What determines the sticking probability of water molecules on ice?

We present both experimental and theoretical studies of the sticking of water molecules on ice. The sticking probability is unity over a wide range in energy (0.5 eVâ€"1.5 eV) when the molecules are incident along the surface normal, but drops as the angle increases at high incident energy. This is explained in terms of the strong orientational dependence of the interaction of the molecule with the surface and the time required for the reorientation of the molecule. The sticking probability is found to scale with the component of the incident velocity in the plane of the surface, unlike the commonly assumed normal or total energy scaling.