  Photoionized and electronically excited ethylene \ce{C2H4+} can undergo
\ce{H}-loss, \ce{H2}-loss, and ethylene-ethylidene isomerization, where the
latter entails a hydrogen migration. Recent pioneering experiments with
few-femtosecond extreme ultraviolet pulses and complementary theoretical
studies have shed light on the photodynamics of this prototypical organic
cation. However, no theoretical investigation based on dynamics simulations
reported to date has described the mechanisms and time scales of dissociation
and isomerization. Herein, we simulate the coupled electron-nuclear dynamics of
ethylene following vertical ionization and electronic excitation to its four
lowest-lying cationic states. The electronic structure is treated at the CASSCF
level, with an active space large enough to describe bond breaking and
formation. The simulations indicate that dissociation and isomerization take
place mainly on the cationic ground state and allow the probing of previous
hypotheses concerning the correlation between the photochemical outcome and the
traversed conical intersections. The results, moreover, support the
long-standing view that \ce{H2}-loss may occur from the ethylidene form.
However, the ethylene-ethylidene isomerization time predicted by the
simulations is considerably longer than those previously inferred from indirect
experimental measurements.
