  We reproduced the initial sticking probability of O$_{2}$/Al(111) by use of
spin-polarized density functional theory and quantum dynamics calculations. We
found a large activation barrier when the molecule is dissociatively adsorbed
through $abstraction$ on the top site unlike on the hollow and bridge sites.
This barrier is sensitive to the molecular orientation and surface site. The
vibrational motion of the molecule facilitates the abstraction due to the $late
barrier$. Finally, we suggest a possibility on how widely separated oxygen
atoms adsorbed on the surface could be realized.
