Nuclear Dynamics of H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ Fragmentation in NH$_{\mathrm{3}}$ Following Direct Single-Photon Double Photoionization at 61.54eV.

We report measurements on the H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ fragmentation pathways in neutral NH$_{\mathrm{3}}$ following direct single-photon double photoionization at 61.54eV, where the two photoelectrons and two protons are measured in coincidence using 3-D momentum imaging. We observe four dication electronic states which contribute to H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ fragmentation, three of which exhibit equal energy sharing between the two protons, while the third exhibits strongly unequal energy sharing. We tentatively attribute the unequal proton energy sharing feature to multi-step fragmentation, while the other three equal proton energy sharing features are attributed to single-step fragmentation. Molecular plane proton momentum distributions for these states provide insight into the nuclear dynamics that emerge following direct single-photon double photoionization.