Jessica F. Lockyear

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

The nitrogen molecular dication (N22+) has been proposed as a minor but significant component of the ionosphere of Saturns 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 Titans 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+.

Single-electron transfer between Ar2+(3P, 1D) and He at low collision energies

The single-electron transfer reactions between helium atoms and the ground (3P) and first (1D) excited states of Ar2+(3p−2) have been investigated at a state-resolved level employing a position-sensitive coincidence mass spectrometer. Using this apparatus, the complete angular distributions of the Ar+(2P) and He+(2S) product ions arising from this electron transfer reaction have been determined at centre-of-mass collision energies ranging from 0.4 to 1.2 eV. The Ar+ product ions formed by the reaction of the 3P state of Ar2+ are predominantly forward scattered at collision energies between 1.2 and 0.6 eV, the distribution broadening with decreasing collision energy, and are scattered isotropically at the lowest collision energy investigated (0.4 eV). Product Ar+ ions arising from the reaction of He with the 1D state of Ar2+ have angular distributions which vary strongly with the collision energy over the range studied. No reactivity of the second (1S, 3p−2) excited state of Ar2+ is observed, allowing an upper limit of 0.02 to be placed on the relative reaction cross-section for this state with respect to that of the 1D state of Ar2+.