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PRELIMINARY RESEARCH ON A MOLECULAR BEAM FOR THE 1-10 ev RANGE

George T. Skinner, Charles E. Treanor

Published Dec 1, 1959• 0 citations
OpenAlex

Abstract

The design and construction of a molecular-beam apparatus for the production of atomic and molecular beams in the 1 to 10 ev energy range are discussed. A shock tube is used as the source of high-temperature, high-pressure gas, and a hypersonic expansion is used to obtain a high-energy, nearly mono- energetic beam. Use of a two-center potential for the treatment of atom-molecule collisions is discussed. (auth)

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The design and construction of a molecular-beam apparatus for the production of atomic and molecular beams in the 1 to 10 ev energy range are discussed. A shock tube is used as the source of high-temperature, high-pressure gas, and a hypersonic expansion is used to obtain a high-energy, nearly mono- energetic beam. Use of a two-center potential for the treatment of atom-molecule collisions is discussed. (auth)
We realize a continuous, intense, cold molecular and atomic beam source based on buffer-gas cooling. Hot vapor (up to 600 K) from an oven is mixed with cold (15 K) neon buffer gas, and then emitted into a high-flux beam. The novel use of cold neon as a buffer gas produces a forward velocity distribution and low-energy tail that is comparable to much colder helium-based sources. We expect this source to be trivially generalizable to a very wide range of atomic and molecular species with significant vapor pressure below 1000 K. The source has properties that make it a good starting point for laser cooling of molecules or atoms, cold collision studies, trapping, or nonlinear optics in buffer-gas-cooled atomic or molecular gases. A continuous guided beam of cold deuterated ammonia with a flux of 3×10\\(^{11}\\) ND\\(_{3}\\) molecules s\\(^{−1}\\) and a continuous free-space beam of cold potassium with a flux of 1×10\\(^{16}\\) K atoms s\\(^{−1}\\) are realized.
We realize a continuous, intense, cold molecular and atomic beam source based on buffer-gas cooling. Hot vapor (up to 600 K) from an oven is mixed with cold (15 K) neon buffer gas, and then emitted into a high-flux beam. The novel use of cold neon as a buffer gas produces a forward velocity distribution and low-energy tail that is comparable to much colder helium-based sources. We expect this source to be trivially generalizable to a very wide range of atomic and molecular species with significant vapor pressure below 1000 K. The source has properties that make it a good starting point for laser cooling of molecules or atoms, cold collision studies, trapping, or nonlinear optics in buffer-gas-cooled atomic or molecular gases. A continuous guided beam of cold deuterated ammonia with a flux of 3 × 10 11 ND 3 molecules s −1 and a continuous free-space beam of cold potassium with a flux of 1 × 10 16 K atoms s −1 are realized.
A molecular beam source for high-energy metastable atoms, utilizing a corona discharge, is described. It was designed for scattering experiments where accurate alignment of beam components is required. The metastable atoms produced in this source have kinetic energies from 5 to 74 eV dependent on the species and sufficient velocity selection to reveal structure in the differential cross sections. These high-energy metastables were previously reported as photons. When the molecular beam source was operated at 200–300 Torr, a supersonic beam of metastables was produced. An optimization of operating conditions produced a supersonic beam flux two orders of magnitude greater than previously reported.
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)
When a gas or vapour at a higher pressure passes through an impedance to the ow into a region of lower pressure, then in general a beam is formed. Whether the beam is composed mainly of atoms or of molecules, the beam-forming process is the same, and without loss of generality; we will mainly use the term ‘atomic beam’ to mean either an atomic or a molecular beam. We justify this preference since atomic beams are much more common than molecular beams. However, we note that many authors use ‘molecular beam’ to cover atomic beams as well and ‘particle’ is also used. It is also more usual to refer to gas ow through a bounded space such as a tube as ‘molecular ow,’ and ‘molecular beam epitaxy’ (MBE) is a well-established expression.
We have developed an apparatus capable of performing intrabeam and near-copropagating beam scattering experiments at collision energies from room temperature to below 1 K where interesting quantum phenomena can be observed. A detailed description of the major components of the apparatus, single and dual molecular beam valves, high speed chopper, and the discharge source, is presented. With the intrabeam scattering setup, a novel dual-slit chopper permits collision energies down to millikelvins with a collision energy spread of 20%. With the near-copropagating beam configuration, state-to-state differential cross sections for rotationally inelastic collisions of highly vibrationally excited NO molecules with Ar have been measured at broadly tunable energies documenting the versatility of the instrument. Future applications in stereodynamics and cold state-to-state collisions of vibrationally excited polyatomic molecules are briefly discussed.