  We analyze the displacements of the particles of a liquid perpendicular to an
attractive confining surface, using extensive molecular dynamics simulations.
Because of the attraction of the surface, the particles are trapped in its
vicinity. This leads to a three-step relaxation of the liquid close to the
surface. The first step is related to the rattling motions of the particles in
the cages formed by their nearest neighbors; the second step is related to
small-scale movements close to the surface, and the third step is associated
with the process of escaping from the surface. The three-step relaxation is
seen through the self-intermediate scattering function or the mean squared
displacement of the particles. Upon reducing temperature, the second and third
relaxation steps seem to be gradually merged. The solid surface induces also a
strong heterogeneity in the mobility of its adjacent particles. This can be
seen from the large peak of the non-Gaussian parameter for the particle
displacements, which concurs with the appearance of the third step in the
relaxation functions. The surface-induced dynamical heterogeneity increases
with decreasing temperature.
