Strong Anisotropic Interaction Controls Unusual Sticking and Scattering of CO at Ru(0001)

Complete sticking at low incidence energies and broad angular scattering distributions at higher energies are often observed in molecular beam experiments on gas-surface systems which feature a deep chemisorption well and lack early reaction barriers. Although CO binds strongly on Ru(0001), scattering is characterized by rather narrow angular distributions and sticking is incomplete even at low incidence energies. We perform molecular dynamics simulations, accounting for phononic (and electronic) energy loss channels, on a potential energy surface based on first-principles electronic structure calculations that reproduce the molecular beam experiments. We demonstrate that the mentioned unusual behavior is a consequence of a very strong rotational anisotropy in the molecule-surface interaction potential. Beyond the interpretation of scattering phenomena, we also discuss implications of our results for the recently proposed role of a precursor state for the desorption and scattering of CO from ruthenium.

Figure 1

Upper panel: Calculated sticking probabilities at normal incidence (solid lines and circles) compared to the experimental results (dashed lines) of Kneitz, Gemeinhardt, and Steinrück [18] (squares) and Riedmüller et al. [20] (triangles). Lower panel: Initial orientation ${\theta }_i$ of scattered (blue circles) and adsorbed (purple triangles) molecules for $E_i=0.1\mathrm{eV}$ and $T=273\mathrm{K}$. The black line represents the initial distribution of orientations given by $\frac{1}{2}\sin ({\theta }_i)$.

Figure 2

The color map shows the PES with the center of mass of CO placed at the top site as a function of the molecule-surface distance $Z$ and the orientation angle $\theta$. The other degrees of freedom are optimized. The contour lines are separated by 0.1 eV (blue for negative potential values, black for zero, and red for positive values).

Figure 3

Angular distributions of directly scattered molecules for an incidence angle of ${\alpha }_i=60°$ (marked by a vertical black line). Our results (solid line with circles) are compared with experiments of Riedmüller et al. [20] (dashed lines with triangles). Distributions corresponding to trapped molecules are shown with dotted lines. Inset: Inverse square root of the translational energy of the scattered molecules that is proportional to the time of flight (TOF) of the molecule to the detector. The vertical line separates directly scattered molecules (left peak) from trapped molecules (right peak) set at $1/\sqrt{E}=1.2{\mathrm{eV}}^{-1/2}$.

Figure 4

Dependence of the translational energy loss (shown as the ratio between the initial and final translational energy of the molecule) on the scattering angle. Our results are shown with solid lines and symbols and the experimental results of Riedmüller et al. [20] are shown with dashed lines and empty symbols. Only directly scattered molecules are taken into account in the analysis.