  We report the results of an x-ray scattering study that reveals oxidation
kinetics and formation of a previously unreported crystalline phase of SnO at
the liquid-vapour interface of Sn. Our experiments reveal that the pure liquid
Sn surface does not react with molecular oxygen below an activation pressure of
\~5.0*10-6 Torr. Above that pressure a rough solid Sn oxide grows over the
liquid metal surface. Once the activation pressure has been exceeded the
oxidation proceeds at pressures below the oxidation pressure threshold. The
observed diffraction pattern associated with the surface oxidation does not
match any of the known Sn oxide phases. The data have an explicit signature of
the face-centred cubic structure, however it requires lattice parameters that
are about 9% smaller than those reported for cubic structures of high-pressure
phases of Sn oxides.
  Keywords: X-ray scattering, diffraction, and reflection; Oxidation; Surface
chemical reaction; Surface structure, morphology, roughness, and topography;
Tin; Tin oxides; Liquid surfaces; Polycrystalline thin films
