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Journal of Physics E Scientific Instruments

A pulsed superthermal beam source for metastable atomic and molecular species

John Philip Simons, Kaoru Suzuki, Clive Washington

Published Jul 1, 1984• 4 citations
Publisher

Abstract

Describes the construction and characterisation of a pulsed, superthermal molecular beam source incorporating a rotor accelerator and a synchronously pulsed electron bombarder. The addition of the pulsed bombarder greatly improves the kinetic energy resolution of the superthermal metastable atomic and molecular beams, particularly for light particles at low velocities and eliminates problems of 'wrap-around' at low velocities. The optimum conditions of resolution at a desired signal level are discussed: a typical velocity distribution for Ar( 3 P 2 ) at a most probable beam velocity of 1500 m s -1 would have a standard deviation sigma approximately=0.07.

DOI
10.1088/0022-3735/17/7/012

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Describes the construction and characterisation of a pulsed, superthermal molecular beam source incorporating a rotor accelerator and a synchronously pulsed electron bombarder. The addition of the pulsed bombarder greatly improves the kinetic energy resolution of the superthermal metastable atomic and molecular beams, particularly for light particles at low velocities and eliminates problems of 'wrap-around' at low velocities. The optimum conditions of resolution at a desired signal level are discussed: a typical velocity distribution for Ar( 3 P 2 ) at a most probable beam velocity of 1500 m s -1 would have a standard deviation sigma approximately=0.07.
A metastable atomic beam source was developed utilizing pulsed helium and argon nozzle beams. A coaxial electron gun, which can be used in a pulsed as well as in a continuous mode, excites the noble gases to their metastable states by electron bombardment in the direction of propagation of the ground-state atoms. In contrast to former work, the electron beam is controlled and accelerated by a combination of three hemispherically shaped tungsten grids. With continuous electron gun operation the source produces a metastable flux of about 1015 atoms/sr s for helium and argon under optimum expansion conditions. Applying short electrical pulses to a control grid yields supersonic metastable atom pulses, with velocity distributions having a relative width as low as 3.8% for helium and 6.5% for argon ( FWHM ) with a 100% modulation of the metastable beams. These features make the source applicable for velocity analyzed scattering experiments using the time-of-flight technique.
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.
We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2 × 10(11) molecules/sr/pulse in the X(2)Σ(+)(v = 0, N(rot) = 0) state, with 140(m/s) forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.
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.
Mounting a molecular beam source near the tip of a hollow high-speed rotor provides a means to shift the velocity distribution of the beam downward or upward over a wide range. We describe the construction of such a device and experiments and model calculations characterizing its operation, for both supersonic and effusive beams of rare gases, O 2, CH 3 F, and SF 6 . For example, the flow velocity of a rotating supersonic beam of O 2 was accelerated to above 1000 m/s (corresponding to a kinetic energy of 2200 K and deBroglie wavelength of 0.1 Å) and decelerated (when seeded in Xe) to below 70 m/s (corresponding to a kinetic energy below 10 K and deBroglie wavelength of nearly 2 Å). With improvements in prospect, the rotating beam source offers a versatile and relatively simple way to enhance techniques for manipulating molecular trajectories.
We describe an improved experimental method for the generation of cold, directed particle bunches, and the highly accurate determination of their velocities in a pulsed supersonic beam, allowing for high-resolution experiments of atoms, molecules, and clusters. It is characterized by a pulsed high pressure jet source with high brilliance and optimum repeatability, a flight distance of few metres that can be varied with a tolerance of setting of 50 μm, and a precision in the mean flight time of particles of better than 10(-4). The technique achieves unmatched accuracies in particle velocities and kinetic energies and also permits the reliable determination of enthalpy changes with very high precision.