Photoelectron and fragmentation dynamics of the H^{+}+H^{+} dissociative channel in NH_{3} following direct single-photon double ionization

We report measurements on the H^{+}+H^{+} fragmentation channel following direct single-photon double ionization of neutral NH_{3} at 61.5 eV, where the two photoelectrons and two protons are measured in coincidence using three-dimensional (3D) momentum imaging. We identify four dication electronic states that contribute to H^{+}+H^{+} dissociation, based on our multireference configuration-interaction calculations of the dication potential energy surfaces. The extracted branching ratios between these four dication electronic states are presented. Of the four dication electronic states, three dissociate in a concerted process, while the fourth undergoes a sequential fragmentation mechanism. We find evidence that the neutral NH fragment or intermediate NH^{+} ion is markedly rovibrationally excited. We also identify differences in the relative emission angle between the two photoelectrons as a function of their energy sharing for the four different dication states, which bare some similarities to previous observations made on atomic targets.