  Experiments are conducted to investigate the role of the avoided crossing
seam in the photodissociation of H$_3^+$. Three-dimensional imaging of
dissociation products is used to determine the kinetic energy release and
branching ratio among the fragmentation channels. Vibrational distributions are
measured by dissociative charge transfer of H$_2^+$ products. It is found that
the photodissociation of hot H$_3^+$ in the near ultraviolet produces cold
H$_2^+$, but hot H$_2$. Modelling the wavepacket dynamics along the repulsive
potential energy surface accounts for the repopulation of the ground potential
energy surface. The role of the avoided crossing seam is emphasized and its
importance for the astrophysically relevant charge transfer reactions is
underlined.
