Ion pair dissociation from N₂O followed by autodetachment of N^-(¹D_g)

A novel kind of ion pair dissociation, where autodetachment from the negative ion follows charge separation, has been observed in N₂O by velocity imaging photoionization coincidence spectroscopy. At a photon energy of 16.85 eV, electrons with a kinetic energy of ∼1.5 eV are found in coincidence with vibrationally excited NO⁺(X¹∑⁺). The formation mechanism is assigned as N₂O(X¹∑⁺) + hv → NO⁺(X¹∑⁺) + N^-(¹D_g) → NO⁺(X¹∑⁺) + N(⁴S_u) + e^-, on the basis of the energetics of the system and the kinetic energy distribution of the NO⁺(X¹∑⁺). Asymmetry parameters β for the electrons and NO⁺ ions are -0.12 ± 0.02 and -0.03 ± 0.01 respectively. Lack of angular correlation between the electron and NO⁺ directions is interpreted as showing autodetachment at very long range, consistent with the very long calculated lifetime of the excited N^- ion.