NO+ formation pathways in dissociation of N_2O ~+ ions at the C~2∑~+ state revealed from threshold photoelectron-photoion coincidence velocity imaging

摘要

Using the novel threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging technique, the dissociative photoionization of N_2O molecule via the C_2∑~+ ionic state has been investigated. Four fragment ions, NO~+, N_2~+, O~+, and N~+, are observed, respectively, and the NO ~+ and N~+ ions are always dominant in the whole excitation energy range of the C_2∑~+ ionic state. Subsequently, the TPEPICO three-dimensional time-sliced velocity images of NO~+ dissociated from the vibrational state-selected N_2O ~+(C_2∑~+) ions have been recorded. Thus the kinetic and internal energy distributions of the NO~+ fragments have been obtained directly as the bimodal distributions, suggesting that the NO ~+ fragments are formed via both NO~+(X~1∑ ~+) N(2P) and NO~+(X~1∑ ~+) N(2D) dissociation channels. Almost the same vibrational population reversions are identified for both dissociation pathways. Interestingly, the obtained branching ratios of the two channels exhibit some dependence on the excited vibrational mode for N_2O~+(C ~2∑~+), in which the excited asymmetrical stretching potentially promotes dissociation possibility along the NO~+(X ~1∑~+) N(2D) pathway. In addition, the measured anisotropic parameters of NO~+ are close to 0.5, indicating that the C_2∑~+ state of N_2O~+ is fully predissociative, indeed, with a tendency of parallel dissociation, and therefore, the corresponding predissociation mechanisms for the N _2O~+(C_2∑~+) ions are depicted.