Dissociative recombination of vibrationally cold CH+3 and interstellar implications

Abstract

CH+3 is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the major destruction mechanisms of CH+3, dissociative recombination (DR), has long been uncertain, hindering the use of CH+3 as an astrochemical probe. Here, we present the first absolute measurement of the DR of vibrationally cold CH+3, which has been made using the heavy storage ring CRYRING in Stockholm, Sweden. From our collision-energy- dependent cross sections, we infer a thermal rate constant of k(T) = 6.97(± 0.03) × 10-7(T/300) -0.61(± 0.01)cm3 s-1 over the region 10 K ≤ T ≤ 1000K. At low collision energies, we have measured the branching fractions of the DR products to be CH3 (0.00+0.01-0.00), CH2 + H (0.35+0.01- 0.01), CH + 2H (0.20+0.02- 0.02), CH + H2 (0.10 +0.01-0.01), and C + H2 + H (0.35 +0.01- 0.02), indicating that two or more C-H bonds are broken in 65% of all collisions. We also present vibrational calculations which indicate that the CH+3 ions in the storage ring were relaxed to the vibrational ground state by spontaneous emission during the storage time. Finally, we discuss the implications of these new measurements for the observation of CH+3 in regions of the diffuse interstellar medium where CH+ is abundant. © 2012. The American Astronomical Society. All rights reserved.