  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.
