H. Bruhns

Columbia University, New York City, New York, United States

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Publications (54)111.97 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report experimental and theoretical results for associative detachment (AD) of D−+D→D2+e−. We compare these data to our previously published results for H−+H→H2+e−. The measurements show no significant isotope effect in the total cross section. This is to be contrasted with previously published experimental and theoretical work which has found a significant isotope effect in diatomic systems for partial AD cross sections, i.e., as a function of the rotational and vibrational levels of the final molecule formed. Our work implies that though the rovibrational distribution of flux is different for AD of H− + H and D− + D, the total flux for these two systems is essentially the same when summed over all possible final channels.
    Physical Review A 09/2012; 86(3). · 3.04 Impact Factor
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    ABSTRACT: Molecular hydrogen plays a central role in the cooling and formation of protogalaxies and first stars in the early universe. The dominant H2 formation mechanism during this epoch is the associative detachment (AD) reaction H- + H → H2 + e-. Previously published values for this process differ by almost an order of magnitude. These uncertainties hinder our ability to reliably model this epoch of the universe, limiting our ability to understand the formation of protogalaxies, the characteristic masses of the first stars, and the cooling times for formation of the first stars. We have developed a novel merged-beams apparatus to measure, for the first time, the energy resolved cross section for this reaction. Beginning with an H- beam, we use an infrared laser to convert ∼ 10% of the beam into ground state H via photodetachment. This generates a self-merged, anion-neutral beams arrangement. Laboratory energies are in the keV range; but because the beams co-propagate, center-of-mass energies from the meV to keV range are achievable. We have measured the cross section for energies from 3.7 meV to 4.8 eV. Our results confirm recent non-local calculations but are not in agreement with other previously published theoretical results or with published flowing afterglow measurements. This work was supported in part by the NSF Divisions of Chemistry and Astronomical Sciences.
    05/2012;
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    ABSTRACT: Using a merged-beams method, we have performed absolute, energy-resolved measurements for the associative detachment reaction H− + H → H2 + e−. Our results remove a long-standing discrepancy between theory and experiment for this fundamental reaction. In particular, we find excellent agreement with theoretical results which previously seemed to be ruled out by a recent flowing afterglow experiment.
    Journal of Physics Conference Series 01/2012; 388(8).
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    ABSTRACT: A novel technique for absolute wavelength determination in high-precision crystal x-ray spectroscopy recently introduced has been upgraded reaching unprecedented accuracies. The method combines visible laser beams with the Bond method, where Bragg angles (θ and -θ) are determined without any x-ray reference lines. Using flat crystals this technique makes absolute x-ray wavelength measurements feasible even at low x-ray fluxes. The upgraded spectrometer has been used in combination with first experiments on the 1s2p(1)P(1) → 1s(2)(1)S(0) w-line in He-like argon. By resolving a minute curvature of the x-ray lines the accuracy reaches there the best ever reported value of 1.5 ppm. The result is sensitive to predicted second-order QED contributions at the level of two-electron screening and two-photon radiative diagrams and will allow for the first time to benchmark predicted binding energies for He-like ions at this level of precision.
    The Review of scientific instruments 01/2012; 83(1):013102. · 1.52 Impact Factor
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    ABSTRACT: Using a merged-beams apparatus, we have measured the associative detachment (AD) reaction of H−+H→H2+e− for relative collision energies up to Er≤4.83 eV. These data extend above the 1-eV limit of our earlier results. We have also updated our previous theoretical work to account for AD via the repulsive 2Σg+ H2− potential energy surface and for the effects at Er≥0.76 eV on the experimental results due to the formation of long-lived H2 resonances lying above the H+H separated atoms limit. Merging both experimental data sets, our results are in good agreement with our new theoretical calculations and confirm the prediction that this reaction essentially turns off for Er≳2 eV. Similar behavior has been predicted for the formation of protonium from collisions of antiprotons and hydrogen atoms.
    Physical Review A 11/2011; 84(5). · 3.04 Impact Factor
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    ABSTRACT: Using a merged beams apparatus we have measured the associative detachment reaction of H^- + H -> H2 + e^- for relative collision energies Er <= 4.8 eV. These data extend above the 1 eV limit of our previous results. We have also investigated and ruled out several possible sources of systematic error in our previous work. Merging both data sets these results are in excellent agreement with recent theoretical calculations and confirm the prediction that this reaction essentially turns off for Er >= 2.25 eV. Similar behavior has been predicted for protonium formation from collisions of antiprotons with hydrogen atoms.
    06/2011;
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    ABSTRACT: The research program at the Heidelberg Electron Beam Ion Trap (EBIT) has concentrated mainly on precision measurements relevant to quantum electrodynamics (QED) and nuclear physics. Spectroscopic measurements in the optical region have delivered the most accurate wavelengths ever reported for highly charged ions, extracting even isotopic shifts. The forbidden transitions of B-like Ar XIV and Be-like Ar XV ions were studied. They are especially interesting, since the QED contributions are as large as 0.2%. Improved atomic structure calculations allowed for the determination of their values with growing accuracy. The lifetimes of the corresponding metastable levels have also been measured with an uncertainty of less than 0.5% thus becoming sensitive to the influence of the bound electron anomalous magnetic moment, so far an almost experimentally unexplored QED effect. A new laser spectroscopic setup aims at facilitating future studies of the hyperfine structure of heavy hydrogenic ions. Through the study of the dielectronic recombination, information on rare processes, such as two-electron-one-photon transitions in Ar16+, or the interference effects between dielectronic and radiative recombination in Hg77+, and accurate values for the excitation energies of very heavy HCI have been obtained. A novel X-ray crystal spectrometer allowing absolute X-ray wavelength measurements in the range up to 15 keV with very high precision and reproducibility is currently used to study the Lyman series of H-like ions of medium-Z ions and the 2s–2p transitions of very heavy Li-like ions. PACS Nos.: 31.30.Jv, 32.80.Fb, 32.80.Dz, 32.30.Jv, 32.30.Rj, 95.30.Dr
    Canadian Journal of Physics 02/2011; 83(4):387-393. · 0.90 Impact Factor
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    ABSTRACT: During the epoch of protogalaxy and first star formation, H2 is the main coolant of primordial gas for temperatures below ~8,000 K. The H2 is formed via associative detachment (AD) of H- and H. Uncertainties in the rate coefficient for this reaction have limited our understanding of protogalaxy formation during this epoch and of the characteristic masses and cooling times for the first stars. Recently we have carried out a series of laboratory measurements which remove these uncertainties. Here, we present the cosmological motivation for our work, describe the experimental approach, and point the reader to the relevant works where our AD results are reported and their cosmological implications explored.
    11/2010;
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    ABSTRACT: Molecular hydrogen plays a central role in the cooling of primordial gas. The creation of molecular hydrogen in the early universe during the epoch of first star formation proceeds predominantly through the associative detachment (AD) reaction H- + H --> H2- --> H2 + e. Despite being the most fundamental anion-neutral reaction in chemistry, no agreement has yet been reached between theory and experiment for this process. The uncertainty in the H2 creation rate severely limits our understanding of the formation of the first stars and protogalaxies. We have developed a new merged beams apparatus to measure the H2 AD rate coefficient as a function of the collision energy. We will present the experimental approach and show results of the first energy-resolved measurement of the H2 AD reaction which we use to derive an experimentally confirmed thermal rate coefficient. We will also show results of new cosmological models, demonstrating the implications of our measurements for the evolution of primordial gas in an initially ionized protogalactic halo.
    11/2010;
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    ABSTRACT: During the epoch of first star formation, molecular hydrogen (H2) generated via associative detachment (AD) of H- and H is believed to have been the main coolant of primordial gas for temperatures below 10(4) kelvin. The uncertainty in the cross section for this reaction has limited our understanding of protogalaxy formation during this epoch and of the characteristic masses and cooling times for the first stars. We report precise energy-resolved measurements of the AD reaction, made with the use of a specially constructed merged-beams apparatus. Our results agreed well with the most recent theoretically calculated cross section, which we then used in cosmological simulations to demonstrate how the reduced AD uncertainty improves constraints of the predicted masses for Population III stars.
    Science 07/2010; 329(5987):69-71. · 31.20 Impact Factor
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    ABSTRACT: We have developed an electrostatic, double-focusing 90 degrees deflector for fast ion beams consisting of concentric cylindrical plates of differing heights. In contrast to standard cylindrical deflectors, our design allows for focusing of an incoming parallel beam not only in the plane of deflection but also in the orthogonal direction. The optical properties of our design resemble those of a spherical capacitor deflector while it is much easier and more cost effective to manufacture.
    The Review of scientific instruments 06/2010; 81(6):063304. · 1.52 Impact Factor
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    ABSTRACT: We have performed the first energy-resolved measurement of the associative detachment (AD) reaction H^- + H → H_2+e^-. This reaction is the dominant formation pathway for H_2 during the epoch of first star formation in the early universe. Despite being the most fundamental anion-neutral reaction in chemistry, experiment and theory have failed to converge in both magnitude and energy dependence for this process. The uncertainty in the rate coefficient of the AD reaction severely limits our understanding of the formation of the first stars and protogalaxies. To address this issue we have developed a dedicated merged beams apparatus utilizing photodetachment to create a strong ground state H atom beam. Kinematical compression in a collinear beams arrangement allows us to cover the entire relevant collision energy range from 4 meV to 1 eV. We will give an overview of the technique and compare the experimental results to theoretical calculations. We will present a new experimentally confirmed thermal rate coefficient for the AD process and outline its implications for early universe cosmological models.
    06/2010;
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    ABSTRACT: We have developed a novel laboratory instrument for studying gas phase, anion-neutral chemistry. To the best of our knowledge, this is the first such apparatus which uses fast merged beams to investigate anion-neutral chemical reactions. As proof-of-principle we have detected the associative detachment reaction H(-)+H-->H(2)+e(-). Here we describe the apparatus in detail and discuss related technical and experimental issues.
    The Review of scientific instruments 01/2010; 81(1):013112. · 1.52 Impact Factor
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    ABSTRACT: Using a merged-beam configuration, we have performed absolute measurements for the associative detachment reaction H+HH+e. Our energy-resolved measurements for this process remove a long-standing discrepancy between theory and experiment for this fundamental reaction. In particular, we find excellent agreement with theoretical results which previously seemed to be ruled out by earlier experiments performed using a flowing afterglow technique.
    Physical Review A 01/2010; 82(4):042708-042708. · 3.04 Impact Factor
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    ABSTRACT: We have developed a novel apparatus at the Columbia Astrophysics Laboratory to study anion-neutral reactions. Beginning with an anion beam, we use photodetachment to generate a self-merged, anion- neutral beams arrangement. Laboratory beam energies are in the keV range. Because the beams run co-linear, center-of-mass energies from the meV to keV range are achievable. Our proof-of-principle measurement is the associative detachment (AD) reaction H^- + H ->H2+ e^-. Published values for this process differ by almost an order of magnitude. With theory and experiment unable to reach a consistent description for this fundamental molecular formation reaction, it raises questions of how can we expect to do better for anion-neutral reactions involving more complicated systems? Measurements using our novel apparatus will help to resolve this fundamental issue in physics and chemistry. We observe the AD reaction by detecting fast H2^+ ions formed through ionizing collisions of the AD-generated H2 with He inside a gas cell. Here we present the current status of the project and discuss our future plans.
    05/2009;
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    ABSTRACT: Synopsis We have developed a novel apparatus to study associative detachment of H -and H forming H 2 . Beginning with an H -beam, we use laser photodetachment to neutralize a fraction of the beam. This generates self-merged, anion-neutral beams. Laboratory beam energies are in the keV range. Because the beams run co-linear, center-of-mass energies from the meV to keV range are achievable. Our measurements will help to resolve the nearly order of magnitude scatter in previously published results. Resolving this issue will have major implications for understanding the early universe chemistry which led to the formation of the first stars and protogalaxies. We have developed a novel merged beam apparatus to study anion-neutral reactions. The proof-of-principle measurement is the associative detachment (AD) reaction H -+ H → H 2 -→ H 2 + e -. (1) Despite over 40 years of theoretical study, the various published results have yet to converge in either the magnitude or energy dependence for this most basic molecular formation process in negative ion chemistry. The one measurement for this system, a flowing afterglow study, has at least a factor of 2 uncertainty. A more detailed discussion of these previous studies is given in Glover et al. [1] Uncertainties in reaction 1 have major implications for our understanding of the formation of the first stars and protogalaxies. The nearly order of magnitude uncertainty in the rate coefficient affect predictions of whether or not a given protogalactic halo can cool and condense before being gravitationally disrupted by a collision with another halo [1]. Our novel laboratory apparatus begins with an H -beam produced using a duoplasmatron source and selected with a Wien filter. Typical beam energies used are eU source ~ 10 keV. The extracted beam is bent around a 90° turn by a spherical deflector in order to prevent source-generated UV photons and neutrals from entering the interaction region of the apparatus and possibly altering the chemistry studied. After the 90° bend the ions pass through a floating cell in which they are crossed with a 1.4 kW IR laser (975 nm) at an angle of 2.7°. Nearly 10% of the beam is neutralized by photodetach-ment to create self-merged anion-neutral beams. The floating cell itself is biased at a potential, U float . Neutral H atoms leave the floating cell with laboratory energy of e(U float + U source). The remaining anions leave the floating cell and return to their initial energy eU source . By selecting the potential on the floating cell we can set the center of mass energy E cm between the ions and neutrals with an accuracy of a few meV. After leaving the floating cell, the anions and neutrals interact for about a meter after which the anions are removed from the beam by a quadrupole deflector. The remaining H beam and any AD-generated H 2 then pass through a gas cell of He at a pressure of ~0.2 mTorr. Stripping collisions within the gas cell ionize a few percent of the AD-generated H 2 . The cross section for this reaction is well known [2]. The resulting H 2 + ions enter an analyzer where two sequential cylindrical deflectors direct the H 2 + onto a channel electron multiplier. Here we will report on the current status of the apparatus development and present preliminary results for reaction 1. To the best of our knowledge, this work represents the first use of merged, fast anion-neutral beams to study anion-neutral chemistry. With minor upgrades, we can use this apparatus to study AD reactions for a range of homo-nuclear systems. Carrying out such measurements would dramatically enhance our experimental knowledge of the AD processes.
    Journal of Physics Conference Series 05/2009;
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    ABSTRACT: We report about high-precision wavelength determination of H-like and He-like ions at the Heidelberg Electron Beam Ion Trap (HD-EBIT). The experiment was carried out with a novel flat crystal (Si-111) spectrometer applying a reference technique without collimation. The result for the transition energy of the 1s2p 1P1→ 1s2 1S0 resonance line in He-like S14+ agrees with theoretical predictions with an experimental accuracy five times higher than earlier experiments. The same line in Ar16+ was measured with a relative uncertainty of δλ/λ = 2 × 10-6, a factor of 2.5 more accurate than any X-ray wavelength in highly charged ions ever reported, and for He-like ions probes QED two-electron and two-photon radiative corrections. Beside relative wavelength measurements absolute ones were carried out using the Bond method. The results point at the possibility of establishing absolute Lyman-α1 transition X-ray wavelength standards in the future.
    Journal of Physics Conference Series 01/2009;
  • K. Kubiček, H. Bruhns, J. Braun
    Journal of Physics: …. 01/2009;
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    ABSTRACT: The dominant formation mechanism of H2 during the epoch of first star and protogalaxy formation is the associative detachment (AD) reaction H + H- forming H2 + e-. Published values for this process differ by almost an order of magnitude. These uncertainties limit our ability to reliably model this epoch of the universe. For example, recent studies have shown that the effect of these uncertainties is particularly large for protogalaxies forming in previously ionized regions, affecting predictions of whether or not a given protogalaxy can cool and condense within a Hubble time and altering the strength of the ultraviolet background that is required to prevent collapse. We have built a novel apparatus at the Columbia University Astrophysics Laboratory to study this reaction. Beginning with an anion beam, we use photodetachment to generate a self-merged, anion-neutral beams arrangement. Laboratory energies are in the keV range. Because the beams co-propagate, center-of-mass energies from the meV to keV range will be achievable. We observe the AD reaction by detecting the fast H2+ ions formed through ionizing collisions of the AD-generated H2 with the background gas in the vacuum chamber. Here we present our recent results and discuss our future plans. This work has been supported in part by the NSF Chemistry Research Instrumentation and Facilities: Instrument Development program.
    12/2008; 41:414.
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    ABSTRACT: Cross sections of charge transfer for Si3+ ions with atomic hydrogen at collision energies of &ap;40-2500eV/u were carried out using a merged-beam technique at the Multicharged Ion Research Facility at Oak Ridge National Laboratory. The data span an energy range in which both molecular orbital close coupling (MOCC) and classical trajectory Monte Carlo (CTMC) calculations are available. The influence of quantum mechanical effects of the ionic core as predicted by MOCC is clearly seen in our results. However, discrepancies between our experiment and MOCC results toward higher collision energies are observed. At energies above 1000 eV/u good agreement is found with CTMC results.
    Physical Review A 06/2008; 77(6):064702. · 3.04 Impact Factor

Publication Stats

187 Citations
111.97 Total Impact Points

Institutions

  • 2007–2012
    • Columbia University
      • Columbia Astrophysics Laboratory
      New York City, New York, United States
  • 2004–2012
    • Max Planck Institute for Nuclear Physics
      Heidelburg, Baden-Württemberg, Germany
  • 2009
    • Catholic University of Louvain
      Walloon Region, Belgium
  • 2005
    • Vinča Institute of Nuclear Sciences
      Beograd, Central Serbia, Serbia
    • University of California, Berkeley
      • Department of Physics
      Berkeley, MO, United States