Ab initio calculations of the dispersion of surface phonons of a c(2 x 2) CO overlayer on Ag(001)

    Authors

    M. A. Ortigoza; T. S. Rahman; R. Heid;K. P. Bohnen

    Comments

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    Abbreviated Journal Title

    J. Phys.-Condes. Matter

    Keywords

    HELIUM-ATOM SCATTERING; GENERALIZED GRADIENT APPROXIMATION; MIXED-BASIS; APPROACH; CARBON-MONOXIDE; ADSORBED CO; PHOTOELECTRON-SPECTROSCOPY; LATTICE-DYNAMICS; LINEAR-RESPONSE; METAL-SURFACES; ADSORPTION; Physics, Condensed Matter

    Abstract

    We examine the phonon dispersion of c(2 x 2)-CO on Ag(001) by applying density functional perturbation theory with the generalized-gradient approximation. Our calculations indicate that the c(2 x 2)-CO overlayer on Ag(001) is dynamically stable. We find that the bond length of CO is expanded and its stretch mode (nu(1)) softened by similar to 9 meV upon adsorption on Ag(001), in excellent agreement with experiments. We show that nu(1) at (Gamma) over bar alone cannot gauge the metal-CO interaction since it is not entirely determined by the C-O intramolecular force constant. Further softening of nu(1) on Ag(001) is obtained outside (Gamma) over bar, indicative of CO-CO interactions even at a distance of similar to 4 angstrom. The frequency of the Ag-CO stretch mode (nu 2) is similar to 30 meV and it is nearly dispersionless, implying that the perturbation corresponding to this mode is short-ranged. The frustrated rotation mode of CO (nu(3)) overlaps with the bulk band and mixes with substrate modes inside the SBZ, suggesting this as one of the key features for the enhanced diffusivity of CO on Ag surfaces over that on Cu surfaces. The frustrated translation mode of CO (nu(4)) is everywhere below similar to 2.8 meV and therefore mixes with substrate modes in the region of the SBZ around (Gamma) over bar. Depending on the q vector, vertical and in-plane surface modes may soften or stiffen with respect to their counterpart on clean Ag(001). Although the response of most Ag(001) modes to CO adsorption is similar to that of corresponding Cu(001) modes, there are some contrasting features between the dynamics of the two surfaces concerning the changes in the surface force constants and mixing of S(6) with nu(3) on Ag(001).

    Journal Title

    Journal of Physics-Condensed Matter

    Volume

    22

    Issue/Number

    39

    Publication Date

    1-1-2010

    Document Type

    Article

    Language

    English

    First Page

    15

    WOS Identifier

    WOS:000281958500003

    ISSN

    0953-8984

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