Photoelectron and fragmentation dynamics of the H$^{+}$ + H$^{+}$ dissociative channel in NH$_3$ following direct single-photon double ionization

TL;DR

This study investigates the fragmentation dynamics of NH$_3$ following single-photon double ionization at 61.5 eV, revealing multiple electronic states, dissociation mechanisms, and electron emission characteristics using advanced momentum imaging and theoretical calculations.

Contribution

It provides the first detailed analysis of the electronic states and fragmentation pathways of NH$_3^{2+}$ after single-photon double ionization, combining experimental and theoretical approaches.

Findings

Identified four dication electronic states contributing to dissociation.

Discovered three states dissociate concertedly, one sequentially.

Observed differences in electron emission angles related to energy sharing.

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.