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Journal of Physics B Atomic Molecular and Optical Physics

Collisional production of fast metastable hydrogen atoms from cold H2: toward twin atoms

Aline Medina, G. Rahmat, C. R. de Carvalho, Ginette Jalbert, Fábio Zappa, R F Nascimento, R. Cireasa, Nicolas Vanhaecke, I. F. Schneider, N. V. de Castro Faria, Jacques Robert

Published Oct 20, 2011• 13 citations
Publisher

Abstract

We have investigated the production of fast metastable H(2 2 S) coming from the dissociation of cold H 2 molecules produced in a Campargue nozzle beam after crossing with an electron beam emerging from a high intensity pulsed electron gun. We have achieved dissociation by electron impact in order to avoid limitations due to the selection rules governing radiative transitions. Two detectors, placed 230 and 254 mm away from the collision centre, have analysed the neutral fragments as a function of their time-of-flight through localized quenching of H(2 2 S) in H(2 2 P) and Lyman-α detection. Performing coincidence experiments in order to ascertain the existence of the H(2 2 S)–H(2 2 S) dissociation channel is appealing due to the anlysis of the spectra.

DOI
10.1088/0953-4075/44/21/215203

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We have investigated the production of fast metastable H(2 2 S) coming from the dissociation of cold H 2 molecules produced in a Campargue nozzle beam after crossing with an electron beam emerging from a high intensity pulsed electron gun. We have achieved dissociation by electron impact in order to avoid limitations due to the selection rules governing radiative transitions. Two detectors, placed 230 and 254 mm away from the collision centre, have analysed the neutral fragments as a function of their time-of-flight through localized quenching of H(2 2 S) in H(2 2 P) and Lyman-α detection. Performing coincidence experiments in order to ascertain the existence of the H(2 2 S)–H(2 2 S) dissociation channel is appealing due to the anlysis of the spectra.
It is a difficult task to obtain “twin atoms”, i.e. pairs of massive particles such that one can perform experiments in the same fashion that is routinely done with “twin photons”. One possible route to obtain such pairs is by dissociating homonuclear diatomic molecules. We address this possibility by investigating the production of metastable H(2s) atoms coming from the dissociation of cold H2 molecules produced in a Campargue nozzle beam crossing an electron beam from a high intensity pulsed electron gun. Dissociation by electron impact was chosen to avoid limitations of target molecular excited states due to selection rules. Detectors placed several centimeters away from the collision center, and aligned with respect to possible common molecular dissociation channel, analyze the neutral fragments as a function of their time-of-flight (TOF) through Lyman-α detection. Evidence for the first time observed coincidence of pairs of H(2s) atoms obtained this way is presented.
A beam source of atomic hydrogen is described which produces metastable atoms in the 2S1/2 state by optical pumping. A beam flux of 1016 atoms/s is generated in the ground state. The atoms in the beam pass in front of a lamp producing Lyman-β (1026 Å) radiation, where some of them are excited to the 3P level and cascade with a branching ratio of 12% to the 2S1/2 state. The number of metastable atoms produced is measured by quenching them with an electric field and detecting the emitted Lyman-α (1216 Å) radiation. Beams of 106 metastable atoms/s were obtained. Using the Bethe-Lamb theory for the quenching process, a metastable beam effective temperature of 100 K was measured.
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.
Abstract An intense source of cold hydrogen atoms in a supersonic beam is presented. H and D atoms are produced from pure samples of H2 and D2 using a barrier discharge near the orifice of a cryogenic pulsed valve. The atoms are entrained in the supersonic expansion of the precursor molecules and cool down to translational temperatures as low as ∼ 15 mK. The H-atom density at a distance of 1.2 m from the valve orifice is estimated to be around 108 cm−3. The mean velocity of the supersonic beam containing the H (D) atoms can be adjusted between 2600 and 1200 m s−1 (between 1850 and 820 m s−1). The barrier discharge also produces H n + and D n + ions with n ranging from 2 to 8, which can be deflected off the beam by electric fields. The use of this H-atom source in spectroscopic measurements is demonstrated by an investigation of the Stark effect in the 1s–2s two-photon transition of H and D. A near-Fourier-transform-limited pulsed laser generating pulses of UV radiation of adjustable pulse length between 10 and 200 ns near 243 nm is used to record spectra with full resolution of the hyperfine structure in the s states, providing a textbook example of the Stark effect in the simplest set of near-degenerate states of one-electron atoms.