  We present ab initio calculations of transport properties of atomic-sized
aluminum contacts in the presence of oxygen. The experimental situation is
modeled by considering a single oxygen atom (O) or one of the molecules O2 and
O3 bridging the gap between electrodes forming ideal, atomically sharp
pyramids. The transport characteristics are computed for these geometries with
increasing distances between the leads, simulating the opening of a break
junction. To facilitate comparison with experiments further, the vibrational
modes of the oxygen connected to the electrodes are studied. It is found that
in the contact regime the change of transport properties due to the presence of
oxygen is strong and should be detectable in experiments. All three types of
oxygen exhibit a comparable behavior in their vibrational frequencies and
conductances, which are well below the conductance of pure aluminum atomic
contacts. The conductance decreases for an increasing number of oxygen atoms.
In the tunneling regime the conductance decays exponentially with distance and
the decay length depends on whether or not oxygen is present in the junction.
This fact may provide a way to identify the presence of a gas molecule in
metallic atomic contacts.
