Reaction of NH+, NH2+, and NH3+ ions with H-2 at low temperatures The pathway to ammonia production in the interstellar medium
Abstrakt
Aims. We present an experimental investigation of the exothermic reactions of NH+, NH2+, and NH3+ ions with H-2 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 H-2 has two channels, which lead to NH2+ (similar to 97%) and H-3(+)(similar to 3%) with nearly constant reaction rate coefficients (k(NH+)(a) (17 K) = 1.0 x 10(-9) cm(3) s(-1) and k(NH+)(b) (17 K) = 4.0 x 10(-11) cm(3) s(-1), respectively). The reaction of NH2+ with H-2 produces only NH3+ ions.
The measured rate coefficient monotonically decreases with increasing temperature from k(NH+2) (17 K) = 6 x 10(-10) cm(3) s(-1) to k(NH +2) (300 K) = 2 x 10(-10) cm(3) s(-1). The measured rate coefficient of the reaction of NH3+ with H-2, producing NH4+, increases with decreasing temperature from 80K down to 15 K, confirming that the reaction proceeds by tunnelling through a potential barrier.