Aims. We present an experimental investigation of the exothermic reactions of NH+, NH2+, and NH3+ ions with H2 at temperatures relevant for interstellar clouds. Methods. The reactions were studied using a variable-temperature 22-pole radio frequency ion trap instrument. Results. The temperature dependences of rate coefficients of these reactions have been obtained at temperatures from 15 up to 300 K. The reaction of NH+ with H2 has two channels, which lead to NH2+ ( ∼ 97%) and H+3 ( ∼ 3%) with nearly constant reaction rate coefficients (kaNH+(17 K) = 1.0 × 10−9 cm3 s−1 and kbNH+(17 K) = 4.0 × 10−11 cm3 s−1, respectively). The reaction of NH2+ with H2 produces only NH3+ ions. The measured rate coefficient monotonically decreases with increasing temperature from kNH2+(17 K) = 6 × 10−10 cm3 s−1 to kNH2+(300 K) = 2 × 10−10 cm3 s−1. The measured rate coefficient of the reaction of NH3+ with H2, producing NH+4, increases with decreasing temperature from 80 K down to 15 K, confirming that the reaction proceeds by tunnelling through a potential barrier.

and cations play a significant role in the chemistry of the cold interstellar medium and hence their hydrogen abstraction reactions with have to be included in ion chemical models. The reactions lead directly or indirectly to ions that subsequently recombine with electrons and dissociate into H atoms and . The experiments described in this paper provide rate coefficients ( and ) for the reactions of and with over a wide temperature range (from 15 to 300 K). A cryogenic 22-pole RF ion trap instrument was employed for this purpose. It was found that increases from at 17 K to at 263 K while is nearly constant, varying from at 17 K to at 218 K.

No abstract

Aims. This paper presents experimentally obtained rate coefficients for the weakly endothermic reaction OD − + H 2 → OH − + HD with ortho-and para-hydrogen at astrophysically relevant temperatures between 10 and 300 K. Methods. The reaction was studied with normal and para-enriched (99.5% para-H 2 ) hydrogen in a 22-pole ion trap. The measured temperature dependencies of reaction rate coefficients are analyzed using a model which assumes that the rotational energies of the two reactants are equivalent to the translational energy in driving the reaction. Results. At room temperature, the rate coefficients of reactions with both nuclear spin variants reach 7 × 10 −11 cm 3 s −1 , which is in good agreement with the previous results from ion trap and swarm experiments with normal hydrogen. Cooling down the trap slows down the reaction and leads, at a nominal trap temperature of 11 K, to a rate coefficient below 10 −14 cm 3 s −1 for paraenriched hydrogen. The fitted reaction endothermicity of 25.3 ± 2.2 meV agrees well with the literature value calculated in the Born-Oppenheimer approximation, ∆H 0 = 24.0 meV. A simpler evaluation procedure, fitting the data with Arrhenius functions, results in p k = 16.8 × 10 −11 exp(−234 K/T ) cm 3 s −1 for pure para-hydrogen and o k = 9.4 × 10 −11 exp(−101 K/T ) cm 3 s −1 for pure orthohydrogen.