Reaction of NH+, NH+2 , and NH+3 ions with H2 at low temperatures the pathway to ammonia production in the interstellar medium

Abstract

Aims. We present an experimental investigation of the exothermic reactions of NH+, NH+2 , and NH+ 3 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 NH+2 (∼97%) and H+ 3 (∼3%) with nearly constant reaction rate coefficients (ka NH+(17 K) = 1:0 × 10-9 cm3 s-1 and kb NH+ (17 K) = 4:0 × 10-11 cm3 s-1, respectively). The reaction of NH+2 with H2 produces only NH+ 3 ions. The measured rate coefficient monotonically decreases with increasing temperature from kNH+2 (17 K) = 6 × 10-10 cm3 s-1 to kNH+2 (300 K) = 2 × 10-10 cm3 s-1. The measured rate coefficient of the reaction of NH+ 3 with H2, producing NH+ 4 , increases with decreasing temperature from 80K down to 15 K, confirming that the reaction proceeds by tunnelling through a potential barrier.