Mechanisms and dynamics of the NH$_2^{+}$ + H$^{+}$ and NH$^{+}$ + H$^{+}$ + H fragmentation channels upon single-photon double ionization of NH$_3$

TL;DR

This study investigates the detailed mechanisms and electronic states involved in the fragmentation of NH$_3$ upon single-photon double ionization, revealing state-specific dissociation pathways and the role of autoionization.

Contribution

It provides the first state-selective measurements of NH$_3$ double ionization channels, identifying multiple dication states and their dissociation dynamics with momentum imaging.

Findings

Identification of three dication states contributing to NH$_2^{+}$ + H$^{+}$ dissociation.

Observation of intersystem crossing leading to cold NH$_2^+$ fragments.

Evidence of autoionization in double ionization channels.

Abstract

We present state-selective measurements on the NH$_2^{+}$ + H$^{+}$ and NH$^{+}$ + H$^{+}$ + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH$_{3}$, where the two photoelectrons and two cations are measured in coincidence using 3-D momentum imaging. Three dication electronic states are identified to contribute to the NH$_2^{+}$ + H$^{+}$ dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold NH$_2^+$ fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited NH$_2^+$ fragment with roughly 1 eV of internal energy. The NH$^{+}$ + H$^{+}$ + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the NH$_2^{+}$ + H$^{+}$ channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states.