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 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.