Alkali-Induced Enhancement of Surface Electronic Polarizibility

From results of ab initio electronic structure calculations based on density functional theory for a set of prototype systems, we find alkali adsorbates to cause a dramatic enhancement of the electronic polarizability of the metal surface extending it several angstroms into the vacuum. This phenomenon is traceable to an unusual feature induced in the surface potential on alkali adsorption. The effect appears to be general, as we find it to be present on metals as varied as Pd and Cu, and helps explain the observed substantial decrease in the vibrational frequency of molecules when coadsorbed with alkalis on metal surfaces. Specifically, for two dissimilar molecules CO and ${\mathrm{O}}_2$, we trace the softening of the frequencies of their stretching mode when coadsorbed with K on Pd(111) to the enhanced electronic polarizability.

Figure 1

Top view of the $A_{0.25}/\mathrm{M}(111)$ surface ($A=\mathrm{Na}$, K; $\mathrm{M}=\mathrm{Cu},\mathrm{Pd}$). The large balls represent alkali atoms, small dark balls are for the first layer M atoms and small gray balls are for the rest of the M atoms. The dashed line represents the projection of the plane along which $w^N(r)$ is plotted in Fig. 2.

Figure 2

Plot of $w^N(r)$ for clean Pd(111) (left panel) and for ${\mathrm{K}}_{0.25}/\mathrm{Pd}(111)$ (right panel) along the plane perpendicular to the surface. The Pd1 labels mark the positions of the topmost Pd atoms and K that of potassium atoms. A drastic decrease in $w^N(r)$ is seen for the clean surface [the white area corresponds to $w^N(r)=0$, while adsorption of K induces a huge increase and delocalization of $w^N(r)$ even far from the adsorbate].

Figure 3

Plot of $w^N(r)$ along the surface normal. The upper panel shows $w^N(r)$ for Pd(111) (dashed line) and ${\mathrm{K}}_{0.25}/\mathrm{Pd}(111)$ (solid line). The lower panel shows $w^N(r)$ for Cu(111) (dashed line) and ${\mathrm{Na}}_{0.25}/\mathrm{Cu}(111)$ (solid line).

Figure 4

The self-consistent total potential plotted along the surface normal. The upper panel displays the potential for Pd(111) (dashed line) and ${\mathrm{K}}_{0.25}/\mathrm{Pd}(111)$ (solid line). The lower panel provides the same for Cu(111) (dashed line) and ${\mathrm{Na}}_{0.25}/\mathrm{Cu}(111)$ (solid line). Arrows indicate the heights of the alkali atoms.