  Vapour deposition can directly produce ultrastable glasses, which are similar
to conventional glasses aged over thousands of years. The highly mobile surface
layer is believed to accelerate the ageing process of vapour-deposited glasses,
but its microscopic kinetics has not been experimentally observed. Here we
studied the deposition growth kinetics of a two-dimensional colloidal glass at
the single-particle level using video microscopy. We found that newly deposited
particles in the surface layer (depth $d<14$ particles) relaxed via frequent
out-of-cage motions, while particles in the deeper middle layer
($14<d\lesssim100$ particles) relaxed via activation of cooperative
rearrangement regions (CRRs). These CRRs were much larger, more anisotropic and
occurred more frequently than CRRs in the bulk ($d\gtrsim100$ particles) or
after deposition. Their centers of mass moved towards the surface, while the
particles within moved towards the bulk, causing free-volume "bubbles" to move
towards the surface to give a more compact bulk glass. This two-step relaxation
in two surface layers is distinct from the previously assumed relaxation in one
surface mobile layer
