Translational Inelasticity of NO and CO in Scattering from Ultrathin Metallic Films of Ag/Au(111)

We investigated translational inelasticity in molecular beam surface scattering of NO and CO from ultrathin metallic films of Ag with atomically defined thicknesses grown on single-crystal Au(111). For both molecules, we observe a gradual decrease of the mean final translational energy for Ag film thicknesses between 0 and 3 ML after which no thickness dependence is seen. For Ag films with thicknesses greater than 3 ML, observations are indistinguishable from those of scattering experiments performed on pure Ag crystal surfaces. The similar behavior of both molecules suggests that translational inelasticity is dominated by the mechanical properties of the surface. Theory predicts a thickness-dependent trend of the phonon spectrum that can qualitatively explain the observed behavior.