Ion-pair dissociations of BrCN by electron impacts^†

Ion-pair dissociation is an important molecular process and frequently happens when the target molecule is pumped to its electronically superexcited states. In contrast to the experimental studies of photoexcitation ion-pair dissociation, there are some experimental challenges in the electron-impact ion-pair dissociation study, in particular, on determination of the energetic threshold. Here we report an experimental development for the ion-pair dissociation study by using the monochromized electron impacts. As an example, the threshold of BrCN→Br⁻+CN⁺ is determined as 13.78 eV according to the appearance energy of CN⁺ signals, meanwhile, the time-sliced ion velocity image of CN⁺ is recorded at 16.09 eV and indicates an anisotropic distribution of the CN⁺ momentum.