The Low-temperature nuclear spin equilibrium of H+3 in collisions with H2

F. Grussie, M. H. Berg, K. N. Crabtree, S. Gärtner, B. J. McCall, S. Schlemmer, A. Wolf, H. Kreckel

Research output: Contribution to journalArticlepeer-review


Recent observations of H2 and H+3 in diffuse interstellar sightlines revealed a difference in the nuclear spin excitation temperatures of the two species. This discrepancy comes as a surprise, as H+3 and H2 should undergo frequent thermalizing collisions in molecular clouds. Non-thermal behavior of the fundamental H+3/H2 collision system at low temperatures was considered as a possible cause for the observed irregular populations. Here, we present measurements of the steady-state ortho/para ratio of H+3 in collisions with H2 molecules in a temperature-variable radiofrequency ion trap between 45 and 100K. The experimental results are close to the expected thermal outcome and they agree very well with a previous micro-canonical model. We briefly discuss the implications of the experimental results for the chemistry of the diffuse interstellar medium.

Original languageEnglish (US)
Article number21
JournalAstrophysical Journal
Issue number1
StatePublished - Nov 1 2012
Externally publishedYes


  • ISM: molecules
  • astrochemistry
  • molecular processes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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