Direct Single-Photon Double Photoionization of NH$_{\mathrm{3}}$ at 61.54eV Investigated Using Coincidence 3-D Momentum Imaging.

We present state-selective measurements on various H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ dissociation channels in neutral NH$_{\mathrm{3}}$ following direct single-photon double photoionization (DPI) at 61.54eV. Here, the two photoelectrons and two protons are measured in coincidence using 3-D momentum imaging, providing insight into the details of the electron and nuclear dynamics that ensue following direct single-photon DPI. Results indicate that four dication electronic states contribute to H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ dissociation. Three of these states result in equal energy sharing between the two protons, while the fourth results in unequal energy sharing between the two protons. Molecular plane proton momentum distributions suggest the three former states dissociate in a single step, while the later state fragments in multiple steps. Complementary photoelectron momentum distributions and singly differential cross sections for these states provide information on the mechanisms and energetics involved in the direct single-photon DPI process.