  We numerically investigate the impact of Coulomb collisions on the ion
dynamics in high-$Z$, solid density caesium hydride and copper targets,
irradiated by high-intensity ($I\approx2{-}5\times10^{20}{\rm\,Wcm^{-2}}$),
ultrashort (${\sim}10{\rm\,fs}$), circularly polarized laser pulses, using
particle-in-cell simulations. Collisions significantly enhance electron
heating, thereby strongly increasing the speed of a shock wave launched in the
laser-plasma interaction. In the caesium hydride target, collisions between the
two ion species heat the protons to ${\sim}100{-}1000{\rm\,eV}$ temperatures.
However, in contrast to previous work (A.E. Turrell etal., 2015 Nat. Commun. 6,
8905), this process happens in the upstream only, due to nearly total proton
reflection. This difference is ascribed to distinct models used to treat
collisions in dense/cold plasmas. In the case of a copper target, ion
reflection can start as a self-amplifying process, bootstrapping itself.
Afterwards, collisions between the reflected and upstream ions heat these two
populations significantly. When increasing the pulse duration to $60{\rm\,fs}$,
the shock front more clearly decouples from the laser piston, and so can be
studied without direct interference from the laser. The shock wave formed at
early times exhibits properties typical of both hydrodynamic and electrostatic
shocks, including ion reflection. At late times, the shock is seen to evolve
into a hydrodynamic blast wave.
