  The acetylene-vinylidene system serves as a benchmark for investigations of
ultrafast dynamical processes where the coupling of the electronic and nuclear
degrees of freedom provides a fertile playground to explore the femto- and
sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources
both on the table-top as well as free-electron lasers. We focus on detailed
investigations of this molecular system in the photon energy range $19...40$ eV
where EUV pulses can probe the dynamics effectively. We employ
photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto
unrevealed aspects of this system. In this work, the role of excited states of
the $C_{2}H_{2}^{+}$ cation, the primary photoion, is specifically addressed.
From photoelectron energy spectra and angular distributions, the nature of the
dissociation and isomerization channels is discerned. Exploiting the
$4\pi$-collection geometry of velocity map imaging spectrometer, we not only
probe pathways where the efficiency of photoionization is inherently high but
also perform PEPICO spectroscopy on relatively weak channels.
