Structure, bonding, and anharmonic librational motion of CO on Ir{100}

Abstract

A combined low energy electron diffraction (LEED)—density functional theory (DFT) study of the structure of the Ir{100}-c(2×2)-CO phase provides a comparison of the two techniques for a simple molecular adsorbate. Both studies clearly identify atop adsorption and agree on the key structural parameters: a strong buckling of the first Ir layer, a short Ir–C bond length, and a slight lengthening of the CO bond. The molecule is found to be adsorbed in an upright configuration, although an incomplete treatment of the correlated vibrational motion of the CO molecule across the surface in the LEED analysis results in an apparent tilt of 8° from the surface normal. The DFT study determines a high adsorption energy of 2.65 eV for the c(2×2) phase which can be associated with the relief of the high tensile stress of the metastable Ir{100}-(1×1) phase and can be correlated with the short Ir–C bond. The 0.25 ML p(2×2)-CO phase displays an almost identical local bonding geometry but has a slightly lower adsorption energy of 2.61 eV, indicative of an attractive nearest neighbor interaction in the c(2×2) phase. The potential-energy surface for displacement of the CO molecule away from the atop position is found to display quartic anharmonicity. The resulting vibrational amplitude of 0.19 Å can be associated with a harmonic frequency of 8 meV, in good agreement with previous EELS measurement. The level of agreement between the LEED and DFT determined structures is sufficiently good to demonstrate that the two techniques are capable of converging on very similar structures. Furthermore, this study clearly demonstrates the future role for low-temperature LEED measurements and DFT studies in achieving an understanding of the structure, bonding, and energetics of molecules adsorbed at surfaces.