We have studied the trends in CO adsorption on close-packed metal surfaces: Co, Ni, Cu from the 3d row, Ru, Rh, Pd, Ag from the 4d row and Ir, Pt, Au from the 5d row using density functional theory. In particular, we were concerned with the trends in adsorption energy, geometry, vibrational properties and other parameters derived from the electronic structure of the substrate. The influence of specific changes in our set-up, such as choice of the exchange correlation functional, the choice of pseudopotential, size of the basis set and substrate relaxation, has been carefully evaluated. We found that, while the geometrical and vibrational properties of the adsorbate–substrate complex are calculated with high accuracy, the adsorption energies calculated with the gradient-corrected Perdew–Wang exchange–correlation energies are overestimated. In addition, the calculations tend to favour adsorption sites with higher coordination, resulting in the prediction of the wrong adsorption sites for the Rh, Pt and Cu surfaces (hollow instead of top). The revised Perdew–Burke–Erzernhof functional (RPBE) leads to lower (i.e. more realistic) adsorption energies for transition metals, but to the wrong results for noble metals—for Ag and Au, endothermic adsorption is predicted. The site preference remains the same. We discuss trends in relation to the electronic structure of the substrate across the periodic table, summarizing the state-of-the-art of CO adsorption on close-packed metal surfaces.

https://iopscience.iop.org/article/10.1088/0953-8984/16/8/001/pdf

CO adsorption on close-packed transition and noble metal surfaces: trends from ab initio calculations

http://dipc.ehu.es/Echenique/index.htm/admin/documentos/archivos/publicaciones/186SS1999.pdf

Image potential states on metal surfaces: binding energies and wave functions

http://dipc.ehu.es/etxenike/admin/documentos/archivos/publicaciones/257SSR2004.pdf

Decay of electronic excitations at metal surfaces

We have studied the trends in CO adsorption on close-packed metal surfaces: Co, Ni, Cu from the 3d row, Ru, Rh, Pd, Ag from the 4d row and Ir, Pt, Au from the 5d row using density functional theory. In particular, we were concerned with the trends in the adsorption energy, the geometry, the vibrational properties and other parameters derived from the electronic structure of the substrate. The influence of specific changes in our setup such as choice of the exchange correlation functional, the choice of pseudopotential and size of the basis set, substrate relaxation has been carefully evaluated. We found that while the geometrical and vibrational properties of the adsorbate-substrate complex are calculated with high accuracy, the adsorption energies calculated with the gradient-corrected Perdew-Wang exchange-correlation energies are overestimated. In addition, the calculations tend to favour adsorption sites with higher coordination, resulting in the prediction of wrong adsorption sites for the Rh, Pt and Cu surfaces (hollow instead of top). The revised Perdew-Burke-Erzernhof functional (RPBE) leads to lower (i.e. more realistic) adsorption energies for transition metals, but to wrong results for noble metals - for Ag and Au endothermic adsorption is predicted. The site preference remains the same. We discuss trends in relation to the electronic structure of the substrate across the Periodic Table, summarizing the state-of-the-art of CO adsorption on close-packed metal surfaces.

CO adsorption on close-packed transition and noble metal surfaces: Trends from ab-initio calculations

Structural studies of alkali metal adsorption and coadsorption on metal surfaces

Two-photon photoemission spectroscopy was employed for an investigation of the image states on Pd(111) with various coverages of Ag. Image state, from areas with different thicknesses of the Ag layer can be observed simultaneously. This proves the applicability of the concept of a local work function. The decay length of the wave function of the image state on Ag(111) into the metal was determined experimentally to be 7.6\ifmmode\pm\else\textpm\fi{}1.2 \AA{}, and the probability to find the electron in the metal is estimated as 10%.

Image states and local work function for Ag/Pd(111).

Coverage-dependent electronic structure of Na on Cu(111)

High-resolution bichromatic two-photon photoemission spectroscopy was used for an investigation of intrinsic linewidths and, thus, lifetimes of image-potential states on clean and adsorbate-covered metal surfaces. The linewidths of the first image state are 16\ifmmode\pm\else\textpm\fi{}4 and 70\ifmmode\pm\else\textpm\fi{}8 meV for Cu(111) and Ni(100), respectively. Together with the results for Ag(100) and Ni(111) published previously, a systematic comparison with theoretical predictions is presented. The discrepancies between experiment and theory can be understood qualitatively if the actual band structure is taken into account. A high density of unoccupied bulk or surface states offers many channels for the decay of the image state via electron-hole pair production and results in the large linewidth observed on the nickel surfaces. The opposite is found for Cu(111) where the band gap extends below ${\mathit{E}}_{\mathit{F}}$. For oxygen adsorption on Ni(100), an increase of the linewidth is observed, which can be explained by the lateral confinement of the image state and inelastic scattering by the adsorbate atoms.

Lifetime of image-potential states on metal surfaces

Bichromatic two-photon photoemission spectroscopy (Bi2PPES) leads to an increased signal-to-noise ratio compared to conventional two-photon photoemission spectroscopy and therefore allows the observation of the first four image potential states as well as a lineshape analysis with improved accuracy on Ag(100). The n=2 image state with a measured linewidth of 37 meV FWHM is the narrowest unoccupied structure measured on any solid surface so far. The intrinsic linewidths of the first two image states were determined as 21±4 meV and 5±5 meV, respectively, in reasonable agreement with theoretical calculations. Disordered adsorption of oxygen on Ag(100) leads to a linewidth broadening of the first and — to a lesser extent — of the second image potential state. A quantitative analysis of the broadening suggests that the underlying mechanism is lifetime shortening due to scattering of the image state electrons by the adsorbate atoms.

Bichromatic two-photon photoemission spectroscopy of image potential states on Ag(100)

High-resolution photoemission and two-photon photoemission spectroscopy have been used for a comparative study of the electronic structure of a single layer of Na and K on Cu(111), Co(0001), and Fe(110). All systems under investigation exhibit an occupied alkali-induced state near the Fermi energy as well as a series of unoccupied states converging toward the vacuum energy, which are identified as image-potential states. For Na (K) on any of the substrates we find the lowest image state to be located at an energy of 2.04\ifmmode\pm\else\textpm\fi{}0.03 eV (1.62\ifmmode\pm\else\textpm\fi{}0.03 eV) above the Fermi level. From this and from the convergence behavior of the image-state series we conclude that the work function for the perfect single alkali layer is independent of the substrates investigated, and is characteristic for the alkali metal. This is in contrast to results of conventional work-function measurements, but in accordance with the theoretical expectation.

N. Fischer

Papers

Image states and local work function for Ag/Pd(111).

Coverage-dependent electronic structure of Na on Cu(111)

Lifetime of image-potential states on metal surfaces

Bichromatic two-photon photoemission spectroscopy of image potential states on Ag(100)

Image states and the proper work function for a single layer of Na and K on Cu(111), Co(0001), and Fe(110).