  We report the studies of ultrafast electron nanocrystallography on
size-selected Au nanoparticles (2-20 nm) supported on a molecular interface.
Reversible surface melting, melting, and recrystallization were investigated
with dynamical full-profile radial distribution functions determined with
sub-picosecond and picometer accuracies. In an ultrafast photoinduced melting,
the nanoparticles are driven to a non-equilibrium transformation, characterized
by the initial lattice deformations, nonequilibrium electron-phonon coupling,
and upon melting, the collective bonding and debonding, transforming
nanocrystals into shelled nanoliquids. The displasive structural excitation at
premelting and the coherent transformation with crystal/liquid coexistence
during photomelting differ from the reciprocal behavior of recrystallization,
where a hot lattice forms from liquid and then thermally contracts. The degree
of structural change and the thermodynamics of melting are found to depend on
the size of nanoparticle.
