  The mass (volume and areal densities) versus velocity as well as the size
versus velocity distributions of a shock-induced cloud of particles are
investigated using large scale molecular dynamics (MD) simulations. A generic
3D tin crystal with a sinusoidal free surface roughness (single wavelength) is
set in contact with vacuum and shock-loaded so that it melts directly on shock.
At the reflection of the shock wave onto the perturbations of the free surface,
2D sheets/jets of liquid metal are ejected. The simulations show that the
distributions may be described by an analytical model based on the propagation
of a fragmentation zone, from the tip of the sheets to the free surface, within
which the kinetic energy of the atoms decreases as this zone comes closer to
the free surface on late times. As this kinetic energy drives (i) the
(self-similar) expansion of the zone once it has broken away from the sheet and
(ii) the average size of the particles which result from fragmentation in the
zone, the ejected mass and the average size of the particles progressively
increase in the cloud as fragmentation occurs closer to the free surface.
Though relative to nanometric scales, our model reproduces quantitatively
experimental profiles and may help in their analysis.
