Role of Physisorption States in Molecular Scattering: A Semilocal Density-Functional Theory Study on ${\mathrm{O}}_2/\mathrm{Ag}(111)$

We simulate the scattering of ${\mathrm{O}}_2$ from Ag(111) with classical dynamics simulations performed on a six-dimensional potential energy surface calculated within semilocal density-functional theory. The enigmatic experimental trends that originally required the conjecture of two types of repulsive walls, arising from a physisorption and chemisorption part of the interaction potential, are fully reproduced. Given the inadequate description of the physisorption properties in semilocal density-functional theory, our work casts severe doubts on the prevalent notion to use molecular scattering data as indirect evidence for the existence of such states.

Figure 1

Calculated angular distributions for in-plane ${\mathrm{O}}_2$ scattering off Ag(111) for incidence angles ${\mathrm{\Theta }}_i=40°$ (left) and ${\mathrm{\Theta }}_i=70°$ (right) and various $E_i$. All fluxes are normalized to the peak height obtained for ${\mathrm{\Theta }}_i=40°$ and $E_i=0.4\mathrm{eV}$.

Figure 2

Peak height of the distributions of Fig. 1 as a function of incidence energy $E_i$. The experimental data reproduced from Ref. [23] (left) can be directly compared with the results of our adiabatic (full lines) and GLO (dashed lines) simulations (right). Lines through the data points are guides to the eye.

Figure 3

Upper panels: Distributions of the molecule’s closest approach distance $Z_{\min }$ to the surface for the different incidence conditions. The distribution curve for each ${\mathrm{\Theta }}_i$ and $E_i$ is normalized to the corresponding total number of in-plane scattered molecules. Lower panels: Range of PE values exhibited by the 6D PES $E(X,Y,Z,d,\theta ,\varphi )$ as a function of distance $Z$ to surface (maroon vertical lines) as compared to the PE range that is actually probed by the ${\mathrm{O}}_2$ molecules along their trajectories for $E_i=0.4\mathrm{eV}$ (black vertical lines) and $E_i=1.2\mathrm{eV}$ (green vertical lines); see text. Left panels correspond to ${\mathrm{\Theta }}_i=40°$, right panels to ${\mathrm{\Theta }}_i=70°$. Incidence angles in between (not shown) exhibit a smooth transition between these two limits.

Figure 4

Angularly resolved final-to-initial energy distributions of ${\mathrm{O}}_2$ scattered off Ag(111) under different incidence conditions (${\mathrm{\Theta }}_i$,$E_i$). The experimental data by Raukema et al. [23] (left panels) are compared to the results from the GLO calculations (right panels); see text.