since the energy available for the collisions, or the temperature, is determined only by the relative velocity of the reaction partners this enables the study of chemical processes at very low temperatures without the need to prepare slow molecules in the laboratory frame of reference. This paper describes the method and presents results on the Ne((3)P2)+NH3 Penning ionization.
Molecular collisions can be studied at very low relative kinetic energies, in the milliKelvin range, by merging codirectional beams with much higher translational energies, extending even to the kiloKelvin range, provided that the beam speeds can be closely matched. This technique provides far more intensity and wider chemical scope than methods that require slowing both collision partners. Previously, at far higher energies, merged beams have been widely used with ions and/or neutrals formed by charge transfer. Here, we assess for neutral, thermal molecular beams the range and resolution of collision energy that now appears attainable, determined chiefly by velocity spreads within the merged beams. Our treatment deals both with velocity distributions familiar for molecular beams formed by effusion or supersonic expansion, and an unorthodox variant produced by a rotating supersonic source capable of scanning the lab beam velocity over a wide range.
We present two methods for studying reactive collisions between two atomic or molecular species: a collinear merged-beam method in which two gas pulses from a single supersonic beam source are coalesced and an intrabeam-scattering technique in which a single gas pulse is used. Both approaches, which rely on the laser cooling and deceleration of a laser-coolable species inside a Zeeman slower, can be used for a wide range of scattering studies. Possible experimental implementations of the proposed methods are outlined for autoionizing collisions between helium atoms in the metastable 2 3 S 1 state and a second, atomic or molecular species. Using numerical trajectory calculations, we provide estimates of the expected on-axis detection efficiency, collision-energy range, and collision-energy resolution of the approaches. We have experimentally tested the feasibility of such an experiment by producing two gas pulses at very short time intervals, and the results of these measurements are also detailed.
A study was conducted to demonstrate the manipulation of molecular beams with electric and magnetic fields. Seeded pulsed supersonic expansions were employed to conduct the investigations. The conservative forces exerted further downstream by the electric and magnetic fields enabled the researchers to manipulate and control the shape and the position of the distribution in the six-dimensional phase-space. All of the original experimental geometries were devised to create strong magnetic or electric field gradients to efficiently deflect particles from the beam axis. It was demonstrated that a detailed understanding of the influence of the external field on the energy level structure of the molecules was required for the manipulation of molecules with electric or magnetic fields.
In the last two decades, enormous progress has been made in the manipulation of molecular beams. In particular, molecular decelerators have been developed with which advanced control over neutral molecules in a beam can be achieved. By using arrays of inhomogeneous and time-varying electric (or magnetic) fields, bunches of molecules can be produced with a tunable velocity, narrow velocity spreads, and almost perfect quantum-state purity. These monochromatic or "tamed" molecular beams are ideally suited to be used in crossed-molecular-beam scattering experiments. Here, we review the first generation of these "cold and controlled" scattering experiments that have been conducted in the last decade and discuss the prospects for this emerging field of research in the years to come.