McCall Research Group Illinois

Chemical Physics of H3+

Introduction

H3+ is the simplest polyatomic ion, and is important from both a fundamental and astrochemical perspective. It exists in two nuclear spin configurations, ortho (I=3/2) and para (I=1/2). Owing to symmetry, these two spin configurations exist as separate species; each one has its own manifold of rotational states. In our laboratory, we are interested in studying how the dissociative recombination rate of H3+ depends on the nuclear spin configuration. We are also interested in the processes that interconvert ortho and para H3+. Each of these processes has implications for the chemistry occuring in diffuse cloud envoronments in the interstellar medium.

H3+ and nuclear spin
(left) Nuclear spin configurations of ortho and para H3+. (right) Energy level diagram showing H3+ rotational states belonging to the ortho (red) and para (blue) nuclear spin configurations.

Dissociative Recombination of H3+

When it encounters an electron, H3+ can be destroyed, forming either H2 and H or 3H in a process called dissociative recombination (DR). The H3+ DR rate has had a stormy and controversial history over the past 30 years, with measurements ranging from 10-7-10-11 cm3 s-1, and theoretical calculations falling throughout that range as well. However, in recent years, measurements of the DR rate in ion storage rings have become the accepted value, and theoretical calculations have come into agreement with these measurements. Recent efforts have focused on measuring this rate at the low temperatures characteristic of the interstellar medium, as well as measuring the difference in the DR rates of ortho and para H3+.

H3+ spectroscopy
R(1,0), R(1,1)u, and R(2,2)l transitions of H3+ obtained in a supersonic expansion using normal hydrogen (black) and 99.9% para hydrogen (gray). Click for full size.

In our laboratory, we use a pulsed supersonic expansion discharge source with hydrogen gas to produce rotationally cold H3+. We characterize the rotational temperature of the ions with cavity ringdown spectroscopy. To measure the difference in recombination rates between ortho and para H3+, we feed hydrogen gas enriched in para H2 into the source to produce para-enriched H3+. Using cavity ringdown spectroscopy, we can determine exactly the degree of enrichment.

To measure the DR rate, we take our ion source to a storage ring facility. We have taken measurements at the CRYRING in Stockholm, Sweden (no longer operating), and more recently at the Test Storage Ring (TSR) at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany. In these experiments, the H3+ ions are extracted from the ion source and accelerated into a beam that travels inside a circular storage ring. Inside the ring, the ion beam is overlapped with an electron beam that is precisely matched to the velocity of the ion beam, and the resulting neutral fragments are collected on a detector. From the rate at which neutral particles hit the detector, the DR rate of H3+ can be calculated. Our most recent results show for the first time detailed structural differences between the rate coefficients of ortho and para H3+ at low electron collision energies.

DR spectrum and TSR
(left) The Test Storage Ring. (right) DR spectra showing the structural differences in the ortho and para rate coefficients.

However, there is more to the story. We also performed fragment imaging experiments on the ions inside the storage ring, and these measurements allow us to determine in situ the degree of rotational excitation of the H3+ ions. In principle, this should reflect the rotational temperature of the ions measured in our laboratory with cavity ringdown spectroscopy. The fragment imaging results actually revealed that the temperature of the ions inside the ring was much higher than they were in the supersonic expansion, and analysis of the extraction process suggests that this is due to heating upon acceleration of the ions into the storage ring. Because our DR rate measurements agree so well with previous measurements, we infer that previous measurements may have been affected as well, and that the low-temperature DR rate of H3+ may still be unknown. Also, the difference in recombination rates of the lowest ortho and para H3+ states is not yet known either. Our current efforts are focused on overcoming the acceleration heating and thereby obtaining state-specific DR rate coefficients.

Reaction of H3+ with H2

Identity, hop and exchange reactions
Diagram showing the outcomes of the identity, hop, and exchange pathways in the H3+ + H2 reaction.

As mentioned above, the ortho and para nuclear spin configurations of H3+ act as separate chemical species. A reaction with H2 can transfer population between these two manifolds via proton scrambling in the H5+ collision complex. There are three possible outcomes of this reaction: identity (nuclei remain in their original configuration), hop (a proton from H3+ "hops" onto H2), and exchange (a proton in H3+ changes places with one in H2). The branching fractions for these three processes at conditions relevant for interstellar chemistry are unknown. This process plays an important role in the thermalization of dense and diffuse interstellar clouds, highlighing the need for an understanding of this fundamental reaction.

In the laboratory, we perform high resolution spectroscopy on H3+ formed in a plasma made from pure hydrogen. We produce ions in a hollow cathode discharge cell, and probe them with a difference frequency generation laser using direct absorption multipass spectroscopy. By spectroscopically monitoring the populations of several rotational energy levels, we can measure the H3+ kinetic temperature, rotational temperature, and ortho:para ratio. Measuring these quantities as we vary the para enrichment of the hydrogen precursor gas and the temperature of the discharge cell allows us to extract information about the dynamics of the H3+ + H2 reaction using chemical models.

Hollow cathode discharge
A close-up view of the hydrogen plasma formed in the hollow cathode discharge cell. This photo focuses on the anode.

We have also employed our chemical models towards understanding the observed ortho:para ratio of H3+ in diffuse molecular clouds. In these environments, with densities on the order of 100 cm-3 and temperatures on the order of 70 K, it has been observed that the "excitation temperature" of H3+ (determined from the relative populations of the (J,k) = (1,0) and (1,1) states) is only ~30 K, instead of 70 K. Our recent modeling work suggests that in the reaction of H3+ with H2, the identity branching fraction dominates. As a result, knowledge of the exact difference in the dissociative recombination rates of ortho and para H3+ may be the primary barrier to understanding the chemistry in these regions. Our work in this area is ongoing.

Related Content

Papers

85 A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril and B. J. McCall
"High-precision R-branch transitions in the ν2 fundamental band of H3+"
Journal of Molecular Spectroscopy (2015), 317, 71-73.
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72 K. N. Crabtree, J. N. Hodges, B. M. Siller, A. J. Perry, J. E. Kelly, P. A. Jenkins II, and B. J. McCall
"Sub-Doppler Mid-Infrared Spectroscopy of Molecular Ions"
Chemical Physics Letters (2012), 551, 1-6.
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70 F. Grussie, M. H. Berg, K. N. Crabtree, S. Gärtner, B. J. McCall, S. Schlemmer, A. Wolf, and H. Kreckel
"The Low-Temperature Nuclear Spin Equilibrium of H3+ in Collisions With H2"
Astrophysical Journal (2012), 759, 21.
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68 K. N. Crabtree and B. J. McCall
"The ortho:para ratio of H3+ in laboratory and astrophysical plasmas"
Philosophical Transactions of the Royal Society A (2012), 370, 5055-5065.
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61 K. N. Crabtree, B. A. Tom and B. J. McCall
"Nuclear Spin Dependence of the Reaction of H3+ with H2. I. Kinetics and Modeling"
Journal of Chemical Physics (2011), 134, 194310.
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60 K. N. Crabtree, C. A. Kauffman, B. A. Tom, E. Beçka, B. A. McGuire and B. J. McCall
"Nuclear Spin Dependence of the Reaction of H3+ with H2. II. Experimental Measurements"
Journal of Chemical Physics (2011), 134, 194311.
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51 H. Kreckel, O. Novotny, K. N. Crabtree, H. Buhr, A. Petrignani, B. A. Tom, R. D. Thomas, M. H. Berg, D. Bing, M. Grieser, C. Krantz, M. Lestinsky, M. B. Mendes, C. Nordhorn, R. Repnow, J. Stuetzel, A. Wolf and B. J. McCall
"High-Resolution Storage-Ring Measurements of the Dissociative Recombination of H3+ Using a Supersonic Expansion Ion Source"
Physical Review A (2010), 82, 042715.
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47 B. A. Tom, A. A. Mills, M. B. Wiczer, K. N. Crabtree and B. J. McCall
"Development and Characterization of a Source of Rotationally Cold, Enriched para-H3+"
Journal of Chemical Physics (2010), 132, 081103.
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39 B. A. Tom, S. Bhasker, Y. Miyamoto, T. Momose, and B. J. McCall
"Producing and Quantifying Enriched Para-H2"
Review of Scientific Instruments (2009), 80, 016108.
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38 B. A. Tom, V. Zhaunerchyk, M. B. Wiczer, A. A. Mills, K. N. Crabtree, M. Kaminska, W. D. Geppert, M. Hamberg, M. af Ugglas, E. Vigren, W. J. van der Zande, M. Larsson, R. D. Thomas, and B. J. McCall
"Dissociative Recombination of Highly Enriched Para-H3+"
Journal of Chemical Physics (2009), 130, 031101.
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36 M. Larsson, B. J. McCall and A. E. Orel
"The Dissociative Recombination of H3+ -- A Saga Coming to an End?"
Chemical Physics Letters (2008), 462, 145-151.
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31 B. J. McCall
"Dissociative Recombination of Cold H3+ and its Interstellar Implications"
Philosophical Transactions of the Royal Society of London Series A (2006), 364, 2953-2963.
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Proceedings

14 N. Indriolo, B. D. Fields and B. J. McCall
"Probing cosmic-ray acceleration and propagation with H3+ observations"
Proceedings of the International Conference of Computational Methods in Sciences and Engineering, AIP Conference Proceedings, Eds. T. E. Simos and G. Maroulis, AIP, (2015), 1642, 370.
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15 M. Goto, T. R. Geballe, T. Usuda, N. Indriolo, B. J. McCall and T. Oka
"Gas in the CMZ toward the Galactic nucleus studied by H3+ and CO spectra"
Proceedings of the International Conference of Computational Methods in Sciences and Engineering, AIP Conference Proceedings, Eds. T. E. Simos and G. Maroulis, AIP, (2015), 1642, 377.
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16 K. N. Crabtree, N. Indriolo, H. Kreckel and B. J. McCall
"The ortho:para ratio of H3+ in laboratory and astrophysical plasmas"
Proceedings of the International Conference of Computational Methods in Sciences and Engineering, AIP Conference Proceedings, Eds. T. E. Simos and G. Maroulis, AIP, (2015), 1642, 380.
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Talks

139 C. R. Markus, J. E. McCollum, T. S. Dieter, P. A. Kocheril, and B. J. McCall
"Sub-doppler rovibrational spectroscopy of the H3+ cation and isotopologues"
Seventy Second International Symposium on Molecular Spectroscopy, University of Illinois at Urbana-Champaign, 2017.
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142 C. R. Markus, J. E. McCollum, T. S. Dieter, P. A. Kocheril, and B. J. McCall
"Cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers"
12th International User Meeting on Cavity Enhanced Spectroscopy, Radboud University, Nijmegen, Netherlands, 2017.
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134 J. N. Hodges and B. J. McCall
"Quantitative determination of lineshape parameters from velocity modulation spectroscopy"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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135 J. N. Hodges and B. J. McCall
"Using NICE-OHVMS lineshapes to study relaxation rates and transition dipole moments"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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136 A. J. Perry, G. S. Kocheril, J. N. Hodges, C. R. Markus and B. J. McCall
"Highly accurate and precise infrared transition frequencies of the H3+ cation"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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137 C. R. Markus, J. N. Hodges, A. J. Perry, B. J. McCall
"Improved spectroscopy of molecular ions in the mid-infrared with up-conversion detection"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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128 A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril, P. A. Jenkins II and B. J. McCall
"Progress Towards a High-Precision Infrared Spectroscopic Survey of the H3+ Ion"
Seventieth International Symposium on Molecular Spectroscopy, University of Illinois, Urbana-Champaign, IL, 2015.
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129 J. N. Hodges, B. M. Siller and B. J. McCall
"Elucidating the Complex Lineshapes Resulting from the Highly Sensitive, Ion Selective Technique NICE-OHVMS"
Seventieth International Symposium on Molecular Spectroscopy, University of Illinois, Urbana-Champaign, IL, 2015.
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115 J. N. Hodges, A. J. Perry, B. M. Siller, and B. J. McCall
"Sub-Doppler Spectroscopy of H3+"
Sixty-Eighth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2013.
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98 K. N. Crabtree, N. Indriolo, H. Kreckel, B. A. Tom and B. J. McCall
"Nuclear Spin of H3+ in Diffuse Molecular Clouds"
Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
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91 K. N. Crabtree, B. A. Tom and B. J. McCall
"Modeling the Influence of Nuclear Spin in the Reaction of H3+ with H2"
Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
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92 K. N. Crabtree, C. A. Kauffman, B. A. Tom, E. Beçka, B. A. McGuire and B. J. McCall
"Spectroscopic Measurements of the Reaction H3+ + H2 → H2 + H3+"
Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
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88 H. Kreckel, O. Novotny, K. N. Crabtree, H. Buhr, A. Petrignani, B. A. Tom, R. D. Thomas, M. H. Berg, D. Bing, M. Grieser, C. Krantz, M. Lestinsky, M. B. Mendes, C. Nordhorn, R. Repnow, J. Stützel, A. Wolf and B. J. McCall
"Storage Ring Measurements of the Dissociative Recombination of H3+: a Closer Look"
Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2010.
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87 K. N. Crabtree, N. Indriolo, H. Kreckel and B. J. McCall
"The ortho:para Ratio of H3+ in Laboratory and Astrophysical Plasmas"
Spectroscopy of Molecular Ions in the Laboratory and in Space, Kos, Greece, 2010.
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85 H. Kreckel, O. Novotny, K. N. Crabtree, H. Buhr, A. Petrignani, B. A. Tom, R. D. Thomas, M. H. Berg, D. Bing, M. Grieser, C. Krantz, M. Lestinsky, M. B. Mendes, C. Nordhorn, R. Repnow, J. Stuetzel, A. Wolf and B. J. McCall
"Storage Ring Measurements of the Dissociative Recombination of H3+: A Closer Look"
Eighth International Conference on Dissociative Recombination: Theory, Experiments and Applications, Granlibakken, Lake Tahoe, CA, 2010.
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75 N. Indriolo, T. Oka, T. R. Geballe, K. H. Hinkle, G. A. Blake and B. J. McCall
"Comparing the Ortho-to-Para Ratios of H2 and H3+ in Diffuse Interstellar Clouds"
Sixty-Fifth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010.
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76 K. N. Crabtree, B. A. Tom, C. A. Kauffman and B. J. McCall
"Nuclear Spin Dependence of the Reaction of H3+ with H2"
Sixty-Fifth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010.
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74 K. N. Crabtree, B. A. Tom, C. A. Kauffman, B. A. McGuire and B. J. McCall
"Experimental Measurements of the H3+ + H2 → (H5+)* → H2 + H3+ Reaction"
239th American Chemical Society National Meeting, San Francisco, CA, 2010.
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72 K. N. Crabtree, B. A. Tom, C. A. Kauffman, B. A. McGuire and B. J. McCall
"Implications of the H3+ + H2 → H2 + H3+ Reaction for the ortho- to para-H3+ Ratio in Interstellar Clouds"
239th American Chemical Society National Meeting, San Francisco, CA, 2010.
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68 B. A. McGuire, B. A. Tom, L. E. Moore, T. J. Wood and B. J. McCall
"Spectroscopic Studies of the H3+ + H2 Reaction at Astrophysically Relevant Temperatures"
Sixty-Fourth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2009.
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60 B. J. McCall
"H3+: The Simplest Polyatomic Molecule in the Laboratory and in the Interstellar Medium"
Advancing Chemical Understanding Through Astronomical Observations, Green Bank Telescope, Green Bank, WV, 2009.
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59 B. J. McCall
"Spectroscopic Study of the Astrophysically Important Reaction Between H3+ and H2"
Gordon Conference on Gaseous Ions, Galveston, TX, 2009.
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57 B. A. Tom, M. B. Wiczer, A. A. Mills, K. N. Crabtree and B. J. McCall
"The H3+ + H2 Reaction: A Possible Mechanism for para-H3+ Enrichment in the Diffuse Interstellar Medium"
Inaugural Midwest Astrochemistry Meeting, University of Illinois, 2008.
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56 B. J. McCall
"New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H3+"
Gordon Conference on Atomic and Molecular Interactions, New London, New Hampshire, 2008.
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46 B. A. Tom, M. B. Wiczer, A. A. Mills, K. N. Crabtree, and B. J. McCall
"Observation of Nuclear Spin Selection Rules in Supersonically Expanding Plasmas Containing H3+"
Sixty-Third International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2008.
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45 B. J. McCall
"H3+: A Case Study for the Importance of Molecular Laboratory Astrophysics"
American Astronomical Society National Meeting, St. Louis, Missouri, 2008.
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Posters

26 J. N. Hodges, K. N. Crabtree, B. M. Siller, M. W. Porambo, A. A. Mills, and B. J. McCall
"Indirect THz Spectroscopy of Molecular Ions"
Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2011.
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27 K. N. Crabtree, N. Indriolo, H. Kreckel, B. A. Tom, C. A. Kauffman, E. Becka, B. A. McGuire and B. J. McCall
"Nuclear Spin Dependence of Hydrogenic Plasmas in the Laboratory and the Diffuse Interstellar Medium"
Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2011.
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24 K. N. Crabtree, N. Indriolo, H. Kreckel, B. A. Tom, C. A. Kauffman, E. Beçka, B. A. McGuire and B. J. McCall
"Nuclear Spin Dependence of Hydrogenic Plasmas in the Laboratory and the Diffuse Interstellar Medium"
International Astronomical Union Symposium 280: The Molecular Universe, Universidad Castilla la Mancha, Toledo, Spain, 2011.
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22 C. A. Kauffman, K. N. Crabtree, B. A. Tom, E. Beçka and B. J. McCall
"Experimental Measurements of the H3+ + H2 → H3+ + H2 Reaction in a Hollow Cathode"
Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2010.
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19 H. Kreckel, K. N. Crabtree, C. A. Kauffman, B. A. Tom, N. Indriolo, B. A. McGuire, O. Novotny, M. Berg, D. Bing, H. Buhr, A Petrignani, C. Krantz, M. Lestinky, M. Mendes, C. Nordhorn, J. Stützel, R. Thomas, A. Wolf and B. J. McCall
"Nuclear Spin Effects in the Reactions of H3+ with H2 and Electrons"
Spectroscopy of Molecular Ions in the Laboratory and in Space, Kos, Greece, 2010.
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18 K. N. Crabtree, N. Indriolo, H. Kreckel, C. A. Kauffman, E. Becka, B. A. Tom and B. J. McCall
"Nuclear Spin Effects in Hydrogenic Plasmas in the Laboratory and Interstellar Medium"
Gordon Research Conference on Atomic and Molecular Interactions, Colby-Sawyer College, New London, NH, 2010.
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12 B. A. Tom, B. A. McGuire, L. E. Moore, T. J. Wood and B. J. McCall
"Hydrogenic Plasmas in a Cold Hollow Cathode"
Advancing Chemical Understanding Through Astronomical Observations, Green Bank Telescope, Green Bank, WV, 2009.
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Other Publications

38 C. R. Markus
"Progress in High-Precision Spectroscopy of Molecular Ions in the Mid-Infrared"
Research Prospectus for Preliminary Examination, University of Illinois, 2015.
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45 B. M. Siller and B. J. McCall
"Applications of NICE-OHMS to Molecular Spectroscopy"
Book Chapter, Cavity-Enhanced Spectroscopy and Sensing (Springer), 2014.
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33 B. M. Siller
"New Techniques for Sub-Doppler Spectroscopy of Molecular Ions"
Ph.D Thesis, University of Illinois, 2013.
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29 K. N. Crabtree
"Chemical Physics and Astrochemistry of H3+ and Sub Doppler Mid Infrared Spectroscopy of Molecular Ions"
Ph. D. Thesis, University of Illinois, 2012.
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24 C. A. Kauffman
"The Nuclear Spin Dependence of the Reaction of H3+ with H2 and the Proposed Spectroscopic Investigation of HO2+"
M. S. Thesis, University of Ilinois, 2011.
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19 B. A. Tom
"Nuclear Spin Dependence in Reactions of H3+ in the Laboratory and the Interstellar Medium"
Ph.D. Thesis, University of Illinois, 2009.
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18 B. A. McGuire
"Design and Implementation of a Liquid Nitrogen-Cooled Hollow Cathode Discharge Source for the Study of the Reaction H3+ + H2 → H2 + H3+ at Physically Relevant Temperatures and the Study of the B3Πg → A3Σu+ Band System of N2"
B. S. Thesis, University of Illinois, 2009.
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17 B. A. Tom
"Reactions Involving the Nuclear Spin Modifications of H3+ and H2; Implications for the Interstellar Medium"
Research Prospectus for Preliminary Examination, University of Illinois, 2008.
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