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Journal of Physics E Scientific Instruments

A high-intensity beam of metastable helium atoms with good velocity resolution

Bernhard Brutschy, Hellmut Haberland

Published Jan 1, 1977• 61 citations
Publisher

Abstract

The excitation of a low-energy ground state helium beam to its metastable states by electron impact is analysed. It is shown that the narrowest velocity distribution for the resulting metastable helium beam is obtained by directing the helium and electron beams parallel or antiparallel. A source of metastable helium atoms was constructed according to this analysis which produced a beam of 3*10 14 metastable atoms/s sr with a velocity resolution of 3% FWHM for a room-temperature beam (E=66 meV). A possible source of errors in time-of-flight experiments with metastable atoms is pointed out.

DOI
10.1088/0022-3735/10/1/021

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The excitation of a low-energy ground state helium beam to its metastable states by electron impact is analysed. It is shown that the narrowest velocity distribution for the resulting metastable helium beam is obtained by directing the helium and electron beams parallel or antiparallel. A source of metastable helium atoms was constructed according to this analysis which produced a beam of 3*10 14 metastable atoms/s sr with a velocity resolution of 3% FWHM for a room-temperature beam (E=66 meV). A possible source of errors in time-of-flight experiments with metastable atoms is pointed out.
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.
We have constructed an atomic beam of metastable helium atoms He(2 3S) with a mean velocity of 300 m/s (15 K) and a yield of 3×1012 atoms/s sr. The metastable atoms are produced in a dc discharge in a cryogenic environment cooled by liquid helium. Using a hexapole magnetic lens, we have increased further the beam intensity by focusing the metastable atoms. Initial studies show a factor of 2.5 increase in the beam flux but more is expected when the hexapole is constructed from permanent hexapole magnets. The He(2 3S) atoms are subsequently loaded into a magneto-optical trap.
We present experimental results that demonstrate—for the first time—the Zeeman deceleration of helium atoms in the metastable 2 3 S 1 state. A more than 40% decrease of the kinetic energy of the beam is achieved for deceleration from 490 m s −1 to a final velocity of 370 m s −1 . Metastable atom generation is achieved with an electron-impact-excitation source whose performance is enhanced through an additional discharge-type process which we characterize in detail. Metastable helium is efficiently generated even in a mixture of other noble gases that have much lower excitation energies. Comparison of deceleration data at different electron beam pulse durations confirms that a matching between the initial particle distribution and the phase-space acceptance of the decelerator is crucial for the production of a decelerated packet with a well-defined velocity distribution. The experimental findings are in good agreement with three-dimensional numerical particle trajectory simulations.
Beams of metastable helium produced by electron bombardment generally contain atoms in both the 21S and 23S states. It is important in many experiments to distinguish between these states, or at least to know the population ratio. A beam of 23S and ground state helium atoms has been produced by electron bombardment and subsequent resonant quenching of 21S atoms with 21S−21P 2 μ radiation. This radiation mixes the 21S and 21P states, the latter decaying primarily to the 11S state by emission of 584 Å radiation. The quenching has been studied as a function of beam atom velocity. The results indicate that beams of metastable helium containing less than 1% 21S atoms can be produced.
We describe a technique for producing a high-flux beam of atomic hydrogen with a velocity distribution corresponding to liquid-helium temperatures. We have studied how a gas of hydrogen atoms (H) may be cooled to low temperatures through interaction with cold walls. The gas was analyzed by forming an atomic beam. We obtained fluxes φH≃2.4×1016 atoms/s at T≃8 K, which corresponds to an increase in flux of low-velocity atoms by a factor of 20 over that of the same source operated at room temperature. The degree of dissociation and the translational temperature of the gas were determined using a quadrupole mass spectrometer and time-of-flight techniques. A beam modulation technique advantageous for such a system is discussed and analyzed. General design considerations for the transport and cooling of H are presented and illustrated with examples. The methods of data analyses are discussed in detail.