Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+–e- recombination rate


The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules1,2. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature3,4. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models5,6,7. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations5,6,7 of H3+ can be resolved.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

Figure 1: Spectra of two H3+ transitions arising from the two lowest rotational levels, which are the only levels with significant population in diffuse clouds. R(1,1)u originates from the lowest para level (J = 1, K = 1), while R(1,0) comes from the lowest ortho level (J = 1, K = 0).
Figure 2: Measured dissociative recombination rate coefficient of rotationally cold H3+ as a function of detuning energy.
Figure 3: Calculated thermal rate coefficient ke for the dissociative recombination of rotationally cold H3+ ions as a function of electron temperature, based on CRYRING measurements.


  1. Herbst, E. & Klemperer, W. The formation and depletion of molecules in dense interstellar clouds. Astrophys. J. 185, 505–534 (1973)

    Article  ADS  CAS  Google Scholar 

  2. Watson, W. D. The rate of formation of interstellar molecules by ion-molecule reactions. Astrophys. J. 183, L17–L20 (1973)

    Article  ADS  CAS  Google Scholar 

  3. Geballe, T. R. & Oka, T. Detection of H3+ in interstellar space. Nature 384, 334–335 (1996)

    Article  ADS  CAS  Google Scholar 

  4. McCall, B. J., Geballe, T. R., Hinkle, K. H. & Oka, T. Observations of H3+ in dense molecular clouds. Astrophys. J. 522, 338–348 (1999)

    Article  ADS  CAS  Google Scholar 

  5. McCall, B. J., Geballe, T. R., Hinkle, K. H. & Oka, T. Detection of H3+ in the diffuse interstellar medium toward Cygnus OB2 No. 12. Science 279, 1910–1913 (1998)

    Article  ADS  CAS  Google Scholar 

  6. Geballe, T. R., McCall, B. J., Hinkle, K. H. & Oka, T. Detection of H3+ in the diffuse interstellar medium: the Galactic center and Cygnus OB2 Number 12. Astrophys. J. 510, 251–257 (1999)

    Article  ADS  CAS  Google Scholar 

  7. McCall, B. J. et al. Observations of H3+ in the diffuse interstellar medium. Astrophys. J. 567, 391–406 (2002)

    Article  ADS  CAS  Google Scholar 

  8. Larsson, M. Experimental studies of the dissociative recombination of H3+. Phil. Trans. R. Soc. Lond. A 358, 2433–2444 (2000)

    Article  ADS  CAS  Google Scholar 

  9. Oka, T. Help!!! Theory for H3+ recombination badly needed. in Dissociative Recombination of Molecular Ions with Electrons (ed. Guberman, S. L.) (Kluwer Academic/Plenum, New York, in the press )

  10. Guberman, S. L. Dissociative recombination without a curve crossing. Phys. Rev. A 49, R4277–R4280 (1994)

    Article  ADS  CAS  Google Scholar 

  11. Orel, A. E., Schneider, I. F. & Suzor-Weiner, A. Dissociative recombination of H3+: progress in theory. Phil. Trans. R. Soc. Lond. A 358, 2445–2456 (2000)

    Article  ADS  CAS  Google Scholar 

  12. Schneider, I. F., Orel, A. E. & Suzor-Weiner, A. Channel mixing effects in the dissociative recombination of H3+ with slow electrons. Phys. Rev. Lett. 85, 3785–3788 (2000)

    Article  ADS  CAS  Google Scholar 

  13. Sundström, G. et al. Destruction rate of H3+ by low-energy electrons measured in a storage ring experiment. Science 263, 785–787 (1994)

    Article  ADS  Google Scholar 

  14. Jensen, M. J. et al. Dissociative recombination and excitation of H3+. Phys. Rev. A 63, 052701 (2001)

    Article  ADS  Google Scholar 

  15. Tanabe, T., et al. in Dissociative Recombination: Theory, Experiment and Applications Vol. IV (eds Larsson, M., Mitchell, J. B. A. & Schneider, I. F.) 170–179 (World Scientific, Singapore, 2000)

    Google Scholar 

  16. Kokoouline, V., Greene, C. H. & Esry, B. D. Mechanism for the destruction of H3+ by electron impact. Nature 412, 891–894 (2001)

    Article  ADS  CAS  Google Scholar 

  17. Kokoouline, V. & Greene, C. H. Theory of dissociative recombination of D3h triatomic ions, applied to H3+. Phys. Rev. Lett. (in the press)

  18. Strasser, D. et al. Breakup dynamics and the isotope effect in H3+ and D3+ dissociative recombination. Phys. Rev. A 66, 032719 (2002)

    Article  ADS  Google Scholar 

  19. Kreckel, H. et al. Vibrational and rotational cooling of H3+. Phys. Rev. A 66, 052509 (2002)

    Article  ADS  Google Scholar 

  20. Larsson, M., et al. Studies of dissociative recombination in CRYRING. in Dissociative Recombination of Molecular Ions with Electrons (ed. Guberman, S. L.) (Kluwer Academic/Plenum, New York, in the press)

  21. Plasil, R. et al. Advanced integrated stationary afterglow method for experimental study of recombination processes of H3+ and D3+ ions with electrons. Int. J. Mass Spectrom. 218, 105–130 (2002)

    Article  CAS  Google Scholar 

  22. Savage, B. D., Drake, J. F., Budich, W. & Bohlin, R. C. A survey of interstellar molecular hydrogen. I. Astrophys. J. 216, 291–307 (1977)

    Article  ADS  CAS  Google Scholar 

  23. Cardelli, J. A., Meyer, D. M., Jura, M. & Savage, B. D. The abundance of interstellar carbon. Astrophys. J. 467, 334–340 (1996)

    Article  ADS  CAS  Google Scholar 

  24. Bohlin, R. C., Savage, B. D. & Drake, J. F. A survey of interstellar H I from Lα absorption measurements. II. Astrophys. J. 224, 132–142 (1978)

    Article  ADS  CAS  Google Scholar 

  25. van Dishoeck, E. F. & Black, J. H. Comprehensive models of diffuse interstellar clouds — physical conditions and molecular abundances. Astrophys. J. Suppl. Ser. 62, 109–145 (1986)

    Article  ADS  CAS  Google Scholar 

  26. Lepp, S. in Astrochemistry of Cosmic Phenomena (ed. Singh, P. D.) 471–475 (IAU Symposium 150, Kluwer, Dordrecht, 1992)

    Book  Google Scholar 

  27. Heiles, C. & Troland, T. H. The millennium Arecibo 21-cm absorption line survey. II. Properties of the warm and cold neutral media. Astrophys. J. (in the press); preprint astro-ph/0207105 at 〈〉 (2002)

  28. Lindsay, C. M. & McCall, B. J. Comprehensive evaluation and compilation of H3+ spectroscopy. J. Mol. Spectrosc. 210, 60–83 (2001)

    Article  ADS  CAS  Google Scholar 

  29. Paul, J. B., Collier, C. P., Saykally, R. J., Scherer, J. J. & O'Keefe, A. Direct measurement of water cluster concentrations by infrared cavity ringdown laser absorption spectroscopy. J. Phys. Chem. A 101, 5211–5214 (1997)

    Article  CAS  Google Scholar 

Download references


B.J.M. and A.J.H. thank K. Wilson and C.-Y. Chung for their assistance in the laboratory, D. Lucas for the loan of equipment, and H. Chan and E. Granlund for their support. B.J.M. thanks T. Oka, L. M. Hobbs, D. G. York and T. P. Snow for conversations about the ζ Persei line of sight. We thank C. H. Greene for providing us with the results of his calculations in advance of publication. M.L. thanks D. Zajfman for information on TSR results before publication. The idea of using a supersonic expansion source for dissociative recombination measurements originated during conversations between B.J.M. and C. M. Lindsay, and the experiment was designed by B.J.M., A.J.H. and M.L. The supersonic expansion ion source was built and spectroscopically characterized by A.J.H. and B.J.M. in the laboratory of R.J.S. in Berkeley. The dissociative recombination measurements in Stockholm were carried out by all of the authors except R.J.S. and T.R.G. The UKIRT observations were obtained by B.J.M. and T.R.G. The authors thank the staff of the Manne Siegbahn Laboratory for help with the experiment. B.J.M. is supported by the Miller Institute for Basic Research in Science. A.J.H. and R.J.S. acknowledge support from the AFOSR and NSF. T.R.G. is supported by Gemini Observatory, operated by AURA for the international Gemini partnership. N.D. and G.H. were supported in part by the US DOE, Office of Fusion Energy. J.S. is supported in part by the State Committee for Scientific Research. Support is acknowledged from the EU fifth framework program, the EOARD, the Swedish Research Council and STINT.

Author information

Authors and Affiliations


Corresponding author

Correspondence to B. J. McCall.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

McCall, B., Huneycutt, A., Saykally, R. et al. An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+–e- recombination rate. Nature 422, 500–502 (2003).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing