  The formation of ultrathin oxide layers on metal surfaces is a
non-thermally-activated process which takes place spontaneously at very low
temperatures within nanoseconds. This paper reports mechanistic details of the
initial oxidation of bare metal surfaces, in particular Al(111) and TiN(001),
as obtained by means of first-principles molecular dynamics modelling within
the Density-Functional Theory. It is shown that the reactions of bare metal
surfaces with O2 molecules take place according to a 'hot-atom' dissociative
mechanism which is triggered by the filling of the sigma-star antibonding
molecular orbital and is characterised by a sudden release of a large amount of
kinetic energy. This released energy provides a driving force for metal/oxygen
place-exchange processes which are responsible for the onset of oxide formation
at virtually 0 K and at oxygen coverages well below 1 monolayer (ML). Further
simulations of the oxidation reactions reveal that a disordered ultrathin oxide
forms on Al(111), whereas a rather ordered structure develops on TiN(001)
following a selective oxidation process which leaves clusters of Ti vacancies
in the TiN lattice underneath the oxide layer.
