A model system for the study of structure and dynamics of molecules on surfaces: CO on NaCl(100)

The coverage dependence of the infrared absorption of CO adsorbed to NaCl(100)

CO adsorbed to NaCl(100) exhibits perhaps the weakest possible coupling between the adsorbate and solid. It is, therefore, an ideal system to observe the influence of adsorbate-adsorbate interactions on infrared absorption. In this work, we report polarized FTIR absorption spectra of CO/NaCl(100) as a function of coverage (0.02 ≤ θ ≤ 1 ML), where the coverage has been quantitatively determined by temperature-programmed desorption and molecular beam dosing. We extend a previous semi-empirical model designed to describe the screening of the local electric field due to dipole-dipole interactions in a CO monolayer. The extended model applies to sub-monolayer coverages and describes properly the electric field of the absorbed radiation at the vacuum-substrate interface. Fitting this model to coverage-dependent IR absorption data allows us to derive the vibrational and electronic polarizabilities [χ_v = 0.0435(14) ų, χ_e = 3.30(36) ų] and the integrated absorption cross section of 2.51(8) × 10^-17 cm/molecule for an isolated CO molecule adsorbed at the NaCl (100) surface. The determined integrated absorption cross section is substantially smaller than that of gas phase CO.

Linkage isomerism of CO adsorbed on alkali halides

Variable-temperature FTIR spectra of CO adsorbed on NaCl and KBr films are shown to reveal linkage isomerism, that is, formation of C- and O-bonded adsorption complexes with the same cationic sites. Not all of the sites can be involved in that, only the three- or four-coordinated cations located on kinks, edges, or steps of microcrystals. For NaCl, these adsorption complexes account for the high-frequency (HF) C-O stretching bands at 2175 and 2160 cm(-1). Coresponding LF bands of O-bonded CO were found at 2115 and 2124 cm(-1). In the spectrum of KBr, the HF band is poorly resolved, and only one LF band can be clearly seen, near 2124 cm(-1). The value of the isomerization enthalpy for complexes that account for the bands at 2160 and 2124 cm(-1) of CO adsorbed on NaCl, estimated from the experiment, is 4.0 ± 0.2 kJ/mol. Quantum mechanical calculations by DFT methods applied to NaCl model clusters interacting with the CO molecule confirm the existence of two potential wells with adsorption and isomerization energies close to the experimentally measured values, and enable us to estimate the height of the potential barrier between the two adsorption states.

The structure of carbon monoxide adsorbed on the NaCl(100) surface—a combined LEED and DFT-D/vdW-DF study

The structure of the first layer CO adsorbed on NaCl(100) is investigated experimentally by means of quantitative low-energy electron diffraction at 25 K, and theoretically by means of density functional theory. Consistent with earlier helium atom diffraction results, the monolayer structure has p(2×1) symmetry with a glide-plane along the longer axis of the unit cell. The structure analysis confirms the binding of CO via the carbon end to the NaCl(100) surface. The vertical distance of carbon above Na(+) is 2.58 ± 0.08 Å, in good agreement with geometry optimizations based on dispersion-corrected density functional theory, and 0.15 Å lower than predicted in calculations based on the nonlocal van der Waals density functional.