  High-intensity extreme ultraviolet (XUV) pulses from a free-electron laser
can be used to create a nanoplasma in clusters. In Ref. [Michiels et al. PCCP,
2020; 22: 7828-7834] we investigated the formation of excited states in an
XUV-induced nanoplasma in ammonia clusters. In the present article we expand
our previous study with a detailed analysis of the nanoplasma evolution and ion
kinetics. We use a time-delayed UV laser as probe to ionize excited states of H
and H$_2^+$ in the XUV-induced plasma. Employing covariance mapping techniques,
we show that the correlated emission of protons plays an important role in the
plasma dynamics. The time-dependent kinetic energy of the ions created by the
probe laser is measured, revealing the charge neutralization of the cluster
happens on a sub-picosecond timescale. Furthermore, we observe ro-vibrationally
excited molecular hydrogen ions H$_2^{+*}$ being ejected from the clusters. We
rationalize our data through a qualitative model of a finite-size non-thermal
plasma.
