Abstract
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 3-D 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 ro-vibrationally 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.
Highlights
Using the insights gained from the calculations on dication electronic states described in the previous section, we provide a detailed discussion of the experimental results which has been divided into three subsections
We argue that this feature corresponds to the magenta color-coded (1e−2) 1E state. This feature possesses an electron energy sum of 16.7 eV, which exactly coincides with the electron energy sum measured for the feature in the H+ + H+ dissociation channel corresponding with the (1e−2) 1E state. From this evidence we suggest that the single feature observed in the N+ + H+ channel corresponds with the same dication electronic state that contributes to the sequential H+ + H+ dissociation mechanism
With the assistance of theoretical multireference configuration-interaction (MRCI) calculations of dication potential energy surfaces (PESs) cuts, we identified the four participating dication electronic states that lead to H+ + H+ fragmentation, which correspond with the four features we observed, and we have estimated their branching ratios
Summary
Since measuring body-frame photoelectron angular distributions following PDI poses a great experimental challenge, there exists only a small body of literature covering this topic, primarily focused on H2 [6,7,11,12,13] Various experimental methods such as particle coincidence three-dimensional (3D) momentum imaging, including cold target recoil ion momentum spectroscopy (COLTRIMS), allow measurements to be made in the molecular frame, but they are predicated on the axial recoil approximation, it is useful to first determine which dication states exhibit concerted fragmentation mechanisms.
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