1. Department of Chemistry, Department of Astronomy, University of California at Berkeley, Berkeley, California 94720, USA
2. Gemini Observatory, 670 North A'ohoku Place, Hilo, Hawaii 96720, USA
3. JILA, University of Colorado and National Institute of Standards and Technology, Boulder, Colorado 80309, USA
4. Institute of Physics, wietokrzyska Academy, 25 406 Kielce, Poland
5. Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University Prague V Holesovickach 2, Prague 8, Czech Republic
6. Department of Physics, SCFAB, Stockholm University, S-106 91 Stockholm, Sweden
7. Manne Siegbahn Laboratory, Stockholm University, S-104 05 Stockholm, Sweden

Correspondence to: B. J. McCall^1,2 Correspondence and requests for materials should be addressed to B.J.M. (e-mail: Email: bjmccall@astro.berkeley.edu).

The H₃ ⁺ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules^{1, 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 temperature^{3, 4}. But observations of diffuse clouds have suggested that H₃ ⁺ is considerably more abundant than expected from the chemical models^{5, 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 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 observations^{5, 6, 7} of H₃ ⁺ can be resolved.