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The Journal of Chemical Physics

Preparation of state purified beams of He, Ne, C, N, and O atoms

Justin Jankunas, Kevin S. Reisyan, Andreas Osterwalder

Published Mar 13, 2015• 13 citations
Publisher

Abstract

The production and guiding of ground state and metastable C, N, and O atoms in a two-meter-long, bent magnetic guide are described. Pure beams of metastable He((3)S1) and Ne((3)P2), and of ground state N((4)S3/2) and O((3)P2) are obtained using an Even-Lavie valve paired with a dielectric barrier discharge or electron bombardment source. Under these conditions no electronically excited C, N, or O atoms are observed at the exit of the guide. A general valve with electron impact excitation creates, in addition to ground state atoms, electronically excited C((3)P2; (1)D2) and N((2)D5/2; (2)P3/2) species. The two experimental conditions are complimentary, demonstrating the usefulness of a magnetic guide in crossed or merged beam experiments such as those described in Henson et al. [Science 338, 234 (2012)] and Jankunas et al. [J. Chem. Phys. 140, 244302 (2014)].

DOI
10.1063/1.4914332

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The production and guiding of ground state and metastable C, N, and O atoms in a two-meter-long, bent magnetic guide are described. Pure beams of metastable He((3)S1) and Ne((3)P2), and of ground state N((4)S3/2) and O((3)P2) are obtained using an Even-Lavie valve paired with a dielectric barrier discharge or electron bombardment source. Under these conditions no electronically excited C, N, or O atoms are observed at the exit of the guide. A general valve with electron impact excitation creates, in addition to ground state atoms, electronically excited C((3)P2; (1)D2) and N((2)D5/2; (2)P3/2) species. The two experimental conditions are complimentary, demonstrating the usefulness of a magnetic guide in crossed or merged beam experiments such as those described in Henson et al. [Science 338, 234 (2012)] and Jankunas et al. [J. Chem. Phys. 140, 244302 (2014)].
The key features of the cross sections for electron-impact excitation out of the ground levels of the rare-gas atoms are summarized and application of these cross section data is shown. This is followed by discussions of recent experiments on determination of electron-impact cross sections for excitation out of the metastable levels of He and Ar. The metastiable atom targets are produced by two different methods, i.e., hollow-cathode discharges and charge-exchange collisions of rare-gas ions with Cs atoms. The former method is simple but is limited to low electron energies because of the very small metastable concentration, whereas the latter method yields a fast target beam with predominantly metastable atoms allowing measurements up to the keV range. The measured cross sections show features very different from those of excitation out of the ground levels. Another new development in electron-atom collision is the use of atom traps for measuring ionization cross sections. The experiment is conducted by passing an electron beam pulse through magneto-optically trapped Rb atoms while the trap is momentarily turned off. Turning the trap back on immediately after the electron beam pulse recaptures the unionized atoms allowing only the ions to escape. The ionization cross section is determined bymore » measuring the electron beam current density at the trapped atom region and the fractional loss of trapped atoms due to the electron beam. This method avoids measurements of the absolute number of the target atoms and the overlap between the atomic and electron beams and therefore eliminates the major sources of uncertainty associated with the crossed-beam method for measuring ionization cross section.« less
Generating a controllable and pure source of molecular free-radicals or open-shell atoms has been one of the primary barriers hindering the detailed study of radical processes in the laboratory. Here, we introduce a novel magnetic guide for the generation of a pure beam of velocity-selected radicals-a tuneable source that will enable the study of radical interactions with exceptional control over the properties of the radical species. Only radicals with a selected velocity are transmitted through the guide; all other components of the incoming beam (radical species traveling at other velocities, precursor molecules, and seed gas) are removed. The guide is composed of four Halbach arrays-hexapolar focusing elements-and two skimming blades. The relative positions of these components can be adjusted to tune the properties of the resulting beam and to optimise transmission for a given velocity. Experimental measurements of Zeeman-decelerated H atoms transmitted through the guide, combined with extensive simulations, show that the magnetic guide removes 99% of H-atoms traveling outside the narrow target velocity range.
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.
Nobel gases, with their closed electronic shells, require large energy to transfer the atoms from the electronic ground state to the first accessible excited state. For this reason, it is common to use various discharge sources to transfer atoms from the ground state into excited states, in particular into a long-lived metastable state for optical manipulation. There are specific applications, however, that would benefit from optical methods over discharge methods to produce metastable atoms. These include Atom Trap Trace Analysis and velocimetry/wind tunnel experiments, where a discharge source is impractical.
Discrete atomic energy levels lying between the first and second ionization potentials which are metastable against both auto-ionization and radiative decay have been observed in the alkali elements. These levels arise from the excitation of an electron from the outermost closed shell of the atom and have characteristic lifetimes in the microsecond region. A beam of metastable alkali atoms is produced by electron bombardment of neutral ground-state atoms and is detected by collecting the charged products of the decay. Stern-Gerlach magnetic-deflection experiments have been performed to show unambiguously that the signals arise from paramagnetic atoms and not from possible stray-photon effects in the apparatus. The excitation energies, electron-bombardment production cross sections, natural lifetimes, and the tentative spectroscopic assignments of these atoms are listed below: The energy and lifetime of the metastable lithium atom are in good agreement with the available theoretical estimates. The metastable states in the heavier alkali atoms are classified in relation to the known Ib{\mathrm{I}}^{b}Ib spectral terms. Magnetic and electric fields have been found to be effective in reducing the lifetime of the metastable atoms. The Zeeman quenching is discussed in a separate paper. The Stark quenching rate has been found to be proportional to the square of the electric field and to increase rapidly with atomic number ZZZ. It is suggested that the Stark quenching results from a second-order interaction which involves the product of the electric field and the spin-orbit or other magnetic operators.
A source of electron spin polarized He*(2('3)S) metastable atoms is described that provides a thermal energy beam with a flux up to 8 x 10('12) metastables/sec/sterad at a polarization P(,z) (TURN) 50%. The polarization is created by optical pumping and can be simply reversed or modulated without changing the beam trajectory or flux. The source is housed in a system of three differentially pumped vacuum chambers which allows the beam to be used for surface experiments in an ultra high vacuum environment. The polarization is measured with a Stern-Gerlach type analyzer which also serves as a useful diagnostic of beam composition and verifies the purity of the 2('3)S beam. The beam is demonstrated not to contain any significant admixture of 2('1)S metastables, ions, fast neutrals, or photons. Unless care is exercised, however, it is possible to produce a beam of energetic ground state helium which is of much greater intensity than the metastable beam. The conditions under which this occurs are described. A brief overview of metastable deexcitation spectroscopy (MDS) is given and a proposed first experiment involving the investigation of surface magnetism of Ni(110) and the necessary apparatus for this experiment are described.