The formation of NH+ following the reaction of N22+ with H2

Abstract

The nitrogen molecular dication (N₂²⁺) has been proposed as a minor but significant component of the ionosphere of Saturn's moon Titan with an abundance comparable to that of several key monocations. It has also been suggested that the reactions of N₂²⁺ with H₂ can provide a source of N₂H²⁺ in Titan's atmosphere. This paper reports the results from experiments, using a position-sensitive coincidence technique, which reveal the chemical reactions forming pairs of monocations following collisions of the N₂²⁺ dication with H₂(D₂) at a centre-of-mass collision energy of 0.9(1.8) eV. These experiments show, in addition to single electron-transfer processes, a bond-forming pathway forming NH⁺ + H⁺ + N and allow an estimate to be made of the reaction cross section and the rate coefficient for this reaction. The correlations between the product velocities revealed by the coincidence experiments show that NH⁺ is formed via N atom loss from a primary encounter complex [N₂H₂]²⁺ to form NH₂²⁺, with this triatomic daughter dication then fragmenting to yield NH⁺ + H⁺. A computational investigation of stationary points on the lowest energy singlet and triplet [N₂H₂]²⁺ potential energy surfaces confirms the mechanistic deductions from the experiments and indicates that the formation of NH⁺ occurs solely, and efficiently, from the reaction of the c³Σ⁺_u excited electronic state of N₂²⁺.