Metallic-to-nonmetallic transition of Na coadsorbed with ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2$O on the ${\mathrm{Cr}}_2$${\mathrm{O}}_3$(111)/Cr(110) surface

Coadsorption of Na with ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2$O on a thin ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$(111) single crystal epitaxially grown on a single-crystal Cr(110) substrate has been performed to study the effects of the coadsorbates on the electronic properties of the Na species. High-resolution electron-energy-loss spectroscopy spectra as a function of Na overlayer coverage reveal a continuous attenuation of the Fuchs-Kliewer surface-phonon loss modes with near total attenuation by ∼2 monolayers (ML). Coadsorption with either ${\mathrm{H}}_2$O or ${\mathrm{CO}}_2$ returns the substrate Fuchs-Kliewer modes to a relative intensity approximately equal to that observed on the clean ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$(111) surface. Angle-resolved photoemission spectroscopy (ARPES) measurements from the ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$(111) surface demonstrate almost total attenuation of the ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$(111) valence-band emission by a Na coverage of ∼2 ML with no return of ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$(111) valence-band emission features upon coadsorption. These observations provide strong evidence that the development of a metallic Na overlayer results in a damping of the coupling interaction between the electron and the Fuchs-Kliewer phonons and that upon reaction with the coadsorbed species, a uniform dielectric overlayer is formed which permits the return of the Fuchs-Kliewer losses. The metallic-to-nonmetallic transition of the overlayer is also supported by the ARPES data which reveal suppression of emission from the Fermi level, the disappearance of the LVV Auger transition, and a shifting to higher binding energy of the Na 2p emission upon compound formation during coadsorption.